Robust longitudinal flight control system under weight and center of gravity uncertainty

This paper presents a post-design robustness assessment for flight control systems that have been designed using two different techniques, namely H_∞ and μ-synthesis, for the longitudinal pitch rate control system of the Bombadier Challenger 604. In order to perform the analysis of flight control systems, a linear fractional representation (LFT) of the aircraft model is generated. We consider the longitudinal linear equations of motion (both short-period and phugoid modes of the aircraft). The LFT model presents uncertainty in the aerodynamic parameters due to aircraft mass and center of gravity variations. The closed-loop system combined with each controller is used to determine stability robustness and performance robustness with respect to changes in aerodynamic parameters. We also determine the corresponding stability margins and the worst-case parameter combinations destabilizing the system. The objective is to verify that the control system remains stable and achieves desired performance with predicted Level 1 handling qualities for all possible parameter variations over a specified range. Simulation results are given to validate our approach.