If It Flies - 2011/06

HI, GANG! The literature is full of the tales
of great deeds and glorious victories won by
noble heroes wielding mighty swords. Let’s
face it; this is the sort of stuff makes for
good storytelling!
Those swords were often described in
great detail, with their origins often
shrouded in mystery—sometimes described
as emblems of a craftsman’s devotion to the
craft itself, made to honor ancestors, or even
attributed to deity. Never would the legends
describe these weapons as clumsy in the
hand, crooked, or ill balanced.
Now, that wouldn’t do at all, would it?
Of course not ... It doesn’t work with
swords, real or legendary, and it doesn’t
work with airplanes either.
We aeromodelers sharpen our swords in
two primary ways: with care in the
workshop and with systematic adjustments
or trimming on the flying field. That is what
I’d like to talk about. This isn’t necessarily
difficult or time-consuming stuff; mostly it
consists of little tweaks, the sorts of things
you could do between flights at the field
while watching and helping your clubmates.
Come to think of it, this can be one of
those conversation starters in the pits that
help us all brush the mundane world aside
and focus on having fun with our flying
buddies.
There are five
adjustments—or
misadjustments—
that I see most
often at the flying
field. These are not
typically
showstoppers in
the crash-causing
sense. Instead,
they make an
airplane a little
harder to fly than it
ought to be. That
makes it more
difficult to look
good while doing
your thing—
whatever that is.
Imagine going
into battle, not
with comic strip
hero Prince
Valiant’s Singing
Sword, but instead
with a misbalanced
Stinging Sword that
hurts the hands with
every blow, like a
hit with the end of
the baseball bat.
Two of the Big Five model misadjustments
June 2011 71
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
The figure eight, or “S” seal. does not
tighten at either extreme of throw and
can be used at 3-D throw angles. See
text for details.
Flexible clear vinyl window-sealing tape
makes an excellent seal on painted
surfaces. Applied in 3-inch strips with a
thin credit card-sized piece of 1/32
plywood, the seal can be pushed to the
middle of the gap so that it does not
tighten in one direction.
Three examples of symmetrical airfoils
and airflow. Top: Clean airflow at
neutral elevator is desired for level flight.
Middle: When up-elevator is applied,
airflow on top changes direction and
bottom airflow bends to reattach to it.
This produces maximum control power,
but turbulence can compromise the
result. Bottom: Hinge line gaps allow
high-pressure air to leak from one side
of the surface area to the other. This
weakens airflow on top of the elevator,
partially destroying the bottom airflow’s
ability to rejoin it. The result is poor
elevator response during landing flare.
Top: Sewn hinges are traditional,
inexpensive, and nearly gap-free. Use
heavy Dacron thread (intended for 1/2A
CL) and CA or cellulose-based glue to
prevent loosening. Bottom: Blending
two techniques achieves an inexpensive
and gap-free hinge, which is useful on
smaller sport models. Use a healthy
overlap, such as 3/8 inch, between the
two pieces of iron-on covering and
check the integrity of your ironing!
Lateral balance is necessary for airplanes to fly straight at all
airspeeds, from high-speed passes to landings. The tail is
supported with a “V” of string passed under the middle rudder
hinge and tied to the author’s garage door tracks. The nose is
supported by the crankshaft after rotating so that it turns freely.
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Okay, maybe badly balanced airplanes are
not the same as badly balanced swords, but
both are harder to use than they ought to be.
The big five are:
• Control surface hinge-line gaps
• Heavy wingtip or lateral misbalance
• Balance point, usually referred to as CG
• Engine thrust—both downthrust and right
thrust
• Aileron differential and adverse yaw
We won’t get through all five this month,
but let’s get started.
Hinge Gaps: Air is tricky stuff that resists
all too many of our efforts to tame it; just
ask a race car aerodynamicist. Let’s look at
how a control surface does its thing.
The point of a control surface such as an
aileron, elevator, or rudder is to change the
direction of the flow off of the TE and, in so
doing, change the camber of the flying
surface. That creates lift in the desired
direction, and the force (more properly,
“torque”) needed to deflect the control
surface is called “control effort.”
At the very least, hinge gaps waste
control effort by “leaking” high-pressure air
from one side of the flying surface to the
other. This leakage reduces the control force
that a deflected control surface produces and
has other more undesirable effects.
These ugly side effects become worse at
low speed and large control deflections,
which often means that takeoff and landing
are the most affected flight regimes. That’s
when solid controllability is most important,
right? In particular, I’ll focus on the effects
on pitch and roll control.
Have you ever had an airplane that
landed nicely if you kept the speed up a bit,
but the elevator didn’t have enough control
authority to make the model flare out if you
slowed too much? Loss of elevator power on
landing is a common problem.
I’m not talking about stalling the
airplane; what I mean is that the elevator
effectiveness peters out while the wing still
has plenty of lift, forcing you to land faster
than you would like. There is almost nothing
as pretty as a nose-high full-stall landing,
and that requires ample elevator power
while flying close to the stall speed.
Sometimes this problem is caused by
design (very short tail moment), and
sometimes it’s caused by nose-heaviness.
Short tail moments, as found on many Scale
designs, can make an airplane susceptible to
this problem, and even slight nose-heaviness
will make things much worse.
In either case, the additional loss of
control power caused by hinge-line leakage
will happen at the worst time, and that is
when the control surface deflected the most
and airspeed is the least.
Leakage is at its worst at precisely that
moment. Let’s see why. It might be helpful
to look at the accompanying diagrams,
illustrating leakage that can occur at the
hinge line.
The progression shows a symmetrically
airfoiled stabilizer and elevator in level
flight with no deflection. All is well.
Next is an image showing the elevator
sharply deflected upward for a landing flare.
The abrupt change in airflow direction, on
both the top and bottom sides of the airfoil,
results in a large pressure difference
between the top and bottom of the hinge
line.
The high pressure on top, as shown,
would leak through, given a chance. That
chance would be a gap between the elevator
and stabilizer.
The result is a flat sheet of air that squirts
through the gap and distorts the air as it
flows around the outside of the hinge line.
This reduces the effectiveness of the
elevator and creates extra drag.
We need to seal the hinge gaps. This
includes not only the elevator, but also the
ailerons. Actually, aileron hinge line gaps
can be even more of a problem when it
comes to messing up a model’s friendly
flying characteristics. There is nothing fancy
about this, and it’s simple to do, and it often
fixes several problems.
There are several ways to seal hinge
gaps. Indoor models using Blenderm tape
for hinges are already done! Proving that
nothing is truly new, airplanes using the
traditional figure-eight sewn or alternating
“S”-type cloth hinges are already in the clear.
Many of us use iron-on plastic covering
for at least the wings and tail feathers. Even
with trim schemes that cut across hinge
lines, we can do a pretty job using the same
72 MODEL AVIATION
06sig3.QXD_00MSTRPG.QXD 4/20/11 12:34 PM Page 72
covering material.
To make a seal that does not tighten and
sag when the controls are moved, we have to
make an “S” seal. Where the colors do not
change, cut two 1/2-inch-wide strips that are
long enough to cover all of the hinge lines.
Carefully join those strips, glue side to glue
side, with a 1/8-inch overlap. Cut to lengths
that fit between the hinges, and iron them
down.
The beauty of the “S” is that it does not
tighten and bind the control surface, even at
3-D control throws. You can use clear, ironon
covering if there are too many color
changes near the hinge line.
For small models, with wing areas of less
than 500 square inches or so, consider
cutting the “S” strip into 1/2 or 3/4-inch
lengths. Hinge the entire surface with them,
alternating the direction.
This method is light, simple, and
economical. This method is light, simple,
and economical. I wouldn’t do it with
anything intended to fly very fast, but it
works great for many models.
For painted models or ARFs covered
with low-temperature film, it is probably
best to seal with clear tape. I like to use
pliable clear vinyl window sealing tape.
I used to buy 3M part number 117 (and
still have a several years’ supply), but a
walk through the local home-improvement
store will yield the equivalent. The 117
sticks tenaciously, provided that the surface
underneath is clean.
To apply the seal, cut a credit card-sized
piece of 1/32 plywood that is just long
enough to reach from hinge to hinge. Wrap a
piece of the tape, sticky-side out, around the
card, and keep it taut with your fingers.
With the control surface bent against the
upper stop, stuff the edge of the card as deep
into the underside of the hinge line as you
can. Stick the tape to both the fixed and
movable surfaces by rocking the card, and
leave the free ends.
Using either a vorpal sword (snickersnack!)
or a sharp modeling knife, cut the
loose edges free just inside the hinge line
bevel.
High-tech composite airplanes seldom
need seals on the hinge lines, but some of
them whistle as the controls are deflected,
telling us that the air is still looking for a
way through!
Ailerons need the same treatment,
because uneven air leakage from side to side
will produce an aircraft that changes roll
trim with changes in speed, or G loading.
We want the model to stay in trim whether
it’s flying fast or slow, level, or
maneuvering.
An old contest flying buddy who passed
away a few years ago had a full-scale Piper
J-2 that was slower than a turtle (but oh so
fun to fly!). It also had terrible aileron
response during the stall. That’s how it was
designed!
Pilots who trained on this airplane years
ago were taught to use rudder as the primary
roll control in near-stall conditions. Come to
think of it, it needed lots of rudder input,
coordinated with the ailerons, just to roll
into and out of turns normally.
Some airplanes that were designed in the
same era would enter an opposite-direction
spin if the ailerons were used during the
stall. Spin training was necessary, back then,
to get a full-scale private pilot’s license, but
that is no longer the case.
Cub-yellow duct-tape seals increased the
cruise speed by nearly 4 mph, making that
racing turtle nervous, and suddenly the
ailerons worked all the way through the
stall. It also made the airplane behave better
during takeoff and landing, and, strictly
speaking, was illegal without reregistering
the airplane in the Experimental class.
Lateral Balance: Now that we have sealed
the aileron hinge line, the wings will lift
equally, provided there are no warps or
twists. The next thing we need to do is
balance the model from wingtip to wingtip
so that we are asking for equal lift from both
sides. This way, balance between lift and
weight on both sides of the airplane is
maintained throughout the range of flying
speeds.
Let’s balance those wings. This prevents
an aircraft from consistently wandering off
to one side on landing, even after the
ailerons are carefully trimmed for level
cruise flight.
It is surprising how far off-balance many
models are. The muffler alone could
amount to nearly 1/2 pound in weight
located maybe 4 inches from the center of
the airplane.
Minor differences in the weight of
building materials in one wing panel can
result in a severe imbalance. At high speed
that imbalance can easily be counteracted
with a tiny bit of aileron trim. That’s
typically how we set the transmitter trim: in
cruise-speed level flight.
At landing speed, the imbalanced wing
weight doesn’t change, but the aileron
effectiveness does, so the model starts to
wander off toward the heavy wing.
I like to balance the airplane by
suspending it from the crankshaft and from
one of the rudder hinges. It is important to
balance the whole airplane—not just the
wing—because of the influence of things
such as the muffler or engine hanging out
one side.
I tie a string to the bare crankshaft and
tie it to a nail in one of the rafters above the
worktable. Then I run a piece of string, or
thin wire, under a rudder hinge, roughly
halfway up the rudder, and lift the tail by
both ends of the wire. You can get the most
sensitive measurement of side-to-side
balance by picking the correct hinge. The
lower pickup point gives a more sensitive
balance check.
Balance the airplane by adding weight
to the high wingtip until it is level, and
then find a way to keep the weight from
falling off. Everything from stick-on lead
tire-balancing weights to finishing nails
stuck in the end of the tip-block have been
used.
If you want to patch a shrink-oncovering
job, feel free to put the weight
inside the wing. It looks better too!
Your landings will be prettier, and you
can put more effort into that perfect threepoint
flare (for which you now have enough
elevator control authority) rather than
fighting to keep the aircraft from veering
off of the runway.
I’m out of room, for now, but next time we
get together, we’ll spend more time
sharpening our swords. See you next time …
Until then, have fun and do take care of
yourself. MA
74 MODEL AVIATION
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