A control surface is a hinged or movable panel on a wing, tail, or body that the pilot or flight controller deflects to change the aircraft’s attitude. Deflecting a control surface alters the local camber and angle of attack, changing the lift distribution on that part of the airframe. The resulting asymmetric force creates a moment (torque) that rotates the aircraft around one of its three axes.
The conventional fixed-wing control surfaces are:
- Aileron — on the outer trailing edge of each wing, deflecting differentially (one up, one down) to roll the aircraft.
- Elevator — on the horizontal tail, deflecting symmetrically to pitch the aircraft nose-up or nose-down.
- Rudder — on the vertical tail, deflecting to yaw the aircraft left or right.
UAVs frequently use combined control surfaces to reduce part count:
- Elevon — combines aileron and elevator function on a tailless or delta wing. Symmetric deflection controls pitch; differential deflection controls roll. Standard on delta-winged expendable drones.
- Ruddervator (V-tail mixer) — two surfaces on a V-tail that combine elevator and rudder function.
- Flaperon — combines flap (high-lift device) and aileron function. Used on some UAVs to reduce the number of servos.
Control surface sizing depends on the moment arm (distance from the aircraft’s center of gravity) and the required control authority. Larger surfaces provide stronger moments but add weight and drag. Undersized surfaces cause sluggish response; oversized surfaces cause twitchy handling and increased parasitic drag.
For multirotors, there are no aerodynamic control surfaces — all attitude control is achieved through differential motor speed via the ESCs, with the flight controller managing the mixing in software.