An aileron (French for ‘little wing’) is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Also refers to the extremities of a bird’s wings used to control their flight. Ailerons are used in pairs to control the aircraft in roll, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called ‘rolling’ or ‘banking’.
The aileron was first patented by the British scientist and inventor Matthew Piers Watt Boulton in 1868.
Aileron Flight Dynamics
Pairs of aileron are typically interconnected so that when one is moved downward, the other is moved upward. The down-going aileron increases the lift on its wing while the up-going aileron reduces the lift on its wing, producing a rolling (also called ‘banking’) moment about the aircraft’s longitudinal axis (which extends from the nose to the tail of an airplane). Ailerons are usually situated near the wing tip, but may sometimes also be situated nearer the wing root. Modern airliners may also have a second pair of ailerons on their wings, and the terms ‘outboard aileron’ and ‘inboard aileron’ are used to describe these positions respectively.
Horns and aerodynamic counterbalances
Particularly on larger or faster aircraft, control forces may be extremely heavy. Borrowing a discovery from boats that extending a control surface’s area forward of the hinge lightens the forces needed first appeared on ailerons during World War I. When ailerons were extended beyond the wingtip and provided with a horn ahead of the hinge. Known as overhung ailerons, possibly the best known examples are the Fokker Dr.I and Fokker D.VII. Later examples brought the counterbalance in line with the wing to improve control and reduce drag. This is seen less often now, due to the Frise type aileron which provides the same benefit.
Spades are flat metal plates, usually attached to the lower surface, ahead of the hinge, by a lever arm. They reduce the force needed by the pilot to deflect the aileron and are often seen on aerobatic aircraft. As the aileron is deflected upward, the spade produces a downward aerodynamic force. Which tends to rotate the whole assembly so as to further deflect it upward. The size of the spade determines how much force the pilot needs to apply to deflect the aileron. A spade works in the same manner as a horn but is more efficient due to the longer moment arm.