Flap (aircraft)

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Flaps are hinged surfaces on the trailing edge of the wings of a fixed-wing aircraft which, when deployed, increase the lift and drag of a wing.

Lift is increased by:

increasing the camber of the wing.
increasing the size of the effective lifting surface by increasing the wetted area.

Drag is increased by:

increasing the wetted area of the wing, resulting in more skin friction drag.
increasing the induced drag of the wing.

Flaps are usually fully extended whilst landing to allow the aircraft to fly more slowly (by increasing the lift generated by the wings at slow speeds) and to steepen the approach to the landing site. Depending on the aircraft type, configuration and method of takeoff (e.g., short field, soft field, normal, etc.), flaps are often partially extended for take-off to give the aircraft more lift when trying to leave the ground.

Triple-slotted trailing-edge flaps on a Boeing 747 fully extended for landing at Heathrow Airport, London.

Four types of flaps

Types of flap systems include:

Plain flap — rotates on a simple hinge.
Split flap — upper and lower surfaces are separate, the lower surface operates like a plain flap, but the upper surface stays immobile or moves only slightly.
Fowler flap (Fairey-Youngman flap) — slides backwards before hinging downwards, thereby increasing both camber and chord, creating a larger wing surface better tuned for lower speeds.
Slotted flap — a slot (or gap) between the flap and the wing enables high pressure air from below the wing to re-energize the air blowing over the flap. This helps the airflow to stay attached to the flap, delaying the stall.
Blown flaps — systems that blow engine air over the upper surface of the flap at certain angles to improve lift characteristics.

Slats, also known as leading-edge flaps, have a similar purpose to trailing-edge flaps, save they are located on the leading edge of the wing.

Flap (aircraft)

See also