If It Flies ... - 2010/04

HI, GANG. It’s time to get back to what I
described several months ago as our Walter
Mitty urge to become a test pilot.
Aeromodeling can, if you let it, fully
engage the thrill of being a test pilot: taking
up a radical new prototype for the first
time.
You don’t need the silk scarf and helmet
or the “pocketa-pocketa-queep” of the
engine in the background. All you need to
do is design and create it yourself. Then all
that excitement (and nervousness!) can be
yours.
For many of us, the thrill and
nervousness of test-flying a new airplane is
there even though the designs we fly come
from proven kits. What you do for the first
20 or so flights with your new model is
similar to what most full-scale test pilots
do. They repeatedly check the airplane in a
methodical manner to evaluate whether or
not it could be improved by making a small
change.
A few months ago, the subject was
downthrust and flight-testing you do to
adjust it properly. Before that, I wrote about
the same topic for right thrust and the
“cross-trim” problem.
I spent a fair bit of the last year covering
pitch stability and what goes into fore and
aft balance-point location. Although I went
into detail about the effect that CG
placement has on stability, I did not write
much about flight-testing. I’ll fix that in
these next few months.
Testing and Adjusting a Model’s Roll
Control: I don’t think it’s an exaggeration
that nearly half of the airplanes sitting on
the flightline at the field on a sunny Sunday
afternoon could be substantially improved
by a few adjustments to the ailerons.
Getting solid aileron control at all
flyable airspeeds is not always trivial.
Many years ago, I remember an airplane of
my dad’s that would roll the wrong way
with aileron control if it were near stall, as
in a steep climb after takeoff.
Does anybody out there remember a
design called the “Regulas”? I think it was
an old Royal kit. If you put the nose down
and gained a bit of speed, everything
worked normally. As I remember, one of
our more experienced clubmates fixed the
problem with some tape stretched across
the aileron hinge gap.
Many airplanes have what is called an
“adverse yaw” problem at low speeds and
high angles of attack. This messes up clean
roll control where it counts most: during
takeoff and landing.
Combine this with poor right-thrust
adjustment, and your aircraft turns left over
the pits and spectators on takeoff, even
though you have gobs of right aileron
control cranked in. Sound familiar? I’ll bet!
I have heard about and seen all kinds of
cures for this problem, including packing
up and going home when the wind blows in
How to be a model aircraft test pilot
April 2010 73
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
Also included in this column:
• Understanding roll problems
• Proverse and adverse yaw
• Use aileron differential
correctly
High-Wing Differential
For models 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.
Low-Wing Differential
For models 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.
04sig3.QXD_00MSTRPG.QXD 2/23/10 9:52 AM Page 73
the wrong direction down the runway. But
the two primary causes are inadequate right
thrust and severe adverse yaw, with aileron
application.
What is adverse yaw? Let’s say your
model is climbing steeply, just after takeoff,
and you push right aileron to start a turn.
The left aileron goes down, lifting the left
wing, and the right aileron goes up, dropping
the right wing.
That’s what’s supposed to happen.
Lifting is work, and even though wingtips
aren’t that heavy, to bank to the right 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. It means that the
left wingtip, which is being raised, creates
more drag than the right wing, which is
being lowered. That drag imbalance tries to
yaw the airplane—but in the wrong
direction.
This problem is fundamental. Its cause is
buried in the physics of flight, and we must
deal with it.
There are three ways to take care of this.
The first is piloting technique. We can, and
ought to, do as the full-scale pilots do: use
rudder with aileron all the time. It’s called
coordinated aileron and rudder, and it is a
basic flying skill.
Most RC pilots would do well to develop
the skill of flying coordinated aileron and
rudder, but good airplane setup can alleviate
the worst of the problem immediately.
Clearly asking for coordinated rudder use
would be asking too much of the student
RC pilot, but a dedicated teacher would be
doing his or her students the greatest favor
by teaching it from the beginning.
The second thing we can do is couple
the ailerons into the rudder, using the
control-coupling features found in many,
but not all, radios. This is simply asking the
radio to do, for you, what the full-scale
pilots do. You can do this mechanically too,
though you’ll hardly ever see it.
If you fly Scale, you will probably want
to make sure that your next radio has
control coupling. There are also add-on
control mixers available for a moderate
price, such as the Futaba MSA-10 and the
JR MatchBox.
Full aileron throw typically requires
only approximately one-quarter rudder or
less. I will use the Dutch roll test to check
and adjust the amount of coupling needed.
The third, and most commonly used,
method is aileron differential. Some
coordinated rudder might be necessary
during the steepest climbs, but with most
airplanes it’s fairly easy to find a
differential setting that works well enough
throughout the entire flight.
What is this aileron differential? It’s
simple to describe but requires effort to set
up.
When you move the aileron stick, the
aileron that goes up must travel farther, in
degrees, than the aileron that goes down.
This is true with both left and right aileron
throw. The trick is to offset the linkages in
clever ways to achieve this.
Modern radios also allow for this to be
programmed, provided that you use a
separate servo for each aileron. The add-on
mixers I mentioned could also be used for
this if you have a radio without that feature.
Let’s go flying to see if and how much
adverse yaw I have. I’ll keep adjusting until
the problem goes away, using the Dutch roll
test.
At the Field: Let’s look at the Dutch Roll
method. This test is also a flying skill
builder.
Fly a straight line away from yourself, at
a safe but low altitude. Then smoothly, but
quickly, rock the aileron stick back and
forth, so that the model banks 45° one way
and then the other way. Use as much aileron
throw as you can, while comfortably
keeping up with the airplane. Ideally, the
rhythm will be roughly a half second in one
direction and the same in the other direction.
One of three things will happen.
In the first case, the aircraft will roll back
and forth and the tail will point straight at
you and not wiggle at all. The airplane will
appear to roll as if on a string. This means
that the differential is perfect for level flight.
In the second case we see adverse
yaw. That’s what we expect because of
the physics of flight. The model
“duckwalks,” meaning that as it rolls
right, the tail wiggles right. Then as it
rolls left, the tail wiggles left.
That indicates that the nose is going in
the direction opposite the roll. This means
you need to add differential or more aileroninto-
rudder coupling.
Proverse yaw seldom happens. It’s
where the nose wiggles the same way as the
bank. You’ll see the tail swing out of the
Dutch roll in what looks like the beginning
of a snap roll.
If you are interested in aerobatics, this is
bad; but it is perfectly acceptable for
training. It adds controllability during all
“positive-G” flight.
A moderate amount of proverse yaw
actually helps make a stable model such as
a trainer more controllable. If you want to
get rid of proverse yaw, you must reduce
the differential or the aileron-into-rudder
coupling.
Adverse yaw is worst at low airspeeds
and high angles of attack, such as in a
climb. You’ll want to repeat the Dutch roll
test, in a climb, pointed directly away from
you. Put the model in the steepest climb
angle you normally expect to use. The
corrective actions are the same as with the
level-flight Dutch roll test.
If you fly heavy, slow, or short-tailed
Scale airplanes, you will benefit
tremendously from optimizing their
differential for the takeoff climb out. This
is because good roll authority, especially
to the right, is necessary to help counteract
torque during takeoff and climbout. It
would help to revisit the right-thrust
discussion in the December 2009 “If It
Flies … ” column.
Sometimes the climbing differential test
will indicate that the airplane requires a
great deal of differential. Rather than go
crazy adjusting linkages, consider one of the
following approaches.
1. Learn to move the rudder stick in
unison with the ailerons during takeoff and
landing, when it is needed most.
2. Use coupled aileron into rudder
(CAR).
3. Install two aileron servos, to get more
differential adjustment. Don’t be surprised
if some airplanes need twice as much throw
on the rising aileron as on the dropping one.
Mechanically Adjusting Aileron
Differential: If your radio allows you to
electronically adjust the differential, and
uses separate aileron servos in each wing,
ignore the next couple of 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.
If your aircraft 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 singleservo
low-winger.
It’s that simple. That’s how you put in
differential.
Since this requires a bit of shop time, you
want 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 close to 20% more than the other aileron
goes down. All of these amounts are for
throw angles, in degrees.
A high-wing trainer would like roughly
two-to-one, but the mechanical method
shown in the diagram will get you in the
neighborhood of only one-and-a-half-to-one.
With high-wing strip aileron linkages, it
is best that you use a fitting that does not
move the clevis pin forward of the heavy
wire aileron horn. The plastic part that is
included in most kits moves the clevis pin
more than one-quarter of an inch forward of
the bent wire horn, and that produces
backward differential.
Instead, use an ideal piece of hardware
that both Nelson Hobby Specialties (Rocket
City) and Sonic-Tronics make. These
products place the clevis pin in the middle of
the music-wire aileron horn. It is part Sot115
on the Sonic-Tronics Web site.
The recommendation for how much
differential to put into a trainer may seem to
be a lot, but a full-scale Cessna 150 has a
one-and-a-half-to-one differential; that is,
the up-moving aileron moves 15° while the
other aileron drops 10°. But a 150 still
requires coordinated rudder to make it
respond properly.
If you cannot put more differential into
your model’s linkages but think your highwing
airplane needs more, I have something
for you to try. Most flat-bottom-airfoil
trainers have more lift than they need and
tend to float on landing. If that is so,
lengthen both aileron linkages equally so
that both ailerons are trailed up no more than
5°.
This will reduce adverse yaw a
meaningful amount near neutral, and it will
kill off some of that “float” feeling on
landing. I do not recommend this for highperformance
sport planes.
I’ve run out of space, but we will get
together soon and continue our feet-on-theground
test-pilot school. Until then, have
fun, and do take care of yourself. MA
Sources:
Sonic-Tronics
(888) 721-0128
http://sonictronics.com