‘Blown’ flaps

‘Blown’ flaps are a powered variation of the high lift devices that uses air directed through nozzles over the trailing edge of the wing to delay flow seperation.

In a conventional blown flap, small amounts of compressed air produced by the jet engine is “bled” off from the compressor and directed to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation.

They were capable of increasing lift upto 3 times the maximum lift coefficient. They were most famously used on the F104 ‘Starfighter’ and the MiG 21 ‘Fishbed’. They fell out of favor due to the increased maintenance demands and the fact that engine failure would render them useless.