Going Faster Than the Wind In a Wind-Powered Cart 315
Shawnconna writes "Can a wind cart travel faster than the wind? A group of makers say, 'Yes!' Make: Online has published a story about the Blackbird wind cart that just set a record. This is a follow-up to an earlier story in which Charles Platt built a cart based on a viral video where a guy claimed he'd built a wind-powered vehicle that could travel downwind faster than the windspeed. Charles built one and said it didn't work. Heated debates broke out in forums, on BB, and elsewhere on the Net. In the ensuing time, a number of people have built carts and claimed success, most principally, Rick Cavallaro. He got funding from Google and JOBY to build and test a human-piloted cart. They claim success, with multiple sensor systems on board, impartial judges and experts in attendance."
Two words (Score:2, Funny)
TACO Bell
store and release energy? (Score:3, Insightful)
I probably am just thinking about this too simply, but can't something go faster than the wind if it stores some of that energy and uses it later?
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That's not the point. They show that they can go straight downwind (i.e. in the same direction as the wind) faster than the wind with nothing but the wind at the same time for the energy source.
Re:store and release energy? (Score:5, Informative)
The reason this works is that the propeller is able to "push off" against the tail wind. Think of it like sitting on a skateboard and pushing off from a moving wall behind you with your arm. The difficulty in making it work is that you need very little drag and a very efficient propeller. But the energy equations for traveling faster than the wind do balance and there is no violation of energy conservation.
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Actually, the opposite is the case: The propeller is used to take energy from the wind, which is then used to drive the wheels and move the vehicle forward. This is most easily seen if looking at it on its own frame of reference. At stationary speed the wind comes from the front (because it's moving faster than the wind), while the road underneath goes backwards. The propeller takes energy from that wind and uses that energy to drive the wheels, which then keep the vehicle in place, against the forces of th
Re:store and release energy? (Score:5, Informative)
Nope, you've got it backwards, the GP got it right, and this is absolutely the key to understanding how this works.
The car isn't using the propeller as a turbine as a source of energy to power the wheels. That, indeed, would be impossible, because once you reach wind speed the force exerted on the propeller is zero.
Instead, it works the other way around, as a fan to push air backwards and accelerate the car. The energy is transfered from the wheels to the fan.
Assume that, to begin with, the car is moving at wind speed. The wheels are spinning (because the car is moving) and you can use that energy (i.e. brake the car) to push the propeller. The propeller blows air backwards, which propels the car forwards. If your mechanism is efficient enough, that push more than counteracts the braking action on the wheels and the car actually accelerates forwards. As it accelerates, the efficiency drops and it eventually stabilizes at some speed, faster than the wind.
Now everyone is shouting "Perpetual motion! You're producing more energy with the fan than you're getting out of the wheels!". Nope. That's the final bit. Let's say that wind speed is 10km/h. If the car is moving at 11km/h (faster than the wind), then the motion on the wheels relative to the ground is 11km/h. However, the fan only has to push air backwards at 1km/h, as the wind is doing the rest and providing the base 10km/h of forward motion. This difference in velocity is what offsets the inevitable energy losses: the ground speed is whatever you're generating with the fan plus the velocity of the wind "for free". This "free velocity" goes down (as a fraction of total velocity) as you accelerate, until it matches the (in)efficiency of the system (energy loss), and this is the stable velocity that the car achieves, faster than the wind.
This really isn't an issue with perpetual motion. It's easy to see that you could use a stationary turbine to generate (say, electric) power from the wind, and then use that power to accelerate a car (say, powered by a laser, so it is not tethered) in a different (windless) location faster than the original wind. Output velocity can be greater than input velocity. The difficulty lies in grasping the interesting mechanics and interactions of the downwind-faster-than-the-wind car uses to achieve this within the original wind itself. It's a mechanics puzzle, not an energy conservation puzzle. Another way to look at it is that the energy lies in the difference between the velocity of the wind and the ground, and the car always has access to both of these moving entities via friction (friction with the wind, and friction of the wheels with the ground), and thus can harness that power regardless of what its own velocity is.
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Instead, it works the other way around, as a fan to push air backwards and accelerate the car.
Like any good airfoil, it both pushes behind and pulls forward at the same time.
One simple test the author did for a small rig was to use a treadmill to represent the forward motion from the tailwind. If the propeller is able to spin fast enough to pull the cart forward on the treadmill with no wind, then it can do the same when the treadmill is replaced by a tailwind.
The treadmill test works, so the large-scale test should work as well. To prove it they built the cart and had the NALSA [nalsa.org] run the test.
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That actually depends entirely on which frame of reference you choose to view the problem in. It is therefore both subjective and inaccurate.
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So the bottom line is that it's possible to capture enough energy from the wind to move faster than the wind, but you have to find a way to keep receiving that energy when you exceed windspeed. The mechanism satisfies this by capturing energy based on the ground speed rather than the wind speed.
So here's a question for everyone: could you make it work in a boat? The analogue would be to have the water propel a screw that turns the propellor, which sounds possible, but the efficiency equations might not be w
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Well said. I think it should be possible to gain some amount of velocity greater than wind speed on a sailboat with an added fan connected to an underwater turbine, but I'm not sure if the fractional speed gained will be useful. It would be a very cool demo to try, though, and even achieving 5% over wind speed would be very interesting.
I think the main problem with a boat is that you have a massive amount of friction with the ground (the water), while on a car the axle friction of the wheels can be made ver
Re:store and release energy? (Score:4, Informative)
So here's a question for everyone: could you make it work in a boat?
Yes. [boatdesign.net]
Re: (Score:3, Interesting)
This is what I first thought. But you are thinking of speeds gained in cross-wind situations, where sail-powered crafts easily travel faster than the wind speed.
This is faster than wind-speed in downwind situations, "spinnaker legs", in other words. Took me a few minutes to get my head around the physics, but the concept is simple once you have the idea. The grandparent is a very good, if a little long-winded (oh groan) explanation.
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Even confused a former science fiction writer and current Makezine author into making the following statement:
"I have never denied that a vehicle may be designed that will move into a headwind if the propeller is geared appropriately. What I do not believe is that this vehicle can start from rest with the wind behind it, accelerate until it is moving at the same speed as the wind, and then continue to accelerate faster than the wind, i.e. into a net headwind, without any fluctuations in wind speed, and without any gear-shifting along the way. That is what the original video from Florida purported to demonstrate, and is where all the arguments began. I have always suspected that the Florida video was faked.
I know very little about Rick Cavallaro's cart, and am not very interested, partly because Rick has been extremely abusive, obnoxious, and condescending to me, and partly because, as I say above, I am quite willing to believe that his vehicle can move into a headwind. Indeed, the very primitive cart that I built for my original MAKE article did succeed in edging forward into a strong blast from a large fan. Again, what I do not believe is that his vehicle or any other can start with a steady wind behind it, accelerate to a speed equal to that of the wind, and then continue to accelerate so that it is moving faster than the wind, in one uninterrupted process.
I have repeated myself in an effort to make this clear. -- Charles Platt"
He does not acknowledge the existence of this video:
http://www.fasterthanthewind.org/2010/07/video-from-richard-jenkins-world-land.html [fasterthanthewind.org]
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Assume that, to begin with, the car is moving at wind speed. The wheels are spinning (because the car is moving) and you can use that energy (i.e. brake the car) to push the propeller
No, you can't use that energy to move forward. The experiment boils down to two mediums moving relative to each other (we can choose to tie the coordinate system to the ground, to the wind, or at any constant speed). If you are stationary relative to one of the mediums, you can only use the other medium to change your velocity. You can only use it to change your velocity in the direction of the movement of that medium.
Imagine two opposing winds with between them, moving at the speed of wind1 (in its direc
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First lets imagine zero wind, the car traveling 100 mph, and lets assume that all components of the car operate with 100% efficiency.
We tie a perfect generator to the wheels to extract 100 watts of power. Conservation of energy says it will apply a force slowing the car down. We pipe the 100 watts running a perfect propeller. It applies a force speeding the car up. If all components are perfect, conservation of energy says the car will go at a constant 100 mph forever. Perfect balance.
Now lets break that pe
SImpler; just what sailboats do (Score:2)
More specifically, this works because the "propeller" (rotating sail, really (*)) goes slower than the wind, relative to it. And achieves this by exploiting the resistance of surface (the difference in speed between it and the wind) - just like sailboats do when tackling. (*)In their case the resistance allowing the sail to move sideways comes from the keel & water; here it comes from wheels and ground - and the sail also moves sideways! (relative to the wind, all that matters; don't let the propeller-l
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Oh, this is a much nicer way of understanding what's going on.
Though... you could also think of the sails in a tacking sailboat as being able to extract more energy from the air because they can bring the air to a full stop relative to the water underneath the boat. The keel is necessary to keep the air from pushing the boat sideways instead of coming to a stop.
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There's no "bringing the air to full stop" anywhere... (except when in a boat traveling directly downwind, sort of - but that's actually not the most efficient scenario, and one where keel isn't involved much)
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The blades of the "propeller" (rotating sail) move sideways.
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What you again fail to understand - the sail moves sideways (sure, a quite specific case of sideways - it rotates; but there is no difference from the perspective of the wind)
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Think of it like sitting on a skateboard and pushing off from a moving wall behind you with your arm.
I'm sorry, but my Slash dot number is too low to understand this skateboard analogy thing...
Could you rephrase that as a Car Analogy please?
K, thanks, bye.
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Here is what bothers me about this whole thing.
Although I believe it is theoretically possible, there is a certain whiff of woo about the experimenters. I'm not even saying they didn't achieve their objective--I'm just saying there are a couple of things about the experiment, especially with regard to the stored energy issue, that nearly broke my woo-meter.
From the official rules [nalsa.org]:
Energy shall not be accumulated and later used for propulsion of the yacht or to operate the controls of the yacht.
It seems to me that this would preclude the use of massive windmills (i.e., flywheels), such as the one on the craft. Later, th
Re:store and release energy? (Score:4, Informative)
It's simple: the vehicle must be able to move forwards faster than the wind forever, as long as the wind keeps blowing. In other words, the energy stored in the moving parts must not decrease and eventually cause it to stop working. Or in yet other words: the system must achieve a steady state where energy is flowing in and out at a constant rate, while traveling faster than the wind.
For a race where time matters, energy input initially into the system is relevant. However, for the purposes of proving that DWFTTW is possible, it isn't. Any amount of energy added initially will by necessity be dissipated in the friction losses of the system - you can't run a car forever on a fixed amount of energy. If it can, in fact, run forever on a steady wind, then you can discount any initially applied or stored energy, and conclude that it is being powered solely by the wind. If it does that while going faster than the wind, then you can conclude that DWFTTW is possible.
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Here is what bothers me about this whole thing.
Although I believe it is theoretically possible, there is a certain whiff of woo about the experimenters. I'm not even saying they didn't achieve their objective--I'm just saying there are a couple of things about the experiment, especially with regard to the stored energy issue, that nearly broke my woo-meter.
From the official rules [nalsa.org]:
Energy shall not be accumulated and later used for propulsion of the yacht or to operate the controls of the yacht.
It seems to me that this would preclude the use of massive windmills (i.e., flywheels), such as the one on the craft. Later, the rules specifically prohibit flywheels:
It is not permissible to use stored energy to propel the yacht or operate its controls. This might includes things like compressed gas, stressed springs, batteries, capacitors and flywheels. This includes energy stored before a run or during a run. No pumps, generators or mechanical devices that are intended in part or whole to provide energy to storage devices are permitted. Stored energy in the form of momentum of the yacht, its wheels or other **normally moving** or flexing parts of the yacht is allowed. These forms of stored energy are inherent in the operation of the yacht and either do not add energy useful for increasing the speed of the yacht or **do so in a trivial way**.
(emphasis mine)
What constitutes a "normally moving" part of the yacht? What constitutes a "trivial" use of stored energy to increase its speed?
That's I thought to, but here is how it works with laymen terms for all (including myself). The vehicle is simply geared in a way that the propeller will move air from the front of the vehicle to the back faster than the air moves from the front to the back. So if the vehicle is moving 5 mph forward, the propeller is trying to force air in the same direction at 10 mph. What happens is, as the vehicle is being pushed by the wind, the vehicle moving forward is building kinetic energy. Eventually the vehicle
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.
The vehicle accelerates to a a speed faster than the wind, then stays at that speed forever (as long as the speed of the wind is constant) and does not oscillate. It really will get you to your destination faster than e.g. a balloon traveling at precisely the speed of the wind.
There is a feedback loop, but it works like this: there is a wind velocity X, and a stable ve
Re:store and release energy? (Score:4, Informative)
You do realize that NALSA certified it, right? As in they checked all these things?
They installed a bracket on the shaft to ensure the propeller never drives the wheels, so all the momentum of the propeller is going to be able to do is allow the propeller to continue spinning. It never, ever, drives the wheels.
Anyway, it's way beyond theoretical. The current land sailing speed record is 3.15 times wind speed 126mph in a 40mph crosswind, fast!), set with a traditional land sail in a crosswind. It was set the same day NALSA certified the first DDWFTTW record.
Here's the explanation of the physics: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind [wikipedia.org]
Re: (Score:3, Interesting)
If you look at Rick Cavallero's replies to posts here [makezine.com], you'll see he directly answers that question, clarifying that there is a ratchet to prevent the propeller from directly turning the wheels (i.e. only the wheels can turn the propeller). This was how they proved to NALSA that they were not using stored energy from the propeller as a flywheel to accelerate the vehicle.
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If these device will accelerate to and beyond the average velocity of the volume of gas it sits in, what prevents it from accelerating from a dead stop in a still volume of air?
This really sounds like a perpetual motion scheme.
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So you didn't read the article or watch the video either.....
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Yes there is: the wheels are rotating. If the cart were a balloon, you'd be correct. However, the cart has contact with the ground, and can extract energy from that interface. It uses that energy to rotate the fan, which increases the apparent wind speed on the fan's blades and, thus, propels the cart forwards faster than the external wind.
Yes, these carts do in fact work when they are traveling at precisely the same direction as the wind, and can
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Congratulations, you just invented a perpetual motion machine!
In the example described in the article, the author overlooks one huge fact -- the treadmill is a source of energy, so assuming that a treadmill in a room w
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In the example described in the article, the author overlooks one huge fact -- the treadmill is a source of energy, so assuming that a treadmill in a room with no wind is equivalent to traveling over a road with a wind from behind is fundamentally flawed.
What he really overlooks is the fact that no matter how fast the vehicle moves on a treadmill, the wind will always be faster than the vehicle. This doesn't happen in reality, since eventually the vehicle will catch up with the wind.
Re:store and release energy? (Score:4, Informative)
On a treadmill, if the vehicle is moving forward (relative to the observer, not the treadmill belt), then it is moving faster than the wind (which is moving at velocity zero relative to the observer). It is simply a change of frame of reference. If you place the observer on the treadmill's belt, then the wind is blowing forwards as fast as the outside world is moving forwards, and the vehicle is moving forwards faster than that. On the flip side, if you take the real-world DWFTTW vehicle example, and place the observer on a balloon moving with the wind, then (just as in the treadmill scenario) the wind is moving at zero velocity relative to you, the ground is moving backwards (just like a giant treadmill), and the vehicle is moving forwards faster than you (just like in the treadmill example the car moves forwards relative to an outside observer, even though the treadmill moves backwards).
To answer the GP, see my post above. Everyone (including myself at first) immediately assumes this is a turbine-powered car using a wind turbine to drive the wheels. That's backwards, it's a sailcar (simply pushed by the wind) which in addition to that uses the wheels as generators to drive a fan (not a turbine) to push air backwards and increase thrust, thus actually achieving faster than wind speed.
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But the ground ISN'T moving, the car/wheels are. Someone draw me a free body diagram showing the forces acting on the vehicle allowing it to accelerate to a speed faster than the wind and I'll believe it when the forces don't sum to zero at Vcar = Vwind.
Duh? (Score:3, Interesting)
If sailboats can travel faster than the wind, of course wind-powered carts can.
Re:Duh? (Score:5, Funny)
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Re:Duh? (Score:5, Informative)
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The mechanical connection to the wheels keeps the "propellers" turning ....
It's not the wind which makes energy extraction (and locomotion) possible, it's the speed difference between the wind and the ground ... as long as there is a speed difference between the ground and the wind you can push against the ground to get energy out of the wind.
How to do it is just a technicality.
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I just cannot see what keeps the propeller turning once the cart hits windspeed, as at that point the apparent wind would be 0.
The wheels keep the propeller turning, as they are connected to it.
Re:Duh? (Score:4, Informative)
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The article specifically mentions heading directly downwind.
With a propeller, which is an airfoil and works on the same principle as the sailboat's 45 degree trick. The reason sailboats can't do it directly downwind is because they can't create an airfoil directly downwind with sails, not because it's impossible to create an airfoil that works directly downwind (it's actually more logical for an airfoil to work directly downwind, rather than crosswind).
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The achievement here is going faster than the wind in the direction of the wind. This is something sailboats cannot do. Sailboats can only travel faster then the wind when they are at an angle to the wind (usually going against the wind).
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Well, it depends on the traffic, obviously.
It's funny you should say that because the first thing I noticed was the dust being generated by the vehicles alongside the contraption to film it; i.e. dust being raised by the vehicles alongside creating air currents that appear to be heading towards the contraption
Couldnt you add to this design (Score:2)
Couldnt you build something that oscillates a weight to speed this up?
Have the propeller pull a weight up a 90* triangle as the weight hits the top fold the propeller for increased aero dynamics, then release the weight which adds torque to the wheels.
Then have the triangle tilt to let the weight roll back to the initial position. When it hits the start position do it all over again?
This could add extra turbo boost to the car.
I'm pretty sure this can be done all without electric.
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You are just adding a complicated energy storage mechanism and then having the energy collection mechanism disable itself for part of the time. It would be slower.
You could get the car up to speed faster by having a sail that folded itself as soon as the amount of energy it was extracting dropped off. Maybe a triangle sail with the base of the triangle along the bed of the vehicle and the tip at the propellor axis. Then have it spring loaded in such a way that when wind was pushing into the sail it also
Dupe story (Score:2)
http://tech.slashdot.org/story/10/06/06/0518216/Google-Backed-Wind-Powered-Car-Goes-Faster-Than-the-Wind [slashdot.org]
I don't see much in this article that is new.
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It happens.
The reason that I don't believe it. (Score:2, Insightful)
The main reason nobody believes these clowns, is because they're not good at explaining how it works. I don't even see an attempt at it.
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E = (wind speed * prop thrust) - (cart drag * ground speed).
So if the energy required by the prop is less than E, the system works. You use the difference between cart speed - wind speed for the velocity of the air thru the prop.
Re:The reason that I don't believe it. (Score:5, Informative)
Here's an analysis performed by Mark Drela of MIT (http://web.mit.edu/aeroastro/people/drela.html)
http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf [boatdesign.net]
Re:if this guys from MIT, we should all give up no (Score:3, Informative)
Exactly what everyone wants to see, a mathematical proof. Of course if you look at his free body diagram and his second equation. You'll see that he has his force vector Fp going the wrong way.
Fp is pointing in the correct direction, you merely misinterpreted the meaning of it.
Ft is the drag force on the underwater turbine. It is a drag which tends to slow down the vehicle, but the important point is that we are actively drawing energy from it. And yes, it is preforming exactly the same function as the whee
I don't think you understand science (Score:5, Interesting)
It is not the job of engineers or gods to figure out the science. That is for the scientists. Apples fell from trees long before Newton thought about it.
The scientists can be skeptical, they can demand reproducible tests, but once the tests have been done it is THEIR job to find an explanation, NOT that of the engineers.
These guys build something, they opened themselves up to a lot of tests, so either you make some real accusations and not just "idiot slashdotter doesn't understand so it must be fake" or start to work out the math or just accept that you are an idiot along with everyone else and leave this to smarter people.
But they do NOT have to explain to you how it works, they got far smarter people to convince, not some random kiddie on the net.
Re:I don't think you understand science (Score:4, Informative)
You know, if you read the "fasterthanthewind" website, the story is that the math actually came first, and the first one of these vehicles was built later on, because -- guess what -- there were skeptics who refused to believe the math. In the modern era I'd argue it's rare for something to get invented without the physics having been done before that.
We learned today that Andrew Bauer passed on Sept 6. As our blog followers will recall, Andrew Bauer was not the original inventor of the concept, but did build the first successful DDWFTTW cart that anyone seems to know of. He did this to settle a friendly wager with colleague and notable aero engineer A.M.O. Smith in 1969. As we understand, the wager was based on a claim in a student's paper, written 20 years before, that DDWFTTW should in fact be possible. In some small way JB and I have tried to model ourselves after Andrew by doing the engineering and demonstrating the principle - rather than simply proving it on paper.
http://www.fasterthanthewind.org/ [fasterthanthewind.org]
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How about if you post your math showing that it cannot work?
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So you're saying the North American Land Sailing Association is in the business of rigging official land speed record tests eh?
For Christ's sake, land sails already go 2-3 times faster than the wind using the exact same principles used in these carts. This is not some kind of voodoo physics, it's simply maximizing the available energy.
I'll break it down for you, since you obviously didn't bother to read the article where they already explained it and since I'm such a nice guy:
At a dead stop, the propeller
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From what I saw, it's not unlikely that most of the people taken by this (possibly including at least some early explanations from the creators; I'm not sure, it's been a while) actually don't have very good understanding of what happens - which wouldn't be anything new. For thousands of years sailors and boatbuilders didn't have very precise understanding of how sails work, too (otherwise we would probably have at least gliders a lot sooner), most of them still don't.
But it basically just reshuffles the co
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. Until then, what am I supposed to believe? My gut instinct or my lying eyes?
Which pair of eyes are you planning to read the mathematics with?
old news (Score:2)
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Directly downwind. No windsurfer/sailboat/whatever can do that.
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So it should have been made much more clear that this is about down wind. OK. mentioned later, but who reads that far in a summery.
Summery what? I also didn't read far enough in the inane post above, and provided a not-so-inane comment from my point of view. Punish me as you see fit.
sail carts aren't practical transports (Score:2)
Yachts (I guess they are cailled sailboats in the USA) have been 'sailing faster than the wind' for some time. They have even been sailing 'against the wind'. However to get the best speed, they need to zig-zag a bit depending on the wind direction (Upwind its called tacking, downwind its called jibing).
In order to do this, they need to have a fair bit of room to manouver which they have at sea, but not so good for land navigation. You mostly see sail powered wind vessels on deserts or salt flats, they won'
L/D (Score:3, Informative)
At first blush you would say if the lift/drag ratio of the sail/wing/apparatus is > 1 (plus a bit for drag) then a wind vehicle can go faster then the absolute flow speed.
The complication is that the range of possible angles of attack you can achieve gets dictated to you by trigonometry. Example, if you are on a beam reach (traveling 90 deg to the prevailing wind) and your speed is equal to the prevailing wind, the apparent flow is rotated 45 deg fwd of abeam. Now, a typical wing might give you an L/D of 20 at something like 10 degres AoA, so you would set your wing (sail) at 55 degrees from abeam. Your lift vector would be 55+90+atan(1/20) ~ 148 degrees from abeam, or 58 degrees off your bow.
Well, that's forward of abeam (90 degrees off the bow), so you have a component of lift pushing forward. It's then just a matter of getting the drag of your superstructure and rolling components down low enough to make that component sufficient to accelerate you just a bit, whereupon you are going faster than the wind.
For a boat, the "rolling components" are another wing in the water (the keel) which imposes more trigonometric limitations that make it tricky but not impossible to achieve this. Normally if it is possible it happens on a broad reach. With rolling vehicles it should be easier.
I don't know why people argue about this.
I just made a mechanical demonstration (Score:2)
I can almost see how it works.. almost (Score:2)
We all accept that this vehicle derives it's motion through the rotation of the propeller which drives the wheels.
I think we all accept that the bigger a propeller being driven by the wind, the more energy you extract.
The more energy you have the faster you can drive the wheels.
Ok so far.. but, I just can't get my head around the fact that once the vehicle reaches (or even approaches) the same velocity as the wind, how there is any relative wind left to drive the propeller without reversing the blade pitch.
Re:Mythbusters (Score:4, Funny)
Something almost scientific that ends with an explosion?
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They don't do it directly downwind, however -- they do it at an angle to the wind. This guy says he's doing it directly downwind.
I'm skeptical of this claim -- though I'd like to see their analysis of why they say it works.
Re:Of course (Score:4, Informative)
Re:Of course (Score:5, Informative)
For the more visual people: http://www.youtube.com/watch?v=k-trDF8Yldc [youtube.com]
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+10000000 internets for you, this is the best description of what is going on yet.
+5 informative as well mods...
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it is possible, if what you do is to extract energy from the speed difference between the wind and the ground instead of that between the wind and the vehicle.
Yes, but sails and propellers require airflow and hence derive energy from the air that flows past them. When the propeller moves along at wind speed there is no airflow, and hence no energy to be derived. If the propeller is attached to the vehicle then the relevant base speed is that of the vehicle. The maximum speed will be where the diminished airflow is sufficient to overcome the rolling resistance. If the windmill is separate from the vehicle and energy is provided by some other means, such as a c
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The wheels spin the propeller to generate a forward pull, the propeller doesn't drive the wheels (acceleration would stop at 0 relative wind speed in that case).
Here's the physics: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind [wikipedia.org]
It's exactly the same as tacking from the propeller's perspective, but the propeller allows the cart to move directly downwind instead of at a crosswind.
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Sat things up so that the sail will be closed or parallel to the wind when on top of the wheel, and perpendicular to it when on the bottom
Kind of like... this?
http://en.wikipedia.org/wiki/File:Perpetuum1.png [wikipedia.org]
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If You use a propeller, to do the pushing, then Your airfoil is exactly at such an angle to the wind.
Yes, but when the entire device is moving at the same speed as the wind, then the relative wind speed is zero and you can't extract any further energy from that.
The only way I can see to make this work is to "cheat" somehow -- a string pulling the contraption, storing energy in a spring, a hidden motor, going downhill, a day of gusty wind where the wind was a lot stronger a few seconds ago than it is now, etc.
The video with the toy and the ruler is nice, but if that really explains it -- then we don't even
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...we obey the laws of thermodynamics.
I don't! :-)
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The usual approach is to dismiss the claimant as a crackpot. I was prepared to do exactly that, citing lack of energy or momentum conservation or a violation of Newton's Third Law but it doesn't obviously do any of those things.
The gearing mechanism results in a drag on the wheels but creates an opposing force by spinning the propeller. Does the drag have to be greater than the thrust in that situation? I don't see a clear reason why it would have to be; it would depend on the propeller geometry of course,
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So what happens if we put the wind-powered cart on a treadmill?
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It extracts energy from the fact the ground is moving faster than the air.
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I find it easier to think of speed, momentum, and energy flux relative to the moving thing. Call me Eulerian.
If you have a dragless thing moving through a fluid, it will continue to move at the same speed. Now put a device on it to extract energy from the fluid flowing past it. Call it a propeller if you must. A perfectly efficient device will create drag because it extracts ki
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If a boat had a paddle or propellor in the water to extract energy from its motion relative to the water it could use this to accomplish the same thing with a propellor above the water.
Propellors do not magically generate negative drag. They require energy input to do it. But that energy comes from the wheels interacting with the ground. But the fact they can extract energy this way is because the wind is moving faster than the ground.
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Don't be absurd. Lets say that the wind stopped completely. You are now coasting, and air-resistance/wheel-friction will slow you down, jut like with every other vehicle.
Momentum keeps you from coming to an instant stop, but you will stop.
The maximum speed of a vehicle with that design is roughly a fixed percentage of the wind speed, based on the exact design parameters (coefficients of friction, aerodynamic properties, etc). With the correct parameters you can get the percentage to be greater than 100%.
If
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So you are doing 1.1 times wind speed.
What is 1.1 times 0?
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And if that were how it had been accomplished, it would also technically not have been a successful faster-than-the-wind run.
The criteria for success were as follows:
Directly downwind, level surface, steady-state wind, and no energy storage.
To accomplish the first three, they picked a dry river-bed place known for steady winds (and therefore very popular with cart sailers). The last criteria was accomplished by a bracket that prevented any power from being transferred to the wheels by the propeller.
The way
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That would depend on the wind speed.
In reality this does not function that much different than a boat traveling downwind faster than the wind by not traveling directly down wind, but by jibing back and forth across the path of the wind.
In that case the maximum speed is dependent on the boat's parameters, and the wind speed. The same is true of this vehicle.
The exact maximum one can go at any given fixed wind speed is a function of the materials used, which do have limits, but the exact limits are not curren
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They have been going faster than windspeed for years.
Upon seeing the title I browser-searched for "surfers" and found this post. Traveling at higher-than-windspeed is trivial if you go perpendicular to the wind. From reading TFA it seems they go faster than the wind *going directly downwind*, that's quite a feat.
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It'll slow down and stop. The energy is extracted from the difference in motion between the wind and the ground. No such difference, and the car degenerates into a simple energy feedback loop (i.e. a typical attempt to create a perpetual motion machine) where the wheels drive the propeller which pushes the car forwards. Since the system is (of course) not 100% efficient, it eventually stops.
The cool fact is that, with wind, this energy loss is offset and the system can usefully generate additional thrust, w
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My conclusion: This is a storm in a teapot. The guy duped everyone by using the wrong terminology; he's actually traveling upwind (into the wind) by everyone else's definition. This is confirmed by the direction of the streamers in the video embedded in TFA.
Wrong reasoning, wrong conclusion. The cart is indeed travelling downwind, i.e. in the same direction relative to the ground. Moreover, physics do not state that energy cannot be extracted from the wind when going faster than the wind, because you also need to think about the wind moving relative to the ground. That is the energy difference being extracted. The ultimate theoretical point where the cart cannot possibly accelerate any longer is when the wind speed relative to ground in the wake of the cart is
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Yes, and iceboat races often have top speeds 3 or more times the wind speed. But it's not achieved when running downwind; it's when moving approximately at right angles to the wind. That's when the airfoil effect is the most effective. When you're aimed downwind, the sail is little more than a parachute, and can't move faster than the wind (though with the low friction of an iceboat's runners, you can get a ground speed pretty close to wind speed).
The summary claims a "downwind" speed faster than the win
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What's interesting to me is that argument that
Re:All depends (Score:5, Insightful)
Why is it that you are comfortable with a device that travels upwind at 3 times the wind speed, but think that one that travels downwind at 3 times the wind speed is a perpetual motion machine?
Clearly, any wind powered vehicle that travels faster than the wind in ANY direction must be harvesting energy from the velocity difference between the wind and the ground, not the velocity difference between the wind and the vehicle, or it would be a perpetual motion machine.
Velocity difference between wind and GROUND (Score:3, Informative)
Too bad I haven't got any mod points left. Yours is the best comment in this thread by far, illuminating the essential point :
harvesting energy from the velocity difference between the wind and the ground, not the velocity difference between the wind and the vehicle