Passive Rotor Noise Reduction Through Axial and Angular Blade Spacing Modulation

This study investigates the aerodynamic and aeroacoustic performance of a novel two-stage two-bladed coaxial propeller that is axially and angularly spaced. Aerodynamic propulsive thrust and efficiency are validated and evaluated using a Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) model for the two-bladed APC27x13 propeller. Aeroacoustic assessment is conducted using a Ffowcs Williams–Hawkings integral model. A four-bladed coplanar APC27x13 propeller is simulated and considered as the baseline propeller. The CFD results suggest that changes in the rotor thrust for the coaxial blades are within 3% for propellers with 0.25D axial spacing (where D is the propeller diameter) and 30∘ angular spacing for the advance ratio of J=0.3–0.5. The aeroacoustic assessment for J=0.3 reveals that blades with 30∘ and 60∘ azimuthal spacing and 0.25D axial spacing significantly reduce noise compared to the baseline propeller. The reduction is attributed to the redistribution of tonal noise blade passing frequencies, resulting in a reduction in the A-weighted noise levels by up to 2 dBA. Additionally, the study accounts for the effect of the blade tip Mach number, concluding that a tip Mach number ranging between 0.7 and 0.9 is optimal for noise reduction in the 30∘ configuration. The results highlight the potential noise reduction benefits of uneven axial and angular blade spacing while maintaining similar aerodynamic performance.