In an airscrew wherein the pitch angle varies from root to tip of the blade, openings are provided on the upper surface of each blade and near the trailing edge, and extend from the root for a part of the length of the blade and communicate with an inwardly extending duct to a source of suction or depression. The blade a is provided with a series of openings a1 on its upper surface near the trailing edge, such openings communicating by a central duct a11 with a hollow boss and thence by openings a12 with a carrier box b fixed to the engine. The box b has a passage b1 connected by pipe c with the induction pipe of the driving engine. It may, however, be connected with a suction pump or other means of reducing the pressure. The area of the openings a1 may be varied by means of shutter e, slidable in a slot e3 to control the opening. This shutter e is guided by pin‐and‐slot mechanism e1, e2 and is actuated by rod e4 coupled to a screw e5 movable in a nut member e6 having an external toothed centre e9 engaging a control rack e8. This arrangement may be used with a fixed or with a variable‐pitch propeller.
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