Stick-free stability

In the discussions above, we have assumed that the control stick is being held firmly, so that there is no movement of the control surfaces. However in real­ity, changes in aircraft attitude will alter the loading on the control surfaces; trying to move them.

Some power-operated controls are irreversible; that is, the aerodynamic loads cannot drive them. On most aircraft, however, and invariably on those with servo-assisted or manual controls, the elevators will move in response to changes in pitch, if the stick is not held firmly. The influence on stability of allowing the stick to move freely depends on the design of the surfaces and the control mechanism. The degree of aerodynamic balancing, the stiffness and the inertia of the system components are all important factors. In general the effect of leaving the stick free is to reduce the static stability.