A control device (16) for a wing (10) comprises an airbox assembly (16) connected to an aerofoil wing tip (17). The airbox assembly (16) includes passages (23) that receive air from the lower surface of the aerofoil (10) and accelerate and exhaust the air upwardly, outwardly and rearwardly of the aerofoil (10). This reduces or prevents the formation of wing tip vortices and so reduces induced drag. In addition, the airbox assembly (16) also includes a wing tip (17) of increased camber relative to the wing (10) that changes the flow of air over the lower pressure surface (11) of the wing (10) to mirror that over the higher pressure wing surface (12) so reducing on eliminating bound trailing edge vortices. Such devices can be used on other foils that operate in fluid streams to provide a force.
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