It is well known that when a plane air jet of high velocity blows over a convex surface, such as the upper surface of a lowered trailing edge flap, it flows along the surface, remaining attached for a considerable length; in the case of the flap, it adheres right to the trailing edge. This effect, usually described as the Coanda effect, has been utilised in the attempts to increase lift at constant incidence by “flap-blowing” in which the high velocity air jet is ejected either through slots in the flap shroud, or tangentially out of the flap surface.

The low velocity air stream approaching the flap over the wing upper surface, which in the absence of “blowing” would tend to separate at the flap shroud, is entrained by the high velocity jet, and the incipient flow separation over the flap thereby suppressed. The energy contained in the “blowing” jet can, in addition, also produce “supercirculation.“