35
APPENDIX
FLIGHT TRIMMING
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 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 as certain 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 ever ything. 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
°
degree 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 lose 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 r igging), 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 36.
