Part of the RC Airplane Flying Skills Center
A well-trimmed RC airplane is easier to fly because it is not asking you to correct the same problem every second. With the sticks near center, it should hold a predictable attitude long enough for you to think about the next turn instead of fighting the airplane.
Good trimming starts with a straight airframe, a sensible center of gravity, and mechanically neutral controls. The trim buttons are the last part of the job. They are not a way to hide a warped wing, loose linkage, or bad CG.
Start with the center of gravity
Do not try to trim around a bad CG. A nose-heavy airplane may need a lot of up elevator, feel reluctant to flare, and lose more altitude in turns. A tail-heavy airplane can feel overly sensitive and become less stable as it slows.
Begin at the manufacturer’s recommended location. Check the airplane with the flight battery, fuel system, retract equipment, and anything else installed exactly as it will fly. Use our center-of-gravity guide and nose-heavy versus tail-heavy guide when the symptoms are unclear.
If moving the trim changes the symptom but never makes the airplane feel settled, stop adding trim and check the CG again.
Make the mechanics right before opening the radio menu
- Center the trims and subtrims in the transmitter.
- Power the receiver so every servo moves to its commanded center.
- Install the servo arms as close to 90 degrees as practical.
- Adjust the pushrod or clevis length until each surface is neutral.
- Check that the hinges move freely and the linkages have little unwanted play.
- Secure every clevis and connector so it cannot open in flight.
Use a small amount of subtrim to finish centering a servo arm. If the electronic offset is large, move the arm or adjust the linkage. Starting near mechanical center preserves useful travel in both directions and keeps the geometry more even.
The servo guide explains arm position, control-horn holes, linkage geometry, and installation in more detail.
Look for a problem that trim cannot fix
A rebuilt F-16 once passed its control check, launched cleanly, and then would not climb. The airplane had a little apparent down trim, but it also lost an elevator servo because a connector inside the fuselage had worked loose during the launch. Adding more up trim would not have repaired the intermittent connection.
If a control response changes suddenly, becomes intermittent, or needs an amount of trim that makes no sense, land. Inspect the servo, extension, junction block, linkage, and surface before blaming the trim setting.
Check the surface, direction, and measured travel
- Aileron stick right: the right aileron goes up and the left goes down.
- Elevator stick back: the elevator trailing edge goes up.
- Rudder stick right: the rudder trailing edge moves right.
- Throttle forward: the motor or engine increases power.
Watch the control surface itself. A servo can move while the surface moves in the wrong direction. Repeat the complete check after changing a channel direction, mix, receiver orientation, or flight mode.
Set the throws by measuring the physical movement where the manual specifies. A transmitter percentage does not tell you the actual surface travel because servo arms, control horns, linkage holes, and radio systems all change the result.
Move every control to full travel and watch the linkage. A servo should not buzz, stall, flex the structure, or drive the surface into a mechanical stop. Check combined stick positions and every flight mode because mixes can command more travel than a single input. Use the servo-buzzing diagnostic if a servo complains at the endpoint.
Rates and expo change feel; they do not repair setup
Rates change the maximum available control movement. Expo changes how quickly the airplane responds around stick center while preserving the endpoint. Start with the manufacturer’s recommended settings.
High rates are useful when a maneuver needs more travel, but they can make an ordinary flight tiring. Expo can soften the center without removing the full movement. It should not be used to cover an aft CG or excessive mechanical throw.
Know which switches select the rates and modes before takeoff. Move them on the ground and watch the surfaces. Label the transmitter or standardize the switches across models when that helps you avoid reaching for the wrong control during flight.
Our rates and expo guide explains how to measure travel and build useful flight settings.
Trim high enough to make a mistake
Choose a calm day, climb to a safe altitude, and establish normal cruise power. Let the airplane settle before touching the trim buttons.
- Trim aileron for wings-level flight.
- Trim elevator for the desired cruise attitude and speed.
- Trim rudder only after checking that a warp, misaligned tail, or lateral-balance problem is not causing the turn.
- Make one small change, release the stick pressure, and fly long enough to judge it.
Do not trim during a steep climb, tight turn, gust, or landing approach. If a helper is available, let that person operate the trim buttons while you keep your eyes on the airplane.
Tom once made what he called a low trim pass at about 20 feet with an unfamiliar airplane. It got slow, dropped a wing, and spun before there was room to recover. The cause may have included a wing warp or incorrect washout, but the low altitude removed every option.
A trim pass is still a test flight. Do it high, at a normal speed, and away from the runway until you know how the airplane behaves near a stall.
Read what the airplane is telling you
- Constant aileron trim: inspect lateral balance, wing alignment, warps, aileron neutral, and thrust alignment.
- Large elevator trim: recheck CG, stabilizer alignment, elevator neutral, and thrust angle.
- Pitch change with throttle: inspect the thrust line and setup before adding a mix.
- Very sensitive pitch: check CG and elevator throw before increasing expo.
- Sluggish pitch: check for an excessively forward CG and inadequate elevator travel.
- Trim changes during the flight: land and look for a loose linkage, moving battery, failing servo, weak structure, or electronic problem.
We have a used UMX Ultrix that can change elevator trim while it is flying. Both elevator servos move, which points away from one simple mechanical adjustment and toward a problem in the control system. We can keep correcting it because we know the airplane, but that is not a properly trimmed model. A changing neutral point is a fault to diagnose, not a reason to become faster with the trim buttons.
The Troubleshooting Center has focused procedures for constant roll, pitch sensitivity, sluggish pitch, and pitch change with throttle.
Put large trim corrections back into the linkage
After landing, note the trim positions. If a surface needed more than a small change, adjust the pushrod or clevis so the surface holds the corrected position with the transmitter trim returned near center.
Then verify control direction, endpoints, and stabilization again. A mechanical change can affect travel and a receiver may require its setup to be captured again.
Do not automatically remove every click of trim after one flight. A small offset is normal. Correct the mechanics when the offset is large enough to reduce useful travel, make the setup confusing, or point to an alignment problem.
Treat pilot direction and gyro direction as separate checks
The pilot’s command must move each surface correctly. The stabilization system must also move it in the direction that opposes unwanted airplane movement. One can be correct while the other is wrong.
Follow the receiver manufacturer’s instructions for trim, subtrim, flight modes, orientation, and any setup-capture procedure. Do not assume a gyro will hide poor trim. It may hold the airplane against the error until the mode changes or the gain reaches its limit.
If the model begins to oscillate, reduce gain or disable stabilization if it is safe, slow down, and land. The stabilization guide covers the ground checks and flight modes.
Final trimming checklist
- The CG is within the recommended range.
- The wing, fuselage, and tail are aligned.
- Servo arms and controls begin mechanically neutral.
- Pilot commands and stabilization corrections move in the correct directions.
- Throws are measured at the surface.
- No linkage binds at any stick or flight-mode position.
- The airplane is trimmed high enough, at normal cruise power.
- Sudden or changing trim is treated as a fault.
- Large offsets are corrected mechanically after landing.
A well-trimmed airplane should feel uneventful. Set the mechanics first, make small changes, and listen when the airplane asks for more trim than it should.