It has long been the practice in engineering to use direct manual operation of the controlling parts of a machine, so long as it was within the strength of the operator, and so long as design of the operating gear was relatively simple. In innumerable applications, however, the increased demands for power by the controls, and the need for greater accuracy, have necessitated the development of servo mechanisms to drive them; that is, we have to sever, partially or completely, the direct mechanical link from operator to control, and use the inserted backlash, elasticity, and so forth, to meter power from an external power source.

In the flying control field we have come at this time to the transition stage, in that we have realised that, for a large number of applications, direct manual operation is impractical, and we are feeling our way towards suitable forms of servo controls for our various needs.