How Do Zipline's Silent Propellers Work? 

Hey everyone, for the silver printed propellers, I should have spun them in the clockwise direction, since they are clockwise blades. But luckily the tones that I demonstrated stay the same.

A similar concept is also used in car tires: The tread blocks have slightly different sizes to spread out the rolling noise over a wider frequency range.

As a submarine sonar tech we would analyze the effects except the propeller or screw is in water. One of the first things you learn is how to turn count a merchant using just aural and then using the equipment on board to pick out the individual frequencies, so you can see how many blades the ship has and it's rpm and with some rough turns per knot calculations you can get an estimate of speed

A lot of this discovery and research was done way back in the 40s, but the only thing that was important was efficiency and thrust, not noise. Now it's being re-researched and re-discovered all over again because the focus has now changed to noise :) interesting

i accidentally noticed this while making my first rc plane when i had to stack 2 double props to the motor.I had to find a good angle between those blades ,so i just held the motor in my hands and feel the vibration and noise. some how a 15 degrees angle works best for the least noise and vibration

The issue I'd imagine with the counterbalanced propellers is that there would be an uneven torque on the bearings caused by unequal "lift", even though there is a balanced centripetal force.

Actually, while M.I.T. is getting the credit, I believe Sharrow Marine (Gregory Sharrow - 2012) invented the design. He has a very fascinating story of why and how he developed it. He was a videographer and wanting to video an orchestra, quietly, while using a drone - he developed the Sharrow prop. Look him up.

Channels like yours are the best part of RU-vid. I always learn cool stuff from you!

I want this but for my fan by my bed 👍

I love that you are comparing FDM printed props to FDM printed props. Surface finish also plays a role in perceived loudness of a propeller.

I just started looking up information on how to make quieter propellers as well as for air fluid dynamics sim programs for testing propellers a few hours ago. Impressively good timing.

This is really interesting. The noise has always been an issue to me. Very well presented.

I remember when cars had (engine driven) radiator fans with uneven blade spacing in the '70's. As is so often the case- what's old is new!

I suggest you consider measuring power consumption to windspeed, rather than rpms to windspeed, since different sound profiles also have different efficiency profiles. A quieter blade may not provide as much force per rotation, but it may also spin up more easily due to less turbulence.

Brilliantly presented. I have read several explanations of uneven spacing and torodial props, but none of them mentioned the crucial relationship between frequency and perceived noise level. This was highly useful, as my favourite UAV is deal breakingly noisy. Thank you!

thx for the great work ;)

I'd be interested to see a staggered toroidal blade, just to see if there's any benefits. Toroidal blades themselves are nothing special, they're just a duct integrated into the blades themselves. But there may be interesting occurrences when the toroid is staggered, due to how the blade contorts in 3D space. I'd also be curious to see both the uneven and staggered blades within a normal duct. Maybe Tom Stanton could eventually do a video or series on these.

Brilliant Video. Thank you Sir. Your videos are excellent at quickly distilling to real terms for understanding. I applaud your hands on demos for sure! Cheers

I remember as a kid, we had a toy, that had a pistol grip winder, where you put a propeller on it, that had a ring going all the way around the outside of the blades, you wound the spring up using the propeller as it clicked while you were turning it, once the spring was tight, you pointed it up in the air, and pulled the trigger, the propeller would fly off for some distance, but I noticed how quiet the propellers were, most likely because of the ring around the outside edge of the blade tips, the propellers also had like a saw tooth shape, on the trailing edge of the blades, that made it quieter as well, perhaps we should revisit these old school toys.

Great video, thank you very much.

I watched Rober’s video and thought the blades were extremely interesting. Thanks for making this video.

Props to you on this analysis 😊

Great video, I've been very curious about this since first coming across Ziplines shorts a year or so ago. Very neat stuff.

Thank you! I was really curious about efficiency comparison.

Aerospace propulsion Phd student here. The video was pretty good but I wanted to really highlight the elephant in the room. The big thing us propulsion engineers care about is THRUST. There’s about a million ways to make jet engines and propellers quieter but all of them so far DRAMATICALLY impact thrust. For example, you can make a equivalent size propeller to a classic 4 blade propeller (same pitch and diameter etc) that is wayyy quieter like over 10-20 decibels quieter at 10-m, but a lot of times when you then compare the thrust that new propeller makes you realize it doesn’t make enough. So logically you design your novel propeller geometry to generate more thrust… but then once you get to the thrust output of the classic 4 blade you realize it’s only 1-2 decibels quieter and is significantly less efficient at converting mechanical energy into thrust since you’re doing things to mess with the flow… So, all that to say I REALLY want MIT and Zipline to actually release their white papers on their novel propeller concepts to see how useful they actually are (Thrust vs noise curves). Zipline I understand why they haven’t released anything, since they have a market advantage if it’s legit (and it seems legit), but I am a little frustrated with MIT since they’re a research institution (and they’ve only released a hype video and 1 page of real info)…. But anyway still really cool stuff but it really remains to be seen if it’s the game changer that everyone has hyped them up to be…

This is a perfect video for teaching the dynamics of sound!!!

Awesome that you made this video - I've been thinking about those props since I saw the video and was also surprised there wasn't more detail about them. This is revolutionary.

Cannot tell you how bad I've been wanting a detailed review of this! You read my mind!

Car manufacturers have known this for a long time, radiator cooling fans often have uneven spacing on the blades for noise reduction.

Thank you for this video! I've been waiting for someone to notice and tell me how it works!

This is quality content. Thanks for asking interesting and niche questions and giving us well read answers. Your videos get me thinking.

We appreciate your effort and hard work on this channel. God bless you.

Something that would probably help more to reduce noise from drones would be to have variable pitch blades. This would allow all the rotors to be run at the same speed and have them phased in a way to achieve best cancellation of noise between them. The downside is that it defeats the simplicity of conventional multirotors in which the only moving parts are the motor armatures. Another approach could be to go the opposite way and have a rotor with many more blades, as in a turbine. With enough blades you could have a fundamental frequency above audible. It will however only work for rather small rotors as even with 50 blades you would need to run at 24,000 RPM which somewhat limits the allowable diameter. The efficiency will probably not be too good either but the sound would be like whooshing rather than buzzing or screaming, which might be less annoying even at the same sound level. Dogs and other animals may have a different opinion though. There are also ways of combining with stators with different number of blades that allows a much higher fundamental frequency for a given number of blades, a technique used in many gas turbine engines to reduce noise, at least the noise that is audible to us humans.

do you have the stl files for the zipline propeller? I love your videos. keep up the good work.

Beautiful video, with all the references and also perfect sponsor ♥️

It's so convenient having guys like you around to teach me things I didn't know I needed to know. Thanks

The big blades at the front of turbofan engines are typically designed and limited by the speed at its tip, which is also where most of the noise comes from. Previously the limit was around mach 1, but these days it can be exceeded with careful design of transonic airfoils. The big difference these blades bring to the table of drones is the noise. Blade spacing is one of the factors affecting that, but it also has to do with the wake interaction of the second blade in the series, which is both affected by the radial and vertical spacing between the two blades. Just slapping two props one after the other without careful consideration can make the second blade sit in the wake of the first blade. This may cause the second blade to stall reducing its lift and increasing its drag, in a similar fashion how airplanes can get into a deep stall. An issue these blades may have is damage tolerance, since now only one side of the rotor can take damage, reducing its weight and throwing out its balance. This would lead to vibrations which could take the whole airframe down.

Been waiting for a long time for someone to make a video on this after the referenced video!😅

Your experiments are always interesting and informative. Thanks for making them.

wow that was on my mind looking for a video

You missed a cool feature of the Zipline propellers: The counter weight. It's pretty cleverly designed: It's moved inwards, so while the mass of the counter weight is equal to the propellers on the other side, the propellers are rotating downwards - as they are further away from the center. (Their weight has more leverage on the rotation due to a larger distance from the center) This is necesary because the counterbalance generates no lift. The blades on the other hand will create a upward lift and thus a tipping motion on the bearing. So the counter balance needs to be moved inwards to counter the tipping motion the two blades of the propeller are generating when in hover. By counter I mean, that the propellers create a tipping motion towards them when not hovering. This is equalled out when hovering.

I love that you watched the same video I did and thought the same thing I did and I'm *really* glad you decided to do a video on it; thank you!

Respect! We just started looking into this with our science outreach program, following playing bagpipes with helium. Thank you for giving us an INCREDIBLE head start! You Rock.

Thank God someone finally got the explanation and examples both RIGHT and easily UNDERSTOOD. Lol the toroidal craze has thrown a wrench in the hobbyist drone industries understanding of blade noise reduction. Chris Rosser is makingg and demonstrating more accurate resin prints to test toroidal props, and it would be amazing to see a colab between you guys on testing the zipline prop!

As a U-Controle & R/C hobbyist one of the perks is the propeller noise but the bane of those living close by. I'm fascinated by the sound reduction for our model aircraft and also for boating propeller efficiency and not the sound which is buried in the water. Fishermen may be more interested in the sound reduction though.

Thank you for addressing this! I've been curious about that ever since I watched Mark Rober's video.

Definitely one of the best channels on youtube. Could watch these videos all day.

You sped up the quieter blades to match the air flow by increasing voltage, but the actual power consumed is what you need for calculating efficiency. You'd also need the current, which will depend on the load on the propellers. It could be that both approaches use very similar power because the quieter versions disturb the air less and thus have lower load for a higher rate of spin.

Lose efficiency? Measure the current comparison between the even spaced blades and the uneven spaced blades spinning at a higher RPM. If the current is the same when forcing an equal 3.1 FPS of air flow then efficiency is the same with improved perception in sound reduction.

Always mind blowing videos man, unreal.

Wow! This is something I would never consider to look into. Thank you for keeping me engaged in science.

I've been thinking about Zipline's weird propellers since I saw that same video. It would be awesome if they decided to make their propeller design open source. It looks like a lot of time and engineering went into it.

Awesome analysis. I'm curious whether this would work with cpu or gpu fans given the constraint with the listening position.

Wow, very impressive in so many ways! Thanks! Two sideways questions -- with all the talk about the noise, the actual generated "lift force" (at a given rotational speed) got a bit a brush-off until some quick attention at about the 8:40 to 9:45 mark, where the "wind speed" was measureed. Similarly, the electrical input power kind of got the brush-off, meaning for a given rotational speed, when e.g. comparing traditional blades with "offset" blades, were the motors using the same volts and same amps vis a vis power? Comparisons might be unfair if the electrical power input was higher for one case than the other (at a given rotational speed.) This is one of the best channels on all of RU-vid...wish our high school science classes were far more like the Action Lab.

FINALLY someone is talking about the propellers. Iv been scrolling reddit and youtube and twitter to find someone speculating about the propellers.

Without hearing any more, I think it's to do with interrupting the cumulative oscillation (superposition, phase reinforcement, whatever you want to call it). The same reason marching soldiers step out of time when crossing a bridge.

Another amazing informative video! Keep it up! 💯 ✨

I saw those in the video and wondered why they were shaped like that. Thanks for the video

Thanks, I was wondering how efficiency was affected. It was actually intuitive in this case!

Now we need to try a single counterbalanced toroid prop

MIT did not create the toroidal propeller a few months ago. The design has existed since the 70s and has been known to be more efficient for that long too. Due to manufacturing difficulty it hasn't been mass produced in all the years though. With the rise of better and more accessible advanced machining capabilities like multiple axis CNCs we might just see them become more common.

Dude, i'm a big fan of your work in broadening knowledge and increasing understanding of, at times, complex issues. Thank you. What may also aid comprehension is a standardisation of terms: RPM is an abbreviation of Revs Per Minute. It is colloquially referred to as Rev(olutions)s - revs - but never is it correct to call them RPMs. And, as they are measured in Minutes, your description of '...to measure the RPMs, s 'how many times is it spinning around per second...' could be clearer to your audience. Thanks for making time to read this, and keep up the good work.

Scimitar-shaped propellers are a widely-used intermediate solution between conventional straight props and the toroidal designs that you showed. As with winglets and wing extensions on airliner wings, they work by reducing the energy of the tip vortex.

I like peanut butter

Independent testing found the toroidal props so far are far less efficient than regular props, and the noise isn’t actually better either from memory - their video was a bit dishonest.

Excellent video. I love being able to dip into a 10 minute video on things like this on RU-vid, keep it up.

very nice explanation and demos. thanks

Are they worried about noise because they are flying in areas that they aren't supposed to be there?

Really great explanations, I'd like to compare ducted fan blades In relation to thrust I heard shrouding the blades can improve thrust, but when considering the extra weight of the ducts ie. TWR. It would be interesting to compare any advantages because it also kind of protects the blades also allowing the drone/aircraft to get closer to a mountain side or even touch it.

THANK YOU. I couldn't find any info on this, and I was thinking that this could *literally* be revolutionary for propellers.

first

I did my Ph.D. at UC Irvine in the same lab as the thesis you showed, with Dimitri Papamoschou. Thanks for citing! Those were great explanations.

I'd like to hear about thrust. Also, what about the pressure collapse out at the tips of heli blades? I've never seen staggered blades on a heli like you describe.

I think these Zipline propellers are awesome. So is their idea of lowering their payloads with a retractable line! The brilliant video about these new propellers is much appreciated. I wanted to mention that toroidal propellors have been patented and used to propel commercial boats for many years, so despite their statements, the MIT students didn't invent the idea. On my channel there are a series of ion propelled aircrafts with onboard power that fly almost silently. They are patented for lifting their power supplies against Earth's gravity and for the required efficiency improvements. Please take a look. Thanks for the info!

Very interesting comparisons! Thanks for making this video.

Owl Wings have amazing structures for reducing/countering sound & these are being researched in order to build biomimicry technology - designed by nature, synthesized by us. There are several companies trying to recreate the Stiff Comb Structures along the leading edges of Owl Wings. These structures generate counter-vortices over the top of the wings, which act as inverted/offset soundwaves to significantly nullify the sounds that the wings generate as they fly. (There are many factors why this is as yet in production, with lifespan being at the top. Then the comb's overall structure, tine length, tines/cm, gap size/shape, etc. - all which changes with blade use & rotational frequency) Owl Wings also have highly fractal / fluffy shaped edges along the trailing edge feathers, (along with most body feathers). These act much like the acoustic foam in studio sound booths, trapping several sound frequencies within their structures - making everything much quieter. (Unfortunately, these kinds of structures tend to only work at low frequencies, hence the Owls slow, low flying hunting technique.) These evolutionary adaptations make Owls the quietest flyers in existence, capable of producing less than 15dBa in noise as they fly through testing facilities. This is far less than human ears are capable of hearing (with perhaps some pressure waves on your skin being the only sign an Owl is about to fly past you). [ I can actually attest to this myself as one chased a bat past my where I was sitting one very quiet evening (bush property). I heard the frantic fluttering of the tiny bat as it flew past, then nearly jumped out of my skin as an Owl with a pure white underside & huge 80cm wingspan, silently ghosted out of the night past my face... ]

does it require more energy to spin the uneven blade faster, even if producing the same thrust?

The tail rotor of the AH64 Apache has displaced blades. I assume this is to reduce noise

I love this topic... great video!

Is the thrust performance of each design the same or similar, after all thrust is our main goal is it not?

You are the best science teacher I've ever had.

Wow! A video sponsor that's not genuinely involved with the video, but offers a service I might be interested in. Incredible.

THANK YOU!!! I was trying to find where I saw that prop a couple weeks back and I couldn't find it, I was skimming through Mark's videos, Veritasium and a few other channels I subscribe to.

Very interesting - thanks for the effort!

My question about the design, comes down to the counter weight. Because you're using a counter weight, would it make a difference for the RPM vs. the weight? Or is the counter weight and RPM independent of each other?

and by using different prop angles for each motor you can even spread out the spectrum more.

Cool subject, just up my alley. Thanks.

Great video learnt a lot. Just wondering if frequency is the same as or equates to, actual volume at a distance?

That was a very cool video and what we need in our time. You young people out there: share stuff like this wherever you can!

Excellent and direct explanation of the sound suppression in just 3 minutes! Keep up the good work!

Excellent demo. Some food for thought: I know normalizing by rpm is more practical, but I am inclined to believe it should be normalized by airflow or lift/thrust.

Nice video, very informative. Following this same theme, will you make a video explaining the reazon for using a double propelers one turning clockwise and the other counter clockwise?

Could you combine the two? Have the round MIT blades with two of them at one side and a counter-balanance? I'm guessing that would probably interfere with the consistent flow or something, but IDK

what app are you using on that phone to measure the frequency? I really would like to know.

Very good and explained video.👍We love that kind of basic experiments.

Nicely done review of the blade differences. I liked and subscribed. Hope all your videos are as well done.

Incredibly seamless advert. I was just wondering software what was used to make those simulations.

What about efficiency for these different blade configurations, how do they compare?

For value it would be adjusting blade pitch and amps burn of evaluated props instead of staying with off the shelf standard configurations.

Is the thust afected by change position of blades?