Abstract: A flow control device in a casing through which air passes comprises a baffle or threshold projecting from one inner wall of the casing toward an opposed wall and having an arcuate surface facing upstream, a plate or pressure plane disposed upstream of the threshold and mounted for rotation such that its downstream end moves parallel to the arcuate face of the threshold, and a hollow body disposed transversely of the casing and carried by the downstream end of the pressure plane for movement therewith so that the body serves as a closure member for the space between the free edge of the threshold and the opposed wall of the casing, the body having a transverse edge facing upstream and dividing the air flow into two streams one of which flows to an adjustable gap between the body and the opposed wall of the casing and the other of which flows to a fixed gap located between the body and the pressure plane.