Aileron

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Image:Aileron roll.gif Ailerons are hinged control surfaces attached to the trailing edge of an airplane wing. 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 aileron systems have minimal adverse yaw, such that it is barely noticeable in most turns. Adverse yaw is also caused by the wing on the outside of the turn traveling faster than the inside wing and thus having more lift and drag. Once the desired angle of bank (degree of rotation on the longitudinal axis) is obtained, the pilot uses opposite aileron to prevent the airplane from continuing to roll due to this increased lift. This minor opposite use of the control must be maintained throughout the turn. The pilot also uses a slight amount of rudder in the same direction as the turn to counteract adverse yaw and to produce a "coordinated" turn where the fuselage is parallel to the flight path. A simple guage on the instrument panel called the "ball" indicates when this coordination is achieved.

Modern airliners tend to have a second set of inboard ailerons much closer to the fuselage, which are used at high speeds. If the wings are also in a V shape and the aircraft enters a stall, the ailerons on the inside will still be able to function as the stall will proceed from the tips of the wings to the body. In plane wings, the stall develops the other way so the ailerons are placed towards the tips of the wings. Large jet aircraft often have ailerons towards the tips of the wings to increase the moment of the aircraft so less force is required to pitch the aircraft. 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 Ailerons superseded the earlier wing warping technique, developed by the Wright Brothers.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 an 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.

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