A jet of air is impelled on to the upper surface of a flap in order to increase its lift. Part of the air in this air jet is drawn, through air intakes, from the air stream flowing over an upper surface of the fixed part of the wing, located upstream of the flap. The air jet drawn in by suction by the air intakes is reinforced by a jet of compressed air blown through a passage which opens immediately downstream of the air intakes. This creates a combined jet of the air sucked in and the blown jet, which is ejected on to the upper surface of the flap through outlet apertures located on the rear edge of the fixed part of the wing.