Abstract:

To study the driving rotation characteristics of a compound helicopter’s rotor undergoing the upwash in high-speed flight， a compound helicopter trim model is established on the basis of an existing helicopter flight performance model. With a helicopter example similar to X³ helicopter， the effects of lift share and variable rotor speed on the flight performance of the rotor and helicopter are analyzed in the driving rotation state undergoing the upwash. The results show that the energy absorbed from the airflow increases with the increase of speed due to the upwash， and the rotor drag power increases. The torque distribution is different from that in low speed flight. The blade elements that produce the driving torque increase significantly， and the area with larger resistance torque shifts from the retreating blade to the advancing blade. Decreasing the incidence angle of the wing reduces the proportion of the wing lift， which results in an increase in the rotor drag power. This is beneficial for the rotor to absorb energy from the airflow and improve the performance of the rotor and helicopter as the rotor undergoes an upwash flow. At 400 km/h， the rotor drag power with the wing incidence angle of 8° is 11.2% higher than that of 10°， and the lift-to-drag ratios of the rotor and helicopter increase by 35.7% and 2.6%， respectively. In medium- or high-speed flight， excessive reduction of the rotor speed causes the fuselage to pitch up， and the energy absorbed from the airflow by the rotor increases. As the flight speed is larger than 340 km/h， the fuselage remains horizontal， and the rotor drag power is reduced. Reducing the rotor speed is not beneficial for the airflow to provide energy. However， reducing the rotor speed of the compound helicopter in high-speed flight is beneficial for reducing the rotor power consumption and improving the flight performance.

Key words: compound helicopter, rotor, driving rotation, upwash, lift share, variable rotor speed, lift-to-drag ratio