A Dual-Delta CFD-Coupled Trim Method Based on Dynamic Vortex-Ring Wake for Aerodynamic Loads Study of Coaxial Rigid Rotors

Abstract The coaxial compound helicopter mitigates the dynamic pressure limitations and retreating blade stall issues encountered by conventional helicopters, thereby enabling high-speed flight capabilities. However, the coaxial rigid rotor system introduces complex aerodynamic interactions that complicate trim analysis. To address the computational cost and accuracy challenges in trimming coaxial rigid rotors, this study develops a dual-delta computational fluid dynamics (CFD)-coupled trim method based on a dynamic vortex-ring wake model. This method employs two nested iterative loops: an inner loop that utilizes the dynamic vortex-ring wake model to simulate wake interactions and induced velocities and an outer loop that incorporates CFD to reach the trim solution. This approach efficiently accounts for aerodynamic interference between the upper and lower rotors without relying on experimental interference factors. The method is validated using the Cameron coaxial rotor, demonstrating improved convergence and computational efficiency compared with delta trim methods involving dynamic inflow. This method is subsequently applied to the X2 coaxial rotor to investigate the influence of lift offset on aerodynamic loads in high-speed forward flight. The results indicate that an intermediate lift offset concurrently achieves low vibration loads for coaxial rigid rotors.