Modelling of Flight Control Hydraulic Actuators Considering Real System Effects

The study investigates the effects of servo valve nonlinearity, actuation compliance and friction related nonlinearity on the dynamics of a flight control surface, during its deployment through an electro-hydraulic actuation system. Starting from the pilot command, a realistic model of the electro-hydraulic actuation system is evolved, which includes the command lags, servo valve nonlinearity, actuation chain compliance and friction nonlinearity. A realistic mathematical model for the control surface motion, under the action of the actuator forces and the aerodynamic and inertia forces is postulated, using subsonic incompressible aerodynamics. The above mathematical model is first verified, by carrying out subsystem level simulation wherein effects of servo valve nonlinearity, compliance and friction nonlinearity are studied separately. Next, the combined influence of some of these effects on the resultant control surface response is obtained. Particular attention is paid to the control surface motion during the deployment that can influence the close loop control of the rigid body dynamics. Simulation results show that while friction reduces both transient as well as steady state magnitude of the actuation rate, the compliance influences only the transient response of the actuation rate. The study also provides a methodology for evolving an approximate analytical transfer function in terms of the real system parameters so as to become applicable to a large class of actuation systems.