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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.

APPENDIX

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 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 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 the back cover.