An air control valve (10) is positioned between an entry duct (4) and a fan (6) to control the velocity of air being fed to the fan (6). The valve (10) comprises a motorised flap (12) which is pivoted to restrict the passageway (15) between the duct and fan (6), and second flap (16) which moves with the flap (12) to open a bleed aperture (18). The main air flow (A) is restricted by the flap (12) and draws in an air stream (B) via the bleed aperture (18). Preferably, bleed air is drawn in from an acoustic box (8) surrounding the fan (6).
