L₁adaptive control of a generic fighter aircraft

Abstract: This master's thesis was performed at the section of Flight Control Systems at SAAB Aeronautics in Linköping as a part of my Master of Science in Aerospace Engineering at KTH, Stockholm. This report examines the use of L₁ adaptive control to stabilize the inner longitudinal and lateral loops of a generic fighter aircraft, in the event of failure of the system that measures current speed and altitude. The philosophy of the L₁ adaptive controller is to decouple the adaptation from the control loop by using a state-predictor based adaptation scheme, still only compensating for the uncertainties within the bandwidth of the control channel by the use of low-pass filters. The main goal of the project was to investigate in the tuning of the L₁ adaptive controller with respect to the nonlinear uncertainties related to the failure, and with a limited sampling rate of 60 Hz. The desired closed-loop dynamics for the statepredictor was designed by linearising the aircraft dynamics in a point in the middle of the flight envelope and placing the poles of the system with respect to flying qualities. The modified piecewise constant adaptation law was chosen as adaptation law, which achieves faster adaptation by increasing the sampling rate, yielding better performance at a given sample rate compared to the piecewise constant adaptation law [1]. All the states were transformed to discrete time in order to be implementable digitally. Results have shown that augmenting a state-feedback controller with a L₁ adaptive controller increases robustness in the whole flying envelope, with good flying qualities. Problems were discovered in the low speed regions of the envelope, where the L₁ adaptive controller did not provide the desired performance. A switching scheme between two L1 adaptive controllers was examined. The switch between the controllers was done by knowing when the landing gear was up or down. The second state-predictor was designed with linearised dynamics in landing speed and altitude. The switching scheme was own in a simulator with a nonlinear generic fighter aircraft model with good results