Aeroservoelasticity: key issues affecting the design of flight control systems

The process of designing flight control systems (FCS) for agile combat aircraft is seriously affected by the flexible modes of the structure. The interaction between the triad of structural dynamics, unsteady aerodynamics and the flight control system, which is essential in an unstable aircraft, gives rise to the problem of aeroservoelasticity or structural coupling. The effect of exciting the structural modes is considered as measurement noise in order to show that high loop gains will not produce the desired attenuation. Although the roll-off in the frequency response of the actuation system would give the required attenuation it would place a constraint on the bandwidth thus degrading performance (agility) of the aircraft. As a result, it is necessary to incorporate notch filters to suppress critical structural modes while taking considerable care to avoid increasing the phase lag in the system.