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.