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
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.