Flap (aircraft)
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Flaps are hinged surfaces on the trailing edge of the wings of a fixed-wing aircraft. As flaps are extended, the stalling speed of the aircraft is reduced. Flaps are also used on the leading edge of the wings of some high-speed jet aircraft, where they may be called slats or Krueger Flaps.
Flaps reduce the stalling speed by increasing the camber of the wing and thereby increasing the maximum lift coefficient. Some trailing edge flaps also increase the area of the wing and, for any given aircraft weight, this reduces the stalling speed. The Fowler flap is an example of one which increases the area of the wing.
Extending the flaps also increases the drag coefficient of the aircraft so, for any given weight and airspeed, flaps cause higher drag. Flaps increase the drag coefficient of an aircraft because of higher induced drag caused by the distorted planform of the wing with flaps extended. (Induced drag is a minimum on a wing with elliptical planform.) Some flaps increase the wetted area of the wing and, for any given speed, this also increases the parasitic drag component of total drag.
Depending on the aircraft type, flaps may be partially extended for takeoff. With light aircraft, use of flaps for takeoff may be optional and will depend on the method of takeoff (e.g., short field, soft field, normal, etc.) When flaps are partially extended for takeoff it is to give the aircraft a slower stalling speed but with little increase in drag. A slower stalling speed allows the aircraft to take off in a shorter runway distance. Flaps are usually fully extended for landing to give the aircraft a slower stalling speed so the approach to landing can be flown more slowly, allowing the aircraft to land in a shorter runway distance. The higher drag associated with fully extended flaps allows a steeper approach to the landing site. This is the benefit of the higher drag coefficient of fully extended flaps.
Some gliders not only use flaps when landing but also in flight to optimize the camber of the wing for the chosen speed. When thermalling, flaps may be partially extended to reduce the stalling speed so that the glider can be flown more slowly and thereby turn in a smaller circle to make best use of the core of the thermal. At higher speeds a negative flap setting is used to reduce the nose-down pitching moment. This reduces the balancing load required on the horizontal stabilizer which in turn reduces the trim drag - drag associated with keeping the glider in longitudinal trim. Negative flap may also be 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 without 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.
