Propeller advance paves way for quiet, efficient electric aviation

https://www.chalmers.se/en/current/news/m2-propeller-advance-paves-way-for-quiet-efficient-electric-aviation/

"adjusting pitch angle, chord length and number of blades to optimise propeller design and even out trade-off effect between efficiency and noise... six blades... noise reduction up to 5-8 dBA with only 3.5 % thrust"

Related:

Wingless Transporter Breaks 'Sound Barrier' w/ 300-mi. Range Using NASA-Proven Thrusters

https://www.prnewswire.com/news-releases/wingless-transporter-breaks-sound-barrier-w-300-mi-range-using-nasa-proven-thrusters-301900009.html

Sustainable Skies: NASA and Boeing Unveil the X-66A Aircraft

https://scitechdaily.com/sustainable-skies-nasa-and-boeing-unveil-the-x-66a-aircraft/