Prescribed Performance-Based Envelope Protection Control for Coaxial Compound Helicopter in High-Speed Flight

The control complexity of coaxial compound helicopters (CCH) increases pilot workload and safety risks during aggressive maneuvers, necessitating an envelope protection system for safer operations. However, integrating such systems may degrade handling qualities. In this paper, a prescribed performance-based envelope protection control scheme is proposed, with a focus on both flight handling quality and safety. In this scheme, the required CCH transient performance and flight handling qualities are achieved by developing a command model-based prescribed performance boundary. Subsequently, the barrier Lyapunov method is employed to rigorously enforce the CCH state constraints that reflect the necessary envelope protection requirements. Additionally, an immersion and invariance disturbance observer is utilized to estimate the lumped disturbance, thereby enhancing the capability to manage external disturbances and model uncertainties. The envelope protection simulations for normal acceleration and roll angular rate of the CCH demonstrate that the proposed control scheme effectively limits the protected variables while achieving satisfactory attitude tracking performance.