Last month I went on at length about flaps and airbrakes, and managed to cover just the bare essentials, in answer to a question from Syd Yates. I promised to do the same for leading edge devices, some of which are illustrated in Figure 1. The simplest device 'A' is a small aerofoil section, or even a thin curved plate, fixed ahead of the leading edge to form a converging slot. Instead of adding to the wing, a slot may be cut through the section itself as in 'B' to form the slot. Fixed devices cause unnecessary drag at high speed so the retractable slat was invented. When retracted it forms part of the optimised aerofoil and is intended to be deployed to form the slot only at low speed. A clever variation of this, allegedly invented by Handley Page, is the automatic slat designed to pop open at low speed when the stagnation point moves down to its opening. They also tended to pop open when pulling g so could be locked for aerobatics.
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