Observed Pilot-Vehicle Loop-Closure Characteristics For Hovering Aircraft Control

The parameters in an a priori form of the pilot describing function have been measured for pitch axis stabilization in a hovering aircraft simulation. Controlled element characteristics of static gain and damping were varied, using an input disturbance representative of atmospheric turbulence at very low altitudes. The pilot describing function parameters were measured during each run by an analog mechanization of the method of steepest descent. The results show that, for a given level of damping, the pilot adjusts his gain as the airframe static gain changes in order to hold a constant forward-loop gain. This value of forward-loop gain increases as damping decreases, thus maintaining a relatively well-defined band of open-loop cross-over frequencies between 1 and 2 rad/sec. The pilot utilizes lead in order to provide a phase margin between 20° and 60°.