Development and experimental validation of real-time executable models of primary fly-by-wire actuators

The availability of practical real-time models of primary flight actuators is a key aspect for performing hardware-in-the-loop simulations of fly-by-wire flight control systems. The solution could be offered by empirical models tuned on experimental data, but this approach would imply that hardware-in-the-loop simulations could be performed only after the actuators have been designed, constructed, and tested. The alternative approach, to which this work refers, is to develop high-fidelity actuator models based on the component physics and to reduce their complexity by trying to obtain a compromise between accuracy of results and real-time execution requirements. In the paper, real-time models of a servo-hydraulic actuator for primary flight controls are developed, taking into account the basic features of the fly-by-wire implementation (the non-linear direct-drive servovalve dynamics, the structural compliance, the oil compressibility, the saturation of commands, and the digital controls) as well as other physical phenomena, which are often disregarded in hydraulic actuator modelling (the hinge play, the flow forces on the servovalve spool, and the laminar servovalve flow). The simulation results of models characterized by different levels of complexity are compared with experimental data obtained by testing the aileron actuator of a modern fly-by-wire aircraft, and the relative importance of the model characteristics is highlighted and discussed, providing useful guidelines about actuator model reduction for real-time applications.