Comparison of Acoustic Predictions Using Distributed and Compact Airloads

Abstract

This study examines the performance and acoustics of a co-rotating coaxial rotor in hover with flow simulated using the comprehensive analysis code, RCAS, and high-fidelity CFD/CSD model with Helios. A viscous vortex particle method (VVPM) was used to model the rotor-rotor interference in RCAS. Simulations were conducted at various index angles between the upper and lower rotors. From the results it was observed that the VVPM and Helios simulations both predicted similar trends in rotor performance at various index angles. However, the VVPM simulation predicted generally higher sectional lift at the blade tips compared to the Helios results. Both the VVPM and Helios models were used to predict the acoustic properties of the rotor and the VVPM simulation (compact airloads) was found to over-predict the thickness noise by 1.25 dB at the investigated index angles. Additionally, the VVPM result was within 1 dB of the loading noise predicted by Helios (distributed airloads). In the higher harmonic range (at observer locations directly below the rotor), the VVPM acoustic results are very different than those predicted by Helios.