Other factors affecting longitudinal static stability

In the simple cases shown in Figs 11.5 and 11.6 we conveniently had the drag force passing through the centre of gravity. In practice, the line of action of the tailplane drag must move as the aircraft attitude changes. With the aid of sim­ple sketches, it is easy to work out that for a conventional aircraft this produces a stabilising tendency, while for a canard, the influence is destabilising.

In addition to the factors given above, we also have to consider the influence of fuselage, flaps, undercarriage, external stores (armaments) and any other features that can produce either an aerodynamic force or moment, or a change in the centre of gravity position. It is also very important to take account of the flexibility of the aircraft and control system components.