- All right. (propeller slows) (Xyla laughs) - [Derek] Oh no. - Oh. (laughs) - Nope. - [Derek] Why? What is wrong with me? (Xyla laughs) Ugh. - [Xyla] I got it, I got it. - Ugh. - [Xyla] Oh my god! - [Derek] Doing it for science. (record scratch) - You may be wondering how I got myself into this mess to start with. And since it's Derek's fault,
I'm going to make him answer. - What? - Okay, fine. I'll explain it myself. Basically what happened
is about six weeks ago, Derek texted me out of the blue and asked if I would build him something, and like an absolute idiot, I said yes before ever hearing what it was because I don't know, you like
don't say no to Veritasium. - Derek then sent me
a physics brain teaser that's been disputed online
for nearly two decades. And it's the idea of a
vehicle that's powered only by the wind, travels
directly downwind, and yet can outrun the wind. At first glance, that sounds impossible. That obviously breaks the
conservation of energy, but the reality is it
actually is possible. And Derek drove a real physical
life-size version of it in a recent video, which
I've linked down below. How it works is really counterintuitive, and the hardest part for me to understand was that the wheels are
driving the propeller, and not the other way around. But starting from zero, the
wind pushes the vehicle, and as it approaches wind speed, the wheels are now driving
the propeller so quickly that the propeller is generating
forward thrusts like a fan. And that's exactly why we have
propellers on the front of small airplanes. And so now you can imagine, with the additional
thrusts from the propeller, we exceed wind speed. The problem is that it's
actually a pretty exact science to get it to work. And so, you know, four versions of the car later, a month after Derek posted the video I was supposed to build
this thing for, you know, I finally got it working. I
had a schedule, like always. - Look, I didn't know how
hard that was going to be. Sorry Xyla. (Xyla laughs) - [Xyla] Apology accepted. Mostly. - Good. - [Xyla] 'Cause you give
me free soda waters. - Yeah. And also, you know, I cut myself on the cart that Xyla made, so she got back at me, ultimately. - [Xyla] My fault, my fault. - This is the prop. It's a deadly cart. I don't know why she's
wielding that thing. - [Xyla] What do you think? - Should I show my injury as well? - [Xyla] Oh yeah, Emily got
injured as well. I forgot. - A second casualty of the cart. - [Xyla] All these
people just need to learn not to put their appendages
where propellers are. Anyway, let's rewind to
the part where I said this was version four. Here was my first attempt, and honestly, we're not even going to talk about it. I really didn't understand
the scope of the problem. Here's version two, and this is the one I
took to Derek's house for the first time. I used a Lego gearbox in hopes of making it a little more accessible. - [Derek] Let's test it
out, moment of truth. The suspense is killing me. - [Xyla] I'm kind of scared. - [Derek] It doesn't seem
like it's going to go forward. - [Xyla] Not yet. Nothing has fallen off yet, so. - [Derek] I think this is a win, this totally counts as a win. Still a pretty serious tug. Whoops! I was definitely not thinking
that it was going to be like 10 miles an hour, or whatever. - Hold up. Why are you
testing on a treadmill? You said this was powered by the wind. That is a great question,
and I'm very glad you asked. Think of it like this: take this awesome, beautiful shot of Derek and
Blackbird from his video. The film crew was in a chase car that was going at wind
speed when this was taken. And so what you're seeing is
Blackbird and its surrounding air mass still and the ground
rolling back behind them. And that's exactly what the treadmill is. You're just looking at it from
a different reference frame, and in this case, this reference frame happens to be a lot cheaper than finding an industrial-sized wind tunnel. And by the way, this is what the physics professor from
UCLA offered as his way of settling a $10,000 bet, is
that: prove with a vehicle on a treadmill that this is
generating forward force. - All right. (propeller slows) (Xyla laughs) - [Derek] Oh no. Do you want to go to 12? - Sure. (squeak) One more try? We'll glue everything a little
better and then one more try? - [Derek] Okay. Yeah, it still feels weak. - [Xyla] It still feels really weak. I'm just surprised how much - - [Derek] How far, how far we are? That it's not even like, eh almost. - Yeah. You'd think there are moments
where you're like oh, oh. - [Derek] Yeah, yeah, yeah. - But no. I left Derek's house that night mortified. Like I haven't failed to
build something for someone in a really long time. And of course it had to
be for Veritasium himself. So the next morning I
scrapped everything I had. I started over and I built this cart. I found this helicopter tail rotor gearbox at our local hobby store that was like an hour away from my house, and it's way more legit than the Lego one, But the problem is the axle
only came out one side. So my last-minute solution
was to drill a hole down the shaft and insert a
thinner axle on that side. - [Derek] Okay. - Okay. - [Derek] This is the Mark I? - [Xyla] VI versus VII. It's going to, like, fly
apart, just you watch. It's a lot closer. Closer, maybe, but so unstable that I actually left Derek's
house, went to a hobby store to try to find a way to extend that axle. (Derek laughs) - Yeah, the engineering
process is an iterative thing. - [Derek] I like it. Try version three. Where's the safety glasses? It'll probably be fine, but. - This is definitely the last try, after this I'm like
completely out of ideas. - [Derek] All right. - How to improve. You may notice that I not
only lengthened the axle, but I also got a way bigger prop with the hopes of just pushing more air. Ready? - [Derek] Yep. How's it feeling? It's not feeling like it's
going to push itself forward? Are you going to debunk this whole thing? Is there going to be a
video on your channel? (Xyla laughs) - Derek is lying! - [Derek] Veritasium's
misleading the public. (Xyla laughs) - What a sad, sad axle. - [Derek] That is so sad. - Pitiful. Embarrassing. (whirring) Oh. - [Derek] Nope. (Xyla and Derek laugh) - [Xyla] That was a no. And that kind of brings us to today. At this point, it is a day before Derek's first video
came out, and that's why in that video you see him using my model as an explainer prop, but
not actually functioning. And that kind of broke my heart, but it was too late and
there was nothing I could do. But then, Dr. Alex Kusenko,
a UCLA physics professor, bet Derek $10,000 that this was a hoax and he could not prove that it worked. And so I knew it was my
moral responsibility, my duty as an engineering
YouTuber to not only make a model and a video that shows you it works, but also to provide you with free plans to make sure you can make one at home and prove it to yourself. And for my final design,
I decided that 3D printing is now an accessible enough technology that anyone determined
enough to build this cart is also determined enough to
find access to a 3D printer. So I decided to design a
cart that was entirely a combination of 3D-printed
parts and parts available from McMaster-Carr. Now McMaster-Carr is the
most expensive solution, but they, guaranteed, can
get you parts anywhere, at least in the United States, which means that you are more
than welcome to find these parts cheaper, but you
definitely have the option on McMaster. So the parts list and
the 3D printer files, as well as the CAD model
of my entire design is all down in the description below, and that will make this
build video a little easier. Before we assemble, though, I need to explain to you why
this cart is going to work and none of the other ones did.
So teacher Xyla activate. Okay, so the physics of how
this car works is explained really, really well in Veritasium's video, so I'm not really going to go over that. The thing I'm going to go over
is the engineering concept that you need to make this work, and that is the ratio
of the propeller pitch to the wheel circumference. Technically, it's the propeller
pitch to the drive shaft gearing to the wheel circumference ratio, except that the easiest thing to do is to make your gearing one-to-one, so you can kind of eliminate
that from the equation, and now all we're thinking
about is the pitch to diameter. Let's start with propeller pitch. I like to think of a
propeller like a screw. For one full rotation, it
travels a certain amount through its material. For screw, that would be wood, but for a propeller that's air. If a screw has a really coarse thread, one turn will advance it
much further in the wood, but will require a lot
more force to drive it in, and the exact same thing
goes for a propeller. Keep that in mind, and let's talk about
the wheel for a second. For one full turn of the wheel, it travels the circumference of the wheel. That's sort of self-explanatory. But since this propeller
is geared directly proportionately to the
wheel, those matter a lot. And the ratio of these two factors, the propeller distance
over the wheel distance, is something that the
inventor of this craft calls the vehicle speed ratio or VSR. And for a vehicle like this to work, the VSR needs to be 0.7 or less, which means that the wheel diameter needs to be pretty large. And that was the problem
with my first two carts, other than the fact that
the wheels fell off. But they never would have worked anyway 'cause the VSR was like 1.5 or higher. Now you can do the math
for any cart you build in any wheel diameter and propeller, but what I found the easiest
thing to do is to make the diameter of your wheels 10 divided by pi, which means that the
circumference is just 10. And that means dividing is
really easy and your VSR is going to be basically just whatever
pitch the propeller is. For an ultra light
vehicle, you can get away with a VSR of 0.7, so a
seven-inch propellor pitch, but I found for this one
4.7 was the sweet spot. And that's the key. Class dismissed. And with my pretty purple
parts straight out of the printer - say that three times fast, I recorded that many times - I used a drill bit as kind of
a reamer to make sure that the diameter of that hole
is exactly half an inch. And that will get a nice snug press fit for all those bearings. And you may find that some
sandpaper is involved, which is tragic, but absolutely necessary because these bearings are critical. We need to get this system
as frictionless as possible in order for the cart to work. Like, I'm talking smooth
like butter, people. (snaps) Cut one of your steel
axles down to 12 inches, and then you're very likely
going to have to sand it down. I had to sand mine down
to fit into those bearings because they're 3/16th
bearings, 3/16th axle. And then I marked where it
sits when perfectly centered and CA-glued it in place. Now for the bearings that
go on the propeller axle, since they're both really
close to the ends of the axle, I actually managed to get
away with freezing the axle, heating the bearing,
and just going for it. But you, of course, can just
sand it, if that's easier. I will say though that the
at least lower bearing, the one closer to the bevel
gear on the propeller shaft needs to be a very tight fit, so you'll either need to
glue it or have it press fit like I did. To get a snug fit with the wheels, I just drilled it to 3/16th
and that worked perfectly, although I will say I replaced the wheels we're seeing here with
thicker ones later on. For the center support braces, I used aluminum tubing at
first because it was cheaper, but it ended up being a
really good decision because aluminum tubing can bend ever so slightly, so you can just do really tiny
adjustments to get the cart to run perfectly straight. So those got installed in
their holes and you're going to need to glue them in. I used CA glue. I didn't like how much those
first wheels I put on wobbled, so I redesigned and
3D-printed these wheels. They're much thicker, so the rubber band sits
on them really nicely and they also have a hub which holds them at perfectly 90 degrees from the axle. I CA-glued the beveled gears in place, and you're going to want to make sure there's just the tiniest gap between them and that'll help it run really smoothly. And now it's time for the propeller. Most propellers will come with
these hub adapter thingies. And I used one that was a little bit small and then I drilled it
to be the perfect size. I used a shaft collar a little
bit as a spacer just to hold the propeller out from
the bearing a little bit, and then I also used a shaft
collar on the other side, just for safety and peace of mind. And with that done, it was
time to head back to Derek's. - [Derek] Do you feel redeemed?
Like how confident are you? - [Xyla] I mean, I will be
really sad if it doesn't work, so I don't want to say I feel redeemed until I have redeemed myself. See, this is what you've evolved into. - [Derek] How it started. - [Xyla] How it started. How it's going. (beep) (whirring) - [Derek] Going up to 10 miles per hour. - [Xyla] This is where it should work. - [Derek] No. We're going to 12. (Xyla laughs) It's a close one. - [Xyla] Nice one. - [Derek] Trust me there. (smack) Ow! - [Xyla] Oh! Are you okay? - [Derek] Yes. But have I
done something to the prop? I think we need a
treadmill store treadmill. - [Xyla] Treadmill store treadmill? Okay. - [Derek] Like, it's basically there, it just needs a little extra love. To the treadmill store. - [Xyla] To the treadmill store! - [Derek] For victory. (Xyla laughs) (techno music) - [Derek] Whoa. - [Xyla] Oh! - [Derek] Look at that. (whirring) - It's like almost buoyant. You know when like something will float and then eventually,
like, slowly it sinks? And because we were so close
Derek and I decided to just jump in the car and drive
to the nearest hobby store and buy a couple more propellers
with lower pitch angles to try those out. - [Derek] Yeah, I'm going
to make the claim on camera. I think it's going to work this time. We're changing the propeller. - It has to work before
we get kicked out of the treadmill store. (whirring) Oh! - [Derek] Does it work? - It totally works! - [Derek] Amazing. - Oh my god, it's so good. You want to try it? - [Derek] Yes. (whirring) Oh yeah! Oh! Why? Why? - [Xyla] Are you okay? - [Derek] Why? What is wrong with me? Ugh! Look at that. (Xyla gasps) - [Xyla] Oh my god! - [Derek] Doing it for science. All right, let's do it. - [Xyla] I'm so sorry. - [Derek] It's all good. My main concern is the car. (Xyla laughs) This is dangerous work that we do here. - [Xyla] Ugh, I can't
believe it finally works. - It works! - [Xyla] It finally works. - [Derek] Physics. - I want to see if I
can get this thing to - - [Derek] Run straight? - Minute adjustments. (whirring) Yo! (Derek laughs) (rock music) (Xyla laughs) - [Derek] Amazing. - [Xyla] Victory! (rock music) (whirring) - [Derek] Amazing. - So good. Do you want to play with it? - [Derek] No. (Xyla and Derek laugh) I have the scars to show
I'm not the right one. (whirring) - [Emily] Oh wow. That was so good. Wait that's crazy. It looks so good. It moves in a straight
line and it goes, like, consistently up. It's beautiful. - So proof there's like
no batteries, no motors. (Xyla laughs) - It's beautiful. - [Xyla] Thank you. - Like, you know spherical cow? Like this looks like it's
from the physics problem. Downwind faster than the wind. - Downwind faster than the wind. - [Emily] There it is. - [Xyla] Myth busted. And there you have it. With just a 3D printer and some
simple parts from McMaster, or somewhere cheaper, you can make a cart that some
physicists have been claiming is a hoax for decades. Please subscribe if
you enjoyed this video, and also a massive,
massive thank you to Derek for continually believing
in me as an engineer and letting me keep trying and keep trying until I got it right. Thank you. - I just went to LACMA. There should be an entire floor
just for this in the middle. - [Derek] I love it. - Homies gas up other homies. - [Emily] I'm just speaking,
like, my true emotions right now. There is no
gas in this vehicle. There is truly no gas in this vehicle. I'm just genuinely so impressed
by this, like holy moly. (whirring) - Oh, hey, I didn't see you there. I was too distracted by
my awesome KiwiCo kit. (laughs) Oi. In all seriousness, this
video is sponsored by KiwiCo. KiwiCo creates super cool
hands-on projects and toys designed to expose kids
to the concepts in STEAM: science, technology,
engineering, and math. It's no secret that I'm a
huge advocate for hands-on education, and I think the
only reason I've made it to where I am today is
because I had a small handful of teachers growing up
that acknowledged that I learned through projects, and they'd let me do things
that were more hands-on and less studying from a
book and taking a test. It's summer, which means that
kids aren't really in school, but they still need to be
keeping their brains engaged and focused, and I think
summer is the best time to get our kids building
stuff with their hands. There's no homework to distract them. We can teach them hands-on skills. KiwiCo now ships to more
than 40 countries and you can get 50% off your
first month of any crate by going to KiwiCo.com/XylaFoxlin, which is also linked down
below in the description if you don't like typing. (upbeat techno music)
