Three great drawbacks of modern aeroplanes, from a safety point of view, are their tendency to stall, their high landing speed, and their low gliding angle.

The use of wing-tip slots for preventing lateral instability is well known, but they do not materially reduce the landing speed. Moreover, they cannot be used along the whole span without making the wing unstable. The best length of wing-tip slot for securing stability and maintaining the lift coefficient is about 40 per cent, of the semi-span.