Design of flight control systems to meet rotorcraft handling qualities specifications

This paper describes a methodology for the design of control laws for augmentation of helicopter handling qualities. The design procedure uses eigenstructure assignment techniques for the design of inner loop control laws which decouple roll, pitch and yaw rates and vertical velocity, provide appropriate bandwidths in all channels, and stabilize low frequency open loop instabilities. With the inner loops closed, the angular rates and vertical velocity responses to commands are approximated by four decoupled first order systems. Various response types such as attitude command attitude hold can then be easily realized by simple single loop feedbacks and feedforwards wrapped around these inner loops. Both time and frequency responses show that the closed loop helicopter provides excellent nominal performance in terms of insensitivity to gusts and tracking of pilot commands and achievement of desired response type characteristics (handling qualities). Stability robustness was investigated by approximating unmodeled rotor dynamics, actuators, sensors, filters, sampling and computational delays, etc. by a single time delay. The effect of this uncertainty on the system was evaluated using unstructured singular value techniques.