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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 high-tech 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 old-fashioned 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 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.)
Click on photo to view large image with caption
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 card-sized piece of 1/32 plywood. Make
it 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 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 tape-seal 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
aileron hinge gap. The effect of high-pressure 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.
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