Aerodynamic and structural characteristics of helicopter rotor in circling flight

To investigate the distinct properties of the helicopter rotors during circling flight, the aerodynamic and dynamic models for the main rotor are established considering the trim conditions and the flight parameters of helicopters. The free wake method is introduced to compute the unsteady aerodynamic loads of the rotor characterized by distortions of rotor wakes, and the modal superposition method is used to predict the overall structural loads of the rotor. The effectiveness of the aerodynamic and the structural methods is verified by comparison with the experimental results, whereby the influences of circling direction, radius, and velocity are evaluated in both aerodynamic and dynamic aspects. The results demonstrate that the circling condition makes a great difference to the performance of rotor vortex, as well as the unsteady aerodynamic loads. With the decrease of the circling radius or the increment of the circling velocity, the thrust of the main rotor increases apparently to balance the inertial force. Meanwhile, the harmonics of aerodynamic loads in rotor disc change severely and an evident aerodynamic load shock appears at high-order components, which further causes a shift-of-peak-phase bending moment in the flap dimension. Moreover, the advancing side of blade experiences second blade/vortex interaction, whose intensity has a distinct enhancement as the circling radius decreases with the motion of vortexes.