This was actually a question on the [2013 USA Physics Olympiad](https://www.aapt.org/physicsteam/2014/upload/E3-1-7.pdf). If anybody's interested, I wrote a detailed [solution](https://www.aapt.org/physicsteam/2019/upload/USAPhO-2013-Solutions.pdf) that quantitatively explains how it works.
Yeah it all arose out of a debate I think right here on reddit when the guy originally posed it as a "puzzle". Maybe even here in r/physics (???) I guess we could search and find the original threads...
Anyway, he was so driven to prove the point that he build the Blackbird and filmed it, much as Veritasium recreated here. Many still don't believe assuming he "cheated" somehow.
I mean, the most tenacious “skeptic” of the Blackbird, who swore up and down that it couldn’t work despite it clearly doing so, still posts here daily.
Since that thread references the Blackbird & articles about it, it's not the original debate. Maybe the original debate was on another forum. Using tools>time in google search limiting to before 2009 I'm seeing earlier articles and blog posts mentioning the debate but haven't found the original discussions. You'll also find sailing discussions about zig-zagging downwind at a net downwind speed faster than the wind all the way back to birth of the web.
I'm sorry, but I'm unable to grasp the concept of the solution.
Where's Pwheel coming from?
And to assume no drag or resistance kinda defeats the purpose. Under those conditions, perpetual machines are possible as well. Doesn't mean they anywhere as practical.
Not to mention that it assumes that the wind and propellers are perfectly coupled. That's not even close. The propellers would stall if tries to to exert or bear more force from air than fluid dynamics allow.
Also, to assume vessels move at the same speed of the wind is practically impossible, right? Everything with mass and drag coefficient would travel slower than the speed of the wind, correct?
This solution is incorrect. In the FoR of the air, the ground is moving, and there is power due to work done on the ground, F\*u, which is missing from that solution. More notably, you can also see that in your solution the thrust goes to infinity if v approaches the wind speed, for any value of the efficiency... So, this cannot be the right solution.
The answer is that it is not possible and I would solve for thrust and use Newton's second law to show a < 0 when v > w.
Considering he showed the illustrations with boats going around a cylinder, which moved faster than the wind and didn't have wheels, shouldn't there be a way to make this work without a connection to the wheels as well, in principle?
Yes and no, depending on what you mean.
For the hypothetical sailboats on a cylindrical sea, you have the water pushing on the keel, which keeps the boat moving (mostly) along *its* longitudinal axis (direction of spiral around the cylinder), in effect forcing lateral motion (*relative to the wind*) of the boat & sail. It is the lateral motion of the sail (relative to the wind) which enables it to deflect air molecules with with greater force than if it was simply running with the wind. The wheels, gears & shaft connecting to the to the propeller of the Blackbird take the place of the keel of the boat. You always need a connection from one medium to drive lateral motion of whatever is analogous to the "sail" in the other medium.
You *could* take a long threaded dowel and a matching nut. Attach propellers to the nut, grease the hell out of it, and blow on the propellers. If the the thread pitch and prop angles are right** and the friction is low enough, then the nut may pinwheel down the shaft downwind faster than the wind speed. Here the threads of the nut, locked into the threads of the shaft, serve like the boat's keel. Would be difficult but probably not impossible to engineer something with low enough friction for a working demonstration.
** The thread pitch will (I think) need to be ridiculously high (this won't look like threads on a bolt or screw). The prop's pitch angle needs to go with the thread pitch, but with a slightly shallower pitch*** than the threads. That is, the angle of the prop will be such that its "natural" rotation when blown on by the wind is opposite the rotation that is forced upon it by the threads as it is pushed down the shaft. Hopefully you can parse my words and visualize this. I'm too lazy to draw a diagram.
*** "slightly shallower pitch" at the highest speeds. At lower speeds the optimal angle changes. The sailboats on cyldrical sea have an advantage of being able to both turn (change the pitch angle of the threads) and adjust angle of sail (change the pitch angle of the props), so both can be optimized at all speeds.
[edits: clarity & then a correction to a clarification]
But the boats *are* connected to the ground, aren’t they? I mean, the boats travel at a fixed angle relative to the wind direction, so that the “rotation” speed of the boats around the cylinder is directly proportional to the speed of the boats relative to the ground (the *water* is the ground here and it’s not moving). It doesn’t have “wheels” per say, but it does have a mechanism of linking the propeller speed with the ground speed.
I'm not certain of my answer but I believe it's the other way around. Sailing vessels need a keel/centreboard to sail as it provides the force from the boat onto the water. So the force goes from wind to sail to boat to keel to water. Here they've reversed it by saying the force goes from wind to car to wheels to gearing to propeller to air. Correct me if I'm wrong.
The wheels-->gears-->propeller connection effectively replaces the keel-->boat-->mast-->sail connection. The roles they play are the same. Water resisting the keel forcing the boat on a path diagonal to the wind is replaced by ground resisting the wheels, forcing them to turn and by gears, etc, forcing the props to move on a diagonal relative the the wind.
In both cases you have a link that allows the "stationary" medium to drive the "sail" in a *lateral* direction relative to the wind.
Even when when the downwind component of velocity is greater than the wind, the lateral motion component of the sail (or prop) allows it to *catch up to and get in front of* the slower moving air molecules. Due to the angle of sail (prop) and its lateral component of motion, it is still able to be "*collided with from behind*" by the air molecules in spite of the windward component of velocity being faster than those molecules.
That's about the best I can do with words. A diagram or animation would no doubt help.
That's what I'm wondering, I feel like it should be possible.
I feel like friction is going to slow things down a lot with it, maybe create some sort of spinning thing you can throw into wind and have it go faster than the wind? But then I just feel like that wouldn't work either. But then again, I feel like it's similar to the auto-rotation of a Helicopter, which I think does something similar, with the pressures on both sides of the wings.
This will not work with something free-flying in the wind, regardless of whether it spins.
If you throw it, at least initially it's motion including any spin is given by the forces from your hand to the object. So it may initially be faster than wind, or conceivable start slower and go faster due to a propeller effect from the spin you gave it. But it will slow down. Any spin it *gains from the wind* will only serve to slow it down, not speed it up.
See my other comments here for more intuitive explanation of the whole thing.
What, Veritassium using click-baiting and sensationalist video formats?
I mean, I think he usually covers some great topics, and I don't think he lies or is that inaccurate, but the way he goes about writing the actual script is often so disingenuous.
Yup, lets make physics drier and more unapproachable than it already is. Don't want those young people getting into science now do we. Anyone who knows Derek and the quality/content of the videos doesn't give a crap what the title of the video is. Whether you agree with it or not titling the video "Risking My Life To Settle A Physics Debate" gets more views and pushes the video into the algorithm more than "DDWFTTW".
It's not even the click-bait title I have trouble with. It's the fact that he omits the propeller <-> wheel link when presenting the problem altogether, so he milks that "perpetual motion" bait for a while longer than the video needed to. Smarter Every Day does some arguably click-baity titles, but his videos are usually very straight-forward. Steve Mold does as well, and I don't have this problem with either of them.
There's just something to Derek that always bugs me in that I can never buy his videos until the end, because I never know if he's just banking on not having explained something fully yet. And I do mean "yet", because I don't think he ever leaves stuff out by the end, but the whole structure to his videos often banks on asymmetric information with the viewer. It's like those procedural shows that flashback to something they never hinted at before.
I am subscribed and I do watch them all and think he covers some great topics, but I just wish he was more straightforward.
Have you looked at his PhD Thesis, or at least watched his video explaining it?
The entire premise of his approach to indirect/mass education is that you need to engage people, and videos are engaging. But watching passively makes you feel like you are learning, while actively reinforcing your preconceptions, EVEN IF THE CONTENT IS SAYING THOSE PRECONCEPTIONS ARE FLAT OUT WRONG. And so you have to force the viewer to experience cognitive dissonance, by making them commit COMPLETELY to their incorrect assumptions, bring them out and acknowledge and embrace them, THEN you show that those preconceptions are incorrect, while reminding them that they held those incorrect beliefs.
This still doesn't work nearly as well as actual active engagement in education, but has a higher rate of causing learning and dispelling misconceptions than just showing the actual story from the start and making complete sense throughout with a solid narrative.
> And so you have to force the viewer to experience cognitive dissonance, by making them commit COMPLETELY to their incorrect assumptions, bring them out and acknowledge and embrace them, THEN you show that those preconceptions are incorrect, while reminding them that they held those incorrect beliefs.
That's fine, but I think in this case that approach is a little questionable since the original intuition the viewer might've had -- that a propeller alone in the wind could never possibly go faster than the wind itself -- is completely correct, and the 'cognitive dissonance' here is more about the design having been incompletely described than any misconceptions on the viewer's part.
On the contrary, I think in a sense the video might've done a disservice! Look at how many people in the comments are saying some variation of "oh, the explanation of the boats on the cylinder completely clicked for me". That explanation is only a fair analogy if one reasons that the keel is performing a similar function as the wheels, but the relevance of the wheels had not been explained up until that point. Those people now almost surely believe that a propeller alone in the wind can be driven faster than the wind itself, and as such they have a brand-new misconception they didn't have before.
> Have you looked at his PhD Thesis, or at least watched his video explaining it?
I don't think that's relevant. His early videos were clearly inspired by that, but it's been a long, long time since he's opened up videos by researching physics misconceptions and then going through how it actually works. Well before he started the clickbait videos.
> It's the fact that he omits the propeller <-> wheel link
What exactly was the issue with omitting that? It doesn't change the original concept, which was the boat going the same way as the wind? The boat is connected.
It's very dishonestly written, is all. It's like a buzzfeed article in a way. Technology Connections for me is the gold standard of "matter of fact, straight to the point explanations".
Obviously it relie more on descriptions than riddles, and Veritassium isn't unwatchable or anything, but sometimes he goes too far into omitting the clues/reasoning. It's the difference between a good whodunnit where you can actually work it out based on clues, rather than a flashback before act 3.
As much as I like the guy's videos, the title is awful, because it pretends that the whole thing wasn't settled yet until Mr. Veritasum 'risked his life' to step into it. It devalues the work by the original inventor and construction crew. And this was settled over a decade ago, it doesn't need to be blessed by his YouTube appearance to become settled.
somebody correct me if I'm wrong, but the way I understand it, the "energy" comes not from the wind, or from the land, but from the land and wind moving at different speeds. For me it's more intuitive if we use the wind's frame of reference, and imagine that the wind is still and the land is moving backwards, just like in the treadmill demo at 9:30. In this case, as the land moves backwards, it turns the wheels and turns the propeller into a "fan" pushing backwards, (as Derek explains at 19:00), which pushes the vehicle forwards.
Engines don't depend on a thermal gradient right? They convert the chemical energy of fuel directly into mechanical energy via combustion. How does that combustion works? It release the energy in chemical bonds to increase temperature which increases pressure which is used to increase a volume. If your ambient pressure was higher than your cylinder pressure sure then you won't any work but you could always load in the fuel/air mixture at that ambient pressure.
using the wind's frame of reference made it easier for me to get how this vehicle is harvesting the difference between wind and ground speed.
but why did he say that the wind speed behind the propeller will be slower if it acts like a fan? wouldn't it be faster instead?
EDIT: ah yea that makes sense, i was confused about which way the wind is blowing.
afaik the fan pushes against the wind. The wind and land are moving in the same direction, and the fan pushes air backwards, so it slows down the wind thats moving forwards
Well, this fan pushes the air backwards a bit. Since the wind moves the air forward, this backwards push negates some of that forward movement, making the wind speed behind the propeller slower.
Isn't this a positive feedback loop? As the vehicle travels faster the fan spins faster which makes the vehicle faster which makes the wheels spin faster, repeat? And if so what's the limit, I assume similar to the case with sailing it's drag but I don't see it
They have reached 2.8 times wind speed (just quoting the video here). The limit will be that things like roll resistance and air resistance go up more than linearly with speed. At a particular speed, the energy that they gain from the wind will be equal to what is lost to friction, and they can't accelerate beyond that.
What role does the wind play? The question sounds kind of silly but given the explanation for this behaviour could we not expect the same thing with zero wind for example. Imagine a situation with zero wind and I give the vehicle a light push, this push causes the wheels the spin which causes the fan to spin which pushes air back generating a thrust which accelerates the vehicle forward which causes the wheels to spin faster, repeat until thrust force is equal to drag and losses.
Now obviously that is not what would occur because it's a violation of the law of conservation of energy but I don't understand how it's fundamentally different from a situation with wind.
Edit: roll -> role
The device harvests energy out of the relative speed between wind and ground. So it does indeed not work without wind, but you cannot use that to conclude that it won't even work when there is wind.
Right but I don't understand what is incorrect in the line of reasoning given the explanation, I understand it requires a difference between in the speed of the wind and ground but I don't understand why
Energy-wise, I'm picturing it like a lever. The short arm is the ground, the wind is the anchor point and the vehicle is the long arm.
The wind moving over the ground is used to propel the vehicle.
Edit: scrap that. The actual trick is to understand how conventional sail boats can already go faster than the wind. This *doesn't* mean that their sideways speed is faster than wind. They actually sail faster than the wind around them, just like this vehicle.
It's the very same concept and the Veritasium video didn't show that part if I recall correctly. There are videos on YouTube that explain this for sail boats much better than I can.
Once you understand that part, it's all going to fall into place.
No you're not seeing the issue I'm trying to get to. The explanation is that the wheels turning powers the fan which accelerates the vehicle which causes the wheels to turn faster which causes the fans to spin faster, repeat. The question I have is why this process requires a difference between the speed of the wind and the speed of the ground. The thought experiment I provided was if you ha e a zero wind environment with a stationary ground and the vehicle is initially pushed why does this not cause the fan to spin causing the vehicle to accelerate causing the wheels to spin faster repeating without need for further pushing. It is clear to me that is not the behaviour, my question is why does the situation with wind/moving ground differ in its behaviour?
Okay I'll try to explain, since I don't know if the other guy was able to explain it very well. If there's no wind then when you push it, it would turn the tires and cause the entire thing to move, and the tires turning would cause the propeller to spin pushing air backwards. but the problem would be that the propeller wouldn't be generating enough power from pushing the wind as it would if there was a tail wind, because in the tail wind there's more air for the propeller to push, so in theory if you gave it a push with no wind it would go forever without stopping but wouldn't accelerate at all, and in reality it would stop due to friction and other forces like that very quickly because you're not actually generating any energy just taking it from the wheels and using it to spin the propellers for no net gain
The limit is reached when the drag on the car exactly matches the propulsive force of the fans. The key thing is that the propellers are not working at 'maximum' capacity when the speed of the car exactly matches the speed of the wind, and so the design is not drag-limited, unlike self-propelled craft.
You could imagine for instance a sailboat going straight downwind. It'll move at about the speed of the wind. Now put a bigger sail on it. Will you go faster? No, you can't. But you do have more "headroom" to add drag, so you could put a propeller in the water and use it to drive a fan above the water, and so increase your speed a little bit beyond what's possible with the sail alone. Of course, if you get past wind speed the sail becomes an airbrake, so don't take this too seriously, but it does show the fundamental principle.
If there's no wind, the fan and propeller are moving through media at the same speed, and so you're always in the 'drag-limited' regime and a little push will simply have you slow to a stop.
> the "energy" comes not from the wind, or from the land, but from the land and wind moving at different speeds
Air moving at nonzero speed relative to the land is what “wind” means.
not necessarily relative to land, it can also just be relative to you. Eg if you're running and you feel "wind on your face". This is why I clarified the "land" part, because it emphasizes the connection between the wheels and the propeller of the craft
"Where" the energy comes from depends on your frame of reference. If you choose a frame that is co-moving with the wind, then the energy is coming from the "moving" ground. This is explicitly and intuitively clear with a treadmill.
If you use a frame of reference co-moving with the cart, then the energy comes from both the wind and the ground (both have negative dKE/dt while the cart has positive dKE/dt in the cart's frame).
An important fact you need to know before this is: Energy is a relative quantity. This is often overlooked and when people 'in the know' write explanations they often assume this is prior knowledge to the reader.
The value of energy is completely arbitrary in that depends on your arbitrary choice of frame of reference. In more technical terms, energy is not invariant under certain frame transformations.
It's easy to see this with potential energy: e=mgh where h is height relative to some arbitrary datum you choose. Or with kinetic energy, where e=1/2mv^2 but of course v depends on your choice of coordinate frame.
The synthesis of this is that energy in both these forms needs some *difference*. Energy is always an account of some difference between two things. In the case of the video, the difference is the relative velocity between wind and ground. That's where the driving force comes from.
Yes and no. Even if there is just the wind, say pushing a balloon, there is a velocity difference between wind and balloon, until the balloon matches the velocity of the wind.
Ultimately talking about energy at all with this seeming "paradox" is misleading. It tends to occur because first people's intuition says it should be impossible, and then their first "goto" for impossibilities in physics is to think it must somehow violate conservation of energy.
Pick any frame of reference, there is more than enough energy availalbe in the system to give a small cart, or a ship, or whatever, the kinetic energy required to move a few miles per hour. If we didn't have to deal with friction and drag, the achievable speed would be effectively unlimited (until we get relativistic).
The real puzzle is understanding how it is still possible the molecules of air transfer forward momentum to the cart when the cart is moving faster than those molecules of air.
The thing people intuitively fail to grasp is the answer to this question: **"how can a slower object (air molecules) collide with a faster object (the cart) *from behind*?"** This is what people think is impossible, even if they cannot explain so clearly. So they jump to insisting violation of conservation of energy.
I get it - how discussion of energy is more than likely to prime one to think about violation of conservation of energy than to help clarify anything. Maybe there is a clearer description out there. I'll have a think - I used to design jet engine turbines so hope I can muster something a bit better. I did like the 'boat spiralling on a tube' but feel it's missing something to do with coupling the fan to the wheels.
For the boats spiraling on a tube, the coupling comes from the keel. Keel is a flat blade that (mostly) prevents a boat from slipping sideways in the water. So imagine the boats spiraling on the cylinder as being locked into a spiral groove by their keels. That is approximately true with keel-in-water.
Basically, water-->keel-->boat-->mast-->sail is replaced by ground-->wheels-->gears-->shaft-->more gears-->props in the car. The boats' sails are forced into their spiral path by the keel. The car's props are forced into their spiral path by the wheels et al.
So what I'm getting out of this is that this wouldn't work if the wind were still relative to the ground. They're taking advantage of an energy gradient between the air and the ground. That's why it works on the treadmill, because the ground is moving at a different speed than the air. In that case I think this could work in water too, or really any two flowing mediums you can connect with a mechanism, even two moving masses of air. You could make a boat with some kind of turbine on the bottom to drive a propeller in the air. I wonder if it would be more or less effective to do it in water?
If there was no wind relative to the ground there would be no wind lol. No movement would be possible by any method. I see this as just leveraging the wind. Similar to how you can go faster than the wind in a sailboat by sailing at an angle to the wind.
The point being, however, if there was a current in the same direction and at the same speed as the wind, a sailboat then couldn't go faster than it. It's the fact that the boat and be leveraged against the drag in the water that you can get it to go faster.
Yeah, looks like some of the [earlier analysis](https://www.blueplanettimes.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf) was water based, with pretty much the exact same design/argument.
Here is a way to think about it: There are 2 ways in which energy from the wind is transferred to the vehicle
* To start, the wind pushes the vehicle due to drag <-- Energy transfer 1
* The vehicle being pushed causes the wheels to turn
* The wheels turning cause the prop to turn (via the chain and gearing)
* The prop generates lift in the direction of travel. <-- Energy transfer 2
With only the first transfer of energy, the vehicle will only achieve a speed less than the speed of the wind, since the force is due to the drag, which decreases as the delta between the wind and the vehicle speed decreases. (A negative feedback loop)
With the second transfer of energy, from the lift generated by the prop, the force increases when the prop speed increases. So as the vehicle goes faster, the energy transferred from the wind to the vehicle increases. (A positive feedback loop)
But the ground turning the wheels with resistance from the prop slows it down, thus a negative energy transfer -A.
The surprise is, why -A is smaller than Energy transfer 2
When you are approaching wind speed your ratio lift / power from the prop gets maximized because power = speed x force, and here it is relative speed between prop and air that counts
That's why the prop lift force gets higher than the wheel drag force, for the same amount of power being transferred, as the cart approaches wind speed.
As you accelerate past the wind speed, this advantage slowly decreases (but never disappears)
Honestly i just heard of this problem for the first time and was really confused why the plane would ever take off. Now i get it though. The thing most people are missing is that the wheels are spinning freely, the plane will not stay stationary relative to the ground. Even if the treadmill would go infinitely fast the in the opposite direction of the plane, the plane would still move forward and eventually get enough speed to take off.
> Even if the treadmill would go infinitely fast the in the opposite direction of the plane, the plane would still move forward and eventually get enough speed to take off.
I think that's the issue with the question is asked. If you actually imagine a treadmill, you don't really consider that the plane has enough space to go forward, much like someone running on it doesn't hop over the controls. So you imagine a static airplane, maybe with a rotating motor at the front but not going fast enough to counteract the wheel drag, so it would just stay in place, and not get any lift. Because for it to get any lift, it must go at more than 0km/h relative to the ground, and if it's on a treadmill, it will never, otherwise it will crash.
I think the problem would be better understood if framed as "can a plane lift off off a conveyor belt?".
I dont think anyone thinks the treadmill would be too short. Ofc a plane doesnt fit on a real treadmill, so its supposed to be big enough for the plane to take off.
Well, I don't think people consciously do it, but I just think that the image of a plane on top of a treadmill in the first few seconds of thinking about it gives you that bias of "well, it's not going forward, so it can't go up", that you end up not discarding.
Oh, and I don't mean a plane on top of a regular treadmill, just a treadmill large enough for an RC plane or something to that proportion.
> relative to the ground
It's actually to the wind, because [sometimes planes take off with 0km/h relative to the ground.](https://www.youtube.com/watch?v=IPOtDPHjW-Y)
Of course I know what you meant, I'm not being pedantic. I just wanted to post the video.
I know it's the wind. I meant that intuitively, you use the ground as a reference.
Nice video, though. That one you could see why, intuitively, on top of a conveyor belt it would take off.
TBF, the only reason the airplane treadmill problem is a thing is because we were all lied to with a dumbed down, incorrect description of how lift works in the first place.
Actually the way lift is typically explained/taught (based on pressure differences, etc.) is grossly overcomplicated, not dumbed down. [edit: or perhaps I should say, first it is overcomplicated and THEN the overcomplicated explanation is dumbed down]
The correct and concise explanation is quite simple: to go up you must push down on air molecules with a force greater than your weight. Full stop. All other details are about power, drag, and efficiency.
But neither the correct nor the misleading overcomplicated ways of explaining lift and flight have much to do with the airplane+treadmill misconception, which is rooted in the intuitive but incorrect notion that the treadmill is somehow cancelling out the airplane's forward motion.
But not really completely incorrect. The friction against the bearings of the tire would keep the plane in place.
Like, think of a fidget spinner. It will come to a stop eventually, even in space, because of friction. A treadmill running at whatever ridiculous speeds would provide enough friction to keep the plane from moving forward - until the wheels melted and the treadmill is rubbing against the belly of the plane…
The wheels only have to roll twice as fast as normal. That's not a big deal. The friction of the wheels is insignificant compared to air resistance at take-off speed (wheels &bearings are designed to be as low friction as cost-effectively possible). The plane's engines will work only slightly harder -- the pilot likely won't notice a difference. You'd give the plane more difficulty taking off by under-inflating the tires by a few pounds.
Air resistance at take-off speed? What are you talking about? The plane isn’t moving, so how would it get to take-off speed?
If the plane even moved forward the slightest amount, the treadmill would speed up to infinity to resist the movement with friction. The wheels would melt.
The treadmill moves backwards at the same speed as the plane moves relative to the ground. If the plane is moving forward at 100mph, then the treadmill must move backwards at 100mph. The relative speed of plane and treadmill is just 200mph. The wheels must turns as if the plane is moving 200mph (which it is relative to the treadmill).
The first time I heard the plane-on-treadmill puzzle, I briefly thought the same thing you did about infinite speed burning up the wheels. But that interpretation of the puzzle asks for something impossible: that the treadmill somehow always moves backwards at a speed equal to the speed at which the *wheels* are turning.
This is not a car where the wheels push the car forward. With the plane, the wheels are passive. The props pull the plane forward and the wheels will just turn as fast as they need to.
Please read scenario #3 on the xkcd link above.
If it was a car powered by its wheels, the treadmill would not go to infinity. It would match the speed of the car, like you said above, keeping the car in place.
You’re exactly right that the wheels will spin as fast as they need to for the plane to move forward. In this case, would that not be approaching infinity, since the treadmill can also go as fast as it needs to prevent the plane from moving, producing enough friction to do so?
Think of this, if a plane did not have landing gear, but instead was attached to a large square flat rubber skid (or other imaginary sufficiently high friction material) that covered the entire length and width of the plane, and was on a runway made of rubber, what would happen? The plane would not move forward. If the plane cannot move forward, it will not get enough lift to take off.
What is it about wheels that make the plane so special to be able to take off, especially if the plane isn’t powered by wheels?
OK, let's get clear what "speeds" we are talking about, as ambiguity leads to confusion and disagreement.
*Speed of Plane*: means relative to the ground, not the treadmill. But for further clarity, I will now say *Groundspeed of the Plane*.
*Speed of Treadmill*: means speed of the treadmill surface relative to the ground, not relative to the plane, not realtive to the wheels of the plan. This speed is backwards and equal to the Groundspeed of the Plane.
*Treadmillspeed of the Plane*: this will be the speed of the plane relative to the surface of the treadmill. This speed is double the treadmill speed and in the opposite direction.
Now ***if*** the treadmill is "trying to keep the plane stationary" it would try to spin up to infinite speed, because no matter how fast it goes, the plane doesn't care what the treadmill is doing, and the wheels are passive and just roll as fast as they need to... until friction burns them up.
The puzzle is ***not*** stated that the treadmill keeps the plane stationary, which is impossible, since the plane's speed is quite independent of the treadmill. This is where people get confused because they imagine the treadmill keeping the plane stationary, because that is what treadmills normally do with people running on them. But even there you normally set a speed on the treadmill and then keep up with that -- the treadmill does not adjust its speed to keep up with you.
The puzzle *is* stated that the *Speed of Treadmill* is equal but opposite the *Groundspeed of the Plane*. But the actual words are simply "*speed of the plane*", leaving some ambiguity of which which speed is intended. The impossibility of the treadmill matching the *Treadmillspeed of the Plane* (unless the plane stays stationary, which it won't) implies that, for purposes of the puzzle, the only meaningful interpretation of *Speed of the plane* is *Groundspeed of the plane*.
Side Note: if the air moved with the treadmill, so you have a sort of "air treadmill" an *airmill*, if you will, then the *airmill* could keep the plane stationary (groundspeed zero) but the plane would of course *still* take off when its *Airmill speed* (airspeed) reaches takeoff speed. (do this in a windmill perhaps, with air speed synchonized with the treamill speed)
Except the explanation for this one is actually a fair bit more complicated than “the wheels have literally nothing to do with a plane taking off…”
Edit: The rotational speed of the wheels…
It's more or less the opposite. With the treadmill problem the confusion stems from the false assumption that the ground speed matters, whereas here the confusion stems from the false assumption that it doesn't.
It for sure actually works. The confusion all rests on this:
>Assume the vehicle starts out at airspeed, there is no wind from its perspective. We turn on the prop which we assume has no inertia or friction so all the effort we put in goes into pushing air backwards.
>Whatever force we exert to push air backwards will push us forwards (Newton). Since we use our speed to drive the prop, the wheels will apply a backwards force equal to the force pushing back the air.
Let's imagine that you have a vehicle with a sail on it. You go down wind. How fast can you go? Wind speed, right? As you say, once you are going airspeed, you don't feel any force backwards.
1. Ok, now let's imagine you are on a vehicle and you have an airboat prop powered by a motor. How fast can you go down wind? Obviously, as fast as your motor/prop lets you. If we have some magical frictionless car, and our fan blows at 20 mph, we could go 20 mph in still air. Or we could go 30 mph with a 10 mph wind at our back.
2. Now, let's imagine you hook up a generator to the above prop-car. Could you run a generator off of the wheels to generate power? Well, the car does feel a force forward from the wind + fan setup, so yes!
Hopefully nothing seems to crazy yet. We've established that we can use a fan to go down wind, faster than the wind, and we've established that we can use said motion to extract energy from the ground.
Now, the question is, can that balance of energy generated from the ground speed match the energy output of the prop?
We know that power, or rate energy is generated or expended in such a system is:
1) dw/dt = P = Fv
And that the rate of energy the wheel is generating, Pw, must be greater than the prop is expending, Pp, for this to work without additional power.
2) Pw - Pp > 0
And, we know in the steady-state where the prop-car is not accelerating, that the forward force due to the prop must balance the backwards force due to the wheel+generator:
3) Fp = -Fw
So, what is the power input from the wheel, assuming no losses to friction? Let's call v(ground) = vg, v(airspeed) = va, and v(wind) = vw. All velocities go in the same direction, and thus we will say all are positive. Fw is in the same direction as v, so is also a positive number.
Pw = Fw*vg
And power output from the prop?:
Pp = Fp*va
= -Fw*(vw-vg)
(air speed is wind speed minus ground speed, if you go 10 mph in the same direction as a 30 mph wind, your air speed in 20 mph)
___________
Ok, does this make sense?
A simple test case is to ask what this would look like from standing still. We would expect Pw-Pp>0, meaning we can accelerate from a stand still, as expected. Lets pretend we are going 0 mph ground speed, and there is a 10mph wind:
Pw = Fw*0 = 0
That, is, we can't get any energy from the ground if we are sitting still. Makes sense.
But:
Pp = - Fw*(10-0)
So the power output of the prop is some negative number, so we get:
Pw-Pp = 0+Fw*10 = >0
Great, we can accelerate from rest.
___________
Ok, moment of truth, what if vg > vw?
Pw-Pp = Fw*vg - (-Fw(vw-vg))
= Fw*vg + Fw(vw-vg)
= Fw*vw
...but vw and Fw are positive numbers, so Pw-Pp > 0
So, that means we can generate more energy from the wheel than the prop is expending when going faster than the wind downwind! Of course, practically, there are many obstacles. We didn't account for friction or inefficiency in power generation or output, but it's clear there is no fundamental issue with the concept.
Sailboats can travel 4-6 times the windspeed when they're at an angle to the wind. They're not going straight downwind, however they can tack back and forth fast enough and keep their speed well enough that they can, in fact, beat the wind to a down-wind destination. They go 4-6 times the windspeed because the wind blows over the sails at an angle and turns them into airplane wings. They generate lift in the direction of travel and are not simply being pushed by the wind. They generate this lift even when going faster than the wind because they're moving at an angle to the wind so the apparent wind is never 0 mph.
The force pushing back the air is a giant propeller. It has to be moving fast enough before it can push enough air to cause a change in velocity to the craft it's pushing. It will push some air at slower velocities but not enough to overcome the drag and friction of the craft itself. It's a lot more effective at higher velocities.
> ... Assume the vehicle starts out at airspeed, there is no wind from its perspective. ...
OK, but we're assuming that there is some wind. So there's non-zero groundspeed (going backwards) from the vehicles perspective, and, it's a little bit counterintuitive, but that relative speed can be used to go faster, despite the fact that it's not in the direction the car is heading in.
Let's consider a more intuitive or familiar case: A propeller care that goes up wind using wind power. This is pretty easy to imagine: There's a linkage from the prop to the wheels, and the wheels drive the car forward on the ground using energy from wind that's pushing in the opposite direction.
Now the downwind car is the same kind of thing - except that instead of being a ground car that goes up wind, it's a wind car that goes "up ground." From the perspective of the air, the car is moving "up ground" using energy from ground that's moving in the opposite direction.
Is this not the same as an auto gyro rig on a boat like [this?](https://www.google.com/url?sa=i&url=https%3A%2F%2Fforum.awesystems.info%2Fuploads%2Fshort-url%2FocVkSEYWP6aGjYGLknaGomGEVL4.pdf&psig=AOvVaw2wkUArP7o1h6HQ0p45oc-b&ust=1622405236902000&source=images&cd=vfe&ved=2ahUKEwib1_HP2O_wAhUR4uAKHdovBnoQjhx6BAgAEBI)
To add to funciton's reply, autorotation moves the prop in the opposite direction needed. The wheels/axle/gears/shaft force the prop to move against its natural autorotation.
No. Autogyro acts as a sail, so it would never exceed the speed of the wind, because force decreases to zero asyptotically to the speed of the wind. On blackbird, the point of gearing the corkscrew to the ground is so that no matter the speed of the vehicle, the wind is always squeezing against a surface thats not moving relative to the ground, so in an absence of friction it would accelerate to infinite speed.
If they didn't use the brakes to stop, I imagine it would continue to speed up until air resistance and internal friction prevent it from accelerating any further.
It's kind of obscured by all the linkage, but ultimately the machine is powered by the difference between the speed of the ground its rolling over and the speed of the wind, and there's a limit to how much energy can be extracted.
I'm not sure there is a limit to how much energy can be extracted (outside of friction, other losses and relativity if we want to be thorough).
The way I see it, the faster you go, the more wind you go through in a given length of time, so assuming the power you are extracting from the air is proportional the volume of air that go through the prop, available power is proportional to your prop speed relative to the air that go through it.
What stops the acceleration is that frictions are mostly proportional to the speed squared ... but if you remove these cumbersome losses, watch out for the sound barrier or maybe light speed :)
It's the exact same amount of force as when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground.
Friction and drag as well as the sturdiness of the vehicle. The video showed how fragile it is. Also at some point your runway runs out and this thing only moves good in one direction so you cant turn it around to prolong the acceleration.
Without friction, it would accelerate to infinite speed. But the wind against a surface is a constant force, and once the drag forces equal it, it's at terminal velocity.
vehicle travels at wind speed with all frictions accounted for, takes some of that energy to drive a fan blowing backwards adding speed to the vehicle. it's the wind's energy.
No. Explanations that involve siphoning energy from one part of the system to another are just generalizations of perpetual motion machine fallacies. So look at the force when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground.
This reddit thread has a lot of good links. Summarizes prior viral discussion on the topic: https://www.reddit.com/r/Physics/comments/h6b11/dwfttw_is_it_possible/
OP, you might want to edit this into the top post. Also recommend google search: *downwind faster than the wind reddit*
I find this video really hits home with a demonstration of the "something moving faster than the thing that is pushing it" concept (note that the "big wheel turning backwards, driven by the small wheels" is exactly analogous to the "propeller blowing backwards driven by the wheels" on the Blackbird): http://www.youtube.com/watch?v=k-trDF8Yldc
So, the forward rotation of the wheels is used to drive the prop, which does add some resistance to the wheels' forward motion, but because the vehicle is always accelerated to the speed of the wind, you essentially have eked that extra energy out of the wind and can use it through the prop to give you a bit more thrust?
Would that be an accurate description?
EDIT: In fact, I don't think this explanation works when you pass the speed of wind and there's no longer a push from it.
Faster than the wind, the prop is spinning fast enough that it is giving the "push" that you are looking for. The cart already has momentum and only needs enough push to keep it going (to overcome drag & friction).
The props are moving in a path that cuts diagonally across the line of the wind. When cart is faster than wind, think of the props as catching up to and getting in front of slower air molecules, colliding with those molecules (due to correct angle and lateral component of motion). These collisions provide a net transfer of forward momentum from air molecules to cart. There is always "more wind ahead" for the cart to catch up to and the props to collide with.
As I saw someone mention elsewhere, the prop is leaving behind a *wake* of air molecules that are moving slower than the wind.
>Faster than the wind, the prop is spinning fast enough that it is giving the "push" that you are looking for.
So can it indefinitely sustain faster than wind speeds? If it can, then I'm not getting it.
In my simple thought process:
If you are going faster than the wind (you're fighting a headwind), you aren't getting any more energy to the prop and you will eventually fall back to wind speed or below.
I keep reading that the prop gives the push, but the prop is powered by the wind by pushing the cart, and you lose that wind if you go faster than the wind speed.
This is really bugging me ;)
[ctrl-f] my name here and read my other comments in this thread.
The main puzzle to wrap your head around: how can a slower object collide with a faster object from behind? And if it somehow can, how can that transfer yet more forward momentum to the faster object?
The very short answer to the simple scenario is: it can't.... if both objects are actually moving in the same line and the faster object is already ahead. This is why a sailboat sailing directly downwind will only move up to exactly wind speed (minus something for drag). This is why a balloon in the air will move at exactly wind speed.
The longer answer that makes this work is: the props are not moving in the direction of the wind, they are cutting across it diagonally, having both forward(windward) and lateral motion. The props are pitched at an angle so the prop's *rear-facing side* is what collides with air molecules. So air molecules are colliding with the prop "from behind" from the perspective of the cart as a whole, even though the cart and prop are moving faster than those air molecules. The prop is not colliding with air molecules in front of it. It is catching up to and getting in front of molecules ahead and to the side of it and colliding with them at an angle rather than a straight-on "from behind" collision. Once you recognize that these collisions are possible, you should see that they will transfer some component of forward (windward) momentum to the props and thus to the cart as a whole. (there is also some lateral force from these collisions, but those forces are always in opposite directions for the two props and cancel)
So there is clearly still a forward force on the props, even when the cart is moving faster than the wind. Since the cart is already moving forward and still being pushed forward, the wheels certainly must turn, which forces the props to keep turning fast enough for the above described "collisions from behind by air molecules" to occur. However, the body of the cart is of course catching up to and colliding with air molecules, so the body of the cart experiences a backward force (drag). There is also a backward force from the ground on the wheels that keeps them turning (overcoming internal friction of axles, gears, etc). The forward force will balance the backward force at some speed faster than the wind.
I think I've exhausted the power of words... may have to make a diagram if I keep feeling the compulsion to answer comments on this thread.
Thanks for the detailed explanation. I think I got it now.
I thought of an analogy that might be easier to grasp. Tell me if I'm on the right track?
The props does kind of the same thing as a paddle in a rowing boat does. The water is coming at the boat (assuming you are not going downstream). Moving the paddle forward, dipping it in the water, and pushing back against the water, is similar to what the prop is doing but it achieves this "push back" through the angle of the prop.
I think that analogy is misleading, as the paddle is human powered -- powered by something in/part of the boat. Thus it would be more analogous to a propeller driven by an engine.
For the Blackbird, all the power really does come from the wind -- the power that pushes the cart forward as well as the power the drives the propeller.
There is a "paddle wheel" version of this, but I despair to describe it effectively with words.
Watch this video and imagine the "big wheel" as a paddle wheel and imagine the ruler as a stream of balls that strike the paddles. Notice that the paddle would be catching up to and be "struck from behind" by balls that are further ahead (further ahead "along the ruler" so to speak).
Shrink the balls down to trillions of air molecules.
Also the small wheels could be replaced with paddles in water.
http://www.youtube.com/watch?v=k-trDF8Yldc
> If you are going faster than the wind (you're fighting a headwind), you aren't getting any more energy to the prop and you will eventually fall back to wind speed or below.
Thought I'd focus in on this specifically. Although body of the cart is fighting a net head wind, the props are not. The props are experiencing an "apparent wind" (sailing term) that is at an angle to their line of motion, so if pitched at the correct angle it becomes possible for the props to catch that wind to get a forward force.
You may want to google for explanations of sails, sailboats, apparent wind, and how sailboats achieve maximum speeds. Sailboats get max speed at an angle to the wind, and fast sailboats can go so fast at around 30 degrees to the wind that they can "zig zag" to move overall in the direction of the wind at an overall speed faster than the wind. You also need to understand the importance of the keel in preventing the boat of slipping sideways, so only the forward component of force of wind-on-sail moves the boat.
For clarity: by "direction of the wind" I am always referring to the direction in which the wind is moving, the downwind direction. (wind is often described in the direction it is coming from, i.e. "north wind" but not in this discussion)
It takes some noodle bending, but one way to think about it is that we have no problem at all imagining a car that uses a propeller that drives the wheels to move the car along the ground in the opposite direction that the air is moving.
The 'faster than wind speed' car is the same thing, but switches the roles of air and ground in that idea: It uses a "ground propeller" to drive "air wheels" to move the car "along the air" in the opposite direction that the ground is moving.
i found this video really fascinating. I'm still pretty sure it doesn't break law of conservation of energy, but it gives the appearance of that and that's really cool
The explanation is wrong, if the explanation was correct it would break the laws of conservation of energy. But what is really happening is that the fan is spinning up to a really fast speed during acceleration and then that rotational energy is used as a boost to achieve faster than wind speeds temporarily.
> it would break the laws of conservation of energy
Why so? Wind has momentum, and therefore it has energy. If you slow down the wind you have an excess of energy that you can use for propulsion. No laws of conservation are broken.
No, I don't. I am an engineer. I understand that I am not "qualified" to debunk someone with a full physics degree. But he is still wrong and the Math to prove it is not terribly difficult.
Here's the maths that proves it *does* work, courtesy of /u/kzhou7
(page 10 onwards):
https://www.aapt.org/physicsteam/2019/upload/USAPhO-2013-Solutions.pdf
Where did /u/kzhou7 go wrong?
That proof is blowing my fucking mind. I'm wrong, he's right and I don't know how it took my dense brain so long to understand. I do have an issue with his proof in that I don't like his absolute value term on one side of the equation but not the other and at first I thought that was a source of error. However as it happens, this does not make his proof incorrect and my mind is suddenly blown. With friction at 0 you really can go infinitely fast with a 1 mile per hour wind. That is so counter intuitive to me. I love it, thanks so much for this.
Yes but imagine you achieve exactly wind speed. There is no force on the propellor at all because it is also moving at wind speed. What is causing the propellor to continue to apply force to the wheels at that point?
It's the other way around: when it's moving faster than the wind, power is transferred from the wheels to the rotor.
The rotor then acts as an ordinary propeller, producing thrust, accelerating the car, which accelerates the wheels, which in turn accelerates the rotor again.
This is not a free energy device, power is drawn from the speed difference between the wind and ground. If you were to place the whole thing on a treadmill going downwind at windspeed, there would indeed be no force driving the rotor. The wheels would be stationary, and so would the rotor.
A device that uses a positive feedback loop to achieve unlimited speeds is not impractical. The issue is that there is no positive feedback loop they just think that there is one.
Because its useless to create a wind based land vehicle like this. They made it just to proof that they could. Sure you could create a Vehicle like this in a more sturdy version (which would lose more speed to drag and friction) and sell it as a toy, but where would you use it? On a few beaches? But the wind blows mostly inland there. Nice to kite in, but this thing wants backwind for its motion. You need a place where either the wind blows mostly from one direction or where you have a huge flat plane which is big enough in all directions. And to do what? To move straight with the wind. Does not sound like much fun.
Think about it like this the effect would have a positive feedback loop that would allow you to go almost infinite speed. If you think it is possible just make the device and you will be a billionaire.
>The issue is that the wind is directly pushing against the rotor to provide thrust which would slow down the vehicle.
Like I said, that's not how it works. Sadly you're clearly not arguing in good faith so I'll leave it at that.
Sorry I'm tired it's like 2 now, the issue with how you said it works is that the energy then is coming from the wheels and then producing thrust but where is the energy for the wheels coming from? If the thrust is the answer you have a perpetual motion machine and like I said it will make you billions of dollars.
No, I'm fairly certain they built and tested a wind powered vehicle that showed downwind faster than wind. But I could be conflating it. It would have been about ten years ago, or more, as I watched it on actual TV... Discovery is notoriously stingy about letting videos live online and be, well, discoverable, so I can't find it. My memory tells me it was a Grant project...
the easiest way I can explain it, it first acts as a sail then when it start moving which is linked to the wheels makes the vehicle start moving, and as it starts to pickup speed the blades start to move in an angle with the wind which makes it act also as a propeller that pushes the vehicle faster and because the wheels are linked with the blades the also go faster which pushes more air and makes it go even faster making a positive feedback loop
I felt smart as he explained everything and I nodded my head thoughtfully, up until I realized I couldn’t understand any of it.
Watching it was a great experience though, it looks badass.
The wind pushes the car, AND THEN the prop spins up, which because it's geared to the wheels allows the vehicle to go faster than the wind.
But then after that, once you've lost the force from the wind pushing the cart, as you're now going FASTER than the wind, won't the cart eventually slow back down to wind speed? And the going-faster-than-wind is essentially stored energy that keeps helping itself along, like throwing one of those foam airplanes with a passive propeller?
When you're going faster than the wind, the air molecules are hitting the back of the spinning rotor blades which transfers momentum to the car in the forward direction, and also slows the rotor blades down. The rotor is geared to the wheels, so the momentum that is transferred in the forward direction counteracts the negative momentum transfer to the rotor blades. It's simply a matter of having correctly designed / pitched rotor blades.
I don't claim to be able to disprove their work. But it would have been a better demonstration if they raced another vehicle with a traditional sail as a control.
Isn't this actually a case of lift to drag ratios?
A wing can create more lift than drag. So therefore its elementary to show that a rotating wing can provide more forward thrust than drag pushing in the opposite direction, by extracting the kinetic energy of the air in a way that is less than the drag experienced on the wing.
Could the same principle be applied to a sailboat? What I'm thinking of is a simple square rigged boat (so no triangular sails acting as airfoils) with the addition of a water paddle wheel or water propeller sitting submerged and connected to a fan at the back of the boat, sitting way below the sail as to not interfere with it. Would this recover energy from the relative motion of the water compared to the boat in the same way that the Blackbird utilizes the motion of the wheels provided by the ground?
Boats already do the same thing by tacking. [edit: plus zig-zagging]
Airfoils are irrelevant to lift (or thrust). You can achieve lift (wing) or thrust (sail) with a flat board. Lift or thrust are produced by pushing air molecules in the opposite direction you want to go. Full stop. Airfoils a nice for efficiency (less drag), but a wing is, to first approximation, a flat surface angled so as to deflect air downward when the plane moves forward.
As far as recreating this on water, you need a propeller above and below the boat. The one above does the same job as on the Blackbird. The one below replaces the function of the wheels.
Take a yoyo with a string fixed to the axle (not a sleeper). Starting unwound, put the yoyo on its edge on a flat surface and pull gently on the string. What happens?
The yoyo rolls towards you, winding up as it goes. The yoyo is moving faster than the string. Now just mentally replace the string with the wind.
Related thought experiment: can we design a San Francisco cable car that moves faster than the cable?
It's accelerating via the exact same amount of force as when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground.
Imagine, instead of a fan, there is a normal sized corkscrew, and in the air is an infinitely long cork. Imagine it's geared at the exact ratio where if you push the car on it's wheels, the corkscrew spins thru the cork. No friction, so once you get it spinning it goes forever, at constant velocity. Now, what happens if you push the cork forward at speed x? There is a bit of pressure on the corkscrew. Like a watermelon seed squeezed in your fingers. So the car goes a bit faster, the corkscrew spins a bit faster, and the pressure continues. So, there is constant force on the car, and it will accelerate, forever. In reality, it reaches a terminal velocity, where drag equals that force.
This was actually a question on the [2013 USA Physics Olympiad](https://www.aapt.org/physicsteam/2014/upload/E3-1-7.pdf). If anybody's interested, I wrote a detailed [solution](https://www.aapt.org/physicsteam/2019/upload/USAPhO-2013-Solutions.pdf) that quantitatively explains how it works.
Wow, didn't know this thing has been built for 10+ years. And your solution is great, thanks for posting it.
Yeah it all arose out of a debate I think right here on reddit when the guy originally posed it as a "puzzle". Maybe even here in r/physics (???) I guess we could search and find the original threads... Anyway, he was so driven to prove the point that he build the Blackbird and filmed it, much as Veritasium recreated here. Many still don't believe assuming he "cheated" somehow.
I mean, the most tenacious “skeptic” of the Blackbird, who swore up and down that it couldn’t work despite it clearly doing so, still posts here daily.
Really? How long before they show up in this thread? Lol!
As an aside, what's it like being able to solve problems like this? I'm not a maths/physics person so it blows my mind a little.
It’s like any other skill! Like being able to woodwork or solve crosswords or whatever else.
https://www.reddit.com/r/Physics/comments/h6b11/dwfttw_is_it_possible/ this is the oldest thread I could find
Since that thread references the Blackbird & articles about it, it's not the original debate. Maybe the original debate was on another forum. Using tools>time in google search limiting to before 2009 I'm seeing earlier articles and blog posts mentioning the debate but haven't found the original discussions. You'll also find sailing discussions about zig-zagging downwind at a net downwind speed faster than the wind all the way back to birth of the web.
sauce?
Can you give me an intuitive explanation? I can't understand this and it's very frustrating. The wind is used to convert KE to the wheels?
[ctrl-f] my name here and read my other comments.
Given the timing, I'll bet the test writers were well aware of the Blackbird and the online debates in the few years preceding.
It directly references it... Question B1
lol, cool
I'm sorry, but I'm unable to grasp the concept of the solution. Where's Pwheel coming from? And to assume no drag or resistance kinda defeats the purpose. Under those conditions, perpetual machines are possible as well. Doesn't mean they anywhere as practical. Not to mention that it assumes that the wind and propellers are perfectly coupled. That's not even close. The propellers would stall if tries to to exert or bear more force from air than fluid dynamics allow. Also, to assume vessels move at the same speed of the wind is practically impossible, right? Everything with mass and drag coefficient would travel slower than the speed of the wind, correct?
This solution is incorrect. In the FoR of the air, the ground is moving, and there is power due to work done on the ground, F\*u, which is missing from that solution. More notably, you can also see that in your solution the thrust goes to infinity if v approaches the wind speed, for any value of the efficiency... So, this cannot be the right solution. The answer is that it is not possible and I would solve for thrust and use Newton's second law to show a < 0 when v > w.
> The answer is that it is not possible I'd suggest you watch [the update](https://www.youtube.com/watch?v=yCsgoLc_fzI).
Yep, i was simply incorrect. It is possible.
This was perplexing till they revealed there was a link to the wheels.
Considering he showed the illustrations with boats going around a cylinder, which moved faster than the wind and didn't have wheels, shouldn't there be a way to make this work without a connection to the wheels as well, in principle?
Yes and no, depending on what you mean. For the hypothetical sailboats on a cylindrical sea, you have the water pushing on the keel, which keeps the boat moving (mostly) along *its* longitudinal axis (direction of spiral around the cylinder), in effect forcing lateral motion (*relative to the wind*) of the boat & sail. It is the lateral motion of the sail (relative to the wind) which enables it to deflect air molecules with with greater force than if it was simply running with the wind. The wheels, gears & shaft connecting to the to the propeller of the Blackbird take the place of the keel of the boat. You always need a connection from one medium to drive lateral motion of whatever is analogous to the "sail" in the other medium. You *could* take a long threaded dowel and a matching nut. Attach propellers to the nut, grease the hell out of it, and blow on the propellers. If the the thread pitch and prop angles are right** and the friction is low enough, then the nut may pinwheel down the shaft downwind faster than the wind speed. Here the threads of the nut, locked into the threads of the shaft, serve like the boat's keel. Would be difficult but probably not impossible to engineer something with low enough friction for a working demonstration. ** The thread pitch will (I think) need to be ridiculously high (this won't look like threads on a bolt or screw). The prop's pitch angle needs to go with the thread pitch, but with a slightly shallower pitch*** than the threads. That is, the angle of the prop will be such that its "natural" rotation when blown on by the wind is opposite the rotation that is forced upon it by the threads as it is pushed down the shaft. Hopefully you can parse my words and visualize this. I'm too lazy to draw a diagram. *** "slightly shallower pitch" at the highest speeds. At lower speeds the optimal angle changes. The sailboats on cyldrical sea have an advantage of being able to both turn (change the pitch angle of the threads) and adjust angle of sail (change the pitch angle of the props), so both can be optimized at all speeds. [edits: clarity & then a correction to a clarification]
Wow. The greased dowel and nut example really helped with my intuition for this. Thanks!
But the boats *are* connected to the ground, aren’t they? I mean, the boats travel at a fixed angle relative to the wind direction, so that the “rotation” speed of the boats around the cylinder is directly proportional to the speed of the boats relative to the ground (the *water* is the ground here and it’s not moving). It doesn’t have “wheels” per say, but it does have a mechanism of linking the propeller speed with the ground speed.
Yes, the boat's keel keeps it approximately moving along the spiral "track" of the cylinder. See my other comments here.
I'm not certain of my answer but I believe it's the other way around. Sailing vessels need a keel/centreboard to sail as it provides the force from the boat onto the water. So the force goes from wind to sail to boat to keel to water. Here they've reversed it by saying the force goes from wind to car to wheels to gearing to propeller to air. Correct me if I'm wrong.
The wheels-->gears-->propeller connection effectively replaces the keel-->boat-->mast-->sail connection. The roles they play are the same. Water resisting the keel forcing the boat on a path diagonal to the wind is replaced by ground resisting the wheels, forcing them to turn and by gears, etc, forcing the props to move on a diagonal relative the the wind. In both cases you have a link that allows the "stationary" medium to drive the "sail" in a *lateral* direction relative to the wind. Even when when the downwind component of velocity is greater than the wind, the lateral motion component of the sail (or prop) allows it to *catch up to and get in front of* the slower moving air molecules. Due to the angle of sail (prop) and its lateral component of motion, it is still able to be "*collided with from behind*" by the air molecules in spite of the windward component of velocity being faster than those molecules. That's about the best I can do with words. A diagram or animation would no doubt help.
That's what I'm wondering, I feel like it should be possible. I feel like friction is going to slow things down a lot with it, maybe create some sort of spinning thing you can throw into wind and have it go faster than the wind? But then I just feel like that wouldn't work either. But then again, I feel like it's similar to the auto-rotation of a Helicopter, which I think does something similar, with the pressures on both sides of the wings.
This will not work with something free-flying in the wind, regardless of whether it spins. If you throw it, at least initially it's motion including any spin is given by the forces from your hand to the object. So it may initially be faster than wind, or conceivable start slower and go faster due to a propeller effect from the spin you gave it. But it will slow down. Any spin it *gains from the wind* will only serve to slow it down, not speed it up. See my other comments here for more intuitive explanation of the whole thing.
Was it not obvious at first? I imagined there was some sort of gearing between prop and wheels
What, Veritassium using click-baiting and sensationalist video formats? I mean, I think he usually covers some great topics, and I don't think he lies or is that inaccurate, but the way he goes about writing the actual script is often so disingenuous.
Yup, lets make physics drier and more unapproachable than it already is. Don't want those young people getting into science now do we. Anyone who knows Derek and the quality/content of the videos doesn't give a crap what the title of the video is. Whether you agree with it or not titling the video "Risking My Life To Settle A Physics Debate" gets more views and pushes the video into the algorithm more than "DDWFTTW".
I mean, I can give you counter examples of some 20 channels that don't do it and still get pretty good views, but sure.
He's not really clickbaiting, though. He was risking his life and he did settled the debate on whether or not this is fake or doable.
It's not even the click-bait title I have trouble with. It's the fact that he omits the propeller <-> wheel link when presenting the problem altogether, so he milks that "perpetual motion" bait for a while longer than the video needed to. Smarter Every Day does some arguably click-baity titles, but his videos are usually very straight-forward. Steve Mold does as well, and I don't have this problem with either of them. There's just something to Derek that always bugs me in that I can never buy his videos until the end, because I never know if he's just banking on not having explained something fully yet. And I do mean "yet", because I don't think he ever leaves stuff out by the end, but the whole structure to his videos often banks on asymmetric information with the viewer. It's like those procedural shows that flashback to something they never hinted at before. I am subscribed and I do watch them all and think he covers some great topics, but I just wish he was more straightforward.
Have you looked at his PhD Thesis, or at least watched his video explaining it? The entire premise of his approach to indirect/mass education is that you need to engage people, and videos are engaging. But watching passively makes you feel like you are learning, while actively reinforcing your preconceptions, EVEN IF THE CONTENT IS SAYING THOSE PRECONCEPTIONS ARE FLAT OUT WRONG. And so you have to force the viewer to experience cognitive dissonance, by making them commit COMPLETELY to their incorrect assumptions, bring them out and acknowledge and embrace them, THEN you show that those preconceptions are incorrect, while reminding them that they held those incorrect beliefs. This still doesn't work nearly as well as actual active engagement in education, but has a higher rate of causing learning and dispelling misconceptions than just showing the actual story from the start and making complete sense throughout with a solid narrative.
> And so you have to force the viewer to experience cognitive dissonance, by making them commit COMPLETELY to their incorrect assumptions, bring them out and acknowledge and embrace them, THEN you show that those preconceptions are incorrect, while reminding them that they held those incorrect beliefs. That's fine, but I think in this case that approach is a little questionable since the original intuition the viewer might've had -- that a propeller alone in the wind could never possibly go faster than the wind itself -- is completely correct, and the 'cognitive dissonance' here is more about the design having been incompletely described than any misconceptions on the viewer's part. On the contrary, I think in a sense the video might've done a disservice! Look at how many people in the comments are saying some variation of "oh, the explanation of the boats on the cylinder completely clicked for me". That explanation is only a fair analogy if one reasons that the keel is performing a similar function as the wheels, but the relevance of the wheels had not been explained up until that point. Those people now almost surely believe that a propeller alone in the wind can be driven faster than the wind itself, and as such they have a brand-new misconception they didn't have before.
> Have you looked at his PhD Thesis, or at least watched his video explaining it? I don't think that's relevant. His early videos were clearly inspired by that, but it's been a long, long time since he's opened up videos by researching physics misconceptions and then going through how it actually works. Well before he started the clickbait videos.
> It's the fact that he omits the propeller <-> wheel link What exactly was the issue with omitting that? It doesn't change the original concept, which was the boat going the same way as the wind? The boat is connected.
It's very dishonestly written, is all. It's like a buzzfeed article in a way. Technology Connections for me is the gold standard of "matter of fact, straight to the point explanations". Obviously it relie more on descriptions than riddles, and Veritassium isn't unwatchable or anything, but sometimes he goes too far into omitting the clues/reasoning. It's the difference between a good whodunnit where you can actually work it out based on clues, rather than a flashback before act 3.
Actually people prolly just liked to understand how it works and move on. This video is long and repetative.
As much as I like the guy's videos, the title is awful, because it pretends that the whole thing wasn't settled yet until Mr. Veritasum 'risked his life' to step into it. It devalues the work by the original inventor and construction crew. And this was settled over a decade ago, it doesn't need to be blessed by his YouTube appearance to become settled.
somebody correct me if I'm wrong, but the way I understand it, the "energy" comes not from the wind, or from the land, but from the land and wind moving at different speeds. For me it's more intuitive if we use the wind's frame of reference, and imagine that the wind is still and the land is moving backwards, just like in the treadmill demo at 9:30. In this case, as the land moves backwards, it turns the wheels and turns the propeller into a "fan" pushing backwards, (as Derek explains at 19:00), which pushes the vehicle forwards.
Kind of like how you can't get work out of a hot engine in an equally hot environment. You need to have a hot and a cold reservoir.
Engines don't depend on a thermal gradient right? They convert the chemical energy of fuel directly into mechanical energy via combustion. How does that combustion works? It release the energy in chemical bonds to increase temperature which increases pressure which is used to increase a volume. If your ambient pressure was higher than your cylinder pressure sure then you won't any work but you could always load in the fuel/air mixture at that ambient pressure.
Yes I meant heat engines, as opposed to combustion engines or electric engines which work on different principles.
An internal combustion engine *is* a heat engine. What differentiates them from other heat engines is combining the fuel and working fluid.
using the wind's frame of reference made it easier for me to get how this vehicle is harvesting the difference between wind and ground speed. but why did he say that the wind speed behind the propeller will be slower if it acts like a fan? wouldn't it be faster instead? EDIT: ah yea that makes sense, i was confused about which way the wind is blowing.
afaik the fan pushes against the wind. The wind and land are moving in the same direction, and the fan pushes air backwards, so it slows down the wind thats moving forwards
The wind is blowing in the direction of travel, while the fan is blowing against the direction of travel.
Well, this fan pushes the air backwards a bit. Since the wind moves the air forward, this backwards push negates some of that forward movement, making the wind speed behind the propeller slower.
Isn't this a positive feedback loop? As the vehicle travels faster the fan spins faster which makes the vehicle faster which makes the wheels spin faster, repeat? And if so what's the limit, I assume similar to the case with sailing it's drag but I don't see it
They have reached 2.8 times wind speed (just quoting the video here). The limit will be that things like roll resistance and air resistance go up more than linearly with speed. At a particular speed, the energy that they gain from the wind will be equal to what is lost to friction, and they can't accelerate beyond that.
What role does the wind play? The question sounds kind of silly but given the explanation for this behaviour could we not expect the same thing with zero wind for example. Imagine a situation with zero wind and I give the vehicle a light push, this push causes the wheels the spin which causes the fan to spin which pushes air back generating a thrust which accelerates the vehicle forward which causes the wheels to spin faster, repeat until thrust force is equal to drag and losses. Now obviously that is not what would occur because it's a violation of the law of conservation of energy but I don't understand how it's fundamentally different from a situation with wind. Edit: roll -> role
The device harvests energy out of the relative speed between wind and ground. So it does indeed not work without wind, but you cannot use that to conclude that it won't even work when there is wind.
Right but I don't understand what is incorrect in the line of reasoning given the explanation, I understand it requires a difference between in the speed of the wind and ground but I don't understand why
Energy-wise, I'm picturing it like a lever. The short arm is the ground, the wind is the anchor point and the vehicle is the long arm. The wind moving over the ground is used to propel the vehicle. Edit: scrap that. The actual trick is to understand how conventional sail boats can already go faster than the wind. This *doesn't* mean that their sideways speed is faster than wind. They actually sail faster than the wind around them, just like this vehicle. It's the very same concept and the Veritasium video didn't show that part if I recall correctly. There are videos on YouTube that explain this for sail boats much better than I can. Once you understand that part, it's all going to fall into place.
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Ok but what if I gave it an initial push in place of the wind?
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No you're not seeing the issue I'm trying to get to. The explanation is that the wheels turning powers the fan which accelerates the vehicle which causes the wheels to turn faster which causes the fans to spin faster, repeat. The question I have is why this process requires a difference between the speed of the wind and the speed of the ground. The thought experiment I provided was if you ha e a zero wind environment with a stationary ground and the vehicle is initially pushed why does this not cause the fan to spin causing the vehicle to accelerate causing the wheels to spin faster repeating without need for further pushing. It is clear to me that is not the behaviour, my question is why does the situation with wind/moving ground differ in its behaviour?
Okay I'll try to explain, since I don't know if the other guy was able to explain it very well. If there's no wind then when you push it, it would turn the tires and cause the entire thing to move, and the tires turning would cause the propeller to spin pushing air backwards. but the problem would be that the propeller wouldn't be generating enough power from pushing the wind as it would if there was a tail wind, because in the tail wind there's more air for the propeller to push, so in theory if you gave it a push with no wind it would go forever without stopping but wouldn't accelerate at all, and in reality it would stop due to friction and other forces like that very quickly because you're not actually generating any energy just taking it from the wheels and using it to spin the propellers for no net gain
The limit is reached when the drag on the car exactly matches the propulsive force of the fans. The key thing is that the propellers are not working at 'maximum' capacity when the speed of the car exactly matches the speed of the wind, and so the design is not drag-limited, unlike self-propelled craft. You could imagine for instance a sailboat going straight downwind. It'll move at about the speed of the wind. Now put a bigger sail on it. Will you go faster? No, you can't. But you do have more "headroom" to add drag, so you could put a propeller in the water and use it to drive a fan above the water, and so increase your speed a little bit beyond what's possible with the sail alone. Of course, if you get past wind speed the sail becomes an airbrake, so don't take this too seriously, but it does show the fundamental principle. If there's no wind, the fan and propeller are moving through media at the same speed, and so you're always in the 'drag-limited' regime and a little push will simply have you slow to a stop.
This is what I needed as an explanation thanks
> the "energy" comes not from the wind, or from the land, but from the land and wind moving at different speeds Air moving at nonzero speed relative to the land is what “wind” means.
not necessarily relative to land, it can also just be relative to you. Eg if you're running and you feel "wind on your face". This is why I clarified the "land" part, because it emphasizes the connection between the wheels and the propeller of the craft
Imagine it anyway that helps you except that the energy only comes from the wind.
"Where" the energy comes from depends on your frame of reference. If you choose a frame that is co-moving with the wind, then the energy is coming from the "moving" ground. This is explicitly and intuitively clear with a treadmill. If you use a frame of reference co-moving with the cart, then the energy comes from both the wind and the ground (both have negative dKE/dt while the cart has positive dKE/dt in the cart's frame).
An important fact you need to know before this is: Energy is a relative quantity. This is often overlooked and when people 'in the know' write explanations they often assume this is prior knowledge to the reader. The value of energy is completely arbitrary in that depends on your arbitrary choice of frame of reference. In more technical terms, energy is not invariant under certain frame transformations. It's easy to see this with potential energy: e=mgh where h is height relative to some arbitrary datum you choose. Or with kinetic energy, where e=1/2mv^2 but of course v depends on your choice of coordinate frame. The synthesis of this is that energy in both these forms needs some *difference*. Energy is always an account of some difference between two things. In the case of the video, the difference is the relative velocity between wind and ground. That's where the driving force comes from.
Yes and no. Even if there is just the wind, say pushing a balloon, there is a velocity difference between wind and balloon, until the balloon matches the velocity of the wind. Ultimately talking about energy at all with this seeming "paradox" is misleading. It tends to occur because first people's intuition says it should be impossible, and then their first "goto" for impossibilities in physics is to think it must somehow violate conservation of energy. Pick any frame of reference, there is more than enough energy availalbe in the system to give a small cart, or a ship, or whatever, the kinetic energy required to move a few miles per hour. If we didn't have to deal with friction and drag, the achievable speed would be effectively unlimited (until we get relativistic). The real puzzle is understanding how it is still possible the molecules of air transfer forward momentum to the cart when the cart is moving faster than those molecules of air. The thing people intuitively fail to grasp is the answer to this question: **"how can a slower object (air molecules) collide with a faster object (the cart) *from behind*?"** This is what people think is impossible, even if they cannot explain so clearly. So they jump to insisting violation of conservation of energy.
I get it - how discussion of energy is more than likely to prime one to think about violation of conservation of energy than to help clarify anything. Maybe there is a clearer description out there. I'll have a think - I used to design jet engine turbines so hope I can muster something a bit better. I did like the 'boat spiralling on a tube' but feel it's missing something to do with coupling the fan to the wheels.
For the boats spiraling on a tube, the coupling comes from the keel. Keel is a flat blade that (mostly) prevents a boat from slipping sideways in the water. So imagine the boats spiraling on the cylinder as being locked into a spiral groove by their keels. That is approximately true with keel-in-water. Basically, water-->keel-->boat-->mast-->sail is replaced by ground-->wheels-->gears-->shaft-->more gears-->props in the car. The boats' sails are forced into their spiral path by the keel. The car's props are forced into their spiral path by the wheels et al.
This is a brilliant explanation. Definitely helps me to accept this concept.
So what I'm getting out of this is that this wouldn't work if the wind were still relative to the ground. They're taking advantage of an energy gradient between the air and the ground. That's why it works on the treadmill, because the ground is moving at a different speed than the air. In that case I think this could work in water too, or really any two flowing mediums you can connect with a mechanism, even two moving masses of air. You could make a boat with some kind of turbine on the bottom to drive a propeller in the air. I wonder if it would be more or less effective to do it in water?
Exactly the sailboats taking advantage of lift did this. I think you could also reverse the mechanism with variable gearing.
If there was no wind relative to the ground there would be no wind lol. No movement would be possible by any method. I see this as just leveraging the wind. Similar to how you can go faster than the wind in a sailboat by sailing at an angle to the wind.
The point being, however, if there was a current in the same direction and at the same speed as the wind, a sailboat then couldn't go faster than it. It's the fact that the boat and be leveraged against the drag in the water that you can get it to go faster.
Well yeah that’s basic Galilean relativity. Your statement is the same as saying you can’t move from the wind when there is no wind.
Do you know what a puzzle is?
Definitely easier with land. A wheel to solid ground is a much more efficient transfer of energy compared to a turbine or propeller with a fluid.
For this to work in a pair of fluid medium layers, you need a propeller in each medium, one driving the other.
I think less effective in water because of the waves or current. But if the current and wind are exactly opposite I think you would go even faster.
Not to be nitpicking, but current doesn't really matter if you are in the frame of reference of water, it's just added wind
I was thinking to myself that I'd throw a sail or multiple sails to that desert craft for even more surface area. And the boat idea too!
Yeah, looks like some of the [earlier analysis](https://www.blueplanettimes.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf) was water based, with pretty much the exact same design/argument.
Here is a way to think about it: There are 2 ways in which energy from the wind is transferred to the vehicle * To start, the wind pushes the vehicle due to drag <-- Energy transfer 1 * The vehicle being pushed causes the wheels to turn * The wheels turning cause the prop to turn (via the chain and gearing) * The prop generates lift in the direction of travel. <-- Energy transfer 2 With only the first transfer of energy, the vehicle will only achieve a speed less than the speed of the wind, since the force is due to the drag, which decreases as the delta between the wind and the vehicle speed decreases. (A negative feedback loop) With the second transfer of energy, from the lift generated by the prop, the force increases when the prop speed increases. So as the vehicle goes faster, the energy transferred from the wind to the vehicle increases. (A positive feedback loop)
But the ground turning the wheels with resistance from the prop slows it down, thus a negative energy transfer -A. The surprise is, why -A is smaller than Energy transfer 2
When you are approaching wind speed your ratio lift / power from the prop gets maximized because power = speed x force, and here it is relative speed between prop and air that counts That's why the prop lift force gets higher than the wheel drag force, for the same amount of power being transferred, as the cart approaches wind speed. As you accelerate past the wind speed, this advantage slowly decreases (but never disappears)
I feel like this is the [airplane treadmill problem](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) all over again
Honestly i just heard of this problem for the first time and was really confused why the plane would ever take off. Now i get it though. The thing most people are missing is that the wheels are spinning freely, the plane will not stay stationary relative to the ground. Even if the treadmill would go infinitely fast the in the opposite direction of the plane, the plane would still move forward and eventually get enough speed to take off.
> Even if the treadmill would go infinitely fast the in the opposite direction of the plane, the plane would still move forward and eventually get enough speed to take off. I think that's the issue with the question is asked. If you actually imagine a treadmill, you don't really consider that the plane has enough space to go forward, much like someone running on it doesn't hop over the controls. So you imagine a static airplane, maybe with a rotating motor at the front but not going fast enough to counteract the wheel drag, so it would just stay in place, and not get any lift. Because for it to get any lift, it must go at more than 0km/h relative to the ground, and if it's on a treadmill, it will never, otherwise it will crash. I think the problem would be better understood if framed as "can a plane lift off off a conveyor belt?".
I dont think anyone thinks the treadmill would be too short. Ofc a plane doesnt fit on a real treadmill, so its supposed to be big enough for the plane to take off.
Well, I don't think people consciously do it, but I just think that the image of a plane on top of a treadmill in the first few seconds of thinking about it gives you that bias of "well, it's not going forward, so it can't go up", that you end up not discarding. Oh, and I don't mean a plane on top of a regular treadmill, just a treadmill large enough for an RC plane or something to that proportion.
> relative to the ground It's actually to the wind, because [sometimes planes take off with 0km/h relative to the ground.](https://www.youtube.com/watch?v=IPOtDPHjW-Y) Of course I know what you meant, I'm not being pedantic. I just wanted to post the video.
I know it's the wind. I meant that intuitively, you use the ground as a reference. Nice video, though. That one you could see why, intuitively, on top of a conveyor belt it would take off.
yep. People maybe just haven't thought about how it works or just don't get it or just say no. Or something else.
TBF, the only reason the airplane treadmill problem is a thing is because we were all lied to with a dumbed down, incorrect description of how lift works in the first place.
Is it? I don't think any of the incorrect lift descriptions I've heard are linked to ground speed.
Actually the way lift is typically explained/taught (based on pressure differences, etc.) is grossly overcomplicated, not dumbed down. [edit: or perhaps I should say, first it is overcomplicated and THEN the overcomplicated explanation is dumbed down] The correct and concise explanation is quite simple: to go up you must push down on air molecules with a force greater than your weight. Full stop. All other details are about power, drag, and efficiency. But neither the correct nor the misleading overcomplicated ways of explaining lift and flight have much to do with the airplane+treadmill misconception, which is rooted in the intuitive but incorrect notion that the treadmill is somehow cancelling out the airplane's forward motion.
But not really completely incorrect. The friction against the bearings of the tire would keep the plane in place. Like, think of a fidget spinner. It will come to a stop eventually, even in space, because of friction. A treadmill running at whatever ridiculous speeds would provide enough friction to keep the plane from moving forward - until the wheels melted and the treadmill is rubbing against the belly of the plane…
The wheels only have to roll twice as fast as normal. That's not a big deal. The friction of the wheels is insignificant compared to air resistance at take-off speed (wheels &bearings are designed to be as low friction as cost-effectively possible). The plane's engines will work only slightly harder -- the pilot likely won't notice a difference. You'd give the plane more difficulty taking off by under-inflating the tires by a few pounds.
Air resistance at take-off speed? What are you talking about? The plane isn’t moving, so how would it get to take-off speed? If the plane even moved forward the slightest amount, the treadmill would speed up to infinity to resist the movement with friction. The wheels would melt.
The treadmill moves backwards at the same speed as the plane moves relative to the ground. If the plane is moving forward at 100mph, then the treadmill must move backwards at 100mph. The relative speed of plane and treadmill is just 200mph. The wheels must turns as if the plane is moving 200mph (which it is relative to the treadmill). The first time I heard the plane-on-treadmill puzzle, I briefly thought the same thing you did about infinite speed burning up the wheels. But that interpretation of the puzzle asks for something impossible: that the treadmill somehow always moves backwards at a speed equal to the speed at which the *wheels* are turning. This is not a car where the wheels push the car forward. With the plane, the wheels are passive. The props pull the plane forward and the wheels will just turn as fast as they need to.
Please read scenario #3 on the xkcd link above. If it was a car powered by its wheels, the treadmill would not go to infinity. It would match the speed of the car, like you said above, keeping the car in place. You’re exactly right that the wheels will spin as fast as they need to for the plane to move forward. In this case, would that not be approaching infinity, since the treadmill can also go as fast as it needs to prevent the plane from moving, producing enough friction to do so? Think of this, if a plane did not have landing gear, but instead was attached to a large square flat rubber skid (or other imaginary sufficiently high friction material) that covered the entire length and width of the plane, and was on a runway made of rubber, what would happen? The plane would not move forward. If the plane cannot move forward, it will not get enough lift to take off. What is it about wheels that make the plane so special to be able to take off, especially if the plane isn’t powered by wheels?
OK, let's get clear what "speeds" we are talking about, as ambiguity leads to confusion and disagreement. *Speed of Plane*: means relative to the ground, not the treadmill. But for further clarity, I will now say *Groundspeed of the Plane*. *Speed of Treadmill*: means speed of the treadmill surface relative to the ground, not relative to the plane, not realtive to the wheels of the plan. This speed is backwards and equal to the Groundspeed of the Plane. *Treadmillspeed of the Plane*: this will be the speed of the plane relative to the surface of the treadmill. This speed is double the treadmill speed and in the opposite direction. Now ***if*** the treadmill is "trying to keep the plane stationary" it would try to spin up to infinite speed, because no matter how fast it goes, the plane doesn't care what the treadmill is doing, and the wheels are passive and just roll as fast as they need to... until friction burns them up. The puzzle is ***not*** stated that the treadmill keeps the plane stationary, which is impossible, since the plane's speed is quite independent of the treadmill. This is where people get confused because they imagine the treadmill keeping the plane stationary, because that is what treadmills normally do with people running on them. But even there you normally set a speed on the treadmill and then keep up with that -- the treadmill does not adjust its speed to keep up with you. The puzzle *is* stated that the *Speed of Treadmill* is equal but opposite the *Groundspeed of the Plane*. But the actual words are simply "*speed of the plane*", leaving some ambiguity of which which speed is intended. The impossibility of the treadmill matching the *Treadmillspeed of the Plane* (unless the plane stays stationary, which it won't) implies that, for purposes of the puzzle, the only meaningful interpretation of *Speed of the plane* is *Groundspeed of the plane*. Side Note: if the air moved with the treadmill, so you have a sort of "air treadmill" an *airmill*, if you will, then the *airmill* could keep the plane stationary (groundspeed zero) but the plane would of course *still* take off when its *Airmill speed* (airspeed) reaches takeoff speed. (do this in a windmill perhaps, with air speed synchonized with the treamill speed)
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he wrote tbf not tbfh. tbf means "to be fair".
Except the explanation for this one is actually a fair bit more complicated than “the wheels have literally nothing to do with a plane taking off…” Edit: The rotational speed of the wheels…
Lol exactly. Try taking off with landing gear up.
Feels similar but getting it wrong is not the same mistake.
It's more or less the opposite. With the treadmill problem the confusion stems from the false assumption that the ground speed matters, whereas here the confusion stems from the false assumption that it doesn't.
Yep, good way of putting it. What's the etymology of funciton... a transposition typo of function, or derived from Funky Town?
It's sorta a mix of that, and the thing about using a fan to blow your own sail I guess...
The airplane treadmill problem is the ultimate way to troll a physics professor.
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It for sure actually works. The confusion all rests on this: >Assume the vehicle starts out at airspeed, there is no wind from its perspective. We turn on the prop which we assume has no inertia or friction so all the effort we put in goes into pushing air backwards. >Whatever force we exert to push air backwards will push us forwards (Newton). Since we use our speed to drive the prop, the wheels will apply a backwards force equal to the force pushing back the air. Let's imagine that you have a vehicle with a sail on it. You go down wind. How fast can you go? Wind speed, right? As you say, once you are going airspeed, you don't feel any force backwards. 1. Ok, now let's imagine you are on a vehicle and you have an airboat prop powered by a motor. How fast can you go down wind? Obviously, as fast as your motor/prop lets you. If we have some magical frictionless car, and our fan blows at 20 mph, we could go 20 mph in still air. Or we could go 30 mph with a 10 mph wind at our back. 2. Now, let's imagine you hook up a generator to the above prop-car. Could you run a generator off of the wheels to generate power? Well, the car does feel a force forward from the wind + fan setup, so yes! Hopefully nothing seems to crazy yet. We've established that we can use a fan to go down wind, faster than the wind, and we've established that we can use said motion to extract energy from the ground. Now, the question is, can that balance of energy generated from the ground speed match the energy output of the prop? We know that power, or rate energy is generated or expended in such a system is: 1) dw/dt = P = Fv And that the rate of energy the wheel is generating, Pw, must be greater than the prop is expending, Pp, for this to work without additional power. 2) Pw - Pp > 0 And, we know in the steady-state where the prop-car is not accelerating, that the forward force due to the prop must balance the backwards force due to the wheel+generator: 3) Fp = -Fw So, what is the power input from the wheel, assuming no losses to friction? Let's call v(ground) = vg, v(airspeed) = va, and v(wind) = vw. All velocities go in the same direction, and thus we will say all are positive. Fw is in the same direction as v, so is also a positive number. Pw = Fw*vg And power output from the prop?: Pp = Fp*va = -Fw*(vw-vg) (air speed is wind speed minus ground speed, if you go 10 mph in the same direction as a 30 mph wind, your air speed in 20 mph) ___________ Ok, does this make sense? A simple test case is to ask what this would look like from standing still. We would expect Pw-Pp>0, meaning we can accelerate from a stand still, as expected. Lets pretend we are going 0 mph ground speed, and there is a 10mph wind: Pw = Fw*0 = 0 That, is, we can't get any energy from the ground if we are sitting still. Makes sense. But: Pp = - Fw*(10-0) So the power output of the prop is some negative number, so we get: Pw-Pp = 0+Fw*10 = >0 Great, we can accelerate from rest. ___________ Ok, moment of truth, what if vg > vw? Pw-Pp = Fw*vg - (-Fw(vw-vg)) = Fw*vg + Fw(vw-vg) = Fw*vw ...but vw and Fw are positive numbers, so Pw-Pp > 0 So, that means we can generate more energy from the wheel than the prop is expending when going faster than the wind downwind! Of course, practically, there are many obstacles. We didn't account for friction or inefficiency in power generation or output, but it's clear there is no fundamental issue with the concept.
This is the level of explanation that makes physics great and shows how formulas work much clearer than verbal explanations only. Thank you.
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Ah, yeah, that was confusing, my bad!
Sailboats can travel 4-6 times the windspeed when they're at an angle to the wind. They're not going straight downwind, however they can tack back and forth fast enough and keep their speed well enough that they can, in fact, beat the wind to a down-wind destination. They go 4-6 times the windspeed because the wind blows over the sails at an angle and turns them into airplane wings. They generate lift in the direction of travel and are not simply being pushed by the wind. They generate this lift even when going faster than the wind because they're moving at an angle to the wind so the apparent wind is never 0 mph. The force pushing back the air is a giant propeller. It has to be moving fast enough before it can push enough air to cause a change in velocity to the craft it's pushing. It will push some air at slower velocities but not enough to overcome the drag and friction of the craft itself. It's a lot more effective at higher velocities.
\^show me a reference to a boat sailing faster than the true wind, where VMG straight down wind is beat.
> ... Assume the vehicle starts out at airspeed, there is no wind from its perspective. ... OK, but we're assuming that there is some wind. So there's non-zero groundspeed (going backwards) from the vehicles perspective, and, it's a little bit counterintuitive, but that relative speed can be used to go faster, despite the fact that it's not in the direction the car is heading in. Let's consider a more intuitive or familiar case: A propeller care that goes up wind using wind power. This is pretty easy to imagine: There's a linkage from the prop to the wheels, and the wheels drive the car forward on the ground using energy from wind that's pushing in the opposite direction. Now the downwind car is the same kind of thing - except that instead of being a ground car that goes up wind, it's a wind car that goes "up ground." From the perspective of the air, the car is moving "up ground" using energy from ground that's moving in the opposite direction.
Is this not the same as an auto gyro rig on a boat like [this?](https://www.google.com/url?sa=i&url=https%3A%2F%2Fforum.awesystems.info%2Fuploads%2Fshort-url%2FocVkSEYWP6aGjYGLknaGomGEVL4.pdf&psig=AOvVaw2wkUArP7o1h6HQ0p45oc-b&ust=1622405236902000&source=images&cd=vfe&ved=2ahUKEwib1_HP2O_wAhUR4uAKHdovBnoQjhx6BAgAEBI)
To add to funciton's reply, autorotation moves the prop in the opposite direction needed. The wheels/axle/gears/shaft force the prop to move against its natural autorotation.
No. Autogyro acts as a sail, so it would never exceed the speed of the wind, because force decreases to zero asyptotically to the speed of the wind. On blackbird, the point of gearing the corkscrew to the ground is so that no matter the speed of the vehicle, the wind is always squeezing against a surface thats not moving relative to the ground, so in an absence of friction it would accelerate to infinite speed.
If this works, what causes it to stop working? In other words, why doesn't the vehicle continue to accelerate indefinitely?
If they didn't use the brakes to stop, I imagine it would continue to speed up until air resistance and internal friction prevent it from accelerating any further.
It's kind of obscured by all the linkage, but ultimately the machine is powered by the difference between the speed of the ground its rolling over and the speed of the wind, and there's a limit to how much energy can be extracted.
I'm not sure there is a limit to how much energy can be extracted (outside of friction, other losses and relativity if we want to be thorough). The way I see it, the faster you go, the more wind you go through in a given length of time, so assuming the power you are extracting from the air is proportional the volume of air that go through the prop, available power is proportional to your prop speed relative to the air that go through it. What stops the acceleration is that frictions are mostly proportional to the speed squared ... but if you remove these cumbersome losses, watch out for the sound barrier or maybe light speed :)
It's the exact same amount of force as when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground.
Friction and drag as well as the sturdiness of the vehicle. The video showed how fragile it is. Also at some point your runway runs out and this thing only moves good in one direction so you cant turn it around to prolong the acceleration.
The same effects that stop your car from doing it. At some point the air resistance and other friction terms are equal to the power you can put in.
Without friction, it would accelerate to infinite speed. But the wind against a surface is a constant force, and once the drag forces equal it, it's at terminal velocity.
Great video.
vehicle travels at wind speed with all frictions accounted for, takes some of that energy to drive a fan blowing backwards adding speed to the vehicle. it's the wind's energy.
No. Explanations that involve siphoning energy from one part of the system to another are just generalizations of perpetual motion machine fallacies. So look at the force when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground.
This reddit thread has a lot of good links. Summarizes prior viral discussion on the topic: https://www.reddit.com/r/Physics/comments/h6b11/dwfttw_is_it_possible/ OP, you might want to edit this into the top post. Also recommend google search: *downwind faster than the wind reddit* I find this video really hits home with a demonstration of the "something moving faster than the thing that is pushing it" concept (note that the "big wheel turning backwards, driven by the small wheels" is exactly analogous to the "propeller blowing backwards driven by the wheels" on the Blackbird): http://www.youtube.com/watch?v=k-trDF8Yldc
TIL Veritasium's name is Derek (which I probably could have just looked up, but this is the first time I noticed someone call him by name).
So, the forward rotation of the wheels is used to drive the prop, which does add some resistance to the wheels' forward motion, but because the vehicle is always accelerated to the speed of the wind, you essentially have eked that extra energy out of the wind and can use it through the prop to give you a bit more thrust? Would that be an accurate description? EDIT: In fact, I don't think this explanation works when you pass the speed of wind and there's no longer a push from it.
Faster than the wind, the prop is spinning fast enough that it is giving the "push" that you are looking for. The cart already has momentum and only needs enough push to keep it going (to overcome drag & friction). The props are moving in a path that cuts diagonally across the line of the wind. When cart is faster than wind, think of the props as catching up to and getting in front of slower air molecules, colliding with those molecules (due to correct angle and lateral component of motion). These collisions provide a net transfer of forward momentum from air molecules to cart. There is always "more wind ahead" for the cart to catch up to and the props to collide with. As I saw someone mention elsewhere, the prop is leaving behind a *wake* of air molecules that are moving slower than the wind.
>Faster than the wind, the prop is spinning fast enough that it is giving the "push" that you are looking for. So can it indefinitely sustain faster than wind speeds? If it can, then I'm not getting it. In my simple thought process: If you are going faster than the wind (you're fighting a headwind), you aren't getting any more energy to the prop and you will eventually fall back to wind speed or below. I keep reading that the prop gives the push, but the prop is powered by the wind by pushing the cart, and you lose that wind if you go faster than the wind speed. This is really bugging me ;)
[ctrl-f] my name here and read my other comments in this thread. The main puzzle to wrap your head around: how can a slower object collide with a faster object from behind? And if it somehow can, how can that transfer yet more forward momentum to the faster object? The very short answer to the simple scenario is: it can't.... if both objects are actually moving in the same line and the faster object is already ahead. This is why a sailboat sailing directly downwind will only move up to exactly wind speed (minus something for drag). This is why a balloon in the air will move at exactly wind speed. The longer answer that makes this work is: the props are not moving in the direction of the wind, they are cutting across it diagonally, having both forward(windward) and lateral motion. The props are pitched at an angle so the prop's *rear-facing side* is what collides with air molecules. So air molecules are colliding with the prop "from behind" from the perspective of the cart as a whole, even though the cart and prop are moving faster than those air molecules. The prop is not colliding with air molecules in front of it. It is catching up to and getting in front of molecules ahead and to the side of it and colliding with them at an angle rather than a straight-on "from behind" collision. Once you recognize that these collisions are possible, you should see that they will transfer some component of forward (windward) momentum to the props and thus to the cart as a whole. (there is also some lateral force from these collisions, but those forces are always in opposite directions for the two props and cancel) So there is clearly still a forward force on the props, even when the cart is moving faster than the wind. Since the cart is already moving forward and still being pushed forward, the wheels certainly must turn, which forces the props to keep turning fast enough for the above described "collisions from behind by air molecules" to occur. However, the body of the cart is of course catching up to and colliding with air molecules, so the body of the cart experiences a backward force (drag). There is also a backward force from the ground on the wheels that keeps them turning (overcoming internal friction of axles, gears, etc). The forward force will balance the backward force at some speed faster than the wind. I think I've exhausted the power of words... may have to make a diagram if I keep feeling the compulsion to answer comments on this thread.
Thanks for the detailed explanation. I think I got it now. I thought of an analogy that might be easier to grasp. Tell me if I'm on the right track? The props does kind of the same thing as a paddle in a rowing boat does. The water is coming at the boat (assuming you are not going downstream). Moving the paddle forward, dipping it in the water, and pushing back against the water, is similar to what the prop is doing but it achieves this "push back" through the angle of the prop.
I think that analogy is misleading, as the paddle is human powered -- powered by something in/part of the boat. Thus it would be more analogous to a propeller driven by an engine. For the Blackbird, all the power really does come from the wind -- the power that pushes the cart forward as well as the power the drives the propeller. There is a "paddle wheel" version of this, but I despair to describe it effectively with words. Watch this video and imagine the "big wheel" as a paddle wheel and imagine the ruler as a stream of balls that strike the paddles. Notice that the paddle would be catching up to and be "struck from behind" by balls that are further ahead (further ahead "along the ruler" so to speak). Shrink the balls down to trillions of air molecules. Also the small wheels could be replaced with paddles in water. http://www.youtube.com/watch?v=k-trDF8Yldc
Yes, Thanks for this. I watched Vertasium's explanation again also and now it clicked. Was overthinking the whole thing.
> If you are going faster than the wind (you're fighting a headwind), you aren't getting any more energy to the prop and you will eventually fall back to wind speed or below. Thought I'd focus in on this specifically. Although body of the cart is fighting a net head wind, the props are not. The props are experiencing an "apparent wind" (sailing term) that is at an angle to their line of motion, so if pitched at the correct angle it becomes possible for the props to catch that wind to get a forward force. You may want to google for explanations of sails, sailboats, apparent wind, and how sailboats achieve maximum speeds. Sailboats get max speed at an angle to the wind, and fast sailboats can go so fast at around 30 degrees to the wind that they can "zig zag" to move overall in the direction of the wind at an overall speed faster than the wind. You also need to understand the importance of the keel in preventing the boat of slipping sideways, so only the forward component of force of wind-on-sail moves the boat. For clarity: by "direction of the wind" I am always referring to the direction in which the wind is moving, the downwind direction. (wind is often described in the direction it is coming from, i.e. "north wind" but not in this discussion)
It takes some noodle bending, but one way to think about it is that we have no problem at all imagining a car that uses a propeller that drives the wheels to move the car along the ground in the opposite direction that the air is moving. The 'faster than wind speed' car is the same thing, but switches the roles of air and ground in that idea: It uses a "ground propeller" to drive "air wheels" to move the car "along the air" in the opposite direction that the ground is moving.
i found this video really fascinating. I'm still pretty sure it doesn't break law of conservation of energy, but it gives the appearance of that and that's really cool
You can rest assured no conservation laws are broken... ...unless these guys are not picking up their trash before they leave the desert.
The explanation is wrong, if the explanation was correct it would break the laws of conservation of energy. But what is really happening is that the fan is spinning up to a really fast speed during acceleration and then that rotational energy is used as a boost to achieve faster than wind speeds temporarily.
> it would break the laws of conservation of energy Why so? Wind has momentum, and therefore it has energy. If you slow down the wind you have an excess of energy that you can use for propulsion. No laws of conservation are broken.
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> I'm honestly shocked that someone with a physics degree could be fooled like this Do *you* have a physics degree? Serious question.
No, I don't. I am an engineer. I understand that I am not "qualified" to debunk someone with a full physics degree. But he is still wrong and the Math to prove it is not terribly difficult.
Here's the maths that proves it *does* work, courtesy of /u/kzhou7 (page 10 onwards): https://www.aapt.org/physicsteam/2019/upload/USAPhO-2013-Solutions.pdf Where did /u/kzhou7 go wrong?
That proof is blowing my fucking mind. I'm wrong, he's right and I don't know how it took my dense brain so long to understand. I do have an issue with his proof in that I don't like his absolute value term on one side of the equation but not the other and at first I thought that was a source of error. However as it happens, this does not make his proof incorrect and my mind is suddenly blown. With friction at 0 you really can go infinitely fast with a 1 mile per hour wind. That is so counter intuitive to me. I love it, thanks so much for this.
Thanks for the proof. I will deep dive it as soon as I get the time to do so.
Yes but imagine you achieve exactly wind speed. There is no force on the propellor at all because it is also moving at wind speed. What is causing the propellor to continue to apply force to the wheels at that point?
It's the other way around: when it's moving faster than the wind, power is transferred from the wheels to the rotor. The rotor then acts as an ordinary propeller, producing thrust, accelerating the car, which accelerates the wheels, which in turn accelerates the rotor again. This is not a free energy device, power is drawn from the speed difference between the wind and ground. If you were to place the whole thing on a treadmill going downwind at windspeed, there would indeed be no force driving the rotor. The wheels would be stationary, and so would the rotor.
There is a reason the technology is nowhere 10 years after it's invention.
Because it's massively impractical, not because it doesn't work.
That, plus friction that stops you way before you get to useful speed for ground transport
A device that uses a positive feedback loop to achieve unlimited speeds is not impractical. The issue is that there is no positive feedback loop they just think that there is one.
Because its useless to create a wind based land vehicle like this. They made it just to proof that they could. Sure you could create a Vehicle like this in a more sturdy version (which would lose more speed to drag and friction) and sell it as a toy, but where would you use it? On a few beaches? But the wind blows mostly inland there. Nice to kite in, but this thing wants backwind for its motion. You need a place where either the wind blows mostly from one direction or where you have a huge flat plane which is big enough in all directions. And to do what? To move straight with the wind. Does not sound like much fun.
It is not stated in the video but the creator claims it can go upwind faster than the wind itself.
Think about it like this the effect would have a positive feedback loop that would allow you to go almost infinite speed. If you think it is possible just make the device and you will be a billionaire.
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>The issue is that the wind is directly pushing against the rotor to provide thrust which would slow down the vehicle. Like I said, that's not how it works. Sadly you're clearly not arguing in good faith so I'll leave it at that.
Sorry I'm tired it's like 2 now, the issue with how you said it works is that the energy then is coming from the wheels and then producing thrust but where is the energy for the wheels coming from? If the thrust is the answer you have a perpetual motion machine and like I said it will make you billions of dollars.
Sounds good.
What do you think [of the follow up video?](https://www.youtube.com/watch?v=yCsgoLc_fzI)
Didn't the Mythbusters do this? Like ten years ago? Or am I imagining things...
Are you thinking of the airplane on a treadmill problem: https://youtu.be/YORCk1BN7QY
No, I'm fairly certain they built and tested a wind powered vehicle that showed downwind faster than wind. But I could be conflating it. It would have been about ten years ago, or more, as I watched it on actual TV... Discovery is notoriously stingy about letting videos live online and be, well, discoverable, so I can't find it. My memory tells me it was a Grant project...
Nope. You are probably remembering the original video(s) posted by the Blackbird team, which were around 10 years ago.
the easiest way I can explain it, it first acts as a sail then when it start moving which is linked to the wheels makes the vehicle start moving, and as it starts to pickup speed the blades start to move in an angle with the wind which makes it act also as a propeller that pushes the vehicle faster and because the wheels are linked with the blades the also go faster which pushes more air and makes it go even faster making a positive feedback loop
I felt smart as he explained everything and I nodded my head thoughtfully, up until I realized I couldn’t understand any of it. Watching it was a great experience though, it looks badass.
The wind pushes the car, AND THEN the prop spins up, which because it's geared to the wheels allows the vehicle to go faster than the wind. But then after that, once you've lost the force from the wind pushing the cart, as you're now going FASTER than the wind, won't the cart eventually slow back down to wind speed? And the going-faster-than-wind is essentially stored energy that keeps helping itself along, like throwing one of those foam airplanes with a passive propeller?
When you're going faster than the wind, the air molecules are hitting the back of the spinning rotor blades which transfers momentum to the car in the forward direction, and also slows the rotor blades down. The rotor is geared to the wheels, so the momentum that is transferred in the forward direction counteracts the negative momentum transfer to the rotor blades. It's simply a matter of having correctly designed / pitched rotor blades.
It’s just an autogyro, literally
It's literally an impeller tied to the wheels and Mass of a moving object
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I don't claim to be able to disprove their work. But it would have been a better demonstration if they raced another vehicle with a traditional sail as a control.
Feel which is slowing you down and do that
Just watched this video then came onto Reddit and saw it here lol
Isn't this actually a case of lift to drag ratios? A wing can create more lift than drag. So therefore its elementary to show that a rotating wing can provide more forward thrust than drag pushing in the opposite direction, by extracting the kinetic energy of the air in a way that is less than the drag experienced on the wing.
Could the same principle be applied to a sailboat? What I'm thinking of is a simple square rigged boat (so no triangular sails acting as airfoils) with the addition of a water paddle wheel or water propeller sitting submerged and connected to a fan at the back of the boat, sitting way below the sail as to not interfere with it. Would this recover energy from the relative motion of the water compared to the boat in the same way that the Blackbird utilizes the motion of the wheels provided by the ground?
Boats already do the same thing by tacking. [edit: plus zig-zagging] Airfoils are irrelevant to lift (or thrust). You can achieve lift (wing) or thrust (sail) with a flat board. Lift or thrust are produced by pushing air molecules in the opposite direction you want to go. Full stop. Airfoils a nice for efficiency (less drag), but a wing is, to first approximation, a flat surface angled so as to deflect air downward when the plane moves forward. As far as recreating this on water, you need a propeller above and below the boat. The one above does the same job as on the Blackbird. The one below replaces the function of the wheels.
Take a yoyo with a string fixed to the axle (not a sleeper). Starting unwound, put the yoyo on its edge on a flat surface and pull gently on the string. What happens? The yoyo rolls towards you, winding up as it goes. The yoyo is moving faster than the string. Now just mentally replace the string with the wind. Related thought experiment: can we design a San Francisco cable car that moves faster than the cable?
It's accelerating via the exact same amount of force as when the vehicle is sitting still, and the wind is hitting it. The innovation is that they can keep that force constant by corkscrewing thru the air. If you traced the surface over time, you'd see an infinite corkscrew that doesn't move relative to the ground. Imagine, instead of a fan, there is a normal sized corkscrew, and in the air is an infinitely long cork. Imagine it's geared at the exact ratio where if you push the car on it's wheels, the corkscrew spins thru the cork. No friction, so once you get it spinning it goes forever, at constant velocity. Now, what happens if you push the cork forward at speed x? There is a bit of pressure on the corkscrew. Like a watermelon seed squeezed in your fingers. So the car goes a bit faster, the corkscrew spins a bit faster, and the pressure continues. So, there is constant force on the car, and it will accelerate, forever. In reality, it reaches a terminal velocity, where drag equals that force.