An investigation of noise from electric, low-tip-speed aircraft propellers

This paper focuses on propeller noise considerations that would be appropriate for a 6-9 passenger low tip speed, electric propeller driven aircraft. A baseline aircraft and propeller is used for reference, but all the propellers evaluated in this work have been designed for appropriate tip speed considered. Isolated propellers designed for rotational tip Mach numbers 0.3, 0.4, are compared with a baseline propeller designed for a rotational tip Mach number of 0.7 are compared acoustically for the same thrust and forward aircraft speed. XROTOR is used to design all the propellers in this work. The lower tip speed reduces the noise considerably, but the individual noise sources do not scale with tip Mach number in the same way. Unsteady loading, implemented as an approximation to the loading a propeller experiences when it operates at angle of attack, is shown to change the noise dramatically on the axis of the propeller and to impact the ultimate noise reduction achievable through tip speed reduction. In particular, along the propeller axis of rotation, the noise levels do not change dramatically with tip speed, while in the plane of the propeller, the noise reduction with reduced tip Mach number is considerable (>20 dB). Increasing the number of blades can also result in significant noise reduction (up to 10 dB), but there may be practical limits on the number of blades that can be used. With electric propulsion, it is possible to use several smaller propellers, so configurations with 1, 2, 4, and 6 propellers were considered. In the plane of the propellers the noise directivity becomes increasing complex with an increasing number of propellers and the noise directivity can be shaped into "loud" and "quiet" directions. Along the aircraft X-Z plane (symmetry plane), the noise levels ahead and behind the aircraft tend to increase with increasing number of propellers, while in the plane of the propellers (e.g., directly below) the noise does not change substantially by changing the propeller count. These results are intended to give some guidance that may be helpful to designers and to demonstrate how current tools can be used in a fast and simple manner to obtain propeller noise predictions.