Post-damage reconfiguration for rotorcraft with control redundancy

Abstract

Aircraft survivability in the event of component failure or some loss of control effectiveness is a critical area of research, particularly with regards to the control system design. While this has been thoroughly researched for fixed-wing aircraft since the 1980s, there has been a lack of a similar body of work for rotorcraft. This lack is mainly due to the absence of control redundancy on a conventional single main rotor helicopter. A fully compounded helicopter bridges this gap by adding fixed-wing control surfaces (flaps, ailerons, stabilator, and rudder) and compound-specific auxiliary controls (propeller thrust and main rotor speed). When the platform of interest possesses a significant amount of control redundancy, the allocation of these controls plays a vital role in the system's performance and longevity. The design choices made can allow for tolerance to a range of different control failures. The availability of redundant control effectors allows for an exploration of its ability to tolerate damage on an aircraft, thereby improving its survivability. Furthermore, there is little understanding of a rotorcraft's flight dynamics and transient behavior when damage occurs, and the controls are reconfigured to tolerate such damage. This exploration forms the crux of this dissertation. On a UH-60 Black Hawk, the stabilator can act as a redundant control effector at moderate to high-speed flight conditions. A steady-state trim analysis is performed to demonstrate the feasibility of trimmed flight in various conditions with different locked servo actuator positions for the forward, aft, and lateral actuators. After failure, the controls are reconfigured to partially reallocate the control authority in the longitudinal axis from the main rotor longitudinal cyclic to the stabilator. Flight simulation results demonstrate the ability of this reallocation to compensate for locked-in-place failure of the forward main rotor swashplate servo actuator, as well as the ability of the aircraft to recover safely through a rolling landing maneuver. A similar range of locked positions of main rotor swashplate actuators is demonstrated to be feasible for aircraft recovery using control of the stabilator. So far, stabilator use has been shown to work in an adaptive sense, where the control mixing is remapped in flight once failure is detected. Next, it is shown to perform well when the defined mixing utilizes the stabilator even on the undamaged aircraft, removing the need to detect and identify specific failures on the aircraft. Further investigation considered the benefit of allowing for more or less longitudinal authority to be given to the stabilator in different flight conditions in the context of handling qualities ratings for the aircraft in pitch attitude and vertical rate response. Stabilator hardover failure is also examined. This work is then extended onto the compound helicopter platform. A reconfigurable control allocation method is applied on a compound helicopter in order to utilize the redundant control effectors in the feedback loop to compensate for locked-in-place actuator failures. A range of tolerable positions for locked-in-place actuator failures is established for the aircraft at a cruise speed of 150 knots. A full authority model following linear dynamic inversion control architecture is implemented for the nonlinear simulation model. It is shown to successfully compensate for actuator failures when the feedback control and the pseudoinverse control allocation method redistributes the control authority to the working actuators, assuming fault detection has taken place. Finally, a comparison of the robustness of a baseline pseudoinverse control allocation to an adaptive redistributed pseudoinverse method when the aircraft is subjected to different actuator failure at their extreme positions is examined. This is carried out through handling qualities based assessment and dynamic nonlinear flight simulations.