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In an aircraft in combination, a wing or supporting plane, a pair of ledges arranged at the underside of said wing and extending substantially transversely to the direction of flight and spaced from the front edge of said wing by a distance not exceeding forty per cent. of the chord length of said wing, one ledge being arranged on cither side of the plane of symmetry of the craft, the axes of rotation of said ledges being so arranged that in the inoperative position, in which the ledges lie about flush with the underside of the wing, the free edge of one ledge is disposed in front, the free edge of the other ledge to the rear of its axis of rotation, and means for coupling said ledges for simultaneous rocking movement in relatively opposite sense.