An alternative approach to aeroservoelastic design and clearance
The interaction between an aircraft's structural dynamics, unsteady aerodynamics and flight control system is known as aeroservoelasticity. The problem can occur because the control system sensors are of sufficient bandwidth to sense the structural vibrations as well as the rigid-body motion of the aircraft. This sensed structural vibration can result in further excitation of the structure through both aerodynamic and inertial excitation, leading to a potential closed-loop instability. At present, such an unstable interaction is prevented by the inclusion of notch filters within the feedback path which have a detrimental effect on the aircraft's rigid-body performance. The current clearance procedure is restricted by a poor understanding of the array of complex issues involved. The aim of the project was to develop a clearer understanding of the interactions between system components leading to a reduction in the clearance requirements. Work has concentrated on the effects of system nonlinearities and on the digital nature of modem control systems. A major source of nonlinearities within the control system are the servo-hydraulic actuators. Through detailed actuator modelling confirmed by rig testing of actual hardware, these nonlinearities are analysed and a method for predicting the response of the actuators in the presence of two input signals proposed. As a result, it is demonstrated that an unstable structural oscillation would cause a limit-cycle oscillation as opposed to an unbounded response. Through nonlinear system theory the criteria for the existence of such limit-cycles are obtained, enabling them to be predicted and therefore prevented. Consideration of the true nonlinear nature of the aeroservoelastic system has enabled an alternative design and clearance procedure to be proposed which reduces the attenuation requirements of the structural-mode filters whilst ensuring satisfactory aircraft performance even in the presence of modelling errors. This design procedure is demonstrated on both a model of the aircraft system and a simple test system enabling verification of the nonlinear analysis and comparison between the current and proposed alternative procedures. As a result, it is demonstrated that consideration of the true nonlinear nature of the aeroservoelastic interaction has the potential for allowing a significant reduction in structural filter attenuation requirements. Consequently, a reduction in the phase lag due to the filters is possible resulting in an improvement in closed-loop system performance whilst ensuring the safe operation of the aircraft.