"'''Blown flaps'''" are an [[aerodynamic]] device used on the [[wing]]s of aircraft to improve low-speed [[lift]] and take-off characteristics. The process is sometimes called a '''boundary layer control system''' ('''BLCS''').
Blown flaps bleed air from the engines and force it through slots in the [[Flap (aircraft)|wing flaps]] of the aircraft when the flaps reach certain angles. The bleed air prevents the boundary layer (slow-moving air that accumulates on the airframe surface) on the upper surface of the flap from stagnating, improving lift.
Boundary layer control systems usefully lower the [[stall]] speed of an aircraft, making them useful for [[STOL]] aircraft (like cargo transports intended for use on short fields) and high-performance [[fighter]]s with poor low-speed characteristics. Their disadvantage is that they rob the engine of some thrust while in use, which can take-off performance, particularly in "hot and high" conditions.
The use of BLCS began in the late [[1950s]] on aircraft like the [[A-5 Vigilante]] and the [[F-104 Starfighter]], and later found use on civilian [[airliner]]s.
