Trim State Discovery with Physical Constraints

Abstract

Failure or damage events that degrade performance pose significant risk to aircraft in flight. Adaptive control and system identification may stabilize a damaged aircraft, but identified models may be valid only near each local operating point. This paper presents a novel guidance strategy designed to discover a set of feasible flight states sufficient to enable a safe landing given an unknown degradation event. Rather than persistently exciting the aircraft with the potential to cause further damage, the aircraft may instead be progressively guided through a sequence of trim states that are stabilizable given local envelope estimates. The proposed guidance strategy progressively explores trim state space rather than three-dimensional physical space, identifying a set of low-speed gentle-descent trim states appropriate for landing approach. A potential field method is adapted to steer exploration through trim state space, modeling envelope constraint boundaries as obstacles and desirable approach trim states as attractors. Identified paths through trim state space may present physical-space conflicts such as flight into terrain. To cope, the family of trajectories emanating from the current trim state is propagated in physical space to identify and eliminate physical conflicts. F-16 aileron and rudder jam performance degradation scenarios are presented, showing that the trim state discovery algorithm can effectively explore trim state space without violating vehicle performance or environmental constraints.