Great Planes Tracer 46 Kit - GPMA0400 - Manual (Page 44)

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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 off the controls. The model should go a
reasonable distance with the fuse at an even keel. If the
nose pitches down, remove some nose weight and the
opposite if the nose pitches up. The trick is to use only the
ailerons to get the model up at a 45° bank. We almost
automatically start feeding in elevator, but that’s a no-no. Do
the bank in both directions, just to make sure that you are
getting an accurate reading of the longitudinal balance.
We now want to test the correct alignment of both sides
of the elevator (even if they aren’t split, like a Pattern ship’s,
they can still be warped or twisted). Yaw and lateral balance
will also come into play here, so be patient and eliminate
the variables, one-by-one. The maneuver is a simple loop,
but it must be entered with the wings perfectly level.
Position the maneuver so that your assistant can observe it
end-on. Always loop into the wind. Do several loops and
see if the same symptom persists. Note if the model loses
heading on the front or back side of the loop. If you lose it
on the way up, it’s probably an aileron problem, while a loss
of heading on the way back down is most likely a rudder
situation.
Note that the Yaw test is the same looping sequences.
Here, however, we are altering rudder and ailerons, instead
of the elevator halves. We must repeat that many airplanes
just will not achieve adequate lateral trim without sealing the
hinge gaps shut. The larger you make the loops (to a point),
the more discernable the errors will be.
The Lateral Balance test has us pulling those loops very
tightly. Pull straight up into a vertical and watch which wing
drops. A true vertical is hard to do, so make sure that your
assistant is observing from another vantage point. Note that
the engine torque will affect the vertical fall off, as will
rudder errors. Even though we balance the wing statically
before leaving for the field, we are now trimming it
dynamically.
The Aileron Coupling (or rigging), is also tested by doing
Hammerheads Stalls. This time, however, we want to
observe the side view of the model. Does the plane want to
tuck under a bit? If so, then try trimming the ailerons down
a small bit, so that they will act as flaps. If the model tends
to want to go over into a loop, then rig both ailerons up a
few turns on the clevises. Note that drooping the ailerons
will tend to cancel any washout you have in the wing. On
some models, the lack of washout can lead to some nasty
characteristics at low speeds.
Again, we reiterate that all of these controls are
interactive. When you change the wing incidence, it will
influence the way the elevator trim is at a given C.G. Re-
trimming the wing will also change the rigging on the
ailerons, in effect and they may have to be readjusted
accordingly.
The whole process isn’t hard. As a matter of fact it’s
rather fun — but very time consuming. It’s amazing what
you will learn about why a plane flies the way it does and
you’ll be a better pilot for it. One thing we almost guarantee,
is that your planes will be more reliable and predictable
when they are properly trimmed out. They will fly more
efficiently and be less prone to doing radical and surprising
things. Your contest scores should improve, too.
We wish to acknowledge the Orlando, Florida, club
newsletter, from which the basics of the chart presented
here were gleaned.
Reprinted in part by Great Planes Model Manufacturing
Company, courtesy of Scale R/C Modeler magazine, Pat
Potega, Editor, August 1983 issue.