Flight Trimming


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 the 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 fuz. Make the string into a bridle harness and suspend the entire model inverted (yes, with the wing on!). If the right wing 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.

You 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 insure consistent results. These trim flights must be done in calm weather. Any wind will only make the model weatherweave. 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 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 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 downthrust, 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 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, and 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 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 trims at the middle.

The next maneuver is somewhat trickier than it looks. To verify the C.G., we roll the model up to a 45-degree bank, then take our hands off the controls. The model should go a reasonable distance with the fuz 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 wind. Do several loops and see if the same symptom persist. 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. 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 myriad combination of things we can do just with the ailerons. Each change you make will effect all other variables!

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. 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 that your assistant is observing from another vantage point. Note that the engine torque will effect 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, 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, the rig both ailerons up a few turns on the clevises. Note that drooping the ailerons will tend to cancel and washout you have in the wing. On some models, the lack of washout can lead to some nasty characteristics at low speeds.

The effects noted with the 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, i.e., 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 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 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.

Use the following chart to keep in your flight box.

TRIMMING CHART

Trim Feature Maneuvers Observations Corrections
CONTROL CETERING Fly general circles and random maneuvers. Try for hands off straight And level flight. Readjust linkages so that TX trims are centered.
CONTROL TROWS Random maneuvers. A. Too sensitive, jerky controls.
B. Not sufficient control.
If A, change linkages to Reduce throws.
If B, increase throws.
ENGINE THRUST
LINE
From straight flight
chop throttle quickly.
A. Aircraft continues level
B. plane pitches nose up
C. Nose drops.
If A, trim correct.
If B, decease downthrust.
If C, increase downthrust.
CENTER OF GRAVITY
LONGITUDINAL BALANCE
From lever flight roll to 45-degree bank and neutralize controls. A. Continues in bank for moderate distance.
B. Nose pitches up.
C. Nose drops.
If A, trim is good.
If B, add nose weight.
If C, remove nose weight.
YAW Into wind, do open loops Using only elevator, Repeat tests doing outside loops from inverted entry. A. Wings are level throughout.
B. Yaws to right in both inside and outside Loops.
C. Yaws to left in both Inside and outside loops
D. Yaws right on insides And left on outside loops
E. Yaws left in inside and Right on outside loops.
If A, trim is correct.
If B, add left rudder trim.
If C, add right rudder trim.
If D, add left aileron trim.
If E, add right aileron trim.
LATERAL BALANCE Into wind, do tight inside loops. A. Wings are level and Plane falls to either side Randomly.
B. Falls off to left in loops. Worsens as loops tighten.
C. Falls off to right in Loops. Worsens as loops tighten.
If A, trim is correct.
If B, add weight to right wing tip.
If C, add weight to left wing tip.
AILERON RIGGING With wings level, pull to vertical climb and Neutralize controls. A. Climb continues along same path.
B. Nose tends to go to inside loop.
C. Nose tends to go to outside loop.
If A, trim is correct.
If B, raise both ailerons very slightly.
If C, lower both ailerons very slightly.
  • Engine thrust angle and C.G. interact. Check both.
  • Yaw and lateral balance produce similar symptoms.

Note that fin may be crooked. Right and left references are from the plane’s vantage point.