Author: Albert Glenn

Edition: Model Aviation - 2010/05
Page Numbers: 94,95
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Radio Control Aerobatics

Be an aircraft "Whisperer"

Following is the last installment on trimming by Bryan Hebert, who is a model designer and builder.

You will need an incidence meter and a throw meter. Meter readings are used only as a baseline to let you know where you began and to make adjustments according to what your model requires to fly true.

If we look at an airplane’s surfaces and triangulate the information from the flight and where the surfaces rest, we can decipher deficiencies in the setup. Then we can make improvements from precise, repeatable measurements and by using feedback from maneuvers after we have flown the aircraft.

Knowing the thrustline on the model is crucial. It is the baseline for all measurements and is necessary to be able to duplicate this zero measurement; we will call this the “zero line.” The dynamic between the wing incidence and the CG is the most important adjustment—95% of all trimming.

Initial baseline settings

  • Set the wing incidence 0.5° positive to the thrustline (zero line).
  • Set the motor 0.5°–1.0° negative (downthrust) from the zero line.
  • Set motor right thrust at 0.5°–1.0° right of the fuselage centerline.
  • Set the CG at 25% of the Mean Aerodynamic Chord (MAC).
  • If adjustable, set the stabilizer at 0.0° (it will most likely end up with a slight positive incidence after trimming).

These are required to establish a baseline for the triangulation method of trimming. This method will not work properly with other settings. Avoid picking and choosing which parameters to follow; doing so can make this trimming procedure confusing.

Fixed-gear drag causes at least 1°–2° of up-trim measured from the normal total elevator throw, and this is where most trouble begins in the up-lines. It is also the reason why a rearward CG is so prevalent on many setups. It can be overcome by increasing positive incidence in the wing.

When you increase incidence, you will usually have to move the CG forward. This increases the effective power of all control surfaces (decreasing their sensitivity), therefore generating more lift while flying level. Since a positive incidence adjustment makes the model climb on horizontal lines, this is where we see the effect of the wing adjustment on the airplane’s flight.

Down-trim will correct canopy pull in verticals, up-lines, and down-lines; however, it can be frustrating to get a perfectly pull-free down-line.

Fundamental trimming philosophy

  • The wing is the most powerful and important adjustment; we want the stabilizer and rudder to lift or compensate as little as possible.
  • Trim the attitude and flying direction by adjusting the wing; allow the stabilizer and rudder to steer the wing and fuselage.
  • Triangulate flight observations and surface positions to identify causes and select adjustments that fix multiple issues.

Setup and diagnostic guidelines

  • If you need more than 1.0° of downthrust in the engine to keep your airplane from going to the canopy in the up-lines, you don’t have enough positive incidence in the wing.
  • If you need more than 1.5° right thrust in the engine to keep the airplane from pulling left in an up-line, your elevator halves are off, your wing pins are loose, or you need to move the CG forward.
  • If a stabilizer adjustment affects how the aircraft flies in the up- and down-lines, the CG is too far back.
  • If your model requires a left rudder-to-throttle mix, it is tail-heavy or crooked, or the wing incidence is less than 0.3° positive.
  • If your airplane goes to the belly on a left rudder knife-edge, move the CG forward. If it goes to the belly on a right rudder knife-edge, the same adjustment works.
  • If the aircraft goes to the canopy on left rudder knife-edge, the CG is too far forward. If it goes to the canopy on right rudder knife-edge, the CG is extraordinarily far forward.
  • If you have to use 20% or more exponential in down-elevator, move the CG forward.
  • If you have to use high-rate rudder to do a snap of any kind, the CG is too far back.
  • If you have to do any snap with no elevator, move the CG forward.
  • If you have to do spins with rudder and elevator only, the CG is too far back.
  • If the model continues to rotate when exiting a spin, either the CG is too far forward or the rudder rate is too high; it is also possible the CG is too far rearward. Use information gathered from other indicators to determine the correct adjustment.
  • If the airplane does not rotate up onto the main gear during takeoff, your CG is too far back.
  • If the aircraft requires a larger amount of rudder to rotate to inverted flight and then a larger amount of down-elevator to maintain inverted flight, either the CG is too far forward or the wing incidence is less than the baseline setting.

If your model matches only one of the preceding descriptions, you are not finished; it probably matches others as well. One correction will often fix more than one problem.

The closer you set the wing incidence to 0.0°, the farther back you can run the CG for normal horizontal flying. But a CG at 28%–35% of MAC negatively affects demanding precision aerobatic maneuvers—especially snaps, spins, and 1½ snaps.

Conversely, when you move the CG forward, the model will feel nose-heavy. The only way to move the CG forward in effect is by increasing the incidence in the wing, thereby creating more lift and removing the up-elevator trim that a forward CG induces.

If your model has a fixed wing incidence (no way to adjust it), this method will not work. Wing incidence and CG together trump everything else.

Trimming process — flight checks

  1. Begin with trim passes in level, upright flight. Make sure the model flies horizon to horizon with no need for trim.
  2. Pull to a vertical up-line with hands off and see if the airplane will go straight up with no input.
  3. Fly a straight down-line, hands off, and observe.
  4. Fly left and right rudder knife-edge flights and note what the model does.

Typical initial tendencies and corrections

  • Up-line: may be okay or may pull to the canopy a bit at the end. If it goes to the canopy on up-lines, induce more positive incidence in the wings.
  • Down-line: may have canopy pullout; use down-trim as required.
  • Right rudder knife-edge: often dead on.
  • Left rudder knife-edge: may have a belly tuck if CG is too far aft (not set to 25% MAC).

Be patient and use stick-on weights to get the CG forward if needed. This is crucial.

Trimming sequence and final adjustments

  • Triangulate the information and use an adjustment or two that can fix multiple issues. Because the wing is the most important and powerful part of the airplane, it is the cause of many problems and provides most fixes. Everything else on the aircraft reacts to what the wing requires and can provide clues.
  • Once the aircraft pulls straight on the vertical and the left knife-edge is as close to perfect as you can get, then adjust the stabilizer to match the elevator trims. Adjust stabilizer incidence until the elevator trim is centered. This will require repeated adjustment and flying.

Now you have all the information needed to perfectly trim your airplane, but it takes time and patience.

Thanks, Bryan! All you precision-minded pilots out there can expect more in this column from those we call airplane "Whisperers."

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.