Eugenio Denti

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.

Air Data Computation Using Neural Networks

The paper deals with the use of neural networks for the determination of pressure altitude and Mach number of a fly-by-wire high-performance aircraft during flight. In previous works the authors developed a methodology based on polynomial calibration functions for the determination of such flight parameters, together with the angles of attack and sideslip. Such an approach provided successful results, but the use of different polynomial functions in different areas was needed to map the entire flight envelope. The fading methodologies for the management of polynomial functions overlap and considerably increased both procedure complexity and the time to spent for the proceduretuning.Inparticular,thecalibrationfunctionsrelatedtotheMachnumberandstatic-pressureestimation are susceptible to these problems because of their high nonlinearity. The alternative approach studied in this paper, basedonneuralnetworks,providesalevelofaccuracycomparablewiththatofpolynomialfunctions.However,such anapproachissimpler,becauseitallowstheentire flightenvelopetobemappedbymeansofasinglenetworkforeach outputparameter,andsoiteliminatesthefadingproblems.Inaddition,thenewprocedureisextremelyeasiertotune when new data from flight tests are available. This is a very important point, because several versions of the air data computation algorithms are generally to be developed in parallel with the flight-envelope enlargement of a new aircraft.

Robust force control in a hydraulic workbench for flight actuators

The work deals with the design of the force control in a hydraulic workbench for primary flight actuators, to be used for hardware-in-the-loop simulations of a modern Fly-By-Wire Flight Control System. For this application, a high-bandwidth force response is needed in order to simulate aerodynamic loads on the control surfaces, but plant uncertainties can imply significant limitations. The variation of structural stiffness, due to hinge play and hinge local deformation, the uncertainties related to the flight actuator stiffness and the ones related to the hydraulic plant parameters lead to the necessity of a robust approach to the design of the force control. In the paper, a nominal LTI model of the plant is developed and the closed-loop force control is designed by means of a loop-shaping approach for different values of the bandwidth. The stability of the closed-loop force-controlled system is then verified by a robustness analysis, assuming the structural stiffness, the flight actuator stiffness, and some of the hydraulic plant characteristics as uncertain parameters. The bandwidth of the force control is determined by finding an optimal compromise between dynamic performance and stability margin. Finally, in order to overcome the additional problems related to the flight actuator movements due to aircraft manoeuvres, a compensation feedback based on the flight actuator acceleration is proposed.