"Blown flaps" are an aerodynamic device used on the wings 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 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 aircraft with poor low-speed characteristics. Their disadvantage is that they rob the engine of some thrust while in use, which can harm take-off performance, particularly in "hot and high" conditions.
The BAC TSR-2 featured blown flaps, but the project was cancelled in 1965. The first production aircraft with BLCS was the Lockheed F-104 Starfighter, where after prolonged development problems, it proved to be enormously useful in compensating for the Starfighter's tiny wing service. It was shortly adopted for North American Aviation's A-5 Vigilante and the Blackburn Buccaneer. It later found use on civilian airliners.
More recent designed fighter aircraft achieve the same improved low_speed characteristics using the technically more complex swing-wing design.
Blownflaps are a powered aerodynamic high-lift device the wings of certain aircraft to improve the low-speed lift during takeoff and landing.
Even huge flaps could not offset this to any large degree, and as a result many aircraft landed at fairly high speeds, and were noted for accidents as a result.
Rather than have large flaps, many fighter aircraft of the era used blownflaps, including the F-4 Phantom, some versions of the F-104 and most other aircraft designed in the later half of the 1950s.
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