Flight experiments to evaluate the effect of wing-tip sails on fuel consumption and handling characteristics

Extract

Studies made of the flow about the wing-tip of a wind tunnel model showed that the vortex-type flow generally associated with the rolled-up vortex sheet well behind the tip existed just behind the trailing edge and seemed to be forming over the top of the wing-tip itself. As a result small cambered aerofoils were attached to the wing-tip to redirect the flow locally in such a way that each experienced a force with a significant thrust component. The effect is similar to that generated by the sail of a close-hauled yacht; consequently these small tip extensions were called sails. Reference 1 describes the original wind tunnel and the preliminary flight tests of sails mounted from the tip tanks of the Paris aircraft of the Cranfield Institute of Technology.