A wind-powered vehicle that can travel twice as fast as the wind itself(businessinsider.com)
businessinsider.com
A wind-powered vehicle that can travel twice as fast as the wind itself
https://www.businessinsider.com/youtuber-won-10000-bet-with-physicist-over-wind-powered-vehicle-2021-7
265 comments
https://archive.is/FhZfN
> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward.
That's a terrible explanation - no wonder people doubt the system would work!
That's a terrible explanation - no wonder people doubt the system would work!
It’s really not that complicated - wind speed is not the same as energy. The propellor is a device that pushes against the air to move forward, it takes energy from the wind, produces a force, and the force produces momentum in the vehicle. We know that a stationary turbine can continually take power from the wind. When that wind energy is stored in a battery it can power your car much faster than the wind. This just takes a shorter route and stores the energy in momentum.
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.
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.
This does not explain the most obvious issue (or at least my most obvious issue): when going as fast as the wind, there is no wind anymore. The answer: at that point, your wheels are turning, and so you can get energy from your turning wheels, but not directly from the wind.
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.
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.
I think you are spot on.
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.
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.
> 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?
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?
Forward momentum of the vehicle I guess.
Sailboats can run faster than the wind because of lift (which is not involved here), but not when running away from the wind
the blades move sideways
if it sails away from the wind, the lift is pushing it sideways, not forward
And if you turn a sail sideways and put it on a spindle with several other sails, you have what looks a whole lot like a propeller.
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.
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.
Exactly. The article does not understand this at all.
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Propellers are lift devices. The lift vector is parallel to the axis of rotation.
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.
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.
it's confusing to talk about sailboats and this cart, because this doesn't use lift from the wind to move itself forward, it generates some lift with its propeller
The more important point is conservation of energy. Since the ground is moving backwards faster than the air, you can generate more thrust against the air for the same amount of power than the amount of backwards force exerted by the ground on your wheels (since power = force * velocity). For the same reason, if traveling upwind rather than downwind, you’d need it to use the opposite mode of operation - have a turbine driving the wheels rather than wheels driving a propeller.
Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
I like your point about moving upwind, but think it rather relies on conservation of momentum as that insists the wheels must result in thrust in the opposite direction to the props thrust in order to change the momentum of the earth as much as the momentum of the atmosphere is changed. They must each be reduced, in order to transduce energy. If you have stored energy you can propel mass from rest, but forces can only be applied equal and opposite.
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.
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.
You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.
A better way of looking at this is to not think of 'the propeller' as being in one plane for the wind to 'push'. Since the prop is rotating, consider the 'airspeed' of the surface of a blade. You can have zero airspeed when the vehicle is moving forward faster than the wind. Similarly the airspeed of the blade is slower (actually negative) than the vehicle's airspeed which allows the wind to push the blade surface when the vehicle is moving faster than the wind.
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 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.
Gravity does
Then what is spinning the wheels?
Asking what is spinning what is maybe framing it in a way that impedes understanding.
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.)
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.)
Perhaps another way to think about it - tailwind. Aircraft can derive energy from the wind even when they're going faster than the wind-speed.
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.
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.
Very good explanation. Thanks. I was thinking of some other contraptions to explain this (to myself mostly), but the glider->powered glider made it click. Even if it might not exactly how this vehicle work, at least it demonstrates a system that can obviously go faster than the wind speed.
Sailboats are one I struggled with, especially sailing into the wind.
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.
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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 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
A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for various world records do this somewhat routinely.
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 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.
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It's hard to really explain how boats do it using only text and no good way to draw force diagrams.
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.
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.
I think sailing into the wind works because you divide the force of the wind into components twice. Dividing into components first one way and then another lets you pull some tricks.
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 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...
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The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and prevent it from spinning and it would still move forward.
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.
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.
I may be wrong, but this is how I understand it. The propellers push against the wind. Effectively you’re creating a faster wind by pushing back at it. The energy that drives the propellers still comes indirectly entirely from the wind, but wind across a larger volume behind the propeller so therefore carrying more energy.
You've got it exactly right.
The fact it's going forwards.
So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it?
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?
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?
Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough.
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.
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.
Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?
In the veratasium video [1] you can see it starting just from the wind (first attempt is around 5:20). It works just fine because as you scale up the vehicle you scale up the propeller, so there's plenty of surface for the wind to push against.
1: https://www.youtube.com/watch?v=jyQwgBAaBag
1: https://www.youtube.com/watch?v=jyQwgBAaBag
The 100 pound vehicle also has a much larger cross section to the wind. And its propeller is further off the ground, so it's getting cleaner wind than the little model version.
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.
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.
In the water, there is no static friction and no velocity-independent friction. If you push a floating boat, it moves.
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Fair point. Though, I would guess that Blackbird's internal mechanisms are well-enough engineered that it has very, very little static friction where it counts.
My motorcycle with me on it can weight 700-800 pounds and has a much smaller cross sectional area than blackbird. And yet it still gets pushed around sometimes by crosswinds .
If I understand what you're saying though, you would be in motion on the bike as the crosswind pushes against you- it takes a lot less force to shift your heading while moving forwards than if you were being blown sideways while stationary.
Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)
Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)
I've definitely ridden through some storms that felt that way!
The fan blades are an airfoil, which have a minimum speed to create the lifting effect.
Lift isn't really a part of how this gets started. Regular old pushing works on airfoils, too.
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No, it’s light enough for the wind to start pushing it.
Even with 100+ pounds (50+ kg)?
Yes, at the extreme end you can find videos of 3,000+ lb cars flipped by the wind.
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.
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.
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines?
I would love a diagram showing how the power flows in this system.
I would love a diagram showing how the power flows in this system.
I felt this in a sail boat: when the boat is moving below wind speed, the wind pushes the sail and the boat. At some point, wind and boat equal speeds, so it feels like theres no wind at all except the sail is full.
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.
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.
> as if it was a plane wing.
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...
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...
Obligatory mentioned of the AC72 using rigid wing sails. - https://www.youtube.com/watch?v=hbPTnF6liF8
Veritasium did a video on the Blackbird (plus a bet to prove it does indeed work). https://www.youtube.com/watch?v=jyQwgBAaBag
Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.
Now I think I'm beginning to understand... At first, wind propels the vehicle, moving it forward and turning its wheels. Its wheels turn the propeller which slows down the air, even the air in front of it. The energy lost by the air is now the energy that moves the vehicle!
Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Yeah. Wind pushes the propeller (and attached vehicle) forward, and as the vehicle starts traveling forward, the wheels turn the propeller generating extra thrust (at the expense of the wheels pushing backwards by a smaller amount against the ground), allowing the vehicle to continue to accelerate past the wind speed.
Ah, but not so fast. It actually depends on your frame of reference.
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.
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.
Here's a simple explanation:
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.
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.
Your explanation that the propeller works like a fan implies a sort of perpetual motion, though, since it implies that the source of energy that pushes it past the speed of the wind is somehow coming from inside the car itself.
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.
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.
The problem with this explanation, at least to me, is that it doesn't require any wind at all. Why not just give it a push in still air and have to take off into the distance forever?
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.
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.
Why would you consider those two scenarios analogous? In one scenario, the wind is “giving it a push” constantly, and in the other scenario, you’re giving it a push for only a brief moment.
When it is moving faster than the wind, is the wind still giving it a push?
Absolutely. If the wind stops, the vehicle will soon roll to a stop as well. It's not a perpetual motion machine.
Right. This mechanism doesn’t actually require wind. You get the same effect by placing the vehicle on a treadmill in a windless room: It will move forward on the treadmill.
There is no theoretical limit on the speed of the vehicle though - just a practical one based on limits to propeller efficiency and the fact that drag for the craft keeps going up.
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You’re harvesting the energy from the difference in velocity of the ground and the wind. Sure it is easy to think about that being possible when you’re attached to the ground but it’s perfectly possible when you’re moving too. Regardless of how fast you are moving or in which direction there is always that difference been air and ground.
To go faster than the wind you just have to be a bit clever in how you capture that available energy.
To go faster than the wind you just have to be a bit clever in how you capture that available energy.
The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either increase the area of the cart facing the wind, or increase the pressure. The propeller harnesses part of the wind's energy to increase that pressure thus increasing the total force acting on the cart but the work done on the cart (F*displacement) is always limited by the energy that can be provided by the wind.
It’s not perpetual motion because it’s extracting energy from the fact that the wind is moving relative to the ground. Without wind moving relative to the ground it wouldn’t work.
It’s certainly not perpetual motion, because the vehicle would quickly come to a stop if the wind speed (relative to the ground) went to zero.
The way this clicked for me (it took some thought!) was to see it as a lever, or at least an analogue to one.
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...
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...
I think this is a good intuition about it, and Xyla Foxlin goes into this a bit in her video explaining the build for Veritasium's video.
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).
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).
And one of the Veritasium videos: https://www.youtube.com/watch?v=yCsgoLc_fzI
Yes exactly. And if you were driving the other way into the wind, you’d flip the lever and use a turbine to extract energy from the wind and push against the ground.
Aye, where do the wheels get the energy from? Actually, more seriously, is there any point where the net torque on the drivetrain is zero?
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.
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.
Gravity. Without it the wheels wont have sufficient friction to generate torque
> Gravity
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.
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.
You 're right, gravity provides the force of friction but not energy. The energy is from the pressure of the wind, the device just makes better use of it compared to a fan-less cart.
The energy comes from the air. The air molecules in the wake of the propeller have lower kinetic energy than the rest of the air.
Here's another take:
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.
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.
Its actually the correct explanation tho.
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.
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.
What's not true about it? According to Wikipedia [1] there are a bunch of speed records for sailing faster than the wind by tacking downwind.
[1] https://en.wikipedia.org/wiki/High-performance_sailing
[1] https://en.wikipedia.org/wiki/High-performance_sailing
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tacking is up wind by definition.
I think it's just very poorly worded. Sailboats can go faster than the true wind speed, like in the America's Cup, and typically do it do it going upwind. The addition of tacking is not wrong, it is zigzagging upwind, but just very oddly worded.
> Any sailor worth their salt knows that is not true at all.
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.
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.
Yeah, my first inclination is that since the wind is turning the propeller and the propeller is turning the wheels via a gear system, the speed of the vehicle will be determined by the propeller speed, the gear ratio, and the tire diameter (ignoring friction and air resistance for a moment). The gear ratio is the critical component. It's not like the propeller is a simple sail. I could be completely missing something though.
Most critically: the wind is not turning the propeller.
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.
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.
Gear ratio *is* the critical component, specifically prop pitch vs. forward motion, but not exactly for the reason you describe. Xyla Foxlin built a working model and provides a bit more info here:
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.
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.
No, the wind is not turning the propeller, the wheels are.
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.
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.
The prop is spun by the wheel, not the wind. The tailwind provide push.
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.
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 most surprising thing here is that someone on Twitter made a bet, lost the bet, and then actually paid said bet.
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].
[0] https://www.youtube.com/watch?v=yCsgoLc_fzI
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].
[0] https://www.youtube.com/watch?v=yCsgoLc_fzI
The small demonstration that made it click for me is from that video:
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.
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.
This is simplest and best explanation, no doubt why he got his PhD in physics education.
To be fair, they both agreed to make the whole bet public. They went on video, with "science celebrities" like Neil Degrasse Tyson and Bill Nye, and I believe they even signed something? It's kinda hard to backup from that. It also was a UCLA professor, so not quite some random person on Twitter.
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.
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.
How does a professor have $10,000 sitting around for a bet like this?? I am a nicely-paid tech worker, and $10,000 wouldn't ruin me, but it would be a major setback for that year.
UC salaries are public[0] and it looks like he made $219k in 2019, which is pretty decent for a professor but yeah $10k would still definitely be a significant setback.
[0] https://ucannualwage.ucop.edu/wage/
[0] https://ucannualwage.ucop.edu/wage/
It's not at all counterintuitive. It's just poorly explained.
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.
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.
Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.
Perhaps I can't explain it well either, without pictures.
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.
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.
With due respect to you, and assuming you passed, that's pretty damning of whatever college you went to. I'd want a refund.
The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
I understand what is happening in the vehicle, I don’t understand what is going on with this explanation. We never learned about energy arbitrage when you “have different speeds.” We never learned anything about generating nearly infinite force against still air. These concepts sound really strange to me.
I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.
I actually find the limit to be somewhat less insightful.
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.
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.
A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant.
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
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
> 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.
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.
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.
Side note, this is my favorite mode of exhibiting pedantry. Let the conversation flow while still correcting terminology. Thanks for the info.
But what about once you are going downwind faster than the wind, and thus experience an apparent headwind? I'm not a sailor, but I thought I'd seen people using the term "tacking downwind" to describe that.
You will experience an apparent headwind when sailing on a broad reach faster than the wind, yes. But the jibe involves turning downwind (thus losing lift), as well as bringing the sails across (which will temporarily de-power them). It's been a while since I've been sailing, but I don't think most boats can maintain an apparent headwind through the entire maneuver, even if they are capable of faster-than-wind speed.
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.
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.
The F50 sailing class used in SailGP can remain foiling during jibes in winds as light as 8 knots. That is, the boat always sees an apparent headwind during the entire downwind run.
Amusingly, it's still referred to as jibing[1], even though the apparent wind remains a headwind.
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.
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.
State of the art in crazy expensive racing yachts (think America's Cup) is limited by cavitation on the foils to a bit over 50 knots. Ice boats are setting records over 120 knots. If someone figures out a magic foil, that gap may close, which is a thought that blows my mind.
Wow I had no idea, thank you.
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!
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!
Sport kites can also be great as a sort of demonstration of what a sail's crosswind potential could look like if it didn't have to lug around a vehicle, technically both downwind and upwind since they're essentially tracking a spherical concave shape centered directly downwind of the pilot. Speedkites in particular push the sail speed vs wind speed ratio to ridiculous limits. For scale, this kite is 190cm across.
https://www.youtube.com/watch?v=_quR4xUSrZY
https://www.youtube.com/watch?v=_quR4xUSrZY
"A popular YouTuber filmed himself driving a wind-powered vehicle downwind faster than the wind itself. [...] Muller, the creator of the Veritasium YouTube channel, likes to break down funky science concepts for his 9.5 million subscribers."
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...
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...
Having 9.5 million subscribers on a science-based YouTube channel is definitely more notable than having gone to university, or indeed having a PhD. I think the writer picked the correct description here.
I see your point, but the title and first paragraphs try to paint the picture of a "funky science" youtuber betting a UCLA physicist $10k and winning (which for me is very different to the truth: UCLA physicist bets another physicist $10k and loses)
He did study Physics but his main field is science education, I think it's fair to assume a physics professor would know a bit more. I also think it's a bit of your own preconceived notion at play here assuming that calling someone a "Youtuber" is meant as a negative. This is equivalent to saying "Science communicator vs physics professor", which is a fair portrayal.
I really don't think being called a Youtuber is meant as negative. I take issue with how the writer chose to present this story. If this was about the intricacies of ad revenue on YouTube for example, great, no need to specify he also has a physics degree. But I'm disappointed to see them paint him as just a guy who "likes to break down funky science" (and made 10k off a physicist) when he is more than that, with relevance to the actual story. For someone who doesn't know his work, he might as well be one of the guys burning snow with lighters a few months ago to prove its fake snow...
It's a sad indictment of the times when people feel that having a million subscribers (read: being popular) is more notable than having a PhD (read: expert training) in a relevant field.
It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!
It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!
Subscribers do so for a reason. In this case not insignificantly because of expert training.
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'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.
Re the first statement, that’s complete horseshit. Celebrity culture is all about being part of a cult following and finding like-minded people. Any correlation to objective fact or basis in reality is purely coincidental, and has no bearing on the quality of the person or the content of the movement being followed.
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.
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.
I think it's just a simple matter of the relative exclusivity of those accomplishments.
If I asked you who Woodrow Wilson was, what would you respond with?
If I asked you who Woodrow Wilson was, what would you respond with?
That’s a false dichotomy. Being a president is tangentially related to exceptional intelligence.
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. 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.
The people who have top YouTube channels also have exceptional intelligence. Maybe less so in logical intelligence or rote memorization, but more so in other intelligences.
> 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.
> 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.
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The wind pushes the props because they are sails. Moving sails, but sails none the less. But instead of being windmills, they turn as if they were blowing into the wind. Their sheer sail area moves the car forward. The faster the car goes forward, the faster the blades turn.
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.
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.
Derek from Veritasium asked Xyla Foxlin to build a functional model, and she goes through the trials and tribulations of that in quite a bit of detail in her video here:
https://youtu.be/VUgajGv4Aok
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)
https://youtu.be/VUgajGv4Aok
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)
Although not adding much value to the comment, I concur that her channel is great to watch. She is so enthusiastic she would make even the grumpiest professor smile.
Xyla channel is definitely worth a look.
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.
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.
For those that hate the way news articles are written I found the Wikipedia article a much clearer read https://en.m.wikipedia.org/wiki/Blackbird_(wind-powered_vehi...
Like the professor in TFA I am also confused: how can the ground be imparting power to the propeller while moving downwind?
The original Veritasium video https://youtu.be/jyQwgBAaBag
The video of the bet and the further explanations - https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example without aerodynamics.
The video of the bet and the further explanations - https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example without aerodynamics.
The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground.
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:
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.
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.
In your static example how does the turbine get back to the front of the vehicle? Does the force of the wind push it there? If so, if the wind is blowing at 10 knots but the base of the vehicle is moving at 20 then as far as the turbine is concerned it is actually being pushed backwards, no? Or is the idea that when you lift the anchor the vehicle slows to below wind speed while the turbine gets pushed?
For that 'spherical cow' example, assume that the vehicle has batteries to average out the duty cycle while the turbine is going back to the front. Fold the mast down flat and stow the blades so it has minimal air resistance, assume it has minimal weight, and let an electric motor zip it back to the front of the vehicle at 100 knots ground-speed or 90 knots into the wind, if you're moving forward at 20 knots this gives you an 80% duty cycle.
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!
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!
The wind pushes the propeller, the gearing of the wheels to the propeller cause the overall vehicle to advance faster than the blades on the propeller (as seen by the wind).
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.
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.
Wind does not drive the propeller.
I did not say that. I wrote "Wind pushes the propeller."
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.
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.
> Downwind, when the vehicle is traveling faster than the windspeed, the ground is the fastest-moving medium relative to the vehicle, so the wheels harvest the power and impart it to the rotor, which propels the vehicle.
So the wind pushes the vehicle faster than wind speed to start with? How?
So the wind pushes the vehicle faster than wind speed to start with? How?
The angle of attack of the propeller blades and the gearing relative to the wheels cause the blades to be seen by the wind as moving upwind relative to the ground.
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.
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.
I agree that theoretic top speed is limited only be drag (and other resistive forces) if you are able to adjust propeller pitch, gear ratio, or wheel diameter on the fly.
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.
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.
Propeller pitch is not fixed on Blackbird, that is one of the few controls it has.
wind pushes vehicle. wheels spin. wheels drive propeller (via gears). propeller pushes vehicle.
I first assumed it was the other way around with wind->propeller->wheels, which it is not.
I first assumed it was the other way around with wind->propeller->wheels, which it is not.
I think I have an analogy that actually explains how this works.
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.
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.
This apparently violates conservation of energy as you’re claiming to take energy from one source, put it through some apparatus (i.e two skateboards) to produce more energy without putting anything additional in. Can you account for this?
The people-mover has to expend more energy to push you because you're applying a stronger drag force, so energy is conserved.
Okay so in this case the people mover is drawing the additional energy. Additional energy is being provided from somewhere i.e a diesel generator somewhere up stream has had to increase her fuel consumption to maintain the power on the network.
Right. The people mover is analogous to the wind. You can get to move at the windspeed through drag alone, at which point you no longer move relative to the air. The air is being robbed of only the fraction of energy that you're losing to drag (rolling resistance, as there is no air resistance now). This is you riding along with the peoplemover on the skateboard.
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.
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.
To add on — the power you’re stealing from the ground is the power you can use for propulsion. So before you switch on the handrail skateboard, you have some power, P, available from the generator. You can use that P to power an air conditioner or a light bulb, or to crawl down the rail. The fact that you’ll then use that power for more propulsion is not relevant — Newton’s laws are linear so you can just add up the two scenarios and they don’t have any non-linear interactions.
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.
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.
It’s gearing. Albeit elaborate.
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.
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.
When you're going at exactly wind speed, there basically is no wind at all. If you had frictionless wheels, you could turn off your propellors and you'd just be coasting along, feeling no force whatsoever from the wind, as if there was no wind at all and you were not moving.
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.
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.
What you suggest sounds analogous to a flywheel. You’re using the mass of the vehicle to stabilise your speed and drive the propellor when the energy stored in your momentum > the energy you get from the wind. However, this does not account for the full experience they are describing where you can attain speeds far greater than wind speed. If that’s the case there must be a different “angle” to it, pun intended. He’s not just harnessing the purely linear “push” from the wind. There must be other vectors superimposing. Could well be the lift provided by the Bernoulli effect on the propellor blades. As I mentioned elsewhere once you get to that level it’s all very domain specific.
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This blew my damn mind 10 years ago when it was first written up in Wired - the way the article was written, the build up to the actual run... all of it was so inspirational and today I still refer to it in my mind whenever I think: "People do the impossible all the time and the guidance they get from everyone around them is that they are INSANE"
I think these are the closest we get to moments of 'magic' in real life.
I think these are the closest we get to moments of 'magic' in real life.
The treadmill demos made me think of another counterintuitive possibility that I wondered if anyone had pulled off: sailing a boat indirectly up a river in no (bank relative) wind.
Turns out, it has been done: https://www.youtube.com/watch?v=q2il8Fagbyk
Turns out, it has been done: https://www.youtube.com/watch?v=q2il8Fagbyk
Can it be done with solar sails?
Solar sails only absorb enough energy for tiny crafts in space, a frictionless environment. You overestimate photon flux momentum from the sun.
Solar sails indeed only absorb a tiny amount of energy, BUT per given area.
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.
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 mirrors are extremely heavy and solar sails are exclusively used above our atmosphere, in space, where more flux from the sun is received.
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.
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.
It seems I misunderstood the thread. I somehow thought we were talking about sails in space rather than in atmo.
So what were you originally referring to? Solar wind in space? I'm honestly confused now.
I was actually. I rhought the parent I originally replied to was saying that solar sails aren't useful for large payloads in space.
They overreached a tiny bit in reaction, but they were responding to someone who seemed to be proposing solar sails inside the atmosphere.
I didn't overreach at all. Nearly all the creatures of HN just try to save face after they make invalid statements. This involves acting confused, changing the subject, or a barely clever combination of both. The desire to be correct and win what barely even counts as a debate is a contagion deeply plaguing these creatures.
Not an expert, but I'm going to say no. There is no medium in space, moving relative to the solar wind, against which the "keel" of a solar sail ship could push.
Gravity? I guess something with enough mass to use gravity might be too large to use solar sails.
Gravity does not actually provide a force in the sense that a sailboat would require. (It acts on all parts of the vessel equally.)
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.
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.
This is similar to flying in air.
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.
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.
Not exactly, but you can do a crazy 'sun dive' maneuver: https://aapt.scitation.org/doi/full/10.1119/10.0002178
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
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 think this explains it conceptually best:
https://www.youtube.com/watch?v=k-trDF8Yldc
I'm surprised people have such trouble with this.
I'm surprised people have such trouble with this.
Yeah, this was exactly the mental model I used to simplify it. Nice to see it in action.
I bet your mental model didn't have all the stuffies, though.
Yeah they add a nice touch - might start inserting them in my mental models going forward.
in this case though, it's not the ground that provides the motion but the wind. The mental equivalent of this with wind is not straightforward
In the video, the ruler is the equivalent to the wind. Admittedly he then shows moving the ground, to further illustrate the concept.
I saw the Xyla Foxlin video about this (and her treadmill version) and that was a fun build to watch.
Muller’s bet reminded me of the Mysthbusters episode where they put a fan in a boat that blows on it’s own sail and moves the boat forward, which I’m pretty sure I thought wasn’t possible before it aired.
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.
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.
Whether it's an electric fan or your own lungs, it's pulling energy from storage (a battery or your own chemical energy storage) and converting it to kinetic energy. It's no different than if you put the fan or your face in the water facing backwards to propel the ship.
That seems too dismissive of the part that’s counter-intuitive, and I think this is legitimately counter-intuitive, which is why physics professors are losing bets.
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.
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.
Part of the key to understanding the intuition of this is to recognize wind as a flux field that is an energy source and consider the limits of a sail that has infinite surface area and zero mass, it could slowly accelerate and propel a frictionless wheel close to the speed of light (this would be a fun calculation).
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.
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.
Kusenko "conceded on a technicality — that the vehicle moves marginally faster than the wind temporarily," Cavallaro said. "I offered him another $10,000 bet, because his technicality is entirely wrong, but I know I won't be hearing from him."
Can anyone explain this line's reasoning?
Can anyone explain this line's reasoning?
Kusenko, who conceded the bet to Veritasium's Muller, only conceded that if you accelerate an object to wind speed and then the wind suddenly dies, your (low-drag) vehicle will continue downrange at a speed greater than the (now zero) speed of the wind on momentum alone.
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.
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.
I feel that the Business Insider title is really misleading and counterproductive; because Derek Muller is not 'a youtuber' -- he's a serious physics educator, with a PhD in physics education research, and with probably the best youtube channel (or any channel, in any medium) in the world for explaining physics content; which has been running for a decade; and with nods from multiple famous physicists IN THIS VERY VIDEO saying that he is the best that there is. So the claim that 'A youtuber bet a physicist and won' just makes it seem that experts are wrong, whereas in reality, this was expert vs. expert. That being said, physics is such that if 'some youtuber' bet a physicist, and was right... physicists would concede.
large ocean yacht racing boats can go much faster than the wind:
https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
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.
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.
The difference here is that the vehicle in the article is traveling parallel with the wind
I thought the video below provided a good explanation for why this works. The physics are similar to those of a single sailboat going faster than the wind.
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. :)
https://www.youtube.com/watch?v=jyQwgBAaBag&t=404s
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. :)
https://www.youtube.com/watch?v=jyQwgBAaBag&t=404s
The propeller is being used a forward motion compensator.
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.
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.
I wonder how one would go about making such machine that would work in zero gravity. Without friction, something else would have to convert part of the wind force to propeller torque. Maybe small turbines could be added that would spin according to apparent wind (forward or backward) and spin the propeller to generate additional lift.
Fun thought exercise, but are there any practical scenarios where one has wind but no gravity?
solar wind
Is that really practical considering the already high velocity of the particles? If you matched the downwind particle velocity (>100km/s?) it seems unlikely any mechanical motion compensator could move fast enough to make any significant gain to be worthwhile... but I'm no expert, maybe I'm missing something obvious.
I already commented but have another way to think about this.
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.
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.
I think the key conceptual thing that's difficult to grasp is the propellor's interaction with 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.
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.
Actually, knowing it works now. Even internal combustion engines have only a 40% energy efficiency in converting gas to power, the rest is lost as heat.
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.
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.
> Since the initial wind was enough to overcome the static friction
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?
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?
My first thought about it was that it's awesome. The sad thing is that my second thought about it was that if somehow physics became political, this guy would be banned from Youtube for "physical misinformation" - after disagreeing with a renown expert in the field - and maybe we would never found out about it.
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?
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?
> 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?
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?
The article (and wiki page linked in another comment) seem to indicate that this vehicle design already proved it can go faster than the wind over 10 years ago.
Odd that the professor was so keen on betting in that case.
Odd that the professor was so keen on betting in that case.
The professor's reasoning is near the start of the video about the bet [1]. Basically they thought the makers of blackbird were fooling themselves (e.g. measuring slower wind at the bottom of the car vs the top of the propeller, or getting pushed above the average wind speed by gusts then selectively reporting).
1: https://www.youtube.com/watch?v=yCsgoLc_fzI
1: https://www.youtube.com/watch?v=yCsgoLc_fzI
$10.000 got donated and a lot of publicity created. I think it is just good marketing. Dunno if it is worth 10k but it got on HN a few times, so it undoubtedly works.
And the topic itself is super interesting!
Sailing faster than the apparent wind[0] has been going on for a while, especially on v.high performance land and water yachts. [1]
The physics involved still twist my brain, but there's nothing terribly new here.
[0] https://en.wikipedia.org/wiki/Apparent_wind
[1] https://en.wikipedia.org/wiki/High-performance_sailing
The physics involved still twist my brain, but there's nothing terribly new here.
[0] https://en.wikipedia.org/wiki/Apparent_wind
[1] https://en.wikipedia.org/wiki/High-performance_sailing
No, the issue in question here is sustained sailing faster than the wind directly downwind, which is not what water yachts and sailboats are doing. The connection (belt, gear) between the wheels and the propeller is necessary to do the former.
Yeah. Two boats could sail faster than the wind such that their center of mass is moving directly downwind faster than the wind.
This is not what they are talking about. It's always step one to get past to understand what's going on here.
I don't believe DDWFTTW has been achieved on water.
I don't believe DDWFTTW has been achieved on water.
You are mistaken. It has been achieved, and the fastest race boats today do it routinely. This is one of the reasons you don't see spinnakers on boats like the America's Cup AC72s, AC50s, F50s, huge open class trimarans, etc.
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.
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.
I think we just have different definitions. It's about the "Directly"
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.
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.
I know what you meant. You are mistaken. Look at the 180 degree data on the polars for AC72 or Bank Populaire. They're more efficient at a broad reach, but they can do it a few knots faster than the wind.
Why does the Blackbird use a propeller, why not a simple high-performance land sail boat like the Greenbird, a design that fits with the original idea from the sailing community?
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.
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 sails on those sailboats act like a wing even downwind . That's only usable with racing boats which use light foils fly over water instead of heavy keels , but yes they can move faster than the wind . Curiously i can't find a good explanation of their design.
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
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
You are agreeing current sail boats can go DDWFTTW which is directly downwind not a degree either way, faster than the wind, for a indefinite period of time? Tacking is not allowed, since that's not directly downwind.
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
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
I dont know much about sailboats but i think they mean this: https://en.wikipedia.org/wiki/Wingsail . when they are at a slight angle and in combination with lightweight foils pointing the other way it seems they are able to go faster than wind in any direction, without anything like a propeller. In any case i think talking mixing wings, sailboats and propellers makes it confusing to understand what the blackbird does
You can't go DDWFTTW with sails.
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. )
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. )
There some speed graphs like this online:
https://www.catsailingnews.com/2013/07/ac72-polars-penalties...
I think what you re saying is that you can't be at 0 degrees to the wind, which i guess is true
I think what you re saying is that you can't be at 0 degrees to the wind, which i guess is true
If you believe that sailboats can beat a drifting balloon downwind by zigzagging, then there is a very good explanation in the original Veritasium video that shows that this is essentially the same thing. Here's a link to the section of the video with that explanation [1]. Watch about 2 or 3 minutes starting there and it becomes clear.
[1] https://youtu.be/jyQwgBAaBag?t=404
[1] https://youtu.be/jyQwgBAaBag?t=404
The thing I wonder about stuff like this is that a phenomenon we see in one domain will typically transfer to another domain. For example, the 'hydraulic analogy' [1] for electronic circuits is pretty damn good at least on a macroscopic level. I wonder if there are applications of this phenomenon in other types of physical systems (including electronics).
This seems to be highly domain specific though. It’s reading to me like an example of what you can do when you have highly advanced knowledge of a domain to the point where you can do stuff that looks like magic, specifically because it apparently violates generalised principles established across domains.
Acoustics relies on an analogy to electronic circuit components.
I find it extremely counterintuitive, especially this part:
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...
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...
I posted an explanation for the downwind version above, but here's the gist of why all this is possible:
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.
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.
The key thing to understand this is that it isn’t using a turbine blown by the wind, but rather a propeller that blows the air backwards, with the propeller being driven by the wheels connected to the ground.
As noted elsethread, planes can extract energy from slower tail winds.
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.
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.
Is this similar to sailing? Where the lift generated by wind passing over the curved sail or prop blade produces more lift allowing the boat or car to go faster than the wind speed.
Yes. If I remember right, the angle of attack of the propeller blades when this thing is at speed ends up being about the same as that of the sail when the boat's on a beam reach.
To extend the analogy further, I think that the wheel-and-chain mechanism ends up functioning analogously to the boat's keel.
To extend the analogy further, I think that the wheel-and-chain mechanism ends up functioning analogously to the boat's keel.
Yes, that's where the creator got the idea I believe. He was imagining two sails rotating around a cylinder, which it turns out, is a propeller.
Is there an equivalent for water/hydrofoils maybe?
I think in his original video he described how a tacking boat can go faster than one going straight down wind. As I understand it, that was the inspiration for the propeller. Here is a good site explaining it:
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.
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.
"The physics of a push-me pull-you boat"
http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf
http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf
I never had a good experience with my Joby GorillaPod. All the plastic cup pieces developed cracks and the friction just wasn't there. The structural contraption holding up the propeller looks shotty as well. I'm pretty sure Joby Energy and Joby Photographic equipment are separate entities but that name Joby doesn't instill any confidence.
This is like putting a turbocharger on a sailboat. I love it.
A turbo charger increases fuel consumption though. You can’t just consume more wind.
In this case they are actually "consuming" more wind. A normal sail "consumes" wind by taking moving wind and reducing its speed to 0. This "consumes" wind by taking moving wind and sending it back the other direction, so it creates more thrust per unit wind.
Sounds to me like it improves efficiency rather than consumes more
Yep, I thought about it a bit more and you're totally right.
This was posted about a week ago: https://news.ycombinator.com/item?id=28010133
Very bad article. Watch the 2 videos instead.
Well, 3 https://youtu.be/VUgajGv4Aok Xyla Foxlin's video has the information about the proper ratio of wheel to propeller to make it all work.
Now that this thread is not really active anymore I'd like to comment that Business Insider is the record holder of the year for Surveillance Capitalism Sites for me, with 24 trackers present in this article (after I disabled all the tracking I could in the popup dialog, no less).
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Any sailboat?
"I offered him another 10,000 bet" is such bullcrap though.
For the guys making the vid this is win win due to youtube ads. Kusenko isn't exactly taking a cut from that.
For the guys making the vid this is win win due to youtube ads. Kusenko isn't exactly taking a cut from that.
It's one of the top YouTube channels, they make money off YouTube ads constantly whether or not the professor wants to double down or not. The idea was to point out Kusenko knew it wasn't just a technicality that lost him the original bet even if he didn't want to admit it when conceding.
This is slightly off topic but I have been using this trick lately to try to determine which part of a news article is likely to be BS: read the article in the movie announcer voice (you know the one: “in a world gone mad, one man…”). Whatever headlines or parts of the article sound boring are likely to be legit and whatever parts sound like they would fit in an action movie are likely overblown, underreseaeched, irrelevant, etc. Try it with this passage:
> 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.
> 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.
In this case, your BS detector is giving you a false result: https://youtu.be/yCsgoLc_fzI?t=275
Thanks for the data point!
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.
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.
Science's Biggest Names! It got a chuckle out of me while reading.
I mean they're not entirely wrong in the sense that if you asked a bunch of Americans to name some currently active 'scientists' then those names would almost certainly be at the top of list (I'd guess that those two plus Anthony Fauci would make up the top 3).