Wing-tip sails or feathers

There are several wing-tip devices that have been shown to produce significant reductions in drag. One of these is the wing-tip sail shown in Fig. 4.15. This device has been around for millions of years in the form of the tip feathers on some birds’ wings. At the tip of the wing, there is a strong upflow, as the air spills over from the underside. The feathers, or sails, are angled so that they generate a forward component of force, or negative drag, as illustrated in Fig. 4.16. For optimum effect, the feather angles need to be altered accord­ing to the flight condition. Curiously, quite a bit of research work had been conducted on this idea before anyone spotted that birds were already using the principle. It has been found that beneficial interference effects occur when

several sails (usually three) are used, as in Fig. 4.15. Birds also use several tip feathers; interestingly, always an odd number.