The control system of a helicopter, including a hydraulic servoactuator, HSA, and an autopilot is investigated. The dynamic behavior of HSA is studied theoretically and experimentally. A nonlinear mathematical model and a computer simulation program are developed for the HSA and validated by comparing experimental and theoretical results. A second order transfer function for the HSA is also deduced by using the simulation program. A mathematical model for the hovering helicopter is developed and used to deduce the. helicopter transfer function. The helicopter is found to be unstable in hovering, it could be stabilized either through the pilot action or by using an autopilot. An autopilot is designed to assure the system stability. The transient response of the helicopter equipped with the HSA and the proposed auto pilot is investigated. The effect of some operational parameters on the, HSA and helicopter dynamics is studied theoretically. The results showed that the autopilot insures the required helicopter. stability and reduces substantially the effect of certain. HSA defects, such as the increase in internal leakage.