Fin area and lateral stability

One factor which may have considerable influence on lateral stability is the position of the various side surfaces, such as the fuselage, fin and rudder, and wheels. All these will present areas at right angles to any sideslip, so there will be pressure upon them which, if they are high above the centre of gravity, will tend to restore the aeroplane to an even keel; this applies to many modern types which have a high tail plane on top of a high fin (Figs 9.9 and 9E, later) and such types may have anhedral on the main planes to counterbalance this effect and prevent too great a degree of lateral stability; but if the side surfaces are low the pressure on them will tend to roll the aircraft over still more (Figs 9.10 and 9F, later) and so cause lateral instability, although this must be bal­anced against the effect of high wing compared with the CG position.

The reader will have noticed that, whatever the method of obtaining lateral stability, correction only takes place after a sideslip towards the low wing.

It is the sideslip that effects the directional stability.