Great Planes P-51D Mustang 40 Kit - GPMA0175 - Manual (Page 47)

CAUTION (THIS APPLIES TO ALL R/C AIR-
PLANES): If, while flying, you notice any unusual
sounds, such as a low-pitched "buzz", this may be an
indication of control surface "flutter". Because flutter
can quickly destroy components of your airplane, any
time you detect flutter you must immediately cut the
throttle and land the airplane! Check all servo grom-
mets for deterioration (this will indicate which surface
fluttered), and make sure all pushrod linkages are slop-
free. If it fluttered once, it will probably flutter again
under similar circumstances unless you can eliminate
the slop or flexing in the linkages. Here are some
things which can result in flutter: Excessive hinge gap;
Not mounting control horns solidly; Sloppy fit of
clevis pin in horn; Elasticity present in flexible plastic
pushrods; Side-play of pushrod in guide tube caused by
tight bends; Sloppy fit of Z-bend in servo arm; Insuf-
ficient glue used when gluing in the elevator joiner
wire or aileron torque rod; Excessive flexing of ai-
leron, caused by using too soft balsa aileron; Excessive
"play" or "backlash" in servo gears; and Insecure servo
mounting.
Have a ball! But always stay in control and fly in a safe
manner.
GOOD LUCK AND GREAT FLYING!
SEE THE FULL LINE OF GREAT PLANES AIR-
PLANES AT YOUR HOBBY DEALER.
WE HOPE YOU WILL SELECT ANOTHER "GREAT
PLANE" AS YOUR NEXT PROJECT. THANK
YOU!
FLIGHT TRIMMING
... A model is not a static object. Unlike a car, which 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. bal-
anced according to the manufacturer's directions. There's noth-
ing 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 Monokole 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 scaled. 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. Docs the elevator always come back to neutral
after a 180-degree 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 weathervane. 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
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