Helicopters are extensively utilized in both military and civilian sectors due to their unique flight capabilities. However, their operational safety is constrained by complex flight envelopes and inherent nonlinear, strongly-coupled dynamics. Flight Envelope Protection (FEP) technology, which aims to mitigate these limitations by predicting potential limit exceedances and implementing effective avoidance measures, has emerged as a critical research area in modern helicopter flight control to reduce pilot workload, enhance flight safety, and maximize available maneuverability. A comprehensive review of the key technologies and research progress in helicopter FEP are presented. Firstly, the composition, classification, and typical limit parameters of the helicopter flight envelope are outlined. Subsequently, following the two fundamental stages of “limit prediction” and “limit avoidance”, a variety of envelope protection methods are elaborated. For limit prediction, the principles and applicability of prominent algorithms such as the dynamic trim method, the peak response estimation method, and the data-driven method are analyzed. For limit avoidance, cockpit-cueing-based protection strategies, automatic intervention architectures based on command limiting and control limiting, limit avoidance via state limiting and protected variable feedback methods, and approaches that integrate envelope constraints into controller design are reviewed. A comparative analysis based on simulation results highlights the distinct characteristics of these methods in terms of maneuverability, envelope protection effectiveness, and robustness. Finally, the primary challenges in the field are summarized, including the design of envelope protection for advanced rotorcraft configurations, the trade-off between FEP and handling qualities and the robust fault-tolerant envelope protection under uncertainties and fault conditions. Future trends toward more intelligent, adaptive, and integrated FEP systems are further discussed. This review is intended to provide a valuable reference for both research and engineering applications in helicopter FEP technology.

Multiple flight modes and high maneuverability are typical characteristics of coaxial high-speed helicopters, making conventional separated flight/propulsion control strategies insufficient to satisfy high-performance control requirements. Meanwhile, the pronounced dynamic coupling across the entire flight envelope, together with strong external disturbances and time-varying flight characteristics, poses severe challenges to the robustness of control strategies. To address these issues, this paper proposes an integrated flight/propulsion control architecture based on Robust Explicit Model Following Control (REMFC). First, based on the torque-speed relationships between the rotor and the engine, an integrated flight/propulsion simulation model is established, incorporating the coaxial helicopter, turboshaft engine, and transmission system. Subsequently, by reconstructing the feedforward-feedback structure of explicit model following control and introducing an Extended State Observer (ESO), a robust explicit model following control strategy is developed. The proposed method achieves online estimation and compensation of unmodeled dynamics, aerodynamic disturbances, and parameter uncertainties, and the uniform boundedness of the closed-loop tracking error is theoretically guaranteed. Finally, based on the proposed REMFC strategy, an integrated flight/propulsion control law is designed to realize coordinated control and dynamic regulation of flight manipulation and engine speed. Multi-mission simulation results over the entire flight envelope demonstrate that the REMFC-based integrated control approach for coaxial high-speed helicopters outperforms conventional methods in reference command tracking, disturbance rejection, and rotor speed stabilization, significantly improving flight performance and robustness under complex flight conditions.