Static stability

Solving the problems

The precise analysis of aircraft stability is an extremely complicated process. For conventional straight-winged aircraft in the pre-jet age, it was found that by making a few simplifying assumptions, the problems could be reduced to a form where they could be solved by traditional analysis and hand calculations. Some aspects of this approach are still perpetuated in introductory texts and courses, because the simplification can act as an aid to understanding. The increasing aerodynamic complexity of aircraft has, however, rendered many of the assumptions inappropriate, and for industrial purposes, a more complete solution of the stability equations is normally attempted. This direct approach has been made practical by the advent of the digital computer, but despite the advances that have been made in theoretical methods, the analysis of air­craft stability still represents a considerable challenge, particularly for uncon­ventional types such as the forward-swept X-29 shown in Fig. 9.20.

Although we shall not attempt to describe the process of stability analysis, we can at least explain some of the principles and design features involved in producing a stable and controllable aircraft.

The requirements for trim and stability

For steady flight, the forces acting on an aircraft must be in balance, and there must be no resultant turning moment about any axis. When this condition is achieved, the aircraft is said to be trimmed. In Fig. 11.1 we show an aircraft that is trimmed about its pitching axis.

An aircraft is said to be statically stable if it tends to return to its initial flight conditions; attitude, speed etc., after being disturbed by a gust or a small

Weight

For aircraft to be trimmed L„x a – M0 = L, x b

Fig. 11.1 Forces on an aircraft trimmed for steady level flight

The movements about the centre of gravity due to wing lift, the tail downforce and the pitching couple are exactly in balance

In this simple example we have chosen a case where the thrust and drag forces are on the same line. This is not generally true, and thrust and drag forces normally affect the trim. Fuselage effects have also been ignored impulsive input from the controls. Normally, for steady flight, we require the aircraft to be both trimmed and stable.

There is frequently considerable confusion about the difference between balanced or trimmed, and stable. If you balance a ball on the end of your finger, it may temporarily be perfectly balanced, but it is certainly not in a stable position.

In general, the more stable we make an aircraft, the less manoeuvrable it becomes. A very stable aircraft always tends to continue on its existing path, so excessive stability must be avoided.

We can quickly get some idea of how stable an aircraft is by ignoring inertia or time-dependent effects, and just looking at the balance of the forces and moments acting on the aircraft; in other words, by treating the problem as if it were one of statics. Once it is established that an aircraft is statically stable it is then necessary to go on to investigate the inertia and time-dependent effects; the so-called dynamic stability described in the next chapter. This approach was part of the traditional method of breaking down the complex problem of aircraft stability, and although computational techniques have to some extent rendered it unnecessary, it is still useful, particularly when introducing the subject.