TABLE OF CONTENTS Fundamentally, flight control system structural coupling (simply referred to as structural coupling in the UK, or more commonly in the US, aeroservoelasticity) is a phenomenon associated with the introduction of a closed-loop flight control system into a flexible airframe. The flight control system (FCS) might be provided to enhance the natural stability of the aircraft or, in the extreme case, to provide artificial stability to a configuration which has been purposely designed to be unstable to achieve the required aerodynamic performance.
In each case, the FCS augments the forces and moments produced by the vehicle's aerodynamics by deflecting the control surfaces. In order to supplement inherent forces and moments which are proportional to angle of attack ( ?) for stability, say, or pitch rate ( q) for damping, the FCS-commanded control-surface deflection must be a function of the same quantities. Thus the FCS must comprise a sensor pack, to detect the appropriate measures of aircraft motion, a control algorithm or law, to compute the forces, moments and ultimately control-surface deflections required to augment stability as desired and an actuation system to convert the control law command into physical deflections of the elevator and ailerons.
However, the FCS motion sensors will detect not only the rigid-body motion of the aircraft, but also the superimposed higher-frequency oscillations due to the resonances, or flexible modes, of the structure. It may be appreciated that, if the high frequency components...
