Aerodynamic Characteristics and Mechanisms of the Rotor in the Vortex Ring State

The helicopter rotor may enter a vortex ring state during steep descent, significantly elevating the risk of safety incidents. Consequently, understanding the aerodynamic characteristics and underlying mechanisms of the rotor in this state is essential for ensuring helicopter flight safety. This study employs Reynolds-averaged Navier-Stokes (RANS) equations, sliding mesh techniques, and structured grid modeling, integrated with the k−ε turbulence model, to conduct numerical simulations of the vertical descent process for a helicopter rotor under varying descent rates. The analysis provides detailed insights into the aerodynamic loads and flow field variations associated with the rotor in the vortex ring state. The findings indicate that as the vertical descent rate changes, the rotor's thrust and torque initially increase slightly, then decrease, followed by another increase. Notably, the maximum thrust loss is observed at a vertical descent rate of 0.94, with a reduction of 22.27%. Moreover, the interaction between the free-stream inflow and the induced airflow, which shifts the position of the rotor blade tip vortices, is identified as the primary factor influencing the aerodynamic characteristics of the rotor in the vortex ring state.