Flap (aeronautics)
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A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.
Trailing edge flaps extended on the right on a typical airliner (an Airbus A310-300). Leading edge slats are also extended, on the left.
The three orange pods are fairings streamlining the flap track mechanisms. The flaps (two on each side, on the Airbus A319) lie directly above these.
Flaps during ground roll after landing, with spoilers up, increasing drag.
North American T-6 trainer, showing its split flaps
Stall (fluid dynamics)
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In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.
Airflow separating from an airfoil at a high angle of attack, as occurs at a stall.
A Schweizer SGS 1-36 being used for deep-stall research by NASA over the Mojave Desert in 1983.