Aeroacoustic Wind Tunnel Tests on Full-Scale Propellers with Unsymmetrical Blade Spacing.

Recent theoretical investigations indicated a noise reduction potential of 3 to 4 dB(A) for aircraft propellers with unsymmetrically spaced blades compared to conventional, symmetrical propellers of identical blade geometry and under identical operational conditions. In orderto validate these predictions, aeroacoustic test were conducted on a full-scale 6-blade propeller in the German-Dutch Wind Tunnel (DNW). The results of this study confirm the theoretical results. For the particular test propeller a maximum noise reduction of about 3 dB(A) was determined for helical blade tip Mach numbers in excess of 0.7 and blade spacing angles in the range from 15 to 20 deg. No detrimental effect on the aerodynamic propeller performance was observed within the operational conditions tested