To furnish a control force an aerofoil is formed with a duct connecting an outlet opening at the surface with a forwardly‐directed intake opening at or in advance of the leading edge, so that air entering the intake due to motion of the aircraft issues from the outlet to produce the control force. Normally a pair of superposed ducts 21, 22 are provided as shown in FIG. 1, with inlets 23, 24 at the leading edge and outlets 25, 26 on the opposite surfaces of the aerofoil, pivotal closure members 27, 28 being provided for the inlets and slidable closure members 29, 30 for the outlets. When applied to wing tips for roll control purposes the outlets may be placed side‐by‐side, being of unequal area to compensate for their different distances from the longitudinal axis of the aircraft. FIG. 2 shows an arrangement for use with tail planes 3,90, the outlets 83, 84 communicating through ducts 81, 82 with an annular air inlet formed between a cowl 87 and the fuselage tail 88. Closure members 91 and 92, 93 fitted to the tail of the cowling 87 and the ducts 81, 82, respectively, are so arranged that in normal operation member 91 is in the open position and members 92, 93 in the closed position, this arrangement being reversed when control forces are required.
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