Judging by the number of present-day aircraft which have suffered changes of fin shape, it is clear that there is still some difficulty in estimating the size and shape of fin required on a new project. In this paper some of the principles on which fin design is based will be explained, and some of the difficulties involved described.

It is necessary to have some sort of fin to provide weathercock, or directional, stability. This is illustrated in Fig. 1. This shows the balance of yawing moments, due to sideslipping. In general, the side force acting on the fuselage, represented by the black arrow, has its c.p. ahead of the e.g. of the aircraft, and hence it produces a destabilising moment about the e.g. Sufficient fin area must be provided for the yawing moment due to the fin side force, represented by the white arrow, to at least equal the destabilising fuselage moment. Otherwise, when disturbed laterally due to, say, a gust, the aeroplane will tend to swing still further out of wind, making it difficult to fly accurately. The minimum permissible fin area can, in fact, be fixed simply by this requirement for weathercock stability.