[HN Gopher] Toroidal propeller allows a drone to operate more qu...
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Toroidal propeller allows a drone to operate more quietly [pdf]
Author : lxm
Score : 108 points
Date : 2023-01-29 19:12 UTC (3 hours ago)
(HTM) web link (www.ll.mit.edu)
(TXT) w3m dump (www.ll.mit.edu)
| monocasa wrote:
| 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...
| mLuby wrote:
| 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
| WirelessGigabit wrote:
| That's funny, because I remember watching a documentary about
| some form of caterpillar drive.
| ChrisMarshallNY wrote:
| That was the one that was "narrated" by Sean Connery?
| froh wrote:
| the hishtorically accurate shientific account of a rushian
| boat being reshcued?
| BuckyBeaver wrote:
| And all the while, the captain had a mouthful of shtew.
| jonstewart wrote:
| The order ish, engage the shilent drive.
| petee wrote:
| 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
| dsfyu404ed wrote:
| Even if the design principals are known the acoustics of a
| particular sub are not and can be inferred from the design.
| finnh wrote:
| Also today: https://news.ycombinator.com/item?id=34571282
|
| and a couple months ago:
| https://news.ycombinator.com/item?id=33949895
| moistly wrote:
| 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.
| zawy wrote:
| 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
| zawy wrote:
| 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.
| anigbrowl wrote:
| _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.
| DennisP wrote:
| 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.
| nabla9 wrote:
| 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?
| pacbard wrote:
| 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/
| p_l wrote:
| 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.
| chrisdalke wrote:
| 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.
| someweirdperson wrote:
| > 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.
| SkyPuncher wrote:
| > 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.
| someweirdperson wrote:
| 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.
| peepeepoopoo3 wrote:
| 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.
| mschuster91 wrote:
| 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.
| morcheeba wrote:
| 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.
| Eddy_Viscosity2 wrote:
| 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.
| Havoc wrote:
| 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
| compumike wrote:
| Video demo (with audio) comparing side-by-side with a
| conventional propeller at timestamp 1:12 - 1:22:
|
| https://twitter.com/MITLL/status/1611438712683958306
| cush wrote:
| Wow it's so much quieter in volume, but also less harsh
| sounding without those high frequencies
| O__________O wrote:
| 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.
| jbay808 wrote:
| 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.
| falcolas wrote:
| 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".
| ec109685 wrote:
| They claim where the peaks are is less annoying to humans.
| bbhi wrote:
| 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.
| hdevalence wrote:
| Unfortunately for those who care about noise reduction, LL
| patented the design, preventing its widespread use.
| digdugdirk wrote:
| 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.
| IshKebab wrote:
| How would you get around it? Sounds fairly solidly written to
| me.
| superjan wrote:
| Perhaps if your design does not improve stiffness? The way
| it's phrased suggest that that is a necessary part of the
| design.
| beambot wrote:
| 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/
| BuckyBeaver wrote:
| All of those differ from the one in this ad.
| beambot wrote:
| 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.
| stareatgoats wrote:
| 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!
| cush wrote:
| 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.
| klipt wrote:
| I assume jetpacks have similar issues
| BuckyBeaver wrote:
| "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.
| cush wrote:
| > 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
| keizo wrote:
| Wow like Sharrow boat props.
| [deleted]
| moffkalast wrote:
| Was about to say this too, didn't they already have a patent
| for this already or does it only cover boats?
| b3morales wrote:
| 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.
| macintux wrote:
| 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.
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(page generated 2023-01-29 23:00 UTC)