Knowledge of the motions of aircraft control surfaces due to oscillating hinge moments is of great importance for flutter investigations. While the impedance of the power control unit may play a large part in determining these motions, it is not their only cause. The elasticity of the power unit mounting and the effect on the input of distortion of the aircraft structure, an effect which may depend on air speed, may also be important. Nevertheless the influence of the power unit itself is of major importance. No very satisfactory methods appear to have been devised yet, however, for making such tests. The difficulty lies chiefly in applying the large oscillatory loads to the output of the power control unit, which are necessary if the test is to be realistic. The facts that the oscillating loads must be applied to the jack while it is moving, and that a non-oscillatory opposing load is also desirable, add to the difficulties. For convenience in assessing the characteristics of the unit, it is desirable that the oscillating load should be superimposed on a constant opposing load and that the basic jack motion should be of constant velocity.