Mix and Matching Propellers Reduces Noise

It took some outside-the-box thinking by the folks at Aurora, a Boeing-owned company, to arrive at this invention. They asked the question “but Why are drones so loud?”

The answer is not so simple: constructive interference. When a propeller spins, it has a noise profile. This noise profile is part spinning motor, part propeller’s interaction with air. This profile has peaks and valleys. Add 3 more propellers with identical noise profiles, and the peaks and valleys add up together to form one very large profile. However, constructive interference can be fiddled with.

One solution is to have different kinds of propellers on the drone. According to the inventor, power from a 2-blade vs 3-blade propeller is identical at similar RPM, but their noise profiles are different. The patent reads: “propeller performance is substantially the same at a given speed (i.e., the same RPM); despite the number of blades/propeller blade count. Designing equivalent (matching) propellers with different blade counts reduces the complexity of the flight controller.”

Claim 1. “An aerial vehicle having a reduced noise signature, the aerial vehicle comprising: an airframe; a first electric motor configured to drive a first propeller, wherein the first propeller is oriented to provide lift to the airframe; and a second electric motor configured to drive a second propeller, wherein the second propeller is oriented to provide lift to the airframe, wherein the first propeller employs a first propeller design and the second propeller employs a second propeller design that is different from the first propeller design in terms of blade shape or propeller blade count, and wherein the first propeller and the second propeller are configured to deliver an equivalent amount of thrust when driven at a predetermined revolutions per minute (RPM).”

To translate from Patentspeak, Claim 1 basically says there are two different propellers that have the same performance, on a flying thing.

Title: “Systems And Methods For Reducing The Propeller Noise”

US Patent No: 11008093

Filed (US Reg.): 2018-03-22

Granted: 2021-05-18

Applicant: Aurora Flight Sciences Corporation