Experimental investigation of aerodynamic noise of the drone propeller at low Reynolds numbers

In recent decades, the use of drones as part of urban and regional transport infrastructure, as well as military applications, has developed extensively. Contrarily, their high aerodynamic noise is one of the serious challenges to obtaining a certificate of airworthiness for these flying devices. The most important source of noise in multi-rotors, airplanes, and propeller drones with electric motors or even turboprop engines is the noise caused by propellers. In the present study, to learn more about the mechanisms affecting the propeller's noise at low Reynolds numbers and to investigate the sensitivity of the noise to the propeller rotational speed and the polar angle of its propagation, the far-field noise of a drone propeller has been evaluated experimentally. The results show that the propeller's noise includes tonal and broadband noise, so that at low frequencies, the largest propeller tonal noise occurs at the blade pass frequency, and its harmonics are still visible up to high frequencies. On the other hand, the broadband components of noise predominate in the medium and high-frequency ranges. The increase in propeller rotational speed has led to an increase in the amplitude of both the tonal and broadband noise spectrum, so that the values of the overall sound pressure level and the noise spectra of the propeller at the first blade pass frequency have increased with the increment of the propeller speed. Finally, the results showed that broadband noise has a higher sensitivity to the polar angle of sound propagation than tonal noise.