Difficulties in balancing the four forces

First, the lift. The lift will act through the centre of pressure, which will depend on the position of the wings; so the designer must be careful to place the planes in the correct position along the fuselage. But the problem is complicated by the fact that a change in the angle of attack means a movement of the lift, and usually in the unstable direction; if the angle of attack is increased the pitching moment about the centre of gravity will become more nose-up, and tend to increase the angle even further.

Secondly, the weight. This will act through the centre of gravity, which in turn will depend on the weight and position of every individual part of the aeroplane and the loads that it carries. Here alone is sufficient problem, but again there is a possibility of movement of the centre of gravity during flight caused, for instance, by consumption of fuel, dropping of bombs or movement of passengers. In the Concorde, fuel was actually moved from one tank to another to adjust the position of the centre of gravity.

Thirdly, the thrust. Here the problem is easier. The line of thrust is settled by the position of the propeller shaft or centre line of the jet, which in turn depend on the position of the engine or engines. In this matter the designer has little choice, but has to consider such problems as keeping the propeller clear of the ground and giving the pilot a clear view ahead; new problems arise too when the thrust can be deflected as in certain modern types.

Lastly, the drag. This is, perhaps, the most difficult of all. The total drag is composed of the drag of all the separate parts, and the designer must either estimate the drag of each part separately, and so find the total drag and its line of action, or must rely on wind tunnel experiments on a model or computed predictions; and even when the line of drag has been found it too will be liable to change at different angles of attack.

Arranging the forces

For steady flight along a straight line, whether level or not, it is not only necessary to balance the four forces so that they produce no resultant force; their lines of action must also be arranged so that they produce no resultant moment, otherwise the aircraft will rotate either nose-up or nose-down. When there is no resultant moment, the aircraft is said to be trimmed. Mathematically, if the sum of the moments is M, then the condition for trim is that M=0.

One way to achieve this would be to arrange that all of the forces act through a single point, as in Fig. 5.1. However this is not generally practical, as there are many factors that tend to alter the line of action, apart from those already stated. For example, lowering the undercarriage tends to shift the line of the resultant drag down, and on the floatplane variant of a light aircraft illustrated in Fig. 5B the position of the drag resultant would be very much lower than for the original floatless design.

It is possible to balance the moment produced by the drag and thrust being out of line, by arranging the lift and weight forces to be out of line by an amount that causes them to exactly produce the necessary balancing moment, as in Fig 5.2. However, because of the way that the lines of action of the forces tend to change according to aircraft attitude, fuel weight etc., there is no simple design solution to ensure that the resultant moment will always be zero. As described below, some active involvement of the pilot is required in order to keep the aircraft trimmed.