A Coupled Lateral/Directional Flight Dynamics and Structural Model for Flight Control Design

Abstract

A lateral/directional flight dynamics model which includes airframe flexibility is developed in the frequency domain using system-identification methods. At low frequency, the identified model tracks a rigid-body (static-elastic) model. At higher frequencies, the model tracks a finite-element NASTRAN structural model. The identification technique is implemented on a mid-sized business jet to obtain a state-space representation of the aircraft equations of motion including two structural modes. Low frequency structural modes and their associated notch filters impact the flight control frequency range of interest. For a high bandwidth control system, this frequency range may extend up to 30 rad/sec. These modes must be accounted for by the control system designer to ensure aircraft stability is retained when a control system is implemented to help avoid aeroservoelastic coupling. A control system is developed and notch filters are selected for the developed coupled aircraft model to demonstrate the importance of including the structural modes in the design process.