Aircraft control

Control requirements

An aircraft is free to move in the six different ways illustrated in Fig. 10.1. These are known as the six degrees of freedom, and several aspects of each degree may need to be controlled. For example, we need to be able not only to set the pitch angle, but also to control the rate at which the angle changes. We may even wish to be able to regulate its rate of acceleration, so there can be eighteen or more different aspects to control. To make matters even more complicated, there is often an interaction or cross-coupling between move­ments. As we shall see later, rolling the aircraft invariably causes it to turn (yaw). Since there can be cross-coupling between any pair of factors, there is a large number of possibilities.

With such a vast array of factors to consider, it might seem a daunting task to try to design an aircraft control system. Fortunately, on conventional air­craft, many of the possible cross-coupling effects are insignificant, and can be largely ignored. Traditionally, the important ones were dealt with partly on the basis of experience, and partly by means of simplifications and approxima­tions. Many nicely controllable aircraft were built long before the theoretical design procedures had been fully worked out. Problems always arose, however, when some unconventional configuration was tried, and this led to a very cau­tious attitude to unconventional designs. Very few manufacturers have ever made money out of novel designs, even though they may have benefitted avia­tion in the long term.

Nowadays, it is possible to design very complex control systems with a fair degree of confidence. The most important cross-coupling factors can be predicted, simulated and verified by wind-tunnel experiments, or by test-flying modified aircraft. The introduction of digital electronics into the control sys­tem has made it possible to modify and adjust the control response of the aircraft.