Transition Corridor Estimation for a Tilt-Wing Aircraft with Optimal Control-based Reachability Analysis

Abstract

Tilt-wing aircraft combine efficient cruise flight with vertical take-off and landing capability. This versatility is enabled by the transition flight phases that connect hover and cruise flight states. However, the transition can be considered an unconventional flight maneuver, and its flight physics are associated with high levels of uncertainty, specifically regarding aero-propulsive behavior. Especially the backward transition from cruise to hover is challenging, as the low thrust settings for decelerated flight reduce good-natured slipstream effects, and the wings are pushed towards or beyond flow separation. To ensure safe flight, a sound knowledge of the flight envelope is required, which is expressed by the transition corridor for transformational aircraft. This study estimates the tilt-wing transition corridor for various flight constraints using optimal control-based reachability analysis. Furthermore, the underlying physical causes of the boundaries are examined. While the shape of the forward transition corridor results primarily from the maximum forward acceleration, the backward transition corridor is defined by the aerodynamic decelerating capabilities, the profile stall line, and the tolerated vertical motion. Additional constraints for stall protection or level flight primarily affect the backward transition corridor, but an overlapping area of the forward and backward transition corridors is maintained in both cases. This region of transition flight might be preferred for safety reasons, as the maneuver can be aborted immediately.