Comparison of Acoustic Predictions Using Distributed and Compact Airloads

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. When the index
angle is between -30^o <= phi <= +30^o, the magnitude of the thickness and loading acoustic pressure waves are largest
and the corresponding thickness and loading noise is largest for this range of index angles. As the separation of the
blades increases (|phi|> 30o), the thickness and loading noise tend to decrease. While the thickness noise is primarily
generated in-plane (elevation angle 0o), the loading noise is predominantly at an elevation between 20^o to 50^o.