A hybrid trim strategy for coaxial compound helicopter

Interest in the coaxial compound helicopter (CCH) has been increasing in the civil aviation and engineering community for its high-speed and high-maneuverability features, and is likely to continue to do so for the foreseeable future. Since the control in CCH is totally different from the conventional helicopter, the redundant control strategy design is one of the biggest challenges. In this study, the CCH model based on XH-59A is built to investigate the impact of the propeller and the elevator on the flight performance. Four trim strategies with different objectives are proposed and then compared to find the optimal control allocation. A heuristic descent search method is applied to search the optimal velocity at which the propeller and the elevator are engaged. A significant improvement of power required at medium- and high-speed with acceptable rotor airloads increment was found by using the Hybrid Trim strategy in the speed range of 0–100 m/s, with regard to a pre-configured pitch angle schedule. The corresponding control variables obtained located in a reasonable control range, with a maximum power reduced of 13% at 100 m/s, which showcase the potential of the Hybrid Trim strategy.