Simulation of Tail Rotor Noise Reduction and Comparison with HeliNovi Wind Tunnel Test Data

Abstract

This paper addresses the effects of main rotor (MR) and tail rotor (TR) mutual interaction on the aerodynamics and noise characteristics. A BO105 MR/TR configuration is chosen in the numerical simulations. An unsteady free wake 3-D panel method (UPM) is used to account for the mutual aerodynamic interference between MR and TR. Free wake and rotor noise computations were performed to study the effect of MR/TR mutual interaction on rotor wake development, blade loads and noise radiation. Besides the estimation of noise impact on wind tunnel measurement plane, the impact of TR noise reduction on ground microphone in the real flight procedure is estimated. Several tail rotor noise reduction techniques were investigated and compared with HELINOVI wind tunnel measured data. The investigation included the sense of TR rotational direction and tip speed reduction. Both the test and simulation results indicate that tail rotor noise is most important for climb and high-speed level flight. The study on change TR rotational direction indicated aerodynamic interaction between the main and tail rotors is sensitive to the tail rotor rotational direction. The reduction of TR BVI amplitude and number of TR BVIs on the advancing TR blade side in TR rotating in Advancing Side Up (ASU) is the cause of a significant aerodynamically-induced loading noise reduction. The reduction of the blade tip speed causes in general an increase in the load amplitude and the frequency content which compensate the real reduction of the noise expected by reducing tip Mach number. The comparison between the experimental results and the numerical ones highlighted once more the extreme complexity of the aerodynamic phenomena involved in a complete helicopter configuration operating at different flight conditions.