How Do RC Plane Flaps Work? Setup, Mixing and Flying Tips
Flaps are hinged panels on the inner trailing edge of the wing that move down together. Lowering them increases the wing’s camber, so the wing makes more lift at a lower speed, and adds drag, so the plane comes down steeper without speeding up. On an RC plane that means slower, shorter landings and take-offs. They are set from a switch or slider on the transmitter, usually with a little elevator mixed in to stop the nose from pitching.
By Kyle HilsonUpdated: October 20264 min read
What flaps do
A wing’s lift depends on speed, angle of attack and the wing’s shape. A lowered flap curves the rear of the wing downward, which raises the lift at a given speed and lowers the stall speed. Large deflections mostly add drag. That’s why flaps are set in stages:
| Flap angle | Main effect | Use |
|---|---|---|
| 5–15° | More lift, little drag | Take-off, slow flying |
| 20–35° | Lift and noticeable drag | Approach |
| 40–60° | Mostly drag | Landing, steep approach |
Typical starting values; the model’s manual has the exact throws.
Flap types on RC planes
- Plain flaps: a hinged panel; the most common on foam and balsa models.
- Split flaps: only the lower surface drops; used on some warbird scale models.
- Slotted and Fowler-style flaps: move back and down, opening a gap that keeps airflow attached; found on larger scale models with mechanical tracks.
- Flaperons: full-span ailerons that also drop together as flaps, set up by radio mixing with two aileron servos.
- Crow (butterfly): on gliders, ailerons go up and flaps go down for strong braking on landing.
Setting up flaps
- Use one servo per flap (or a Y-cable if both are on one channel) with enough torque; flaps take high loads at speed. Servo selection is in how to choose servos for an RC plane.
- Set the neutral so both flaps are level with the wing, then set equal down travel using a throw meter.
- Assign a 3-position switch: up, take-off, landing. Slow the servo speed so flaps move over 1–2 seconds.
- Add a flap-to-elevator mix. Most planes pitch up when flaps drop; start with a little down elevator and adjust in flight.
- Test at height: drop flaps at slow speed, watch for pitch and roll changes, and correct trims.
Flying with flaps
Don’t deploy full flaps at full speed; the load can strip servos or flutter the panel. Lower them below cruise speed, keep some throttle on approach, and raise them in steps after a go-around, because the plane sinks as lift drops. An uneven flap causes a roll: if the plane rolls when you deploy, land and check both throws. Planes fly fine without flaps; they just land faster. The wider basics are in how an RC plane works.
Common flap problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Nose balloons up when flaps drop | No or too little elevator mix | Add down-elevator mix in small steps |
| Plane rolls when flaps drop | Unequal throws or one linkage slipping | Measure both flaps, tighten clevises and horns |
| Flap creeps up in flight | Weak servo or stripped gears | Use a stronger metal-gear servo |
| Flutter or buzzing at speed | Slop in the linkage or hinges | Shorten the pushrod, remove play, check hinges |
| Sudden drop after raising flaps | Flaps raised in one go at low speed | Add throttle first, raise in stages |
Always do a range and control check with the flaps in every position before take-off, and make sure the flap switch isn’t the one your thumb finds by mistake. On models with flaperons, check that aileron response still works with the flaps fully down; some radios reduce the aileron travel in that mix.
FAQ
Can an RC plane fly without flaps?
Yes. Most trainers have none; flaps only slow the landing.
Flaps vs ailerons?
Ailerons move opposite to roll the plane; flaps move down together to add lift and drag.
What are butterfly flaps?
Crow braking: ailerons up, flaps down, used on gliders to land steeply.
How many channels do flaps need?
One extra channel, or two if each flap has its own servo for mixing.
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
