Studies begun over ten years ago have shown that the swirling air flow about the tips of a lifting wing can be redirected using small aerofoil surfaces, called sails, to reduce the overall lift-dependent drag of the wing. Tests on Paris, Jetstream and other windtunnel models, confirmed by flight tests on Paris and Centurion aircraft have shown marked reductions in drag and fuel consumption. Handling tests show improvements in roll control and stalling speeds.

Wing tip sails have the effect of reducing the peak adverse effects on other aircraft flying through the wake of a Paris aircraft but reduce the decay rate so the far field effects are worsened. Sails fitted to a crop-spraying aircraft have significantly reduced the amount of spray lifted above the height of the aircraft by the tip vortices and subsequently blown off-target by a cross-wind.

Windtunnel tests on a model with variable incidence sails show that differential sail incidence can be used effectively to provide roll power, reducing the size of the ailerons required, so allowing a greater flap span. The maximum root bending moments of the wings can be reduced by at least 10% by using negative sail movements in positive manoeuvres or gusts.

Sails can reduce the drag of an aircraft at all speeds leading to fuel savings estimated at US$225 per annum per 1000lb take-off weight per 1000 hours annual utilisation. For an airliner of 300 000lb weight and operating for 3000 hours this means a fuel saving of over a fifth of a million dollars per year!