Optimized design of trim strategy for coaxial rigid rotor high-speed helicopter

The maximum cruise speed and maximum level flight speed are among the most important performance indexes for coaxial rigid rotor high-speed helicopters, whose special configuration and operating mechanism lead to serious rotor hub loading problems. The trim strategy design parameters such as high-speed flight pitch attitude, horizontal tail installation angle, differential lateral cyclic pitch, and rotor speed, induce significant cross-coupling effects on the required power, rotor hub loads, controllability, and stability. In this paper, the optimal design method of the trim strategy for high-speed flight is conducted under the handling qualities requirements. The aim is to provide a methodology for achieving the optimal trim strategy that achieves the best trade-off between flight performance and rotor hub loads. Firstly, based on the nonlinear flight dynamics model of the coaxial rigid rotor high-speed helicopter, the impact of design parameters of different trim strategies on the trim characteristics, stability, and controllability was analyzed. Subsequently, the trim strategy design problem was described as a mathematical optimization problem. Lastly, to improve optimization computational efficiency and reduce the risk of optimization failure, the trim strategy optimization design was conducted based on the Kriging surrogate model. The optimization results indicate that, at the maximum cruise speed and maximum flight speed, the required power is reduced by 5.7% and 6.9% under the optimal power trim strategy compared to the baseline strategy; the rotor hub moment load is reduced by 55.6% and 55.2% under the optimal rotor hub moment trim strategy; the required power is reduced by 1.8% and 3.1%, and the hub moment load is reduced by 49.4% and 46.2%, under the integrated optimal strategies for both power and hub moment. These results validate the effectiveness of the proposed optimization design method for trim strategy.