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 (Formula presented.) for propellers with (Formula presented.) axial spacing (where D is the propeller diameter) and (Formula presented.) angular spacing for the advance ratio of (Formula presented.) – (Formula presented.). The aeroacoustic assessment for (Formula presented.) reveals that blades with (Formula presented.) and (Formula presented.) azimuthal spacing and (Formula presented.) 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 (Formula presented.) and (Formula presented.) is optimal for noise reduction in the (Formula presented.) configuration. The results highlight the potential noise reduction benefits of uneven axial and angular blade spacing while maintaining similar aerodynamic performance.