Longitudinal stability

We shall start with longitudinal stability, since this can be considered indepen­dently of the other two. In order to obtain stability in pitching, we must ensure that if the angle of attack is temporarily increased, forces will act in such a way as to depress the nose and thus decrease the angle of attack once again. To a great extent we have already tackled this problem while dealing with the pitching moment, and the movement of the centre of pressure on aerofoils. We have seen that an ordinary upswept wing with a cambered aerofoil section cannot be balanced or ‘trimmed’ to give positive lift and at the same time be stable in the sense that a positive increase in incidence produces a nose-down pitching moment about the centre of gravity.

The position as regards the wing itself can be improved to some extent by sweepback, by wash-out (i. e. by decreasing the angle of incidence) towards the wing tips, by change in wing section towards the tips (very common in modern types of aircraft), and by a reflex curvature towards the trailing edge of the wing section.

But it is not only the wing that affects the longitudinal stability of the air­craft as a whole, and in general it can be said that this is dependent on four factors –

1. The position of the centre of gravity, which must not be too far back; this is probably the most important consideration.

2. The pitching moment on the main planes; this, as we have seen, usually tends towards instability, though it can be modified by the means men­tioned.

3. The pitching moment on the fuselage or body of the aeroplane; this too is apt to tend towards instability.

4. The tail plane – its area, the angle at which it is set, its aspect ratio, and its distance from the centre of gravity. This is nearly always a stabilising influence (Fig. 9.2).