A model is not a static object. Unlike a car, which 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 might look as if 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 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 your 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, and 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 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 linkage, 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 gap you normally leave when you install the hinges you could experience a 20 percent increase in aileron control response just by doing 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-degree 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 ensure consistent results. These trim flights must be done in calm weather. Any wind will only make the model weathervane. 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 the plane will be routinely 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 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 added 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 path for at lease a dozen-plane length 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 manoeuver entry can misled 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 trim 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 oddball combination of trim setting. One bump on 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 trickier than it looks. To verify the C.G., we must roll the model up to a 45-dagree 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 to 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 still can 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 the backside of the loop. If you loose 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.

After you get the inside loops going correctly, do the same maneuver to the outside, entering from an inverted position. Before you make too many dramatic changes, glance at the remainder of the chart and note the many combination of things we can do just with ailerons. Each change you make will affect all other variables

Note 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 lateral trim with sealing the aileron 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. Actually, we prefer the Hammerhead as a better test for a heavy wing. Pull straight up into a vertical and watch which wing drops. A true vertical is hard to do, so make sure 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. This time, however, we want to observe the side view of the model. Does the plane want to tuck under a bit? If so, 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.

The affects noted with aileron coupling tests can also be caused by an improperly set wing incidence. The better test for this is knife-edge flight. If the model tends to pull upward (it swings toward a nose up direction) then reduce the wing incidence. If the model tries to go off heading toward the bottom side of the plane then increase the incidence.

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 at a given C.G.

Re shimming the wing will also change the rigging on the ailerons, in effect, and they may have to be readjusted accordingly.