https://newatlas.com/aircraft/toroidal-quiet-propellers

Toroidal propellers: A noise-killing game changer in air and water
By Loz Blain
January 26, 2023

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These strangely-shaped twisted-toroid propellers look like a revolutionary
(sorry) advance for the aviation and marine sectors. Radically quieter than
traditional propellers in both air and water, they're also showing some huge
efficiency gains.
Propellers are designed to take a fluid, generally air or water, and use a
rotating motion to push that fluid through. They're evolutions, in a sense,
from the Archimedes' screw, which was likely used in ancient Egypt thousands
of years before it was described by Archimedes in 234 BCE.

For devices designed to revolve, however, there's been little in terms of
revolutionary design changes for an awfully long time; prop-driven aircraft
still use twisted-aerofoil bladed props similar in design to the
bamboo-copters Chinese kids were enjoying 2,400 years ago, with surprisingly
slim gains in efficiency over the wooden props the Wright brothers developed
in wind tunnels in 1903. Boats still use screw-style propellers, variants of
which can be found as far back as the 1700s.

So we're fascinated to find a couple of groups claiming they've demonstrated
significant advantages in both air and water using a markedly different
shape – specifically, strange twisted-toroid ring shapes that appear not
only to be much, much quieter than traditional designs, but so much more
efficient, particularly in the marine space, that they could mark a profound
leap forward.

MIT's super-quiet, 3D-printed toroidal propellers fitted to a commercial
drone for testing
MIT's super-quiet, 3D-printed toroidal propellers fitted to a commercial
drone for testingMIT Lincoln Laboratory
A potential game-changer in the air
One key issue with multicopter props is their annoying noise, which is often
described as "whiny," because much of it sits right in the same frequency
range as a baby's cries. Humans tend to be most sensitive to sounds between
around 100 Hz and 5 kHz. This makes evolutionary sense; it's where we hear
vowel sounds that are key to verbal communication. But it's a key issue if
multicopters are going to fulfill their potential and fill our skies with
fast, cheap, clean aerial delivery services. Residents and lawmakers don't
want to add more annoying noises to urban life.

A team working on a silent, ion-propelled plane at MIT's Lincoln Laboratory
found itself wondering whether prop noise in multirotors could be mitigated
with differently-shaped propellers.

"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."

The biggest noise reductions occur right where people will want them: in the
1-5 kHz frequency range
The biggest noise reductions occur right where people will want them: in the
1-5 kHz frequency rangeMIT Lincoln Laboratory
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. Have a listen in the video below:

"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.

While the MIT team hasn't spent much time optimizing these weird-looking
props for efficiency, they're already producing more thrust for a given
power level than standard DJI propellers
While the MIT team hasn't spent much time optimizing these weird-looking
props for efficiency, they're already producing more thrust for a given
power level than standard DJI propellersMIT Lincoln Laboratory
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.

Sharrow's aftermarket toroidal propellers for a range of outboard motors:
expensive, but the claimed benefits are extraordinary
Sharrow's aftermarket toroidal propellers for a range of outboard motors:
expensive, but the claimed benefits are extraordinarySharrow Marine
An even bigger advantage in the water
Drones and aviation are one thing, but aerodynamics and hydrodynamics are
closely related, and it turns out there's already a product close to
production in the marine space that takes a very similar approach.

Sharrow Marine has been getting frankly spectacular results from boat
propellers that use toroidal loops instead of standard blades. After several
years of development, the company has now tested its props against hundreds
of standard propellers, and the difference is incredible. Sharrow's props
simply don't create tip vortices – a major source of energy loss and a
surprisingly large component in the overall noise of an outboard engine.

The gain in efficiency is ridiculous in the water at midrange RPMs, filling
in a pronounced hole in a boat's acceleration curve and conserving huge
amounts of energy
The gain in efficiency is ridiculous in the water at midrange RPMs, filling
in a pronounced hole in a boat's acceleration curve and conserving huge
amounts of energySharrow Marine
Vastly reducing the amount of fluid that "slips" out the sides of a
propeller rather than being pushed through, the toroidal props suck more
water through, and advance a boat further, per turn. They regularly double
the speed a boat can achieve at lower and mid-range RPMs, radically
broadening the effective rev range of the motor. And they reduce fuel
consumption by somewhere around 20% – a seriously big deal given the huge
energy requirements of propeller-driven boats and the scale of the industry.

Sharrow says they have the interesting effect of vastly reducing a boat's
tendency to pitch backward as it accelerates; instead, the entire boat rises
out of the water while staying much more level. On top of all this, the
effect on noise is absolutely profound, as you can see in the video below.