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Ailerons 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.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. The yaw occurs because the down-going aileron will, as well as increasing lift, increase drag. Conversely, the wing with the upgoing aileron will see a reduction in drag as well as the intended reduction in lift.
Adverse yaw can be countered with the aircraft's rudder (a co-ordinated turn), but can also be reduced with clever design. If the upgoing aileron moves further upwards than the downgoing aileron moves down, it will create extra profile drag on that wing and try to yaw the aircraft into the turn. This set-up is known as "differential aileron". Another solution is to use a "Frise aileron", where the up going aileron also projects a section downwards below the wing, again increasing drag on the inside of the turn.
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 by the Aerial Experiment Association, headed by Alexander Graham Bell.
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