Input delay investigation in the short period flying qualities criteria

Abstract

The equivalent systems proposed by Hodgkinson appear viable to predict the flying qualities characteristics of a given aircraft with high order augmentation system . In the mathematical models for flying qualities problems, control time delay is a pure time delay in the resulting low-order models The design of the controller which involve a time delay implies a supplementary condition thus the flying qualities requirements for the closed loop eigenvalues have to be verified on the maximum allowable time-delays. The feedback controls to correct the stability deficiencies will be modified when the time-delay is non zero. For this reason we are interested in the dependence of the eigenvalues with the time delay. In our paper we define the parameters : TO the value of the time delay for which at least one eigenvalue crosses the imaginary axis; TJ the allowable time delay that value for which at least one (closed loop) eigenvalue crosses the boundaries of the specified region. This paper proposes a preliminary design procedure to achieve a controller structure with maximum allowable time-delay. We begin with choosing the entire region of admissible values of gains by using eigenvalues assignment in a way to meet condition imposed on the damping and frequency when timedelay is zero. In the second phase we find domains of delay parameters with respect to gain space. Finally time-delay requirement and time domain criteria are seen to influence selection of controller structure and gain values. A study case( the high manoeuvrability aircraft ) is given to illustrate the computational procedure.