Far Field Acoustic Comparisons of Commercial and Custom Propeller Mounted on a Boom With and Without a Motor Containment Box

Abstract Increasing interest in eVTOL and Advanced Air Mobility (AAM) has caused a new awareness of propellers. As these vehicles often operate in populated environments, the need for quiet and efficient propellers is becoming more important. A new hybrid propeller design, called a double break, has proven itself to satisfy being both quiet and efficient in the near field when compared to a commercial propeller of similar performance. It is especially important to understand how the propeller sound propagates to the far field. Testing this new design in the far field is the subject of this paper. The location of the propeller on the vehicle is also important and will be addressed. For a quadcopter, the propeller can be mounted in a puller/tractor or a pusher configuration which impacts sound generation. Both configurations are tested. A hybrid propeller with an SG6043 hub airfoil and an SG6041 tip airfoil was designed with a 50_94 double break. This custom propeller had a 21.5 inch diameter, a 2.25 inch chord which tapered to an oval tip starting at 0.79 r/R. The commercial baseline propeller tested for comparison was a KDE CF215, a 21.5 × 7.3 propeller which is 21.5 inches in diameter. Both propellers were tested on a boom, such as would be seen on a quadcopter. Pusher and puller configurations, simulating a propeller and motor mounted below or above the boom, were tested and sound pressure levels measured 50 ft from the propeller test stand. At 50 ft the KDE pusher configuration was 2 dBA quieter than the puller configuration. The custom propeller saw little change between the pusher and puller configurations but was another dBA quieter than the KDE pusher. For all locations tested the highest SPL was directly in front of the propeller perpendicular to the plane of rotation and gradually decreasing to the 90 degree location which was the lowest SPL. The sound spectrums showed there was a noticeable high frequency component, between 2,000 Hz and 4,000 Hz, present for all tests. This was believed to be the contribution of the motor noise and a containment box was constructed to shield the motor from the sound measurements. Tests were run again for both propellers in the puller configuration only, with and without the box around the motor. For the KDE propeller, the box reduced the SPL 1 dBA (at zero degrees) to 4 dBA (at 90 degrees). The data for the box/no box SG hybrid propeller recorded no significant difference in SPL. For all measurements, the high frequencies were still present.