How to Trim an RC Airplane: CG, Trims and Test Flights Step by Step
Trimming an RC airplane means setting it up so it flies straight and level hands-off at cruise throttle, and then fine-tuning so it behaves the same in every manoeuvre. Start on the ground with the centre of gravity (CG), control surface neutrals and throws, then fly and use the transmitter trims to correct any drift. Finally, move the trim into the linkages mechanically and use a few test manoeuvres to check CG, wing balance and thrust line.
By Kyle HilsonUpdated: October 20264 min read
On the ground
- CG: balance the plane at the point given in the manual (with the battery in place). Start slightly nose-heavy if unsure.
- Neutrals: with trims centred, all control surfaces should line up with the wing and tail.
- Throws: set the manual’s low-rate throws with a ruler or throw meter.
- Straightness: check the wing for twists and the tail for alignment.
- Wing balance: lift the plane by the prop and tail; heavy wingtips should be balanced with a little weight.
First flight trim
Climb to a safe height and fly straight at a comfortable cruise throttle. If the plane climbs, add down trim; if it dives, add up trim. If it rolls, add aileron trim the opposite way; if the nose wanders left or right, trim the rudder. Make small trim changes, a click or two at a time, and let the plane settle between changes. Land when it flies straight and level hands-off. Afterwards, adjust the clevises so the surfaces hold that position with trims back at centre. This keeps the full trim range available for next time.
Fine-tuning test flights
| Test | What you see | What it means |
|---|---|---|
| Dive test: 45° dive, release sticks | Pulls up strongly | Nose-heavy: move CG back slightly |
| Tucks under | Tail-heavy: move CG forward | |
| Holds line or gentle pull-out | CG about right | |
| Vertical climb | Pulls to canopy/belly | Adjust down/up thrust |
| Yaws left/right | Adjust side thrust | |
| Inside loop | Drops a wing at the top | Wing heavy on that side |
| Knife-edge | Rolls or pitches | Mix rudder-to-aileron/elevator |
Do these in calm air at a safe height, and change one thing at a time.
Why trim matters
An untrimmed plane needs constant small stick corrections, which tires you out and hides how the plane really flies. A well-trimmed plane flies smoothly, makes landings easier, and makes manoeuvres look clean. Trim also changes with speed and throttle, so check it at the speeds you fly most. Understanding why each surface does what it does is covered in how an RC plane works.
Stabilised planes
Planes with gyros (SAFE, AS3X, X-Pilot) still need correct mechanical trim. The gyro corrects disturbances but can’t fix a twisted wing or a wrong CG, and a plane trimmed with gyro on may behave differently with it off. Trim in the mode you fly most, and recheck after crashes. Easy trainers are listed in best slow-flying RC planes.
Common mistakes
- Trimming in wind or turbulence.
- Changing several things at once.
- Leaving large trim offsets instead of adjusting linkages.
- Ignoring the CG and trying to fix it with elevator trim.
- Forgetting to recheck after replacing a battery of different weight.
After crashes and changes
Recheck the trim after any crash, repair, new battery or new propeller. Small changes such as a bent hinge or a shifted servo horn can throw the trim off noticeably.
FAQ
Why does my RC plane keep climbing?
Too much up trim, too much power, or a nose-heavy CG with up trim compensating.
Why does it roll to one side?
Aileron trim, a twisted wing or a heavy wingtip.
What is the dive test?
A 45° dive with sticks released to check the CG.
Should I trim with the gyro on?
Trim mechanically first; then check in your usual mode.
More on this topic: the complete RC Airplanes guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
