Conditions of equilibrium

Now, under what conditions will these four forces balance the aeroplane? That is to say, keep it travelling at a steady height at uniform velocity in a fixed direction, a state of affairs which, in the language of mechanics, is known as equilibrium. It is sometimes hard to convince a traveller by air that he may travel at 200 m/s and yet be in a state of equilibrium; equilibrium simply means that the existing state of affairs is remaining unchanged; in other words, that the aeroplane is obeying Newton’s First Taw of Motion.

In order to do this the forces acting on it must be balanced – the lift must be equal to the weight (this condition will keep the aeroplane at a constant height); and the thrust must be equal to the drag (this condition will keep the aeroplane moving at the same steady velocity).

The idea is often prevalent that the lift must be greater than the weight, or, as it is often expressed, the lift must ‘overcome’ the weight; and when it comes to the question of thrust and drag the author has known students dismiss the idea that the thrust need only be equal to the drag as ‘contrary to common sense’.

There still remains a third condition for equilibrium. In order to maintain straight and even flight, we must prevent the aeroplane from rotating, and this depends not only on the magnitudes of the four forces, but also on the pos­itions at which they act. If the centre of pressure is behind the centre of gravity, the nose will tend to drop and the tail to rise, and vice versa if the centre of pressure is in front of the centre of gravity. But we are also concerned with the lines of action of the thrust and drag, for if the line of thrust is high and the line of drag is low, these two forces also will tend to make the nose drop. Such tendencies could be prevented by the pilot using his controls, but it is the aim of the designer to make an aeroplane which will in the words of the pilot, fly ‘hands off’. Therefore he must see that the forces act in the right places.