Investigation Noise from Electric, Low-Tip-Speed Aircraft Propellers

Open Access
Author:
Zolbayar, Bolor Erdene
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 10, 2018
Committee Members:
  • Kenneth S. Brentner, Thesis Advisor
  • Amy R. Pritchett, Committee Member
  • Sven Schmitz, Committee Member
Keywords:
  • propeller noise
  • low-tip-speed propeller noise
  • unsteady loading
  • electric low-tip-speed propeller noise
Abstract:
This thesis 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 are used for reference, but all the propellers evaluated in this work have been designed for appropriate tip speeds considered. Electric motors are lighter than conventional combustion engines. They also can operate at low-tip-speed more effectively than conventional internal combustion engines. These characteristics allow aircraft to have multiple low-tip-speed propellers, which can result in a reduction in propeller noise. In this research, the noise of isolated propellers designed for low Mtip=0.3 and 0.4 was investigated and compared with noise of a baseline propeller designed for Mtip=0.7 for the same thrust and forward aircraft speed. The design and analysis code CROTOR is used to design all the propellers in this work. The lower tip speed reduces the noise considerably, but the individual noise sources tend to not scale with tip Mach number in the same way. With steady axisymmetric inflow at the level flight condition, the maximum propeller noise is always found in the propeller plane. To study the relationship between propeller noise and tip speed, isolated 3 and 6-bladed propellers are designed for various tip speeds. (Mtip = 0.3, 0.4, 0.5, 0.6, and 0.7). For steady level flight cases, the noise in the propeller plane is found to be the highest and is found to have a linear relationship with blade tip speed. Unsteady loading is implemented as an approximation to the loading propeller experiences when it operates at an angle of attack. The effect of unsteady loading is shown to change the directivity of the noise distribution substantially in the plane of the propeller and to impact the ultimate noise reduction achievable through tip speed reduction, especially for low-tip-speed designs. In particular, along with the propeller axis of rotation, the noise levels do not change significantly with tip speed, while in the plane of the propeller, the noise reduction with reduced tip Mach number is promising. However, with higher angles of attack, the noise below the propeller increases substantially, and its directivity rotates approximately by the angle of attack –. Increasing the number of blades can result in significant noise reduction in the plane of rotation (up to 25 dB), but practical limits on the number of blades should be investigated. Distributed electric propulsion system makes it feasible to use several smaller propellers. So, configurations with 1, 2, 4, and 6-propellers designed in CROTOR code were considered. In the plane of rotation, the noise directivity becomes increasingly complex with an increasing number of propellers and the noise directivity can be shaped into "loud" and "quiet" directions. These results are aimed to give some direction that could be helpful to design engineers and to demonstrate how current design and analysis tools can be used in a fast and simple manner to obtain propeller noise predictions.