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Toroidal Propellers – a Noise-Killing Game Changer

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.”

 video her: https://tinyurl.com/yxwvxkxx 

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,” says 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.”

“We came up with this initial concept of using a toroidal shape, this annular wing shape, to hopefully make a quieter propeller,” Sebastian continues. “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.”

Within a few attempts, the team indeed found a design that reduced not only overall noise levels at a given thrust level, but particularly noise in the 1-5 kHz range.

Indeed, they sound more like a rushing breeze than a propeller, making a much less intrusive sound. Anecdotally, according to the team, a drone running these props makes a level of sound roughly as annoying as a regular drone about twice as far away

“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,” says 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.”

Propeller noise can be somewhat addressed by placing rings of acoustic treatment around the circumference of a prop’s path, which can also act as prop guards from a safety perspective. But these add parasitic mass, reducing battery life, and they can also catch the wind in outdoor situations, making the drone work harder to stay stable.

The team analyzed these weird-looking toroidal props to see whether there would be a thrust efficiency penalty. Apparently not: the team’s best-performing B160 design was not only quieter at a given thrust level than the best standard propeller they tested, it also produced more thrust at a given power level – pretty remarkable given that standard props have more than a century of development behind them and these toroids are at a very early stage, with plenty of optimization yet to come.

What’s more, their looped shape not only adds structural stability, but decreases the chance of a prop cutting, clipping or catching on things it runs into. You’re still not going to want them hitting you in the face, but there’s probably a marginal safety improvement there.

In terms of drawbacks, these are fairly complex shapes, so they’re much harder to manufacture than standard props using cheap and easy injection molding. They’re probably the sort of thing you need to get 3D printed. But even if they double or triple the price of propellers, these are a low-cost part of a drone and the overall impact might not be that tough on the hip pocket.

It’s unclear at this stage whether designs like this might be relevant at a larger scale, replacing traditional propellers on fixed-wing aircraft, or indeed on electric VTOL air taxis. The latter already appear to be significantly quieter than helicopters, but if they end up flooding the urban airspace with fast, cheap, green aerial transport, every decibel of noise will count when it comes to public and regulatory resistance. The question there, really, is what kind of frequencies these larger props will occupy in the audio spectrum, and whether the toroidal props shift the sound in a human-friendly direction.

The team has patented the design, and while it’s not clear whether there are plans to commercialize it, MIT appears to be prepared to license it to interested manufacturers.