To solve the problem of constant velocity control of piston engine for the medium-sized unmanned helicopter,a fuzzy Active Disturbance Rejection Control (ADRC) law is designed with feed-forward compensation.The mathematical model of the engine is established by introducing the rotor load characteristics,and the collective pitch control and forward flight velocity are taken as known disturbances for feed-forward compensation.The fuzzy PID controller is designed to adapt to the nonlinear characteristics in the complicated work process of the piston engine.The rotors aerodynamic change and the wind disturbance are regarded as unknown disturbances,and an Extended State Observer (ESO) is designed to perform estimation and feedback compensation of disturbances in real time.It is verified by numerical simulation that the control law designed in this paper can effectively improve the anti-interference ability and control accuracy of the velocity controller.The ground driving and flight test for the engine are conducted.The results show that the control error of the rotor velocity of the unmanned helicopter is less than 5.0 r/min during the flight test,and the dynamic tracking performance is significantly improved.