Adaptive Backstepping Control and Safety Analysis for Modern Fighter Aircraft

Abstract

There exist many examples of aircraft incidents in which the pilots have successfully used the remaining control authority over an aircraft to save the airframe and its passengers and cargo from apparently hopeless failure conditions. Unfortunately, the opposite is also true. Several accidents happened in which the crew was not able to save the aircraft, although post-flight analysis showed that it was possible with alternative, perhaps unconventional, control strategies. These aircraft accidents indicate that there is a potential benefit of fault tolerant flight control techniques, which are able to accommodate changes in the aircraft’s dynamics due to damage to the aircraft and failures of its systems. In this dissertation a modular adaptive flight control approach was developed based on adaptive backstepping with a recursive least squares estimator. The proposed control design was evaluated in numerical simulations on high-fidelity fighter aircraft models. The performance has been compared in simulation scenarios at several flight conditions with the aircraft model suffering from actuator failures, longitudinal center of gravity shifts and changes in aerodynamic coefficients. Results of the simulations demonstrate that the adaptive flight controller provides a significant performance improvement over classical, non-adaptive flight control designs. Although adaptive flight control techniques have shown that it may be possible to stabilize a damaged aircraft for a variety of faults and failures, it is still unclear what maneuvers are still possible and how much the performance of the aircraft has degraded due to these faults and failures. The safe flight envelope is defined as the region in the state space for which safe operation of the aircraft, and safety of its cargo and passengers can be guaranteed. In this dissertation the level set method was researched to determine the safe region of operation of the aircraft. Application of this method to an F-16 model at different flight conditions showed shrinking of the safe flight envelope and decreased maneuverability with decreasing dynamic pressure.