Great Planes Spitfire 40 Kit - GPMA0179 - Manual (Page 38)

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 39
38