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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.)
Click on photo to view large image with caption
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 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 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 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.
Click on photo to view large image with caption
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!
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