Abstract: In order to analyze the influence of lift offset on the aerodynamic characteristics of coaxial rigid rotors in forward flights, a computational fluid dynamics method based on the Reynolds averaged Navier-Stokes equations is established to solve the coaxial rotor flow field. The nested grid method is used to simulate the blade motion, and the dual-time method is used for time propulsion. A high-efficiency trim strategy based on the "delta method" is adopted to trim the control pitches for coaxial rotor at different lift offset states. The validity of the method is verified by calculating the performance of Harrington-1 rotor. The aerodynamic performance and the flow field characteristics of the coaxial rigid rotor at different advance ratios and lift offset states are calculated and compared. The results show that the control pitches of the coaxial rotor are significantly different at small advance ratio, and the trimmed pitches of the dual rotors tend to be the same at high advance ratio. Under the same thrust condition, with the increase of the lift offset, the lift-drag ratio of the coaxial rotor increases first and then decreases, while the drag as the lift offset increases. At different advance ratios, the maximum lift-drag ratios correspond to different lift offsets. When the dual rotors meet with each other, the blade thrusts fluctuate impulsively. Meanwhile, since the flow field is dominated by the advancing blade, the amplitude of the thrust fluctuation of the retreating blade is larger than that of the advancing blade, and the amplitude increases with the increase of the lift offset.

Key words: rigid coaxial rotor, lift offset, rotor trim, aerodynamic characteristics, computational fluid dynamics, overset grid