Toroidal propeller allows a drone to operate more quietly [pdf] | Hacker News Reader

Toroidal propeller allows a drone to operate more quietly [pdf]

ll.mit.edu

222 pointslxm100 comments

Sort of reminds me of the design of Navy submarine propellers (there known as 'screws'), optimizing for similar noise dampening properties. For a very long time the design of modern submarine screws was one of the closest guarded secrets of submarine design, with submarines docked having cloth shrouds over the screw even in classified facilities.

AFAIK, those weren't torodial, but they had similar long arcs of the blades.

For more information: https://americanhistory.si.edu/subs/anglesdangles/taming.htm...

That's funny, because I remember watching a documentary about some form of caterpillar drive.

That was the one that was "narrated" by Sean Connery?

the hishtorically accurate shientific account of a rushian boat being reshcued?

And all the while, the captain had a mouthful of shtew.

The order ish, engage the shilent drive.

One ping only though

I would have liked to see Montana.

I believe they still shroud them. And I vaguely recall reading about a museum that received a retired prop design; they had to vertically mount it for display partially underground, as the Navy wouldn't let them cut the bottom blades since there was something still secret in the construction of the casting. My guess is it was hollow like jet fan blades

Even if the design principals are known the acoustics of a particular sub are not and can be inferred from the design.

IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops, which you'd expect to benefit from similar noise reduction. Related, I highly recommend HI Sutton's youtube channel if you're into submarines. https://www.youtube.com/watch?v=ugSEIiTZ1Pg

A magnus effect drone may have a decent chance at being quiet. IANAAeroEng but I'd guess that the moving part's interaction with the fluid can be made laminar or at least non-turbulent, and also unlike some propellers, the moving part shouldn't be approaching or interacting with the sound barrier. https://youtu.be/hlmvHfIAszo?t=16

However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126

Ionic propulsion requires massive power. It's cool but you'll probably never see it in real life applications.

Clarke's first law applies here. Maybe beamed power could help around a delivery hub.

I love when primality is useful. Like cicadas. But 9 is not prime.

Whoops I meant to write 11, not 9. I haven't actually heard of an 11-bladed screw, though here's a picture of an 11-bladed pumpjet. https://youtu.be/ugSEIiTZ1Pg?t=386

In wartime it is, and also equals to pi squared.

>IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops,

Might be just odd-numbered ones, not neccesarily prime. But other interesting case is how car tyres have variable feature size on the tyre so the produced sound is more spread over the spectrum and appear quieter

> However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126

That would be literally shocking (and changes properties heavily based on humidity) so I'd think use would be limited, it's not exactly great way to lift anything heavy

That ionic propulsion drone/startup you linked to is a scam. Their drone has a ducted fan in the center that's providing the thrust.

Re: closely guarded submarine secrets, I always found this whole incident and the surrounding events just fascinating [1]; It involves giant machine tools, computer control systems, and a lot of international cold war politics.

[1] https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scan...

From page 2 of that link: "Once they reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles. This is a state known as cavitation."

How does a vacuum result in bubbles? Wouldn't the water just clap back and close the vacuum?

Those bubbles do collapse, but the problem is that as they collapse they generate tiny little shockwaves. Apart from being noisy, these implosions can be quite destructive—the constant bubble collapses are fairly energetic, and cause cyclic stresses on materials. Over time, these can actually eat away at the surface of the propeller [0].

[0]: https://en.wikipedia.org/wiki/Cavitation

Think of what happens to a fluid when exposed to a vacuum.

The water surrounding the vacuum instantly vaporizes and creates bubbles of water vapor. Then these small vacuum bubbles collapse and create small shockwaves that create noise and damage the propeller/screw.

Very curious, since there were commercialized versions reported back in 2019.

https://www.boatindustry.com/news/32122/sharrow-amazing-prop...

https://boattest.com/Sharrow-Propeller

And there are examples from online forums dating back to 2012:

https://www.boatdesign.net/threads/strange-propeller.44740/

All of those differ from the one in this ad.

They are very similar, just with more toroids attached to the central shaft. They also render the patent completely invalid by way of prior art.

Filing date was November 2017.

If you look at computer case fans, you can find commercial examples dating back to at least 2012 that are specifically meant to curb noise:

https://www.overclock.net/threads/noiseblocker-nb-eloop-120m...

Yeah, I missed the 2012 in the other comment.

> Very curious, since there were commercialized versions reported back in 2019.

(1) Their patent application was filed on 11/6/2017. https://patents.google.com/patent/US10836466B2

(2) This propeller is different from those propellers, and the patent office found that the differences were patentably new and non-obvious.

Claim 1 is representative:

> 1. Toroidal propeller comprising: a hub supporting a plurality of elongate propeller elements in which a tip of a leading propeller element curves into contact with a trailing propeller element to form a closed structure with increased stiffness and reduced acoustic signature.

The element "in which a tip of a leading propeller element curves into contact with a trailing propeller element" seems to be common to all the independent claims of the patent.

I was wondering about performance, but I cannot find any data on how these toroidal propellers are preforming compared to traditional airblades.

The two-pager just says:

> Achieves thrust comparable to that of a multirotor drone propeller

which isn't saying anything.

My gut reaction is that these propellers require more material than traditional ones, which makes them weigh more, which should make them perform differently than traditional propellers. At a minimum, they should spin slower and/or strain the motor more for the same RPM. Maybe I'm completely off base.

I found this other website [1] which reports thrust differences between traditional and toroidal propellers for boats. At least under the conditions reported in the graph, it seems that toroidal propellers might outperform traditional boat propellers.

Again, I'm not sure how much air behaves like water. My layman understanding of fluid dynamics tells me that air is different from water just because air propellers don't look like water propellers (e.g., they require longer, thinner, blades; air engines need to spin way faster than water ones; water probably requires more torque) and you can't turn a boat into a helicopter if you turn it sideways.

[1]: https://newatlas.com/aircraft/toroidal-quiet-propellers/

The main difference is compressibility, to the point that liquids are commonly used as air analogs in wind tunnels.

Ship propellers end up working in different density material, with also different speed requirements and that's why they are differently shaped even if the equations are the same.

Related thread from a few months ago discussed the Sharrow propeller mentioned in this article, which applies the same concept to boats: https://news.ycombinator.com/item?id=33949895

The difference in geometry for air vs. water propellers has a lot of complicated reasons but above all the fluid density of water is about 1000x air: ~1000kg/m3 for water vs ~1kg/m3 for air. Air propellers are optimized for very high-speed operation which is needed to produce any significant thrust in low-density fluid. Boat propellers are very "built up" and physically sturdy since they are used in low-speed operation with high torque.

I have seen people use RC airplane propellers in water for small autonomous boats -- They do work but look very comical (think 10x the size of a boat propeller, running at 1/10 intended speed)

Cavitation effects -- microscopic bursts of vacuum at low-pressure boundaries along the propeller -- come into play much sooner than with air propellers.

> Cavitation effects -- microscopic bursts of vacuum at low-pressure boundaries along the propeller -- come into play much sooner than with air propellers.

In liquids vapor-bubbles of the liquid (not vacuum) are caused by low pressure (or high temperature, boiling), and cavitation is the collapse of the vapor back to liquid state when pressure rises (or temperature drops).

Propellers in air do have issues with the speed of sound, but that's a different matter than cavitation in liquids.

Thanks for the corrections: So the bubbles are not a vacuum but low-pressure vapor and cavitation is the name for the collapse, not the bubble itself. And this can only happen in liquids where there is a potential phase transition. I've got a little marine knowledge but my physics is lacking so I appreciate the clarification!

I assumed cavitation had similar parallels in air propellers with high tip speeds (as you approach the speed of sound) but seems that is a different effect?

I would say it's still a vacuum in the more common meaning of the word: A reduction in pressure, not necessarily a complete vacuum.

They should produce more thrust per a given unit of energy than a conventional propeller by eliminating induced drag. Winglets on jet airliners work by the same principle, converting the wingtip vorticity, due to the pressure difference above and below the wing, into an apparent forward thrust.

Propeller efficiency (both and air and water) are factors of torque, RPM, and advance speed (effectively the speed of the vehicle). You can't be efficient for all conditions. For example, a tug boat needs really high thrust at low and even zero advance speeds as it pushes up against a giant ship like a tanker. That design would be very different from what you'd need for a high speed boat with high advance speed. Similarly, props for drones can be designed for efficiency at hover (zero advance speed), or for going fast. Designing for noise will also have trade-offs for different conditions. Like it might be really quiet AND efficient in hover, but then be crap at when moving forward at high speed. Efficiency is always about trade-offs.

And one of those tradeoffs can be complexity. For example, you could integrate a variable pitch propeller with a control computer and your throttle to ensure you are always at the highest efficiency possible within the geometry constraints of the propeller.

In some applications warfare and anything in residential areas (e.g. thermal or roof damage inspection), I'd expect that pilots will gladly go for the lower noise even if it means a bit less flight time.

I'd expect commercial drone pilots to not give shit unless law tells them to give shit.

Less noise makes for less complaints by neighbors during longer flights in my experience.

Right, why would a contractor care? You flying your own drone in backyard, sure.

> At a minimum, they should spin slower and/or strain the motor more for the same RPM. Maybe I'm completely off base.

Only when adjusting RPM. At constant RPM, a heavier properly will actually reduce impulses to the motor.

Someone simply needs to invent a motor that provides constant torque through a full revolution.

To control the amount of thrust changeable rpm is needed, or a changeable geometry, like pitch. I imagine the latter wouldn't be easy with the toroidal propellers.

You could mount each toroidal blade on an angled pivot and use rpm control to change the pitch. This enables thrust vectoring.

This might be what you're referring to, but Tom Stanton built a swashplateless 4 axis R/C helicopter (albeit not with toroidal blades) based on this principle: https://youtu.be/d80oXSCcHTk

I too was also expecting a same-thrust controlled study.

From the fundamental frequencies in the two graphs, we can see the toroidal propeller (72 Hz) is spinning slower than the traditional (88 Hz). Assuming, of course the same fundamental vs. rpm relationship -- the toroidal has twice the number of "blades" as standard, but I'm not sure how that manifests sonically. I wonder how a 3-blade or 4-blade propeller would compare to the standard 2-blade.

Also, a big difference between quadcopter and other propellers (boat, plane, heli) is that they use change speed to carefully control thrust - most quadcopters use fixed pitches. Adding mass to the propeller can reduce responsiveness, which means less stability ... but I don't know enough about the magnitude of this effect to know if that's a problem.

air is compressible, water is not. This effects not only the mechanics, but also the acoustics. Air is actually a much worse conductor of sound so making a quieter propeller should be easier.

The principle of using a toroidal shape to reduce creation of vortices and thus sound should hold for either medium and may even be more efficient since a vortex causes drag and they are known to improve efficiency in boats.

INTERESTED IN ACCESSING THIS TECHNOLOGY? Contact the Technology Ventures Office tvo@ll.mit.edu

LOL good luck trying to corral that. These will be an $11.99 upgrade on Banggood within 3 months.

Especially since they've barely started optimizing it. Should be pretty easy for other people to achieve similar or better performance.

> 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.

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

This article also suggests that 3D printing would be the best way to manufacture these. Just need a file on thingiverse.

I thought it would take a really, really long time for additive manufacturing to become commercially relevant, but I guess people are building rockets with it now. It isn’t about to replace injection moulding and casting completely, but it’s wild that a file on thingiverse can actually be meaningful now.

A toroidal prop could absolutely be shared and optimized there, and metal printers will rapidly improve and yield better products. I won’t have one in my garage any time soon (I can still hope), but I can already hire someone else’s printer online.

I wouldn’t be surprised if toroidal plastic props for drones end up on there in a usable state fairly soon. What a cool thing — I wouldn’t have thought this would be possible in my life time when I was a kid.

Video demo (with audio) comparing side-by-side with a conventional propeller at timestamp 1:12 - 1:22:

https://twitter.com/MITLL/status/1611438712683958306

Wow it's so much quieter in volume, but also less harsh sounding without those high frequencies

The military and civilian applications are huge.

Great marketing for working at the LL lab. They seem really cool.

I noticed he didn't name the "phenomenal intern" who actually did the work to design and make it. Hope it would make others wary to work there.

Yeah I noticed that as well. I don’t think we can read too much into it tho. Did something happen to you—-if you worked there?

Unfortunately for those who care about noise reduction, LL patented the design, preventing its widespread use.

This seems to be the patent in question:

"Claims:

1. Toroidal propeller comprising: a hub supporting a plurality of elongate propeller elements in which a tip of a leading propeller element curves into contact with a trailing propeller element to form a closed structure with increased stiffness and reduced acoustic signature.

2. The toroidal propeller of claim 1 having two or more propeller elements."

That's it. Honestly, that's pretty easy to get around, even without arguing against its validity from prior art in the submarine world.

How would you get around it? Sounds fairly solidly written to me.

Perhaps if your design does not improve stiffness? The way it’s phrased suggest that that is a necessary part of the design.

I don't see how you could use this design without it improving stiffness. And that isn't really a workaround - you're giving up half the benefits of doing it in the first place!

Something other than a toroid? Maybe an 8 shape?

The fun thing is just going word by word and thinking up alternatives. So...

Toroidal: - What adjacent shapes will provide similar efficiency without technically being a toroid?

Hub: - Can we connect it to a "shaft" instead? - What about adding a gearbox in between?

Tip connection: - This specifically calls out connecting the tip of a leading propeller to a trailing body. This is where I'd probably focus first. Can you reverse it to connect a trailing propeller element to a leading element? Do you connect them back to the hub instead? What about a secondary/tertiary element?

Patents are more of a threat than a firm legal protection. A large company can tie up a small business in court until they're bankrupt, so it's really a matter of cost/benefit and how close you want to get to the fire you're playing with.

So basically the rules are, manufacturing in America is about lawyering patents, playing with fire, and being threatened by big companies who can bankrupt you whether you're copying their ideas or not. No wonder China is pulling ahead.

Patent law was mediocre idea turned into atrocity by lawyers. It just stifles the progress at each step with being "beneficial" only to big enough corporations that can brute force the system with lawyers

I disagree. Patents are a good idea and mostly aren't that bad (you mainly hear about the bad ones).

This one seems like a solid idea worthy of some reward.

I think the patent system does need some tweaks but other than that it's not too bad overall:

* Software patents need a shorter term than 20 years. Maybe 10.

* It needs to be way easier to invalidate patents after they have been granted using prior art.

* Ideally they would simplify the crazy language in patents so they are easier to read. Do we really need to put "plurality" in every other sentence?

* The research was funded with public money, so everyone should benefit from the results.

* The one who did all the work (maybe even came up with the shape idea) was one of the interns. From my experience in industry and academia you will give away all your rights with your contract and get some pennies in return, while the department will get all the big bucks. Does it sound fair?

It'll get interesting once AI algos proliferate to other knowledge domains such as physics and engineering and start hallucinating solutions, as does GPT-3 now. Some of those solutions, adjusted and fine tuned, will have real world applications. How will patents be relevant then?

Vast majority of software patents are maybe a month of work.

There is some reason to want something that takes years of research to be protected but basically nothing software is that.

> I disagree. Patents are a good idea and mostly aren't that bad (you mainly hear about the bad ones).

I'd wager if you removed bad and the trivial ones a lot of them would be gone.

Filter out ones that could be clean roomed by competent engineer in a week and 95% of them would be gone.

Also today: https://news.ycombinator.com/item?id=34571282

and a couple months ago: https://news.ycombinator.com/item?id=33949895

These previous discussions have links to papers and videos that are convincing. The sound reduction is astounding. The power graphs are impressive. The price of the boat propellers is gobsmacking… but if the efficiency claim holds true, will easily pay the upgrade, and the dolphins & whales will be ever thankful.

Be careful of the boat propellor claims. For example, they filmed the regular prop in slow motion to hide that it was going at a much faster speed that caused more cavitation to make it look worse. Also when they claimed it needed a LOT less steering, I'm extremely skeptical because that should be mostly caused by the boat riding higher in the water at a faster speed. Also they disabled comments on videos. https://youtu.be/_KkIqC7arnU?t=74

I wonder how the acoustic measurements were taken. From my own experience propeller noise is highly direction-dependant. For a fair comparison a lot of angle-dependant measurements would have to be considered.

Appears primary source of sound in helicopter blades is from blade-vortex interaction:

https://wikipedia.org/wiki/Blade-vortex_interaction

Likely that even a non-toroidal half-crescent shaped tips would significantly reduce sound levels. Here’s comparable design:

https://youtube.com/watch?v=dBS1NRsYuF8

Possible even a vortex diffuser similar to those found on submarines might help too.

This also makes me wonder whether, instead of an entirely toroidal propeller, you could just put a small, appropriately-shaped toroidal loop at the tip of the prop.

At least this PDF shows dB graphs, but the graphs are so noisy that you can't make much out of them. They have fewer harmonic peaks, and the peaks do seem to be up to 10 dB smaller. But the average appears to be pretty similar. Again, hard to be concrete on the differences given the noisy data.

10 dB is a lot (2x the sound pressure) since dB is an exponential scale, but at the peak of 55dB, you're still below the range of a "normal conversation".

They claim where the peaks are is less annoying to humans.

Yeah this is when I don't like it. I can't tell if this is truly an improvement. The graph definitely looks tighter, but the overall level appears the same?

I can agree it is "less annoying" but if the loudness is effectively the same then I don't think it's truly an innovation.

Quieter means more energy is possibly available for more thrust. I noticed SpaceX rockets sound a LOT more quiet and smooth compared to older rocket videos. Notice upward tilting winglets on the tip of large airplane wings these days are to reduce the same vortices that are seen on propellers (wings work on same principle as propellers) in order to be more efficient and more quiet is just a side effect.

Might be but they're not mentioning efficiency at all, which you'd think they would if that was the case (more efficient AND quieter would be even bigger selling point)

There was an article here a couple of months ago [0] on Sharrow Engineering's propeller, which looks very similar. Sharrow were granted their patent [1] earlier the same year as MIT's [2].

I wonder if this is yet another case of people independently reaching similar conclusions based on other work (tip vortex and cavitation research)?

[0] https://news.ycombinator.com/item?id=33949895

[1] https://patents.google.com/patent/US9926058B2

[2] https://patents.google.com/patent/US10836466B2

The flying cars that are in development currently are largely mega-sized drones. Noise is one of the major obstacles to acceptance. Rather than reshaping the propellers I'm thinking active noise reduction might be the solution.

Flying cars that whisk back and forth with a whisper? Could be!

I assume jetpacks have similar issues

"The flying cars that are in development currently are largely mega-sized drones."

Mmmm, I'm not aware of any that are. The Terrafugia wasn't, but it has been abandoned apparently. The Samson Switchblade is a three-wheeled car with a pusher-prop design: https://www.samsonsky.com/models/

The three-wheel strategy is pretty smart, because the vehicle can be licensed as a motorcycle and doesn't have to have all the heavy safety equipment of a car.

> I'm not aware of any that are

1. BCL

2. Boeing AFS

3. Kittyhawk

4. Ehang

5. Workhorse

The list goes on...

You might not have heard of them as they're absolutely unmarketable because of the unbearable noise they make

None of those is a flying car. Flying cars are wheeled vehicles that can drive on the ground like a normal car, but also fly.

Exactly. Drones are so useful, but they screech like banshees, making them effectively useless for consumer applications within 200ft of any humans. Basically only aerial photography.

Wow like Sharrow boat props.

Was about to say this too, didn't they already have a patent for this already or does it only cover boats?

Interesting development, but I'm not certain that I want drones to be quieter. They're not that noisy now, and making them silent would be sort of like taking the bell off the cat.

I was at a ceremony a few years ago for victims of a workplace shooting. There was a drone overhead, I assume one of the local news organizations taking photos, and the noise was quite distracting for such a somber occasion.

Whether or not there should have been a drone overhead, if there's going to be one at a memorial service I'd want it to be quiet.

I’m pretty sure Wing, Walmart and the other very well-capitalised companies developing drone delivery services want quieter drones.

In many ways the toroidal prop is a more of a four-blade than a two blade, so it seems a bit disingenuous to compare it to two-blade DJI variants. (And as some other poster mentioned as well, I would like to see a comparison to prime prime number bladed props.) Finally I would like to see it compared to shrouded props.

I can totally see this taking off (pun intended) - has sufficient weirdness factor that marketing teams can leverage it as a "stealth" design while actually being somewhat credible

Wouldn't be surprised if we see this on a major commercial drone soon

It seems to have smaller diameter and more surface area than same sized normal propeller. This means it could have lower RPM for same power and generate less noise.

Are they better than long-screw propeller with the same diameter, area and RPM?

So, this seems like the performance would be similar to that of a ducted fan, both acoustically and aerodynamically. Would it be better in in either respect? Worse? By how much?

Could these be safer than conventional propellers? From the side, they seem less "sharp" than a conventional propeller, but they're still spinning pretty fast.

Now fix single engine planes, helicopters and lawnmowers.

i'm happy for the science, and super sad at the same time for the people especially UA situation...

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