Effect of pitch attitude on the performance and acoustics of a lift-offset coaxial rotor based on high-fidelity CFD/CSD simulations

This paper investigates the effect of pitch attitude on both performance and acoustics of a lift-offset coaxial rotor based on a first-principles and high-fidelity CFD/CSD loose coupling approach at 150 and 200 knots. The pitch attitudes selected for this research are -5◦, 0◦, and 5◦. The CFD/CSD loose coupling simulations are carried out using the CREATE™-AV software Helios while the coaxial rotor acoustics is simulated using PSU-WOPWOP at eight microphones positioned below the lower rotor. A detailed aerodynamic analysis is performed at 150 knots. A total of six major aerodynamic interactions are identified: 1) hub-wake interaction, 2) self-BVI, 3) parallel rotor-to-rotor BVI, 4) blade-crossover events, 5) root-induced BVI, and 6) reversed-flow-edge-vortex interactions. The strength of these interactions is dependent on the vehicle pitch attitude. The cases with negative pitch attitude show significantly stronger impulsive pressure pulses, which is found to be induced by parallel rotor-to-rotor BVIs of the lower rotor. Moreover, the positive and zero pitch attitude cases tend to dominate the acoustic region on the starboard side of the coaxial rotor, whereas the negative pitch attitude case shows higher acoustic pressure peaks on the port side. Overall, the case with a positive pitch attitude shows significant improvement in rotor aerodynamic efficiency, rotor acoustics, and vehicle power performance at high speed. The hemispherical acoustic simulation results at 150 knots also show that the noise is less likely to propagate in the forward direction with a positive pitch attitude.