Surfaces that extend from the wings to increase lift, allowing flight at slower speeds (such as for takeoff and landing).
Flaps are a type of high-lift-device generally mounted on a wing's trailing (rear) edge. Most types of flap increase lift by increasing the wing's camber (the curvature of its upper surface), its area, or both. A flap can also be combined with an aircraft's ailerons to form a flaperon, which decreases weight compared to having separate flaps and ailerons.
The types of flap include:
- Plain flaps, which are simply a hinged section of the trailing edge that swivels downwards to increase the wing's camber.
- Split flaps, in which the lower surface of the trailing edge swivels downwards, while the upper surface stays put. This deflects air downwards and increases lift, but also considerably increases drag, acting as airbrakes.
- Fowler flaps, which slide backwards from the wing's trailing edge and then swivel downwards, increasing both the wing's area and its camber.
- Slotted flaps, a modification of a plain or Fowler flap where one or more slots open up in the wing when the flaps extend, improving the flap's resistance to flow separation at a high angle-of-attack.
- Blown flaps, where compressed air or engine exhaust is blown over the surface of the flap (and sometimes also the wing itself), dramatically increasing lift, but also dramatically increasing complexity, slightly decreasing maximum engine thrust, and aggravating the consequences of an engine-failure.
- Gouge flaps, which are like Fowler flaps, but swivel down at the same time as they move backwards, instead of after.
- Junkers flaps, which are like plain flaps, but are mounted below the wing's trailing edge rather than attached to it.
- Zap flaps, which behave like Gouge flaps, but where the flap moves only partway backwards as it swivels down.
- Gurney flaps, small perpendicular tabs mounted on the underside of the trailing edge, which cause high-pressure air to pile up underneath the wing, greatly improving said wing's lift.
- Krueger flaps, which, despite their name, are actually a type of high-lift-device mounted on the wing's leading edge, more akin to slats or droops.
For more information, see the Wikipedia page on flaps.