The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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AUCTIONS
requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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AUCTIONS
requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]
Edition: Model Aviation - 2006/09
Page Numbers: 67,68,69,70,72,74,76,78
The solution to keeping the see-saw
balanced at all airspeeds is to have the
weight of the aircraft balanced from side
to side and to make sure both wings gain
and lose lift in exactly the same way as
airspeed changes; that actually takes a
little effort. Following are some possible
causes of airspeed-dependent lift
imbalance.
1) Aileron hinge-line gaps. If air can
Trimming
September 2006 67
by Dean Pappas
Part 3 From the Ground Up
Suspend the model at the front with a string or wire tied to the
crankshaft, and lift the tail with a string under a rudder hinge.
Select a top hinge for a less-sensitive balance and a lower hinge
for greater precision.
IN THE PREVIOUS installment of this
“Trimming From the Ground Up” series I
wrote about improving the ground
handling during takeoff and improving
the controllability of the model in the
critical seconds after liftoff. Right-thrust
and downthrust adjustments figured
prominently.
In this installment I will approach the
largest subject: directional controllability.
I saved the best for last!
In the original list of airplane
personality problems presented in Part 1,
the first two items were devoted to
directional control problems. As with the
pitch discussion we started with two
months ago, there is a balance of trim
forces in roll as well as in yaw. Let’s
address the roll forces.
Roll-Control Balancing Act: There are
fewer actors in this balancing act than in
pitch. There is the wingtip-to-wingtip
weight balance. If the airplane is heavy
on one side, it will tend to roll that way
when in level flight. Because the source
of this force is gravity, it does not change
with airspeed. The other players on the
see-saw are the lift of the left and right
wing panels. (See the Roll See-Saw
diagram.)
Roll See-Saw
When the airplane is balanced from side to side, the CG is in the middle and both
wings lift equally in straight and level flight. When the airplane is imbalanced, one wing
must lift more than the other, making the roll balance airspeed sensitive and adding
asymmetric drag to one side of the airplane compared to the other.
go through the aileron hinge lines, it will.
That represents a loss of lift, and the
leakage is an often unpredictable function
of airspeed, angle of attack, “G” loading,
and aileron-control deflection or trim.
That means the leakage is seldom
balanced from side to side. The leakage
often gets worse at high angles of attack,
such as in a climb. The airplane will turn
to that side.
2) Imperfect airfoils. Tiny differences
Tape Seal
Sealing the aileron hinge line is often done with clear, flexible
tape. Wrap a piece of tape long enough to run from hinge to hinge
around a credit card, sticky side out, and jam it into the underside
of the aileron as far as possible. Trim the loose tape, and voilà!
Photos and drawings by the author
09sig3.QXD 7/25/06 10:39 AM Page 67We have the ideal case; there is no leakage
through the hinge line. Leakage—though
not severe—will occur in normal flight.
As the angle of attack (AOA) increases
during slow flight, the leakage worsens.
Aileron control response suffers.
The worst leakage occurs with high AOA
and a deflected aileron. Notice the sheetof-
air aileron that is pointed in the wrong
direction.
The sealing technique described in the
text can even be used as hinges on
smaller models.
in airfoil shape from side to side
(especially the rounding of the LEs) can
require that the ailerons be trimmed to
counteract. The aileron deflection and
airfoil shape will have different airspeed
characteristics, so the trim will be upset
as the airspeed changes.
3) Wing warps, even subtle ones,
will require the ailerons to be trimmed
to counteract, and these two also vary
with airspeed. The warp usually
maintains its influence at very low
airspeeds better than the aileron
deflection.
4) If the ailerons are trimmed to one
side to counteract a problem caused by
the rudder trim not being centered (or a
crooked fin!), the balance between these
control surfaces will change with
airspeed. We call this condition an
aileron vs. rudder cross-trim.
Let’s cover cross-trim. We typically
trim the ailerons to make the model fly
straight at cruise speed. One of the
hallmarks of a stable aircraft is that the
application of rudder control will yaw
and roll the airplane, in the same
direction.
If the rudder trim is slightly off one
way, the ailerons will have to be
trimmed the other way to make the
model fly in a straight line. We usually
do this trimming at cruise speed. The
balance gets upset at low airspeed (such
as in a climb or glide). The rudder
normally predominates at low airspeed.
Back Into the Workshop! There are a
few things we need to do before we
leave the workshop to make life easier
at the field. As we did in the section on
pitch, we will fiddle around in the shop
for a bit. However, almost all of this
could be done at the field if you don’t
mind wasting daylight on a flying
afternoon.
Let’s cover side-to-side balancing.
First let’s balance those wings. It is
surprising how far off-balance many
airplanes are. The muffler alone can do
that; many are close to a half pound in
weight and maybe 4 or so inches from
the center of the airplane. If there are
one or two heavier sheets of wood in
one wing panel than in the other, the
resulting imbalance can be severe.
When that happens, you have a
difference in the required lift from one
wing to the other. At high speed this
imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
usually how we set the transmitter trims
in our airplanes: in cruise-speed level
flight.
For some of us, cruise speed is at full
throttle. No problem; I like to go fast
too! At landing speed the imbalanced
wing weight doesn’t change, but the
aileron and rudder effectiveness do, so
68 MODEL AVIATION
the model starts to wander off to the
heavy wing.
That’s the why of it; now for the
how. I like to suspend the entire
airplane from the crankshaft and from
one of the rudder hinges. (See the lateral
balance photo.) It is important to
balance the entire airplane—not just the
wing—because of the influence of
things such as the muffler or engine
hanging out one side.
The way I do it is to tie a string to
the bare crankshaft and tie it to a nail in
one of the rafters above a clear area on
the floor. Then I run a piece of string or
thin wire under a rudder hinge,
approximately halfway up the rudder,
and lift the tail by the wire coming out
of both sides.
You can get the most sensitive
measurement of side-to-side balance by
picking the correct hinge. If you start at
the top, a large imbalance will only
cause the model to tilt a bit. As you
move down the balance becomes more
sensitive, and if you pick a hinge that is
too low on the rudder, you won’t be
able to get the airplane to balance at all.
It will just flop over one way or the
other.
Move up one hinge from there and
balance the model by adding weight to
the high wingtip until it balances
properly. Then find a way to keep the
weight from falling off, and you are
finished.
Everything from stick-on lead tire
balancing weights to finishing nails
stuck in the end of the tip-block has
been used. If you feel like patching the
covering job on the wing, feel free to
put the weight inside the wing. It looks
better!
Sealing the Aileron Hinge Line:
Sealing the hinge gaps is a biggie; it
ranks right up there with balancing the
airplane from side to side. Serious
aerobatic types don’t even take the
model out of the workshop before doing
this. (At least they are not supposed to!)
Don’t get the idea that this is a hightech
technique. It is one of the simplest
things in the world to do, and it can fix
all kinds of problems.
There are a couple different ways of
doing this, the first of which is the oldfashioned
method. This is not really a
way to fix the gaps, but rather to
eliminate them. Old-fashioned cloth
hinges and their cousins sewn hinges
don’t have gaps, so all you old-timers
out there were doing it right 40 and 50
years ago—before the hardware
manufacturers made hinging easier for
all of us.
The modern cousin to this hinging
method is sometimes used on park
flyers and small models weighing 4
An iron-on covering hinge. See the text
for assembly instructions.
09sig3.QXD 7/25/06 10:39 AM Page 68For airplanes with the servo mounted to the bottom of the wing, the connection to the servo
should be in front of the center of the wheel and the connection to the aileron horn should
be behind the hinge line, if possible. This produces positive aileron differential.
For airplanes with the servo mounted to the top of the wing, the connection to the
servo should be behind the center of the wheel and the connection to the aileron horn
should be in front of the hinge line.
This shows the non-right angle that produces differential. The angle has its vertex at
the pushrod clevis pin, and the two sides are formed by lines to the center of the hinge
line and to the driving point of the pushrod. If the angle is acute, throw will be greater
on the side away from the horn. If the angle is obtuse, the throw will be greater on the
side with the horn.
September 2006 69
pounds and less. This technique can be
done with tape or iron-on covering.
Short lengths of covering are ironed
together, sticky side to sticky side, with
roughly 1/8 or 1/4 inch of overlap. The
pieces are ironed to the top and bottom
of the fixed surface, in an alternating
fashion, and each piece is fed through
the hinge gap in an “S.”
After a little work with an iron, you
have a gap-free hinge. It’s light, simple,
and economical. I don’t recommend this
for larger models. (See the Iron-On “S”
Hinge drawing.)
Many of us use an iron-on plastic
covering for at least the wings and tail
feathers. Even with trim schemes that
cut across the hinge lines or color
changes from fixed to moving surfaces,
we can do a pretty job with the same
covering material.
To make a seal that does not tighten
and sag when the controls are moved,
we have to make an “S” seal as with the
hinges above. You can even use
different colors in each half of the “S”
bend to match the colors on the top and
bottom of the airplane.
The beauty of the “S” seal is that it
does not tighten and bind the control
surface—even at 3-D control throws.
Clear iron-on covering can also be used
if there are too many color changes near
the hinge line.
For painted models you need to seal
with clear tape. I like to use a pliable
clear-vinyl window-sealing tape. I used
to buy 3M part number 117, but a walk
down the appropriate aisle of the local
home-improvement megastore presented
a variety of brands. This stuff sticks
tenaciously, provided the surface
underneath is clean.
To apply the seal, cut a credit cardsized
piece of 1/32 plywood. Make it just
long enough to reach from hinge to
hinge. Wrap a piece of the tape, stickyside
out, around the card and keep it taut
with your fingers.
With the aileron bent up against the
stop, stuff the edge of the card as deep
into the underside of the hinge line as
you can. Stick the tape to the wing and
aileron by rocking the card, and leave
the free ends. With a sharp knife, cut the
free ends off just inside of the corner of
the beveled edges. (See the two tapeseal
photos.)
Why do we seal the aileron hinge
line? To answer that we have to review
a bit of theory. We don’t need Bernoulli
or any of that fancy stuff; airplanes fly
because the wing pushes down on the air
and the air pushes back up against the
bottom of the wing. The purists out
there are screaming about this
oversimplification. That’s okay.
The high-pressure air on the bottom
wants to leak upward through the
High-Wing Differential
Low-Wing Differential
Horn Angle
09sig3.QXD 7/25/06 10:39 AM Page 69aileron hinge gap. The effect of highpressure
air leaking out from under the
wing, through the gap between the wing
and aileron, is bad. Sometimes it is
really bad. (See the hinge-line leak
drawing.) This leakage causes a loss of
lift and hampers good roll control.
An old friend I lost track of many
years ago had a Piper J-2 Cub. You
could stick your fingers and palm right
through the aileron hinge-line gaps.
The J-2 was slower than molasses in
January and had pitiful aileron response
during a stall. At airspeeds only a few
mph faster than stall speed, the ailerons
worked backward! If overused they
could force the airplane to drop into an
unwanted spin entry. That’s the way the
Cub was designed!
Pilots who trained on this airplane
decades ago were taught to use rudder as
the primary roll control during near-stall
conditions. In those days spin training
was necessary just to get a private pilot’s
license.
Back to the Cub. Yellow duct-tape
seals on the ailerons (they had to be
yellow, didn’t they?) improved the
cruise speed by a whole 4 mph, and the
ailerons worked all the way through the
stall. That is abnormal for any Cub! It
also briefly put the airplane in the
experimental category.
Aileron seals have no bad effects that
I am aware of. They can actually have
good effects such as saving servo power,
preventing flutter, and making the
airplane behave better during takeoff and
landing.
The problem of aileron hinge-line
leakage gets worse when the airspeed is
low and the angle of attack is high, and it
gets even worse when aileron is drooped.
High angles of attack result from pulling
“G”s or from flying slowly. As the angle
of attack increases, the leak worsens.
The leak is further worsened when
you apply aileron control. Picture the left
wing as you roll into a right turn. (See
the drawing.) The depressed aileron
forces the air downward so that the local
air pressure is even greater. The leaking
air squirts out as a “sheet” that
eventually breaks up and joins the
airflow past the wing.
Until it breaks up, that sheet of air
looks like an aileron pointed the wrong
way. It’s not made from wood, but it is
real.
Let’s put this together. Your model is
climbing steeply just after takeoff, and
you push right aileron to start a turn. The
left aileron goes down and the right one
goes up. The sheet of air leaking on the
left wing gets worse, and you have an
airplane with the right aileron going up
and the left aileron going—well, the
wooden aileron goes down, but the
aileron made from a sheet of air goes up
at the same time.
As a result, the left wing has a big
drag brake on it. That doesn’t help when
turning right!
This yaw in the opposite direction of
the desired roll is called adverse yaw,
and it’s bad. Sealing the gaps gets rid of
the leakage problem and reduces (but
not eliminates) adverse yaw. It also
makes the ailerons more powerful, so
you can reduce the aileron throw and
still get the same control effectiveness.
Time to Go Flying Again: In trimming
for good directional control we have two
main goals, the first of which is to trim
the (now sealed) ailerons and rudder so
that the model is not crosstrimmed and
flies straight at all speeds from slow to
fast.
The second goal is to achieve
predictable aileron response at all
speeds—especially slow. The two
critical flight regimes are the steep climb
right after takeoff and the critical lowspeed
turns used to line up with the
runway for landing and to counteract
wind on final approach.
Aileron and Rudder Trim: I shouldpoint out at the start that this topic
overlaps the right-thrust adjustment
discussion. There was no straightforward
way to get a handle on both subjects at
one time, but we will combine the tests
and adjustments at the field.
When an airplane is crosstrimmed it
behaves differently turning left vs.
turning right. Let’s say the model has the
rudder offset to the right. The ailerons
will have to be trimmed left in cruise
flight to fly a straight line. In fact, the
aircraft will be crabbing to the right in
straight flight. The same sort of thing
happens when a car has the rear axle
bolted in crooked.
When this airplane is turned to the left
it will tend to hang its nose “out of the
turn” and may even constantly tend to
roll back to level flight. When turned to
the right, this model will tend to “wind
into the turn” and even try to roll over
into a spiral dive.
You already know the test to detect a
crosstrim: make left and right turns,
always using the same bank angle, and
adjust the rudder trim away from the
direction of turn that winds in. Everytime
you adjust the rudder, go back to
trimming the ailerons for straight and
level flight. As are many other trimming
adjustments, it’s an iterative process and
you’ll have to go back and forth a few
times to get it right.
When you think you have it right, try
a long glide at idle power as a fineadjustment
test. Set up with the airplane
flying straight into the wind, and repeat
the hands-off glide test a few times if
there is any kind of wind out. If the
model wanders off to one side, tweak the
rudder trim to correct and retrim the
ailerons again.
Any difference between this test and
the turn test is generally caused by subtle
wing warps or other assembly issues.
You’ll have to accept any difference that
remains between left and right turns,
although nine out of 10 times the glide
and turn tests agree.
Your aircraft is now really trimmed to
fly straight. Landings can be prettier, and
more effort can be put into that pictureperfect
three-point flare rather than
fighting to keep the model from veering
off the runway.
Rock and Roll—Making the Ailerons
Work Well at All Speeds: Do you
remember the anecdote about the L-19
Bird Dog from Part 2 of this series? That
airplane had a bad adverse yaw problem,
as do many high- and shoulder-wing
models with high-lift airfoils.
During the takeoff climb that turned
left over the pits and spectators, the pilot
had gobs of right aileron control cranked
in but the airplane kept wandering off to
the left. A lack of right thrust might have
been partly to blame, but the aileron
control should have worked well enough
to turn the airplane right. It didn’t, and
the reason was severe adverse yaw with
aileron application.
There’s another scenario. You throttle
back and initiate the turn to your final
approach for landing. As the model lines
up with the runway, you apply opposite
aileron to level off and stop the turn, but
the nose keeps cranking around for just a
heartbeat longer and the ailerons don’t
work immediately.
There is a time lag, and when the
airplane finally responds it wallows as it
rolls. That’s right; it’s adverse yaw. We
have already sealed the aileron hinge
lines, but ...
Adverse Yaw Is Fundamental: Adverse
yaw is not just a problem caused by
aileron hinge gaps; even with perfect
gaps there will be adverse yaw. Again,
the problem gets worse at low speed and
at high angles of attack. Now we need to
look at what is called “aileron
differential.” It’s time to go back to the
theory book.
Let’s say you want your airplane to
roll right to exit a left turn. The right
aileron is raised and the left one is
lowered. The desired result will be to lift
the left wing and lower the rightThe last time I looked, lifting was
work—especially when you’re lifting
furniture. Wingtips aren’t that heavy, but
they do count. So we are asking the left
wing to do more work and the right wing
to do less work. The energy needed to do
this work comes from the creation of
drag.
The force of drag multiplied by the
distance through which it is applied
equals work. This means the wingtip
being raised has more drag than the wing
being lowered. That drag imbalance tries
to yaw the model the wrong way
compared to the desired roll.
How do we fix this? After all, its
cause is buried in the physics and
energetics of flight. It’s not a workshop
problem such as hinge gaps.
Three Ways to Skin This Cat—Piloting
Technique: There are three things we can
do, one of which is to do as the full-scale
pilots do: use rudder with aileron all the
time. It’s called coordinated aileron and
rudder, and it’s a basic flying skill.
In a Piper Cub the pilot needs to apply
the rudder just a little bit before the
ailerons are moved. With a long-winged
sailplane, the rudder-before-aileron lead
may be substantial. That’s how powerful
the adverse yaw can be on an airplane
with a short tail and long wings. That’s
one of the reasons why aerobatic
airplanes these days have long tails and
fuselages that are as long as the wing.
Since those airplanes are required to
roll cleanly over a wide range of
airspeeds, the best way to keep the
aircraft from yawing is to give the fin and
rudder a long moment arm to help keep
things straight. And if the wings are
approximately the same length as the
fuselage, the ailerons can’t apply as much
yawing torque as if the wings were very
long.
Most RC pilots would do well to
develop the skill of flying coordinated
aileron and rudder, but we need to help
ourselves right now. This would clearly be
asking too much of the student RC pilot.
The second thing we can do is couple
the ailerons into the rudder. When you
apply right aileron, right rudder is also
applied. This can be done mechanically
or with a programmable transmitter.
Your radio may or may not have this
feature, although many medium-priced
radios with six channels and more will
do.
If you are a Scale fan, you will
probably want to make sure your next
purchase has this feature. If it is not an
option, aftermarket control mixers are
available for a moderate price.
Typically, full aileron throw only
requires roughly one-quarter rudder or
less. “Roughly” is not a good enough
figure; we need a method to test the
amount of coupling. Give me a few
moments to describe the next plan of
attack, and I will describe the Dutch roll
method.
The third and preferred method is
aileron differential. This is what most of
us will use. Some coordinated rudder
may still be necessary during the steepest
climbs, but a differential setting that is
good for the entire flight profile can
usually be struck.
Aileron differential is easy to describe
but requires a little effort to set up. In
simple terms, when you move the aileron
stick, the aileron that goes up must travel
farther, in degrees, than the one that goes
down. This is true both left and right.
The trick is to do it by offsetting the
linkages in clever ways.
Modern radios also allow for this to
be done with programming, provided you
use an independent servo for each
aileron. I will cover how to adjust aileron
differential later, but for now let’s go
flying to see if and how much adverse
yaw we have. The preferred test method
for airplanes that spend most of their
flight time upright is the …
Dutch Roll Aileron Differential Test
(Also For Coupled Aileron Into Rudder):
Let’s look at the Dutch roll method. This
test is also a bit of a flying exercise (such
as a musician playing scales).
Fly a straight line away from yourself
at a safe but low altitude. Smoothly but
quickly rock the aileron stick back and
forth so the airplane banks 45∞ one way
and then the other way.
You want to use as much aileron
throw as you can while comfortably
keeping up with the airplane. Ideally the
rhythm will be approximately a half
second in one direction and the same
back in the other direction. One of three
things will happen. (Everything comes in
threes!)
1) Axial Rolling. The airplane will roll
back and forth, and the tail will point
straight at you and not wiggle at all. The
airplane will appear to roll on a fixed
axis, as if it were riding on a wire. That
means the differential is perfect for level
flight.
2) Adverse Yaw. This is typical: the
model “duck walks.” By that silly phrase
I mean that as the airplane rolls right, the
tail wiggles right. Then as it rolls left, the
tail wiggles left. That would mean the
nose is going in the direction opposite the
roll—and that’s the wrong way!
This means you need more differential
or more aileron-into-rudder coupling.
3) Proverse Yaw. The nose wiggles
the same way as the bank. You don’t see
it often! You’ll see the tail swing out of
the Dutch roll in what looks like the
beginning of a sudden turn.
This is not great if you are interested
in aerobatics, but it is perfectly
acceptable for training. It adds
controllability during all positive-“G”
flight (upright). A moderate amount of
proverse yaw (opposite of adverse)
actually helps initiate the turn. If you
decide to fix it, do so by reducing the
differential or reducing the aileron-intorudder
coupling.
Let’s Retest in a Climb: As I mentioned,
adverse yaw is worst at low airspeeds,
such as in a climb. You’ll want to repeat
the Dutch roll test, in a climb, pointeddirectly away from you. You should use
the steepest climb angle you normally
expect to use.
The trick to this test is being able to
sight down the tail of the airplane. The
corrective actions are the same as the
level-flight Dutch roll test.
Although this is useful for the student
flier, those of you who fly heavy, slow, or
short-tailed Scale airplanes will benefit
tremendously from optimizing their
differential for the takeoff climbout. That’s
the situation in which so many beautiful
airplanes are lost.
The climbing differential test will often
uncover an adverse-yaw problem that
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AUCTIONS
requires a lot of differential. It may be too
much to practically put into your control
linkages. If so, consider one of several
approaches.
You could learn to move the rudder
stick in unison with the ailerons. You
could use coupled aileron into rudder
(CAR) or you could install two separate
aileron servos to get more differential
adjustment.
This works nicely, but only if your
radio is programmable and has an aileron
differential menu. Don’t be surprised if
some airplanes need twice as much throw
on the rising aileron as on the dropping
one.
Feeling Cranky—How to Mechanically
Adjust Aileron Differential: The
differential crank is an ancient mechanical
device; that means it is deceptively simple
and sophisticated at the same time. The
methods described work with one servo
driving both ailerons or with a separate
servo for each aileron. If you have a radio
that allows you to electronically adjust the
differential and used separate aileron
servos in each wing, you might skip the
next couple paragraphs.
If your airplane has the servo(s) and
control horns on the bottom of the wing,
the proper differential happens if the
aileron horns are behind the hinge line
and/or the connections to the servo wheel
are in front of the center of the wheel. This
is typically the situation on a high-wing
airplane or a two-servo low-winger. (See
the High-Wing Differential drawing.)
On the other hand, if your airplane has
the servo(s) and control horns on top of the
wing, the aileron horns need to be angled
forward and/or the connections to the servo
wheel need to be behind the center of the
wheel. This is usually the situation on a
single-servo low-winger. It’s that simple.
A careful look at the drawings should help
untangle the whole mess. (See the Low-
Wing Differential drawing.)
That’s how you put in differential.
Since it requires a bit of shop time, wewant to leave the workshop with the
differential set to a good guess for
starters. Your typical low-wing sport
model is usually happy when the rising
aileron goes up approximately 20% more
than the other goes down. All these
amounts are for throw angles, in degrees.A high-wing trainer would like
approximately two-to-one, but the
mechanical method shown in the diagram
will only get you close. My
recommendation for trainers, especially
the ones with flat-bottom airfoils, is to
connect to the servo wheel roughly 30∞ in
front of the hold-down screw and to rake
the aileron horns back so that the angle of
the control horn is 90∞.
Let me define the control-horn angle
clearly. If you draw a line from the middle
of the hinge line through the little hole
that the clevis pin goes through, it makes
an angle with the clevis pin at the vertex
with the pushrod. (See the Horn Angle
Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip
aileron horn, this is easier when you use a
fitting that does not move the clevis pin
forward of the heavy wire horn. The
plastic part that is often included in the kit
moves the clevis pin more than 1/4 inch
forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and
Sonic-Tronics make an ideal piece of
hardware. These products place the clevis
pin directly in the middle of the musicwire
aileron horn.
The recommendation for how much
differential to put into a trainer may seem
to be a lot, but a full-scale Cessna 150 has
one-and-a-half-to-one differential; the upmoving
aileron moves 15∞ while the
other one drops 10∞.
Even so, in cruise flight the aileron
response still requires coordinated rudder
to make the airplane respond properly. On
takeoff and in landing trim it definitely
needs aileron-rudder coordination. You
wouldn’t expect a high-wing model to be
much different from a Cessna 150, now
would you?
Remember that if a stable trainer-type
model has inadequate differential, the
aileron response will have an initial lag,
after which the control effectiveness will
still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for
churning butter, but not for flying.
Tidying Up: This concludes my collection
of trim techniques for training and Sunday
flying. I hope I have given you not just a
cookbook method for trimming, but a good
start in understanding the whys and hows
of trimming an airplane.
As it turns out, there is a whole body of
advanced trimming techniques for sport,
Aerobatics, and 3-D flight regimes. We
have a reason to get back together. MA
Dean Pappas
[email protected]