Great Planes Extra 300S 1.20 ARF Patty Wagstaff - GPMA1305 - Manual (Page 35)

Make a copy of the identification tag shown below and
place it on or inside the model.
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 your 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 ascertain 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 combination of trim settings.
One bump of the transmitter and you have lost everything. The trim
must be repeatable, and the only sure way to do this is to always
start with the transmitter control trims at the middle.
The next maneuver is somewhat more tricky than it looks. To
verify C.G., we roll the model up to a 45° bank, then take our hands
Flight Trimming
APPENDIX
35