Ailerons are hinged control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll. The two ailerons are typically 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. As the aircraft rolls, adverse yaw is caused primarily by the fore-aft tilting of the lift vectors on the left and right wings. The rising wing has its lift vector tilt back, producing an aft force component. The descending wing has its lift vector to tilt forward, producing a forward force component. The fore/aft forces on the opposite wingtips produce the adverse yaw. There is also often an additional adverse yaw contribution from a profile drag difference between the up-aileron and down-aileron wingtips.
Adverse yaw is effectively compensated by the use of the rudder, which results in a sideforce on the vertical tail which creates an opposing favorable yaw moment. Another method is by differential ailerons, which have been rigged such that the downgoing aileron deflects less than the upward-moving one. In this case the opposing yaw moment is generated by a profile drag imbalance between the left and right wingtips. Frise ailerons accentuate this profile drag imbalance by protruding beneath the wing of an upward-deflected aileron, most often by being hinged slightly behind the leading edge and near the bottom of the surface, with the lower section of the leading edge protruding slightly below the wing's undersurface when the aileron is deflected upwards, substantially increasing profile drag on that side. Ailerons may also use a combination of these methods.
With ailerons in the neutral position the wing on the outside of the turn develops more lift than the opposite wing due to the variation in airspeed across the wing span, and this tends to cause the aircraft to continue to roll. Once the desired angle of bank (degree of rotation on the longitudinal axis) is obtained, the pilot uses opposite aileron to prevent the aircraft from continuing to roll due to this variation in lift across the wing span. 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 gauge on the instrument panel called the inclinometer, also known as "the ball", indicates when this coordination is achieved.
Since the need for roll control on aircraft was not as obvious as the need for heading and pitch control, the aileron came into widespread use well after the rudder and elevator. The Wright Brothers used wing warping instead of ailerons for roll control, and initially, their aircraft had much better control in the air than aircraft that used movable surfaces; however, as aileron designs were refined, it became clear that they were much more effective and practical for most aircraft.
There are conflicting claims over who first invented the aileron. In 1868, before the advent of powered aircraft, English inventor M.P.W. Bolton patented the first aileron-type device for lateral control. New Zealander Richard Pearse may have made a powered flight in a monoplane that included small ailerons as early as 1902, but his claims are controversial (and sometimes inconsistent), and even by his own reports, his aircraft were not well controlled. The aircraft 14 Bis by Santos Dumont was modified to add ailerons in late 1906, though it was never full controllable in flight, likely due to its unconventional wing form. Ailerons were also developed independently by the Aerial Experiment Association, headed by Alexander Graham Bell and by Robert Esnault-Pelterie, a French aircraft builder. Henry Farman's ailerons on the Farman III were the first to resemble ailerons on modern aircraft, and have a reasonable claim as the ancestor of the modern aileron. Other claimants include American William Whitney Christmas, who claimed to have invented the aileron in the 1914 patent for what would become the Christmas Bullet (built in 1918), and American Glenn Curtiss, who flew an aileron-controlled aircraft in 1908.
Engineer Leslie George Frise (1897-) developed an aileron shape which is often used due to its ability to counteract adverse yaw. The aileron is pivoted at about its 20% chord line and near its bottom surface. The leading edge of the aileron is bluntly rounded, so that when the aileron is deflected up (to make that wing go down), the leading edge of the aileron dips into the airflow beneath the wing surface and adds significant drag to that wing. The resulting drag causes the aircraft to pivot (turn) in the desired direction.