That's a terrible explanation - no wonder people doubt the system would work!
See sailboats faster than wind
https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
> The America’s Cup sailboats are sleek and fast. The AC72, the type of catamaran used in this year’s race, can travel almost three times the speed of the prevailing wind. On June 18th Emirates Team New Zealand recorded a speed of 50.8 mph (44.1 knots), with a wind speed of about 18 mph (15.6 knots).
When sailing directly in line with the wind, the apparent wind in a sail is equal to the actual wind, so there’s no gain. A propellor reshapes the direction of the apparent wind to be in the same direction of the wind.
More interesting is that blackbird can go 2 times as fast as the wind running directly_into_ the wind. That ought to help understand how this is possible.
So instead of sailing sideways so that your air foil sail can move sideways through the wind, the Blackbird harnesses the power of sales moving sideways through the wind while pushing the craft directly into the wind.
It all works the same downwind, and the crazy fast boats like the AC72s do in fact sail faster than the wind when running away from the wind.
Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
It is extremely enigmatic and curious that the engine can work to synergistically blow air into the atmosphere in one direction and resist the inertia of the earth in the other direction with the same force, without expending any energy reserves in the engine. Ive not seen a model or comment or article which fully illuminates the situation, although it is well demonstrated. Perhaps there is not language or a familiar concept which yet suffices. A sane and experienced physicist lost 10k bet on its plausibility.
This is how I explain it in my head:
1. When going slower than the wind, the wind pushes you forward. Propeller at this point is useless.
2. When going as fast as the wind, the wheels will drive a propeller that pushes wind back (same as an airplane)
3. When going faster than the wind, all your energy will come from the turning wheels.
But why do the wheels keep turning in this scenario? Is there somehow a net-force on the back-side of the vehicle from the wind?
IMO it is just positive feedback: the faster you go the more wind is being pushed back by the propeller. At some point the system reaches a state of saturation or equilibrium (the wheels turning no longer can speed up the propeller enough to generate more push). This equilibrium is going back and forth between phases 2 and 3 as you described it.
The thing that makes it a little bit hard to reason about is that all three components (wind, wheel, propeller) are interdependent and influence each other in a more or less fluid way. To me it feels similar like reasoning about active filter topologies.
After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed.
But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.
A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else.
I think that what this all adds up to is that, by introducing this mechanical linkage, the cart is extracting energy from the difference between the wind speed and the ground speed. This runs counter to our intuition, which assumes that it should be getting its energy from the difference between the wind's speed and its own speed.
That's what allows it to go faster than the wind. If it were based off the difference between the wind speed and the cart speed, then the forward force would go to zero as the cart's speed approaches the wind speed. But the difference between the wind speed and the ground speed is not related to how fast the cart itself is moving. So the forward force on it doesn't disappear as it approaches the speed of the wind, and instead it will continue to accelerate until the forward force balances with drag and rolling friction.
A kite might be a good starting intuition pump here? Kites generally don't do much of anything useful unless they're mechanically tethered to the ground.
From there move on to sailboats. The reason why this cart can go downwind faster than the wind isn't all that far off from the reason a sailboat can sail into the wind (albeit not directly into the wind), but, critically, only if it has a keel or centerboard.
(Edit: Another detail to point out is that the linkage causes the propeller to turn in the opposite direction it would if it were spinning freely. And remember that Newton's third law works in both directions at the same time.)
(Edit again -- Another observation that might help change one's intuition is that, while the cart as a whole may be moving relative to the ground, the part of the cart that's currently touching the ground at any given moment is more-or-less stationary with respect to the ground. And is also mechanically linked to the propeller.)
I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well.
the closer you get to perpendicular to wind, the faster you can go.
I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x.
__edit__
oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could
I blasted through this video pretty quickly, but it appears to explain things well, including how it's possible for a sailboat to exceed wind speed: https://www.youtube.com/watch?v=jJtvGF8vZbE
The cart adds some moving parts, but I am pretty sure that all they're doing is ensuring that the blades' angle of attack is about the same as it would be for a sailboat's sail when it's sailing into the wind, by turning the propeller at an appropriate speed.
The first time comes from the sail, you divide the wind into a component flowing along the sail (doing nothing), and one that is perpendicular to it (pushing it)
Then you take the perpendicular-to-the-sail part and divide that into a side-ways (drift) and a forwards component (propulsive). The side ways one is neutralized by the keel.
Btw I kinda wonder if a rotatable keel tuned just right would allow sailing right into the wind...
The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind.
This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that.
People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.
The propeller arrangement is a glider that is being pushed by the wind. The glider is tethered to a cart whose wheels in-turn drives the propeller to become a "powered" glider. Without a tether, the relative speed of the glider with respect to the wind would be zero. With the tether powering the propeller, the relative speed of the glider is greater than zero, which makes the cart go faster than the wind.
Thinking about the "glider" and the tether as two separate systems makes more intuitive sense, imo.
Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?
It looks quite aerodynamic, but by comparison I can start pushing a 3,000lb car on flat level ground. So this thing probably needs less than 10 pounds of force to start moving.
The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.
I can get a boat that weighs more and has less sail area going on less wind than that. And that thing has to be dragged through the water on top of everything else.
Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)
If you think of a longitudinal vertical plane you can calculate the speed of a point on the cross-section of the blade moving backward relative to the vehicle as a function of the rotation speed. As the vehicle goes faster, so too does the prop surface 'move' backward.
I would love a diagram showing how the power flows in this system.
Some well built boats can generate pull (instead of push) in the sail, as if it was a plane wing. The air passing on the convex (forward) side travels faster than the air on the concave, making the pressure lower on the forward side and pulling the boat, and you feel again the wind but against your face (as if you were running forward). It's a bit counterintuitive, because what your eyes see is the sail pushing the boat. So it's not perpetual motion, because the wind needs to keep blowing or the boat slows again. What slows the boat is the friction boat-vs-water.
In the case of the car above, the wind needs to reach the point of wind speed == car speed, and beyond that the wheels keeps rotating the "sails", and thus keeping the pull. If the wind stops, the car can keep going for some time, but as the speed of the wind through the sails is lower, the pull is also lower and at some point it stops. But in this case, the friction is wheels-vs-ground, much lower than the boat-vs-water above, so the car can keep going (same what happens with a bike when you stop pedaling in a flat surface, the bike can keep going for long, and even a faint pedaling or tail winds keep the thing going). This car is a clever construction to keep the wind at high speeds through the blades.
This is how nearly all sailing works. Excepting the case of sailing directly downwind, the curve of the sail works like a wing, and the keel provides something to "push" against - the effective "lift" (if it were a wing) drives you forward.
This is why you can sail into the wind at all...
Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind.
This isn't perpetual motion, though, because eventually the thrust generated by the propeller is insufficient to overcome the additional drag created by the vehicle's faster speed.
The truth is that the car is always getting its energy from the wind, and not expending any of its own. It's just that it's been set up in a clever way that breaks our intuition. We tend to want to mentally simplify the propeller down to a disk whose motion vector is the same as that of the car. But here you've really got to think about the rotation of the propeller and the helical path its blades travel along. The wind is interacting with surfaces whose momentary motion relative to it at any given moment in time are very poorly modeled by the disk abstraction.
EDIT: I think it's clicking. It's because the air and the ground are moving relative to the craft at different speeds. So if the ground and air were still, there would be no speed difference to draw energy from.
Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Suppose the vehicle is facing the +x direction. Suppose the wind is travelling at +10m/s relative to the ground. Suppose the (experimentally determined) maximum velocity of the vehicle under these conditions is +12m/s. Now consider an inertial reference frame which, compared to the ground's reference frame, is moving at +11m/s. (I.e., pick the frame in which the ground moves at -11m/s.) Now let the vehicle run. There will come an instant when the vehicle is stationary in this reference frame (when the vehicle has accelerated to +11m/s relative to the ground, not yet having reached its maximum velocity).
Consider the instantaneous change in kinetic energy at this instant. The air about to be pushed by the propeller has velocity -1m/s and will accelerate in the -x direction. The vehicle has velocity 0m/s and will accelerate in the +x direction. In both cases, the kinetic energy is actually increasing! So in this frame, where does the increasing kinetic energy of the air-vehicle system come from?!
The answer is the ground. It has velocity -11m/s. When it pushes against the contact point of the wheels, it is therefore pushing the contact point in the -x direction. But this means the contact point is pushing against the ground in the +x direction. And therefore the ground is undergoing a minuscule +x acceleration. That acceleration is therefore decreasing the kinetic energy of the ground — by a lot, because of the mass factor.
And this is why we say the energy is derived from the difference in velocity between the ground and the wind — because depending on your inertial reference frame, the kinetic energy might be coming from one or the other. In the frame of reference of the vehicle, it starts out coming from both, but when the (relative) direction of the wind shifts (so that relative to the ground the vehicle is travelling faster than the wind), the kinetic energy starts coming from the ground only, and the vehicle is forced to transmit some of it to the air to keep moving. But this might actually be the nicest explanation of how the vehicle outpaces the wind: in the faster-than-the-wind regime, the vehicle is transmitting energy from the ground into the air — which is exactly what you would expect to happen, given the mechanical linkage between the wheels and the propeller! — but some of this energy bleeds off into increasing the velocity of the vehicle itself, because it's on wheels and that's what happens when wheeled vehicles push against something behind them, no matter what the ground happens to be doing (such as moving backwards like a treadmill, in this frame of reference).
Disclaimer: I did not do well in college-level Mechanics. But I think I have convinced myself of the above explanation, just barely.
To go faster than the wind you just have to be a bit clever in how you capture that available energy.
Imagine a lever with the fulcrum on one end, attached to the ground. Wind pushes on the middle, and the car is at the other end of the lever. So wind exerts more force with less speed, and the car moves forward with more speed but less force.
That's basically what the linkage and gear ratio between the wheels and propeller do; it's the counterintuitive power flow that makes it so confusing...
https://youtu.be/VUgajGv4Aok?t=432
The essential 'gear' ratio is between forward motion of the vehicle and the propeller pitch (essentially how far the prop would screw through a tub of jello for a given amount of rotation). The vehicle has to move *faster* on the ground than the prop does through the air for this to work at all, and it sounds like it should be roughly twice as fast for it to work on something that is reasonably simple to engineer. So if your prop pitch is 5", your wheel size and gearing should allow the vehicle to move forward approximately 10" for each rotation of the prop.
(Note that her video does not explain 'why' this is the case, just that it is a phenomenon that the Blackbird inventors had determined/discovered and is an essential design criteria).
Assuming you're happy that, so long as the wheels are low-friction, that's no difference between "Downwind faster than Wind" and Upwind, and that a single design of craft can do both Downwind and Upwind, then all you need for the "dead air" part is to store energy in a battery, spring or flywheel.
I'm sorry but you're either answering a different question than "where do the wheels get their energy from" or extremely confused.
We're on a flat plane normal to the direction gravity acts, so I'm going to let you go out and work out how much energy gravity can provide there.
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.
This article sucked by the way. It said "Any sailor worth their salt can tell you that a boat can travel faster than the wind by cutting zigzag patterns; that's called tacking."
Any sailor worth their salt knows that is not true at all. This practically destroys any credibility on this topic by this author.
You sure? Take a gander at this article https://en.wikipedia.org/wiki/High-performance_sailing
You’re right in the sense that tacking is not the word for sailing faster than wind, but it is true that sailors can tack faster than the apparent wind, and that professional sailors know that. The “apparent” part is an important part of the discussion that the quote didn’t include or clarify, but would be implicitly understood by the saltiest of them.
If you watch the videos when the thing is first getting going, it is slow enough to see that the propeller is actually spinning in the opposite direction it would be if it was acting as a windmill.
Think of the wheels as a power bank: It convert and temporarily store linear force to rotational force. As long as it accumulated enough power to counter the rolling resistance, headwind drag, gravity, gearing loss, etc; the excess rotational force can be used to spin the prop. Which in turn generate additional push to be converted and stored.
Within this "loop", the vehicle got two linear input, tailwind and propeller. This will allow continuous acceleration once the vehicle goes faster than tailwind up to the point where the additional force from the prop is completely negated.
The reason the wheels are able to turn the propeller without slowing the vehicle down is that the speed of the wheels over the ground is greater than the speed of the propeller through the air thanks to the tailwind.
https://youtu.be/VUgajGv4Aok?t=432
In short the working model she built has approximately a 2:1 ratio between forward wheel motion and propeller pitch. IOW if the propeller pitch is 5" per revolution, the wheels will require 10" of forward travel to spin it one time.