Flap (aircraft)
From Wikipedia, the free encyclopedia
Flaps are hinged surfaces on the trailing or leading edge of the wings of a fixed-wing aircraft which, when deployed, change the coefficient of 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 while 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.
Some gliders not only use flaps when landing but also in flight to optimize the camber of the wing for the chosen speed. At lower speeds when thermalling, positive flap is used, i.e., flaps are lowered. At higher speeds a negative flap setting is used, i.e. the flaps are raised. Negative flap is also used during the initial stage of an aerotow launch and at the end of the landing run in order to maintain better control by the ailerons.
Types of flap systems include:
- Krueger flap - hinged flap on the leading edge.[1][2] Often called a "droop."
- 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 — slides backwards before hinging downwards, thereby increasing both camber and chord, creating a larger wing surface better tuned for lower speeds.
- Fairey-Youngman flap - moves bodily down before moving aft and rotating.
- Slotted flap — a slot (or gap) between the flap and the wing enables high pressure air from below the wing to re-energize the boundary layer 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.
There are several technology development efforts to incorporate the function of the flaps into a flexible wing, so that the aerodynamic purpose is accomplished withouth the weight and mechanical complexity of a flap system. The X-53 Active Aeroelastic Wing is a NASA effort to incorporate this technology, and the Adaptive Compliant Wing is commercial development effort.
Slats, also known as leading-edge flaps, have a similar purpose to trailing-edge flaps, except that they are located on the leading edge of the wing. Note that a Krueger flap and a leading-edge slat differ in how they are extended (and retracted), but their aerodynamic function is the same.
