Investigation of inter rotor spacing effect on aerodynamics and aeroacoustics of a coaxial rigid rotor helicopters in full configuration using high-fidelity numerical simulations

Abstract

The coaxial rotor system, a key component of high-speed and long-range compound helicopters, eliminates the need for a tail rotor. However, aerodynamic interactions between the upper and lower rotors can substantially influence aerodynamic performance and noise generation. Inter-rotor spacing (IRS), defined as the ratio of the vertical distance between the upper and lower rotors to the diameter is a critical design parameter that affects rotor wake structure and blade-vortex interaction (BVI), particularly during forward flight. This study investigates the effects of IRS and advance ratio on the unsteady aerodynamics and aeroacoustics of coaxial rotors through high-fidelity numerical simulations. The simulations employ the Spalart-Allmaras improved delayed detached eddy simulation model coupled with overset mesh techniques. Aeroacoustic analysis is conducted using the Ffowcs Williams-Hawkings acoustic analogy. The full-configuration X2TD helicopter, excluding the pusher propeller, serves as the baseline model. Results indicate that a greater IRS value leads to higher thrust growth rates under low-speed forward flight conditions and reduces unsteady loading fluctuations. Moreover, a greater IRS value mitigates BVI and loading noise, reducing the overall sound pressure levels. Notably, acoustic differences between the isolated coaxial rotor and full-configuration models highlight the influence of fuselage reflection, with downward noise propagation attenuated by up to 2 dB in the latter. These findings provide valuable insights into IRS optimization for enhanced aerodynamic efficiency and noise reduction in coaxial rotorcraft.