Command limiting for full-envelope guidance and control of rotorcraft

Performance and agility of rotorcraft can be Improved using envelope protection systems (or carefree maneuvering systems), which allow the aircraft to use the full flight envelope without risk of exceeding structural or controllability limits. Implementing the constraints generated by this system in automatic feedback system places saturation limits inside closed loop feedback path which causes problems with integrator wind-up, closed-loop stability, and flight control performance. This paper evaluates the integration of an envelope protection system for a rotorcraft with full-envelope guidance and control using a typical inner loop/ outer loop architecture. A torque limit protection system was implemented to generate constraints on inner loop commands of pitch angle in longitudinal axis and descent velocity in vertical axis. The inner loop constraints were converted to the constraint on the outer loop command of forward velocity and altitude. This procedure eliminates the problems associated with saturation limits in feedback path. The system was tested using a high-fidelity simulation model of the UH-60 helicopter. The performance was evaluated for multiple maneuvers in longitudinal and vertical axis and also for off-axis control input of roll angle. Results showed that the system was effective in avoiding the coupled longitudinal and vertical axis constraints corresponding to the torque limit.