Great Planes Extra 300S 1.60 ARF - GPMA1365 - Manual (Page 26)

APPENDIX
FLIGHT TRIMMING
Note: The following article has been reprinted in part for
future reference and also as a guide for your flight instructor
or experienced flying partner to help you with trimming your
model. If further information is required, please contact your
local hobby dealer, local flying club or call Great Planes at
(217) 398-8970
A model is not a static object. Unlike a car, which you can
only hunt left or right on the road (technically, a car does
yaw in corners, and pitches when the brakes are applied), a
plane moves through that fluid we call air in all directions
simultaneously. The plane may look like it’s going forward,
but it could also be yawing slightly, slipping a little and
simultaneously climbing or diving a bit! The controls interact.
Yaw can be a rudder problem, a lateral balance problem or
an aileron rigging problem. We must make many flights, with
minor changes between each, to isolate and finally correct
the problem.
The chart accompanying this article is intended to serve as
a handy field reference when trimming your model.
Laminate it in plastic and keep it in you flight box. You just
might have need to consult it at the next contest! The chart
is somewhat self-explanatory, but we will briefly run through
the salient points.
First, we are assuming that the model has been C.G.
balanced according to the manufacturer’s directions. There’s
nothing sacred about that spot — frankly, it only reflects the
balance point where a prototype model handled the way the
guy who designed it thought it should. If your model’s wing
has a degree more or less of incidence, then the whole
balance formula is incorrect for you. But, it’s a good ballpark
place to start.
The second assumption is that the model has been
balanced laterally. Wrap a strong string or monofilament
around the prop shaft behind the spinner, then tie the other
end to the tail wheel or to a screw driven into the bottom of
the aft fuse. Make the string into a bridle harness and
suspend the entire model inverted (yes, with the wing on!).
If the right wing always drops, sink some screws or lead into
the left wing tip, etc. You may be surprised to find out how
much lead is needed.
At this point the model is statically trimmed. It’s only a
starting point, so don’t be surprised if you wind up changing
it all. One other critical feature is that the ailerons must have
their hinge gap sealed. If shoving some Scotch tape or
Monokote into the hinge gap to prevent the air from slipping
from the top of the wing to the bottom, and vice-versa,
bothers you, then don’t do it.
To achieve the maximum lateral trim on the model, the hinge
gap on the ailerons should be sealed. The easiest way to do
this is to disconnect the aileron linkages, and fold the
ailerons as far over the top of the wing as possible
(assuming they are top or center hinged). Apply a strip of
clear tape along the joint line. When the aileron is returned
to neutral, the tape will be invisible, and the gap will be
effectively sealed. Depending on how big the ailerons are,
and how large a gaping gap you normally leave when you
install hinges, you could experience a 20 percent increase
in aileron control response just by this simple measure.
Your first flights should be to as certain control centering
and control feel. Does the elevator always come back to
neutral after a 180° turn or Split-S? Do the ailerons tend to
hunt a little after a rolling maneuver? Put the plane through
its paces. Control centering is either a mechanical thing
(binding servos, stiff linkages, etc.), an electronic thing (bad
servo resolution or dead band in the radio system), or C.G.
(aft Center of Gravity will make the plane wander a bit). The
last possibility will be obvious, but don’t continue the testing
until you have isolated the problem and corrected it.
Let’s get down to the task of trimming the model. Use the
tachometer every time you start the engine, to insure consistent
results. These trim flights must be done in calm weather. Any
wind will only make the model weather vane. Each “maneuver”
on the list assumes that you will enter it dead straight-and-level.
The wings must be perfectly flat, or else the maneuver will not
be correct and you’ll get a wrong interpretation. That’s where
your observer comes in. Instruct him to be especially watchful of
the wings as you enter the maneuvers.
Do all maneuvers at full throttle. The only deviation from this
is if the plane will routinely be flown through maneuvers at a
different power setting.
Let’s commence with the “engine thrust angle” on the chart.
Note that the observations you make can also be caused by
the C.G., so be prepared to change both to see which gives
the desired result. Set up a straight-and-level pass. The
model should be almost hands-off. Without touching any
other control on the transmitter, suddenly chop the throttle.
Did the nose drop? When you add power again, did the
nose pitch up a bit? If so, you need some down thrust, or
nose weight. When the thrust is correct, the model should
continue along the same flight path for at least a dozen
plane lengths before gravity starts to naturally bring it down.
Do each maneuver several times, to make sure that you are
getting a proper diagnosis. Often, a gust, an accidental nudge
on the controls, or just a poor maneuver entry can mislead
you. The thrust adjustments are a real pain to make. On most
models, it means taking the engine out, adding shims, then
reassembling the whole thing. Don’t take shortcuts.
Don’t try to proceed with the other adjustments until you
have the thrust line and/or C.G. correct. They are the basis
upon which all other trim settings are made.
Also, while you have landed, take the time to crank the
clevises until the transmitter trims are at neutral. Don’t leave
the airplane so that the transmitter has some odd-ball
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