Conditions for longitudinal static stability

It will be seen that the centre of gravity is further forward in the stable case of Fig. 11.5 than in the unstable one of Fig. 11.6. Also, in the stable case, the wing is set at a higher incidence than the tail. The difference between the incidence angles at which the wing and tail are set is called the longitudinal dihedral. By comparing Figs 11.5 and 11.6 it can be seen that the longitudinal dihedral influences the production of a favourable restoring moment. In the stable case of Fig. 11.5 the longitudinal dihedral angle is positive. In the unstable case of Fig. 11.6 the angle is negative. The A-10 Thunderbolt shown in Fig. 11.7 shows a noticeable degree of positive longitudinal dihedral.

In fact, it is not actually the longitudinal dihedral (the difference between wing and tail incidences) that matters, but the difference between wing and tail lift coefficients in the initial trimmed condition. From mathematical analysis we find that for stability, the tail lift coefficient in the trimmed condition should be less than that of the wing by a sufficient margin to overcome the destabilising effects of the camber etc. The longitudinal dihedral effect, though important, is only one of the many influences on stability that appear in a full analysis.

If the centre of gravity of the aircraft is moved forward, the tail down – force has to be increased, to keep the aircraft trimmed. This requires that the

tail incidence should be made more negative, or that the elevator should be raised. Either of these effects will increase the effective longitudinal dihedral, and increase the static stability. Thus, the further forward the centre of gravity position is moved, the greater will be the longitudinal static stability.

It should be noted that the centre of gravity does not have to be in front of the aerodynamic centre of the wing for stability, although this is a common condition for conventional aircraft.

The rearward CG position at which the aircraft is just on the verge of being unstable or is neutrally stable is called the neutral point.

For a conventional aircraft trimmed for steady level flight, the tailplane normally has to produce very little lift, or even a downforce. For this reason, a symmetrical aerofoil is often used for the tailplane.

In situations where the tailplane has to produce a downforce, the wing and tail are effectively fighting each other, so the overall lift is less than that produced by the wing. The tailplane, however, still produces positive drag, and thus serves no useful purpose other than as a means of controlling and stabil­ising the aircraft. The extra drag produced in this way is called trim drag.