An experimental investigation of noise characteristics of overlapping propellers

Multiple propellers arranged in overlapping configurations can reduce the sizes of unmanned aerial vehicles for compact architectures. However, the resulting aerodynamic interaction between the propellers could lead to extra noise and calls for extra studies on acoustic physics. In this work, we conducted aeroacoustic tests of two overlapping propellers in an anechoic chamber, focussing on the influence of the axial separation distance and lateral tip spacing at different tip Mach numbers, ranging from 0.18-0.23 and Reynolds numbers ranging from 5.2 × 104 to 6.7 × 104. The results indicate that, compared with the coplanar configurations, the propeller thrusts of the overlapping propellers are deteriorated. The rear propeller under the impingement of the upstream wake flows is significantly affected, leading to the mean value of the thrust being reduced by up to 8.2% and the thrust fluctuation being increased. Consequently, the tonal noise is intensified by up to 10 dB at the low observation angles (referenced to the rotation axis). However, the broadband noise is slightly changed within 3 dB. Also, the lateral tip spacing has a primary impact on the noise characteristics, while the effect of the axial separation distance is secondary.