Autorotation path planning using backwards reachable set and optimal control

This paper presents a methodology to compute the backwards reachable set from safe on ground to a trimmed autorotation condition. This backwards reachable set represents the region of the trimmed autorotation state space from which safe paths to touchdown at a specified point are guaranteed to exist. The backwards reachable set is found by computing optimal trajectories from candidate initial states (distance and height above the touchdown point, horizontal speed, descent rate, rotor speed) to the designated touchdown point. In addition, the set of trimmed autorotation conditions which are likely to lead to safe trajectories to ground are computed. As an example, the safe landing set is computed for a generic utility helicopter.