Abstract:
The vortex drag of a lifting surface depends only on its spanwise loading and is usually calculated therefrom. Deflection of a part-span flap gives a theoretical spanwise loading that can only be obtained by numerical approximation in non-analytical form. Subsequent evaluation of vortex drag therefore lacks precision, and two such direct methods are compared. A third method, on the reverse-flow principle, is introduced to determine vortex drag separately from the spanwise loading. For a particular wing-flap combination, the three methods are consistent within ± 1½ per cent; this is more than adequate, and in the present state of knowledge no better accuracy is likely to be achieved. Although the third method is rather cumbersome, it provides a ready means of studying the influence of flap span and chord on vortex drag.
