Flap (aircraft)

Flap (aircraft)

Flaps are hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed at which an aircraft can be safely flown and to increase the angle of descent for landing. They shorten takeoff and landing distances. Flaps do this by lowering the stall speed and increasing the drag.

Extending flaps increases the camber or curvature of the wing, raising the maximum lift coefficient—or the lift a wing can generate. This allows the aircraft to generate as much lift but at a lower speed, reducing the stalling speed of the aircraft, or the minimum speed at which the aircraft will maintain flight. Extending flaps increases drag which can be beneficial during approach and landing because it slows the aircraft. On some aircraft a useful side effect of flap deployment is a decrease in aircraft pitch angle which improves the pilot's view of the runway over the nose of the aircraft during landing, however the flaps may also cause pitchup, depending on the type of flap and the location of the wing.

There are many different types of flaps used, with the specific choice depending on the size, speed and complexity of the aircraft on which they are to be used, as well as the era in which the aircraft was designed. Plain flaps, slotted flaps, and Fowler flaps are the most common. Flaps used on the leading edge of the wings of many jet airliners are called Krueger flaps.

The Fowler, Fairey-Youngman and Gouge types of flap increase the planform area of the wing in addition to changing the camber. The larger lifting surface reduces wing loading and allows the aircraft to generate the required lift at a lower speed and reduces stalling speed. Although the effect is similar to increasing the lift coefficient, increasing the planform area of the wing does not change the lift coefficient which is dependent on the camber, not the chord.