An apparatus is provided for effecting rapid changes in the geometry of a flexible wall structure forming at least a part of the cushion seal on an air cushion vehicle. The apparatus comprises one or more inflatable members co-operating with the flexible wall structure and an air ejector for rapidly introducing pressurized air into each inflatable member. Each inflatable member may be constituted by a wall structure portion of hollow construction. The invention finds particular application in assisting directional control of an air cushion vehicle.
