3D printers. Plane characteristics Number of channels The number of channels (technically, servo channels) a plane has is normally determined by the number of mechanical servos that have been installed, with a few exceptions, such as the aileron servos, where two servos can operate via a single channel using a Y harness (with one of the two servos rotating in the opposite direction). On smaller models, usually one servo per control surface (or set of surfaces in the case of ailerons or a split elevator surface) is sufficient. Generally, for a plane to be considered fully functional, it must have four channels (elevator, rudder, throttle, and ailerons). Basic flight controls A four-channel RC system gives the aeromodeler the same basic degree of control that a full-sized aircraft's primary flight controls do: Elevator (or horizontal stabilizer) controls pitch (up and down). Rudder (or vertical stabilizer) controls yaw (left and right). Throttle controls engine rpm (or thrust for jets and ducted fans, or motor speed for electric RC Aircraft). Ailerons control roll. Extra flight control functionality Gear/retracts controls retractable landing gear (usually in conjunction with gear doors). Flaps Increase lift, but also increase drag. Using flaps, an aircraft can fly slower before stalling. Flaps are often used to steepen the landing approach angle and let the plane land at a slower touchdown speed (as well as letting the aircraft lift off at a slower takeoff speed). In both cases, flaps reduce the wear and tear on the airplane landing gear system, and enable an aircraft to use a shorter runway than would otherwise be required. Auxiliary control Additional channels can control additional servos for propeller pitch (such as on 3D planes), or control surfaces such as slats, spoilers, flaps, spoilerons, flaperons, or elevons. Misc bomb bay doors, lights, remote camera shutter can be assigned to extra channels. Additionally, if there is a flight assist or autopilot module on the craft (more common on the multi-rotor copters), features such as gyro-based stabilization, GPS location hold, height hold, return home, etc., can be controlled. Three channels (controlling rudder or (rarely) ailerons, along with the elevator and throttle) are common on trainer aircraft. Four channel aircraft, as mentioned above, have controls for elevator, rudder, throttle, and ailerons. For complex models and larger scale planes, multiple servos may be used on control surfaces. In such cases, more channels may be required to perform various functions such