A wind-powered vehicle that can travel twice as fast as the wind itself
businessinsider.com
businessinsider.com
That's a terrible explanation - no wonder people doubt the system would work!
After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed.
But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.
A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else.
I think that what this all adds up to is that, by introducing this mechanical linkage, the cart is extracting energy from the difference between the wind speed and the ground speed. This runs counter to our intuition, which assumes that it should be getting its energy from the difference between the wind's speed and its own speed.
That's what allows it to go faster than the wind. If it were based off the difference between the wind speed and the cart speed, then the forward force would go to zero as the cart's speed approaches the wind speed. But the difference between the wind speed and the ground speed is not related to how fast the cart itself is moving. So the forward force on it doesn't disappear as it approaches the speed of the wind, and instead it will continue to accelerate until the forward force balances with drag and rolling friction.
A kite might be a good starting intuition pump here? Kites generally don't do much of anything useful unless they're mechanically tethered to the ground.
From there move on to sailboats. The reason why this cart can go downwind faster than the wind isn't all that far off from the reason a sailboat can sail into the wind (albeit not directly into the wind), but, critically, only if it has a keel or centerboard.
(Edit: Another detail to point out is that the linkage causes the propeller to turn in the opposite direction it would if it were spinning freely. And remember that Newton's third law works in both directions at the same time.)
(Edit again -- Another observation that might help change one's intuition is that, while the cart as a whole may be moving relative to the ground, the part of the cart that's currently touching the ground at any given moment is more-or-less stationary with respect to the ground. And is also mechanically linked to the propeller.)
I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well.
the closer you get to perpendicular to wind, the faster you can go.
I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x.
__edit__
oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could
I blasted through this video pretty quickly, but it appears to explain things well, including how it's possible for a sailboat to exceed wind speed: https://www.youtube.com/watch?v=jJtvGF8vZbE
The cart adds some moving parts, but I am pretty sure that all they're doing is ensuring that the blades' angle of attack is about the same as it would be for a sailboat's sail when it's sailing into the wind, by turning the propeller at an appropriate speed.
The first time comes from the sail, you divide the wind into a component flowing along the sail (doing nothing), and one that is perpendicular to it (pushing it)
Then you take the perpendicular-to-the-sail part and divide that into a side-ways (drift) and a forwards component (propulsive). The side ways one is neutralized by the keel.
Btw I kinda wonder if a rotatable keel tuned just right would allow sailing right into the wind...
The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind.
This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that.
People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.
The propeller arrangement is a glider that is being pushed by the wind. The glider is tethered to a cart whose wheels in-turn drives the propeller to become a "powered" glider. Without a tether, the relative speed of the glider with respect to the wind would be zero. With the tether powering the propeller, the relative speed of the glider is greater than zero, which makes the cart go faster than the wind.
Thinking about the "glider" and the tether as two separate systems makes more intuitive sense, imo.
Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?
It looks quite aerodynamic, but by comparison I can start pushing a 3,000lb car on flat level ground. So this thing probably needs less than 10 pounds of force to start moving.
The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.
I can get a boat that weighs more and has less sail area going on less wind than that. And that thing has to be dragged through the water on top of everything else.
Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)
If you think of a longitudinal vertical plane you can calculate the speed of a point on the cross-section of the blade moving backward relative to the vehicle as a function of the rotation speed. As the vehicle goes faster, so too does the prop surface 'move' backward.
If you watch the videos when the thing is first getting going, it is slow enough to see that the propeller is actually spinning in the opposite direction it would be if it was acting as a windmill.
Think of the wheels as a power bank: It convert and temporarily store linear force to rotational force. As long as it accumulated enough power to counter the rolling resistance, headwind drag, gravity, gearing loss, etc; the excess rotational force can be used to spin the prop. Which in turn generate additional push to be converted and stored.
Within this "loop", the vehicle got two linear input, tailwind and propeller. This will allow continuous acceleration once the vehicle goes faster than tailwind up to the point where the additional force from the prop is completely negated.
The reason the wheels are able to turn the propeller without slowing the vehicle down is that the speed of the wheels over the ground is greater than the speed of the propeller through the air thanks to the tailwind.
https://youtu.be/VUgajGv4Aok?t=432
In short the working model she built has approximately a 2:1 ratio between forward wheel motion and propeller pitch. IOW if the propeller pitch is 5" per revolution, the wheels will require 10" of forward travel to spin it one time.
Step 1: a propeller is like a wall or (straight on, no wing stuff) sail moving forwards forever. When the wind pushes on a propeller it's like it's pushing against a sail or wall, but instead of moving forward it turns in place. Virtually, it's still like a wall moving forward. So it converts linear push to a rotational motion.
Step 2: Wheels are the opposite for solid surfaces: they couple rotational motion to linear motion on the ground.
Step 3: the propeller on the vehicle is connected to the wheels. The gearing makes the "virtual wall" of the propeller move backwards as the vehicle moves forwards. This cancels out the forward motion of the vehicle from the point of view of the air.
So from the point of view of the wind, the propeller is like a wall or sail that isn't moving with the vehicle. It's stuck stationary on the ground, or moving slower.
The reason why the wind can't push a vehicle faster than the wind is that once you get to wind speed, there is no more speed difference to impart a force on the vehicle. Here, even when the vehicle is at wind speed, there is still a difference between the (slower) virtual speed of the propeller and the wind. So it still gets pushed, and can further accelerate.
TL;DR the vehicle uses the wheels and propeller to make itself appear to be running slower relative to the wind, and so it can continue to accelerate when its real speed has reached the wind speed.
Another way to visualize it is: instead of a traditional propeller, imagine it like a water wheel, or an impeller, with vanes the air hits. As the vehicle moves forward, the vanes move backwards. With the vehicle at wind speed, the vanes moving backwards cancel out some of its speed, so that there is still a relative difference between the wind speed and the vane speed, so there is still a force exerted.
I would love a diagram showing how the power flows in this system.
Some well built boats can generate pull (instead of push) in the sail, as if it was a plane wing. The air passing on the convex (forward) side travels faster than the air on the concave, making the pressure lower on the forward side and pulling the boat, and you feel again the wind but against your face (as if you were running forward). It's a bit counterintuitive, because what your eyes see is the sail pushing the boat. So it's not perpetual motion, because the wind needs to keep blowing or the boat slows again. What slows the boat is the friction boat-vs-water.
In the case of the car above, the wind needs to reach the point of wind speed == car speed, and beyond that the wheels keeps rotating the "sails", and thus keeping the pull. If the wind stops, the car can keep going for some time, but as the speed of the wind through the sails is lower, the pull is also lower and at some point it stops. But in this case, the friction is wheels-vs-ground, much lower than the boat-vs-water above, so the car can keep going (same what happens with a bike when you stop pedaling in a flat surface, the bike can keep going for long, and even a faint pedaling or tail winds keep the thing going). This car is a clever construction to keep the wind at high speeds through the blades.
This is how nearly all sailing works. Excepting the case of sailing directly downwind, the curve of the sail works like a wing, and the keel provides something to "push" against - the effective "lift" (if it were a wing) drives you forward.
This is why you can sail into the wind at all...
Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind.
This isn't perpetual motion, though, because eventually the thrust generated by the propeller is insufficient to overcome the additional drag created by the vehicle's faster speed.
The truth is that the car is always getting its energy from the wind, and not expending any of its own. It's just that it's been set up in a clever way that breaks our intuition. We tend to want to mentally simplify the propeller down to a disk whose motion vector is the same as that of the car. But here you've really got to think about the rotation of the propeller and the helical path its blades travel along. The wind is interacting with surfaces whose momentary motion relative to it at any given moment in time are very poorly modeled by the disk abstraction.
EDIT: I think it's clicking. It's because the air and the ground are moving relative to the craft at different speeds. So if the ground and air were still, there would be no speed difference to draw energy from.
Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Suppose the vehicle is facing the +x direction. Suppose the wind is travelling at +10m/s relative to the ground. Suppose the (experimentally determined) maximum velocity of the vehicle under these conditions is +12m/s. Now consider an inertial reference frame which, compared to the ground's reference frame, is moving at +11m/s. (I.e., pick the frame in which the ground moves at -11m/s.) Now let the vehicle run. There will come an instant when the vehicle is stationary in this reference frame (when the vehicle has accelerated to +11m/s relative to the ground, not yet having reached its maximum velocity).
Consider the instantaneous change in kinetic energy at this instant. The air about to be pushed by the propeller has velocity -1m/s and will accelerate in the -x direction. The vehicle has velocity 0m/s and will accelerate in the +x direction. In both cases, the kinetic energy is actually increasing! So in this frame, where does the increasing kinetic energy of the air-vehicle system come from?!
The answer is the ground. It has velocity -11m/s. When it pushes against the contact point of the wheels, it is therefore pushing the contact point in the -x direction. But this means the contact point is pushing against the ground in the +x direction. And therefore the ground is undergoing a minuscule +x acceleration. That acceleration is therefore decreasing the kinetic energy of the ground — by a lot, because of the mass factor.
And this is why we say the energy is derived from the difference in velocity between the ground and the wind — because depending on your inertial reference frame, the kinetic energy might be coming from one or the other. In the frame of reference of the vehicle, it starts out coming from both, but when the (relative) direction of the wind shifts (so that relative to the ground the vehicle is travelling faster than the wind), the kinetic energy starts coming from the ground only, and the vehicle is forced to transmit some of it to the air to keep moving. But this might actually be the nicest explanation of how the vehicle outpaces the wind: in the faster-than-the-wind regime, the vehicle is transmitting energy from the ground into the air — which is exactly what you would expect to happen, given the mechanical linkage between the wheels and the propeller! — but some of this energy bleeds off into increasing the velocity of the vehicle itself, because it's on wheels and that's what happens when wheeled vehicles push against something behind them, no matter what the ground happens to be doing (such as moving backwards like a treadmill, in this frame of reference).
Disclaimer: I did not do well in college-level Mechanics. But I think I have convinced myself of the above explanation, just barely.
To go faster than the wind you just have to be a bit clever in how you capture that available energy.
This article sucked by the way. It said "Any sailor worth their salt can tell you that a boat can travel faster than the wind by cutting zigzag patterns; that's called tacking."
Any sailor worth their salt knows that is not true at all. This practically destroys any credibility on this topic by this author.
You sure? Take a gander at this article https://en.wikipedia.org/wiki/High-performance_sailing
You’re right in the sense that tacking is not the word for sailing faster than wind, but it is true that sailors can tack faster than the apparent wind, and that professional sailors know that. The “apparent” part is an important part of the discussion that the quote didn’t include or clarify, but would be implicitly understood by the saltiest of them.
Assuming you're happy that, so long as the wheels are low-friction, that's no difference between "Downwind faster than Wind" and Upwind, and that a single design of craft can do both Downwind and Upwind, then all you need for the "dead air" part is to store energy in a battery, spring or flywheel.
I'm sorry but you're either answering a different question than "where do the wheels get their energy from" or extremely confused.
We're on a flat plane normal to the direction gravity acts, so I'm going to let you go out and work out how much energy gravity can provide there.
See sailboats faster than wind
https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
> The America’s Cup sailboats are sleek and fast. The AC72, the type of catamaran used in this year’s race, can travel almost three times the speed of the prevailing wind. On June 18th Emirates Team New Zealand recorded a speed of 50.8 mph (44.1 knots), with a wind speed of about 18 mph (15.6 knots).
When sailing directly in line with the wind, the apparent wind in a sail is equal to the actual wind, so there’s no gain. A propellor reshapes the direction of the apparent wind to be in the same direction of the wind.
More interesting is that blackbird can go 2 times as fast as the wind running directly_into_ the wind. That ought to help understand how this is possible.
So instead of sailing sideways so that your air foil sail can move sideways through the wind, the Blackbird harnesses the power of sales moving sideways through the wind while pushing the craft directly into the wind.
It all works the same downwind, and the crazy fast boats like the AC72s do in fact sail faster than the wind when running away from the wind.
Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
It is extremely enigmatic and curious that the engine can work to synergistically blow air into the atmosphere in one direction and resist the inertia of the earth in the other direction with the same force, without expending any energy reserves in the engine. Ive not seen a model or comment or article which fully illuminates the situation, although it is well demonstrated. Perhaps there is not language or a familiar concept which yet suffices. A sane and experienced physicist lost 10k bet on its plausibility.
This is how I explain it in my head:
1. When going slower than the wind, the wind pushes you forward. Propeller at this point is useless.
2. When going as fast as the wind, the wheels will drive a propeller that pushes wind back (same as an airplane)
3. When going faster than the wind, all your energy will come from the turning wheels.
But why do the wheels keep turning in this scenario? Is there somehow a net-force on the back-side of the vehicle from the wind?
IMO it is just positive feedback: the faster you go the more wind is being pushed back by the propeller. At some point the system reaches a state of saturation or equilibrium (the wheels turning no longer can speed up the propeller enough to generate more push). This equilibrium is going back and forth between phases 2 and 3 as you described it.
The thing that makes it a little bit hard to reason about is that all three components (wind, wheel, propeller) are interdependent and influence each other in a more or less fluid way. To me it feels similar like reasoning about active filter topologies.
Imagine a lever with the fulcrum on one end, attached to the ground. Wind pushes on the middle, and the car is at the other end of the lever. So wind exerts more force with less speed, and the car moves forward with more speed but less force.
That's basically what the linkage and gear ratio between the wheels and propeller do; it's the counterintuitive power flow that makes it so confusing...
https://youtu.be/VUgajGv4Aok?t=432
The essential 'gear' ratio is between forward motion of the vehicle and the propeller pitch (essentially how far the prop would screw through a tub of jello for a given amount of rotation). The vehicle has to move *faster* on the ground than the prop does through the air for this to work at all, and it sounds like it should be roughly twice as fast for it to work on something that is reasonably simple to engineer. So if your prop pitch is 5", your wheel size and gearing should allow the vehicle to move forward approximately 10" for each rotation of the prop.
(Note that her video does not explain 'why' this is the case, just that it is a phenomenon that the Blackbird inventors had determined/discovered and is an essential design criteria).
Playing devil's advocate here a little: The whole machine is very counter-intuitive, and I can see how I myself may disbelieve it if not for the irrefutable demonstration(s)[0].
https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s
This is exactly what the blackbird does, only with a propeller pushing air in place of the big wheel, and it's a much clearer and simpler demonstration of the same idea, which you can trivially verify.
Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you can arbitrage.
If I am pushing against still air, I can generate nearly-infinite force for arbitrarily little energy.
That's why you use wings instead of pointing jet engines downwards. You push a lot of air by a little bit (lots of mv, not a lot of 1/2mv^2, since v is small).
If I am going an epsilon faster than the wind, with very little energy, I can push hard on the air.
But it's not all too complicated. With pencil and paper, I could explain to a high school student. If you've done college level mechanics, that's on me, or on the medium.
Perhaps I need to make a video explaining this clearly, since people are running in circles around this and it's just not very hard.
The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
It sorta shows that it's possible, but it doesn't really show why it works in this case.
1. Air isn't solid.
2. There aren't two rollers.
The analogy is a little bit distant.
I just need to find time to make mine more eloquent.
For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss:
- In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me.
- In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency.
Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However:
- Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head.
- And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding.
I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of:
1. Being 100% precise
2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head
EDIT: As an aside, I would also add that the original video did a poor job at explaining, and the demo had clear flaws which were the points the physics professor was making. More specifically, his 2 main points were
1. It's possible that when the picture of the flag was taken, the wind was slower than the car, but the car was still moving faster due to momentum
2. It's possible that the wind speed up at the propeller level is different from the ground level where the flag was
Those were both very valid criticisms, even though they didn't actually matter in the grander scheme of things.
This is a pedantic point, but when you're cutting zigzag patterns downwind it's called jibing, not tacking.
I guess there are high-performance boats that can go faster than the wind upwind, and that are so fast that they can perform downwind tacking (since apparent wind stays ahead of the mast), but this is the exception, not the rule.
Some can, which is why I mentioned downwind tacking for high-performance boats, but I think it takes a pretty fast boat to manage it.
This is important because sailing has right-of-way rules that are written in terms of the true wind, not the apparent wind.
[1] or gybing if you're using British spelling.
I was in a relative's ice boat once. I was pretty wonderfully terrifying -- awesome in all senses.
To do that on water would be quite something!
The "popular YouTuber" is Derek Muller, who studied Engineering Physics and has a PhD in Physics Education Research... It's so frustrating to see how the writer tried so hard to make as much as clickbait-y title as possible...
It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!
I'd suggest that very few YouTubers are popular because they are popular, in fact I think this is fairly rare in general. I think it's more likely that they provide some value to their audiences. I think looking down on "celebrity culture" often fails to account for the types of value that one doesn't value themselves.
I totally get that you value Gig Hadid’s opinion on wardrobe whereas I don’t, nor do I judge, to each his/her own. But, to claim that her opinion is more accurate/better/truthful to that of a respected fashion designer or industry expert is inherently flawed. Wearing clothes is not the same as designing them.
If I asked you who Woodrow Wilson was, what would you respond with?
A better example would be Brian May (of Queen). No correlation between academic credentials and artistic career. There, if the article were about black holes and May had an opinion, I would expect the reporter to cite his PhD, making it a (potentially) informed opinion.
> A better example would be Brian May (of Queen). No correlation between academic credentials and artistic career
I am not sure why that example is better -- Muller's academic credentials are very well related to his career. As were Wilson's.
Eventually the blades are churning the air in front of them, creating a cushion of air behind the vehicle. The wind is no longer pushing the blades, it is pushing the cushion of air created by the blades.
That air cushion is now part of the vehicle. As the speed increases, the cushion grows in size to the point which it collapses behind the vehicle. At that point, it's self sustaining and the vehicle keeps accelerating until all the forces involved reach equilibrium, which is around 2.8x the wind speed.
This all works because the blades are geared to the rear wheels in the opposite way everyone is expecting.
There's a key aspect discussed in 7:10 relating to the ratio of the propeller pitch to the vehicle forward motion that is necessary for this to work.
(Xyla's channel is a great follow overall btw)
But she is somewhat rubbing me the wrong way (no pun intented) ... I am not sure if she had a channel before this one, but she basically appears out of nowhere with a perfect brand image: https://www.youtube.com/watch?v=3B9XYJJY6IQ
From the get go she is connected into the wider content creation sphere. Appearing in videos from Veritasium, BPS.space, Dolphin Labs, probably some more.
She is every nerds wet dream ... smart, creative and good looking with a overly happy attitude.
I don't want to talk bad about her ... It's just too perfect for me not to be somewhat suspicious.
I first assumed it was the other way around with wind->propeller->wheels, which it is not.
It's an overdrive—the wind pushes the vehicle faster than the wind itself, but the vehicle gets less net propulsion force when you sum up the tractive force and the wind force.
The vehicle is basically extracting energy from the difference in velocity between the ground and the air.
The wind pushes the propeller forward, the ground pushes the wheels backwards.
So the wind pushes the vehicle faster than wind speed to start with? How?
Alternatively, think of it as merely harvesting energy from the velocity difference between the ground and the air. It's connected to the ground by wheels, and to the air by a big prop.
It doesn't matter how fast the vehicle itself is moving as long as the gearing is right.
Ultimately, the top speed is limited to when the friction the vehicle encounters from drivetrain losses and aero drag exceeds the power it can extract from the wind/ground speed difference.
But if those values are fixed, as with Blackbird, then you are of course limited to a top speed that is some constant multiple of wind speed.
The video of the bet and the further explanations - https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example without aerodynamics.
Energy can be extracted from the interface between the ground at one velocity and the air moving at another velocity. A sail tied to the ground and extending into this wind will exert a force according to the drag equation:
Force = 1/2 x drag coefficient x density x area x (differential velocity)^2
For a more static example, imagine an enormously long vehicle with a linear bearing running the entire axis from the front to the back. Assume for the sake of argument the vehicle is ultra-light and has very low rolling resistance, some kind of cross between a top-thrill dragster and a train, hypothetically a kilometer long. On the axial bearing, mount a wind turbine with a cable and an anchor that can be fixed to the ground. Start with the wind turbine at the front of the vehicle, then drop anchor to fix the wind turbine in place. There is a velocity differential between the ground-referenced turbine and the air that will impart force on the turbine blades, use this to generate electricity. Use an electric motor to drive the bottom part of the vehicle forwards. When the turbine reaches the back of the vehicle, lift anchor - remember we're in spherical cow territory, so assume it's foldable and super-light so this part takes little energy and can be done arbitrarily fast - and push it back to the front of the vehicle, where you can drop anchor again. In this system, the energy you can extract only depends on the difference between the ground speed and wind speed, not the vehicle speed.
Blackbird is just like this repetitively anchored wind turbine concept in that the point of reference for the wind energy extractor is the ground instead of the vehicle. But instead of dropping an anchor, you're tying to the ground speed through tires, and instead of shuttling the turbine from the back to the front, you're continuously advancing it with the gear mechanism, and instead of driving with an electric motor you're directly using the drag force.
The critical thing to glean from the example is that when it's anchored, the 10 knot ground speed wind is generating power at the turbine, and this has absolutely nothing to do with the speed of the vehicle.
Yes, in the real world, there are all kinds of problems with actually building such a vehicle, and the folding and weight and efficiency and timing and air resistance and rigidity and friction and so on would make it hard to actually get it to work. But these are all purely engineering problems, the physics obviously work!
Imagine one of those people-mover walkways like they have at the airport. Standing next to it on a skateboard, you grab the handrail. Even though the skateboard has energy loss to friction, you go exactly as fast as the handrail.
Now imagine you put a little electric generator in the wheels of the skateboard. Now, not only are you moving at the speed of the handrail, you're also generating some electric power, sourced from the handrail.
Now, instead of grabbing the handrail with your hand, yo hold another electric skateboard, and use the power from your feet's skateboard to power that, so you are actually being pulled along the handrail. You'd be inching along the handrail, going faster than it.
But then increase the rolling resistance - the generators on skateboard wheels -- but you still, by drag, move at the wind speed, you're just stealing more energy from the wind/peoplemover now.
Then you use that energy (simply by a chain linkage) to spin the propeller, blowing air aft. The propeller pushes air backward, changing the speed of the air molecules in the region behind the vehicle from whatever they were moving at, e.g. 10 knots relative to the ground, to 0 relative to the ground. That energy pushes the propeller and with it the vehicle forward, by Newton's 3rd law. This is the second electric skateboard driving forward, in the frame of reference of the peoplemover.
So now you're moving faster than wind speed, in the direction of the wind, stealing more power from the airstream (peoplemover) than you could steal by the (original) drag of your craft alone. You got there by increasing your rolling resistance, which won't slow your craft until the rolling resistance exceeds the available force from the wind. In the analogy, this would be equivalent to jamming your brakes so hard against the floor that you stop the peoplemover rail from moving at all. You use the extra energy -- the amount being spent by the wind to counteract your increased rolling resistance -- to push yourself faster forward.
The power input, P, is constant—you’re not going to magically get free energy from the skateboard system. With a constant power source, so your velocity increases but in a sqrt(x) fashion, with diminishing returns. With real-world constraints like friction you’d hit an equilibrium.
Intuitively what’s difficult to get the head around is sailing directly into the wind.
However the penny dropped when somebody says it’s like sailing into the wind and that basically the propellers act like triangular sails that are continuously reorienting.
The only tricky part bow is the “faster than wind” bit. Presumably this “dynamic sail” is better at harnessing any divergent energy coherently. Kind of like how a LASER coaxes her photons out in phase to produce a coherent beam. The balancing coefficient is the (variable) pitch of the blades.
The difference here -- you ARE moving, relative to the ground. You can, instead of coasting, reach down and grab some power from the huge mass rolling by below you. That'll try to slow you down -- it'll apply a force in the direction of the ground speed -- but if you harness some power and use it to drive a high efficiency propellor, you might be able to get a little extra speed, but as you speed up, you have more resistance both from the wind and the ground, so you can't do it forever.
I think these are the closest we get to moments of 'magic' in real life.
Turns out, it has been done: https://www.youtube.com/watch?v=q2il8Fagbyk
They can theoretically made to be extremely large without much weight relative to the size. Which in turn allows you to accelerate even quite large payloads in a reasonable amount of time.
The flux even at earth orbit is a measly 47e-7 Newtons per square meter. That's 0.00016905632 pounds per square inch.
You still stand by what you just stated?
It's not feasible to do below clouds, in an environment with constant friction, air or water resistance.
The only reason you can do it in space is because that tiny slow acceleration can accumulate over a long period of time. This precludes frictional losses that would normally be incurred within atmospheres.
There is some work on the idea of using both a photon sail and a magnetic sail on a single vessel. In many interesting locations, these would push in different directions, allowing you to sail.
I suppose when your craft is big (or long) enough you could leverage differences in luminous flux (or intensity?) to generate some thrust. The differences will be tiny and the necessary craft size big. One would have to run the actual numbers, but it _could_ work in principle.
Compressing air by moving through it to leverage the resulting difference in velocities sadly won't work =( The steady state is then just the speed of air. I guess for light this also applies in a similar way.
It takes advantage of two things:
1) The Oberth effect, where thrust is more effective the deeper you go into a gravity well
2) The thrust from a solar increases as you get closer to the Sun
I'm surprised people have such trouble with this.
https://mythresults.com/blow-your-own-sail
It also reminds me of the Brayton Cycle, and turbofan/turbojet engines where the output thrust of the blades is applied to subsequent blades that turns the shaft, causing the first fan to spin even faster.
To me this feels similar to blowing on your own sails, and to Veritasium’s Blackbird that blows air against the wind to go faster than the wind. These all feel counterintuitive, but they work.
The fan on the boat is blowing air forward from behind the sail, and by it’s very presence on the boat, it applies a force to the boat that is trying to move the boat in reverse. Without the sail, the boat does move in reverse. It’s surprising that one can seemingly recover more force to push the boat forward than was already used to push the boat in reverse. IIRC Myth Busters confirmed the result but didn’t come to a detailed conclusion as to why, which could involve the shape of the hull, directions of the hull & sail relative to the fan direction, or some aerodynamics of the air blocked by the sail.
In any case, I think it’s quite different from putting the fan in the water facing backwards to propel forward.
Starting from that point, you realize that by shrinking the surface area to human sizes of interest, you can utilize a larger surface area of wind energy to move mass to larger speeds with some limitations based on friction of the wheels and so forth which is a function of gravity and materials available, etc.
Can anyone explain this line's reasoning?
That technicality, however, is a complete misunderstanding of how the vehicle works. The designer of the vehicle, Cavallaro, apparently only built the vehicle to prove the soundness of the physics to doubters, according to the narrative given by the Veritasium video. That designer offered a second $10,000 bet to Kusenko presumably simply excluding the technicality that Kusenko 'lost' to.
It would be a simple bet: that the vehicle can accelerate to and sustain a speed faster than any momentary peak wind speed during the race. Since the vehicle can reach 2.8x the wind speed, all it would take is the vehicle doing so while no gusts exceeded, say, 2.5 the average wind speed during the run.
The actual explanation, which apparently still escapes Kusenko:
If the wind speed is 10 knots from west to east, then this vehicle can accelerate to 10 knots going east by drag (east) alone. At that speed the apparent flow of air (from the reference frame of the vehicle) is zero, and so the drag (east) force on the vehicle is also now zero. At that point, you can rob energy from the wheels (manifesting as increased rolling resistance or west drag) to drive the propeller. This increased rolling resistance doesn't slow the craft, as long as the total rolling resistance is less than the drag force of the wind pushing us forward.
We use this energy to rotate a propeller to push air to the west. From the stationary/ground reference frame, the propeller will have the effect of slowing the airflow behind the vehicle from 10 knots relative to ground to 0 knots relative to ground. This energy -- robbed from the air -- pushes the vehicle forward (east) by Newton's third law.
The counterintuition is that the energy that pushes the vehicle forward is robbed from the wheels, i.e. from the vehicle itself. It's correct to understand that the acceleration from such a scheme would be (at best) a net zero.
The vehicle was already travelling at 10 knots, so the additional energy now being extracted from the wind makes the vehicle travel faster than 10 knots. Assuming the mass of the displaced air roughly equals the mass of the vehicle, this would get the vehicle to a total speed of 20 knots. As far as I understand it, you can then get the sqrt(2)~=1.4 multiplier from the pitch of the blades - the propeller blades do not face 'straight downwind' (perpendicular to the wind) but are angled. I think that this 1.4 * 2 * windspeed is what gives the 2.8 figure mentioned in the article.
Also this: "According to the World Ice Racing Circuit, ice boats can sail four to five times wind speed. In March 2009 a land sailboat reached 126 miles per hour on a dry lake bed in the Mojave Desert" from: https://www.straightdope.com/21344013/how-can-racing-yachts-...
I think I read somewhere that an ice sailboat did 143 mph.
Imagine two sailboats on a cylindrical ocean on diagonal tacks, on opposite poles, and rotating around the core with rods connecting them to the core. Now you have a propeller. :)
At a standstill, the wind just acts on the entire vehicle including the stationary propeller, and sets the whole thing in motion.
Without the propeller, going directly downwind would be limited by the speed of the wind acting directly on the vehicle which otherwise lacks any relevant moving parts for the wind to act on.
By adding the propeller, driven by the forward motion via the wheels, a part of the vehicle now moves backwards (imagine the cross-section of a propeller blade as it spins, its intersection with the wind travels backwards into the wind) to compensate for the forward movement, giving the wind something to act on even when the vehicle travels faster than the wind.
Release the Blackbird from rest. It’s easy to visualize what happens: the wind turns the propellers, which turns the wheels, and the Blackbird rolls forward.
While the Blackbird is going slower than the wind, this is a straightforward system.
What happens when the Blackbird is going the same speed as the wind?
In that case, the wind is no longer pushing the rotor, since there’s no difference in speed. However, the rotor is still spinning! So now it acts like a propellor — pushing against the wind.
Forgetting the speed of the car vs the wind for a moment, the propellor is arranged so it is trying to push air into the wind (i.e. it is propelling the craft). So, whatever force the wind might impart on a simple flat disc of similar radius to the prop, the force the prop actually experiences is higher than that because it is pushing against the wind.
For me, this realisation that the prop is pushing against the wind, increasing the overall force, unlocked understanding how this craft is possible.
In this case, I'm trying to think how the wind energy is lost, and it seems that some of that might be lost to the static friction in turning the wheels, and also the attached propeller. However, the energy used to turn the propeller is outputted again, as thrust. Since the initial wind was enough to overcome the static friction, any additional power might be causing it to move faster than the wind. I'm no physicist, but this is pretty cool.
I’ve seen this idea in a couple of comments here, but I’m not sure what it means, can you elaborate?
Rolling wheels are still under static friction. They don’t become dynamic friction unless the wheels are in a skid. Unless what you’re talking about is the static friction of the oiled axle, but I assume that’s not what you meant?
But, apparently, there's a way for people to disagree about science - without any of the sides being evil or stupid - and there's a way to find out who's right. Maybe we could learn something bigger from this?
It seems to me that Derek did several things right here:
- Choose a topic with which he has relevant experience
- Collect experimental data supporting his hypothesis
- Fairly address opposing viewpoints, in this case by providing a platform for a dissenting expert to present his evidence
Are there examples of similar videos being removed from youtube for misinformation?
Odd that the professor was so keen on betting in that case.
The physics involved still twist my brain, but there's nothing terribly new here.
I don't believe DDWFTTW has been achieved on water.
I'm not sure the first boat to achieve it, but it wouldn't be surprising if it was done during the Clipper era. Those boats held 24 hour records from 1860 something all thee way to the 1980s, where modern racing yachts finally eclipsed them.
You need a propeller to go Directly Downwind Faster Than The Wind as far as I know. I think on water it has to be very large. I expect there are other ways other than a propeller but haven't seen them.
A half way definition (This isn't DDWFTTW to me either), I'd like to know if an America's Cup boat could beat a balloon? So it would be allowed off the 'race track' but it has to come back to meet/beat the balloon at the narrow finish line. So it would cover a much larger distance than the balloon and still beat it to the finish line. Is this possible in practice?
The Veritasium video doesn't actually answer this, it kinda goes around it.
[edit] If it's symmetrical which it should be, as long as they are going downwind faster than the balloon, they just reverse it at the halfway point and get back inline with the balloon but ahead.
There is nothing online that says a sail boat can go DDWFTTW. DDWFTTW is at least a 4 decades old term in the sailing community looking at magazines.
Here's a design from 1985 using a propeller https://issuu.com/latitude38/docs/latitude3894apunse/162
Sorry, you are mistaken.
The Blackbird cannot use a land sail to go faster because it needs something to act like the keel on a sailboat and provide a counterforce. The propeller is an elaborate way to provide that force
This should also mean going directly upwind is practically possible, not a degree either way, directly upwind for a indefinite period of time with sails.
Do you have a link for this?
Here's a link to a working directly upwind rotary sail boat model - https://www.youtube.com/watch?v=j1bR5hb8RCQ
No one has gone DDWFTTW on water. It will take a propeller to do when it happens.
If you want to contradict this you need to post a link of someone agreeing with you, else you misunderstand DDWFTTW.
(The link on Wingsail leaves out 0 and 180 degrees for the Points of Sail which are the only two we care about, directly downwind and directly upwind, the wiki article makes no comment on DDWFTTW. This thread is just people saying random words like AC72 or broad reach. If your ideas correct someone else will sum it up for you and you can link it. )
I think what you re saying is that you can't be at 0 degrees to the wind, which i guess is true
In 2012, Blackbird also demonstrated sailing directly upwind with twice the speed of the wind.[0]
[0] https://en.m.wikipedia.org/wiki/Blackbird_(wind-powered_vehi...
To extract energy from the wind, you need a reference that is slower than the wind. The ground is one such reference. The vehicle gets access to that reference via the wheels. It can therefore extract power from the wind regardless of where it's going or what it's doing.
So it's like a wind turbine on the ground, connected to a car that has nothing to do with it. Of course the car can go faster than the wind in this situation; the wind speed doesn't matter, only the drag of the car and how big your wind turbine is (how much energy you can get out of it).
This vehicle just makes the whole thing self contained with gearing and counterintuitive rotation of the propeller. It's touching the ground and can use that reference via the wheels, so it can pretend to be a stationary wind turbine on the ground this way. That means it can go downwind, or upwind; doesn't matter. As long as the wind is moving relative to the ground, it can harness that energy.
I wonder if a sailplane with a ram air turbine in the front and a propeller on the back can go faster than tail the wind.
To extend the analogy further, I think that the wheel-and-chain mechanism ends up functioning analogously to the boat's keel.
https://newt.phys.unsw.edu.au/~jw/sailing.html
> How can boats sail faster than the wind? Lots of boats can – especially the eighteen footer skiffs on Sydney Harbour. Ask a sailor how, and he'll say "These boats are so fast that they make their own wind", which is actually true. Ask a physicist, and she'll say that it's just a question of vectors and relative velocities.
EDIT it’s more like gearing.
For the guys making the vid this is win win due to youtube ads. Kusenko isn't exactly taking a cut from that.
> They even brought in several of science's biggest names, including Bill Nye and Neil deGrasse Tyson, to help decide who was right.
This part just stands out like a sore thumb.
I would still argue that BN and NdT are only the biggest names in pop science (as in people who wear “I <3 science” t shirts) and not in science as in academia. But that’s cool that they did weigh in on this subject.