Electric propulsion is being considered for a wide range of airframes from large commercial transports to the small Unmanned Aerial Systems (UASs). These electric systems, especially for small fixed wing UASs and quadcopters, need to be both efficient and quiet if they are to operate in an urban/populated environment or used in an Intelligence, Surveillance, and Reconnaissance (ISR) scenario.
A propeller test facility was developed to record propeller performance and sound generation in the near field behind UAS propellers. The question of defining near and far field noise was studied by characterizing sound decay with distance from a UAS propeller. Defining near and far field noise is a subject that is not addressed well in the literature. Far field noise generally follows the 1/r decay rate and near field does not. Behind the propeller there are other flow field interactions that also change the decay rate, which this study illustrates. The data presented in this paper shows the difficulty in measuring sound around a UAS propeller and begins to resolve this topic. Previous UAS propeller design work by the authors resulted in propellers that were quieter in the near field and at the same time more efficient. Their studies showed RPM and tip vortex formation both contribute significantly to propeller sound generation. Disrupting the tip vortex formation should decrease the noise being generated. The current work extends these initial findings and examines the noise generation of a stock quadcopter propeller from a DJI Phantom 2 platform. One inch aft of the plane of rotation, this propeller, a 9.4 × 5.0, has a peak sound pressure level (SPL) of approximately 118 dBA under normal static operation producing 0.7 lbf of thrust at approximately 5900 RPM. Modifications were made to four stock propellers by cutting a notch perpendicular to the leading edge of the propeller at the 0.75 r/R and 0.87 r/R locations. The notches were of different depths and widths. Of the modifications, three of the configurations did not noticeably decrease the sound. However; the final configuration reduced the peak near field SPL to 111 dBA, a 6% reduction in dBA over the stock configuration corresponding to a greater than 50% reduction in sound generation. Smoke visualization confirms that a notch located at 0.87 r/R effectively disrupts the tip vortex formation, causing the tip vortices to dissipate much earlier than the stock propeller without the notch. Examining the noise frequency spectrums associated with both the stock and the modified propeller also confirm that the notch changes the magnitude and frequency distribution of the sound being generated.