A QFT subsonic envelope flight control system design

Abstract

An aircraft's response to control inputs varies widely throughout its flight envelope. The aircraft configuration also impacts control response through variations in center of gravity and moments of inertia. Hence, designing a flight control system (FCS) to accommodate the full flight envelope and configuration set of an aircraft is clearly a complex undertaking. Quantitative feedback theory (QFT) is a robust control design method which provides an avenue of approach to full-envelope flight control design. Furthermore, a QFT-based design method gives the engineer direct control over compensator order and gain. In this paper, a full subsonic flight envelope FCS is designed for the VISTA F-16 aircraft using QFT for four representative aircraft configurations. In addition, flying qualities are imbedded in the longitudinal design by using a control variable which varies with the aircraft's energy state throughout the flight envelope. This variable is a linear combination of the aircraft's pitch channel states and is synthesized to closely reflect the actual control desires of the pilot throughout the aircraft flight envelope. The strict control of the compensator order and gain allowed by QFT facilitates the attainment of desired performance while avoiding physical actuator saturations. Linear simulations with realistically large control inputs are used to validate the design.