How Do Zipline's Silent Propellers Work?

a few weeks ago Mark Roper released a video showing these really cool zipline drones that deliver medical supplies in Rwanda as cool as this is he kind of glossed over one of the most interesting features of these drones look at these propellers he mentions that they're shaped like that to reduce the noise but why does this happen and what is it about the shape that reduces the noise the internet right now is just buzzing waiting for someone to make a 3D printable version of these to see how much quieter they actually are and that's exactly what I have here but before we listen to these let me explain why these propellers are shaped this way in the first place what actually makes sound when a propeller spins really fast well as the blades move past by a stationary point it creates a pressure disturbance that propagates away from the blade the frequency that this happens is called the blade passage frequency and it's equal to the rotational frequency so how fast the propeller is spinning times the number of blades on the propeller so let's say that we have a propeller that has four blades the traditional way to make these is to space them evenly so that they're 90 degrees apart when you have a propeller shaped like this it creates a lot of different frequencies but it creates a loud frequency at the blade passage frequency but also it creates frequencies that the harmonics of the blade passage frequency so if you take an audio recording of this you'll see Peaks at the whole number multiples of the blade passage frequency but something really interesting happens if you don't space out the blades evenly so let's say instead of having the blade space 90 degrees apart we space them 30 degrees apart only so there's now a large gap and then a small gap between them so they aren't evenly spaced well then when you record the sound you get the normal Harmonic it's like before with one two three times the blade passes frequency but you also get 0.5 times the blade passage frequency and 1.5 and 2.5 so for the same acoustic energy you're spreading out some of the sound into lower frequencies the most important frequency is the one that's generated which has half the frequency of the blade passage frequency you can see this here so first we'll start with this four blade propeller I have these little reflective strips on here so I can measure the RPM so how many times per second that it's spinning around in a circle so we're at around 1200 RPM right now so around 85 Hertz is one of the loudest frequencies here but now if I switch out these propellers to around a 30 degree propeller let's get it to the same RPM so that's almost exactly the same RPM around 1200 the other one is around 1249 it's around 1260. so they're going the same speed now but let's measure one of the loudest frequencies so this is crazy we're going the same speed with four propellers but the loudest frequency coming off here is half the frequency of the propeller that has the evenly spaced blades so it's picking up a lower frequency the higher one is still there but the energy is shared between the two frequencies so what does that do for us well humans are less sensitive to low frequency sounds so we perceive them as quieter than they actually are so what this means is when you space the blades unevenly it creates a lower frequency that we perceive as quieter and this fact has been known for a long time for example with our cooling fans and helicopter rotors that use uneven blade spacing but this design still doesn't quite look like the zipline design that we saw from Mark rober's video why is there this gap between the blades so that they're on different levels well when you have a propeller design like this there's an interaction that happens where the blades overlap this causes a lot of broadband noise meaning just a large spread of many frequencies through the Spectrum so any noise reduction that you gain with the uneven blade spacing gets canceled out with this Broadband noise that's created but if you just get rid of that overlap by raising one of the blades a little bit higher than the other one then the Broadband noise just goes away so an easy way to create this is just to take two double blade propellers and stack them on top of each other so I've made two propellers here these are taken from an fpv airplane propeller that I have okay so what I have here are two different spacings of blades stacked on top of each other and I put a little sticker on each of them so that I can measure the rotational rate the RPMs as they're spinning to get them at the same RPM okay so I'm going to turn both of them on 10 300 RPM and this one's at 10 000 RPM 10 200 RPM so they're really close to the same RPM Okay let's listen to both of their sounds now in front now behind [Music] so the high-pitched tones are still there with this one but they're a lot less because of the acoustic noise has been spread out to that lower frequency as well as the high wind whereas this one has most of the high ones you can see that the uneven space blades have a noticeably lower tone than the evenly spaced one what that amounts to is that it sounds quieter and not quite as annoying as the regularly spaced one so zipline has made a version of the uneven space blades but instead of including the four blades they just have the two of them with a counterbalance on the other side so it sounds like this [Music] now mine isn't perfectly balanced so it doesn't sound a lot quieter I'd love for someone to make a better version of this in real life that uses the same design as zipline also a few months ago MIT came up with a propeller called the toroidal propeller this is the two blade version and then this is the three blade version this used to propeller in this shape that reduced the sound in a different way it actually reduced the vortices form and slightly reduce the overall sound levels you can actually see what the flow looks like between a toroidal propeller and a regular one in this simulation here for the standard props we see small detached tip vortices from the air rolling off the tip of the blades these contribute to the high buzz noise that's typical of quadcopter drones in the case of the toroidal props this detached Vortex is contained within the Ring of the prop and the overall jet of air is much more coherent so it's quieter so this will correlate to A reduced overall noise signature due to improved turbulent flow this simulation is done using real physics using the software from our sponsor for this video Sim scale the world's first Cloud native engineering simulation platform simscale provides engineers and designers access to digital prototyping and simulation early in the design stage throughout the entire r d cycle and across the entire Enterprise right in your web browser if you want to learn more about simscale you can click the link in my description and create a free community account and also you can access thousands of public simulations as well you can also view the quadcopter project from this video and post-process the simulation results Yourself by clicking the link in my description and thanks to them for sponsoring this video and providing these nice simulations now let's get back to the experiment one problem with this design is notice that with the four propellers at 1200 RPM we get around 3.1 miles per hour of wind coming off of it but at the same rotational frequency for the unevenly spaced propellers we're only getting like 2.5 2.7 miles per hour so for the same rotational speed with the same weight we get less wind coming off of it which means to get the same wind speed you have to speed this one up a little bit faster but that doesn't actually hurt you sound wise because I can turn up the speed to match the four propeller ones let's turn it up to get like 3.1 so now I turned up the speed so overall you have to spin these a little bit faster than you would the evenly spaced blades so they're less efficient but you still get the lower frequency even at the faster speeds to get the same wind speed so overall these still can be a little bit quieter sounding than the evenly spaced blades so what's interesting about this apparent noise reduction with uneven blade spacing is that it depends on the place where you're listening if you're behind the blades then the noise reduction is worse for uneven spacing but when you're in front of the blades or in the same plane as the blade it's actually better so this is actually beneficial for flyover noise for planes because when they're closest to you you're in about the same plane as the blade above you so it's quieter but unfortunately that means it would be worse for drones because everything's below the blades but I'm not sure how zipline's blades would sound on a drone maybe someone will get a hold of the real blades and do a comparison on a drone also I got a lot of this information in this video from a paper on uneven blade spacing and I'll put a link to that paper in the description as well and thanks again for watching this video If you like this video remember to like this video and also subscribe to my channel if you haven't yet and we'll see you next time

Channel: The Action Lab

Views: 721,020

Keywords: zipline, silent propellers, mark rober, the actionlab, simscale, toroidal propeller, toroidal drone propeller

Id: z58RORCUTao

Length: 10min 12sec (612 seconds)

Published: Fri Apr 14 2023