Development and first successful flight test of a QFT flight control system

This paper is a discussion of the development and successful flight test of a flight control system designed using techniques of Quantitative Feedback Theory. The flight control system was designed for and flew on the Lambda Unmanned Research Vehicle. Lambda is a remotely piloted aircraft with a wingspan of 14 feet. It is operated by Wright Laboratory for research in flight control technology. The developmental process began with the use of Digital Datcom, a computer program which predicts stability and control derivatives for aerospace vehicles based upon geometric data. Datcom information formed the baseline model of the aircraft. This baseline model was refined by using system identification software to estimate the aerodynamic derivatives from actual flight test data. Maximum likelihood identification was used to identify the natural frequency and damping ratios of the short period and roll modes. This information combined with the Datcom information provided a working model for the flight control system design. Much of the preliminary QFT design work was accomplished at the Air Force institute of Technology. During the same period, a nonlinear simulation was developed at Wright Laboratory. This simulation incorporated a six degree of freedom simulation, and automatic trim calculation, air vehicle kinematics, control surface saturation, and sensor noise recorded from the Lambda on-board control system. When placed in this simulation, the original control system exhibited undesirable behavior. The controller was then adjusted prior to implementation. By this time, a new computer aided design program was developed by AFIT for designing Quantitative Feedback Theory control systems. This program allowed for a rapid redesign, which resulted in the successful flight test control system that flew on 20 November 1992