MIT's latest drone propellers are very quiet and efficient

They consist of two blades looping together so that the tip of one blade curves back into the other.
Loukia Papadopoulos
MIT's toroidal propellers
An example of a Sharrow propeller. (Courtesy of Sharrow Engineering. Patented by Sharrow Marine)

Sharrow Marine 

The MIT Lincoln lab has produced some new devices for aircraft and drones that make some impressive efficiency gains and are radically quieter.

“The toroidal propeller allows a small multirotor unpiloted aircraft, or drone, to operate more quietly than current drones that use propeller forms unchanged since the beginning of aviation,” said a statement by the lab.

“By enabling a drone that is less of an acoustic annoyance, this propeller may accelerate the acceptance of such aircraft for a wide range of uses—for example, aerial deliveries, cinematography, industrial or infrastructure inspections, and agricultural monitoring.”

Two blades looping together

The quiet toroidal propeller consists of two blades looping together so that the tip of one blade curves back into the other. This closed-form structure reduces and controls the drag effects of swirling air tunnels (i.e., vortices) created at the blades' tips and strengthens the propeller's overall stiffness. As such, the propeller’s acoustic signature is significantly decreased without affecting its performance.

This has been proven by tests of prototype toroidal propellers on commercial quadcopters that indicated thrust levels comparable to those of conventional propellers at similar power levels. The resulting reduced sound levels allowed toroidal-propeller-equipped drones to function without affecting human hearing at distances half of those encountered in typical operations.

"Propellers, as we know, are pretty loud," told NewAtlas Dr. Thomas Sebastian, a senior staff member in the Lincoln Lab's Structural and Thermal-Fluids Engineering Group. "And we can look at wings to see how that works. Back when people were coming up with all kinds of crazy ideas for airplanes in the early 1900s and during World War 2, there were a couple of designs that were basically these ring wings. So I wondered what it would look like if you took a ring wing and turned something like that into a propeller."

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"We came up with this initial concept of using a toroidal shape, this annular wing shape, to hopefully make a quieter propeller," Sebastian continued. "I had an intern of mine, who was just absolutely phenomenal, run with the idea. He took the concept and created a bunch of iterations using 3D printers."

Reducing noise

With a little effort, the researchers were able to produce a model that reduced noise in the 1-5 kHz range.

“The key thing that we thought was making the propellers quieter, was the fact that you're now distributing the vortices that are being generated by the propeller across the whole shape of it, instead of just at the tip," said Sebastian. "Which then makes it effectively dissipate faster in the atmosphere. That vortex doesn't propagate as far, so you're less likely to hear it."

Some key features of the new propellers are a substantial decrease in signals in the frequency range to which humans are most sensitive and a significant reduction in noise without the requirement of additional components that add weight and increase power draw.

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