Ailerons "Little Wing" are hinged flaps attached to the trailing edge of an airplane wing, usually near the wingtips.
They are used to control the aircraft in roll.
The two ailerons are interconnected so that one goes down when the other goes up: the downgoing aileron increases the lift on its wing while the upgoing aileron reduces the lift on the other wing, producing a rolling moment about the aircraft's longitudinal axis. The word aileron is French for "little wing."
An unwanted side-effect of aileron operation is adverse yaw - a yawing moment in the opposite direction to the turn generated by the ailerons. In other words, using the ailerons to roll an aircraft to the right would produce a yawing motion to the left.
Modern airliners tend to have a second set of inboard ailerons much closer to the fuselage, which are used at high speeds. Some aircraft use spoilers to achieve the same effect as ailerons.
The device was developed independently by the Aerial Experiment Association, headed by Alexander Graham Bell, and by Robert Esnault-Pelterie, a French airplane builder. Ailerons superseded the earlier "wing warping" technique, developed by the Wright Brothers.
Another control surface that combines an aileron and flap is called a flaperon. A single surface on each wing serves both purposes. Used as an aileron, the flaperons left and right are actuated differentially; when used as a flap, both flaperons are actuated downwards. As a example of an aircraft using flaperons, see this RJ.03 IBIS experimental aircraft. Please note that when a flaperon is actuated downwards (i.e. used as a flap) there is enough freedom of movement left to be able to still use the aileron function.
On aileron-equipped model aircraft, the ailerons are controlled with the lateral motion of the right stick on the transmitter. They may be activated by a single servo push-pull system. Most, however, have a servo dedicated to each aileron. These may be operated on a single channel with the use of a "Y-harness" which electrically interconnects the servos or on two separate, intermixed channels with the use of a computerized radio.
- NASA Glenn Research Center aileron article with Java demo and more pictures