Detection and avoidance of main rotor hub moment limits on rotorcraft

Future rotorcraft will require systems that allow carefree maneuvering. This has driven the need for advanced algorithms that predict the onset of structural limits and cue the pilot using tactile feedback. This paper presents a new method for the detection and avoidance of the main rotor hub moment limit. The dynamic nature of the hub moment limit makes it a challenging problem. An algorithm was developed which uses linear models to estimate constraints on longitudinal and lateral cyclic stick positions that ensure the transient response of hub moments remains bounded within prescribed limits. The system was tested using a high-fidelity non-linear simulation of a UH-60A helicopter. The simulation was run using prescribed force inputs and a pilot model to simulate maneuvers. The system was shown to be quite successful in constraining stick travel in multiple axes to prevent hub moment limit violations. The algorithm appeared to be relatively robust and could operate in real-time with a 10 ms time frame. The most critical conditions occurred during control reversals, at which point the system effectively imposed rate limits on the stick motion.