Great Planes Giles G-202 46 Kit - GPMA0238 - Manual (Page 48)

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 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 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.
See the Flight Trimming Chart on Page 49
48