Abstract: A linear step discontinuity on the lower surface of the lifting surface is disclosed extending from the front of the lifting surface, possibly the leading edge, and may extend in the direction of the line of flight towards the trailing edge. The linear step discontinuity represents a step up if moving from the outboard end to the inboard end, such that in use fluid moving over the lifting surface tumbles over the linear step discontinuity to create a vortex in the fluid passing over the upper surface. The lifting surface may be an aircraft wing or in particular a winglet on an aircraft wing. The linear step discontinuity may be provided by the straight edge of a semi-circular object integrated into or adhered to the surface of the lifting surface.