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Going Faster Than the Wind In a Wind-Powered Cart

Soulskill posted more than 3 years ago | from the any-way-the-wind-blows dept.

Power 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."

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All depends (0)

Anonymous Coward | more than 3 years ago | (#34149414)

All depends on whether the first and the second mention of wind in fact refer to the same wind.

Two words (2, Funny)

Anonymous Coward | more than 3 years ago | (#34149422)

TACO Bell

Going Faster Than the Wind In a Wind-Powered Cart (-1, Offtopic)

Anonymous Coward | more than 3 years ago | (#34149430)

Science is all downhill from here.

store and release energy? (2, Insightful)

BBrown (70466) | more than 3 years ago | (#34149450)

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?

Re:store and release energy? (2, Funny)

Anonymous Coward | more than 3 years ago | (#34149474)

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? (5, Informative)

Whammy666 (589169) | more than 3 years ago | (#34149490)

Not necessary to store energy to go faster than the wind.

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.

Re:store and release energy? (0)

Anonymous Coward | more than 3 years ago | (#34149604)

also,
OOOOOOLD

http://tech.slashdot.org/story/10/06/06/0518216/Google-Backed-Wind-Powered-Car-Goes-Faster-Than-the-Wind?from=rss

Re:store and release energy? (2, Informative)

maxwell demon (590494) | more than 3 years ago | (#34149728)

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 the wind and the road, which both try to move it backwards.

Re:store and release energy? (5, Informative)

marcansoft (727665) | more than 3 years ago | (#34150030)

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.

SImpler; just what sailboats do (1)

sznupi (719324) | more than 3 years ago | (#34149780)

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-like look trick you)

But, people don't really "feel" how even sailboats propel themselves while tackling...

Re:SImpler; just what sailboats do (0)

Anonymous Coward | more than 3 years ago | (#34149978)

No, tacking has nothing to do with it. The principle still works if nothing on the cart moves sideways. A propeller is used because it is mechanically simple and efficient. The sideways movement is coincidental and irrelevant.

Re:SImpler; just what sailboats do (1)

Omnifarious (11933) | more than 3 years ago | (#34150142)

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.

Re:store and release energy? (1)

icebike (68054) | more than 3 years ago | (#34149790)

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.

Re:store and release energy? (1)

l00sr (266426) | more than 3 years ago | (#34149804)

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?

Re:store and release energy? (1)

cetitau (951106) | more than 3 years ago | (#34149880)

Let's remember that in the description, the word "downwind" is used, if that is strictly true, then the answer is no. Downwind is very specific. If there is an angle between the track of the cart and the wind then possibly but in true downwind, when the cart speed equals the wind speed, there is no differential to be stored or used in any fashion because as far as the cart is concerned, there is no wind. Downwind cart speed must be velocity of the propellent - drag.

Re:store and release energy? (1)

icebike (68054) | more than 3 years ago | (#34149912)

So you didn't read the article or watch the video either.....

Re:store and release energy? (1)

cetitau (951106) | more than 3 years ago | (#34149964)

I responded to the report. The report speaks for itself. If if didn't adequately describe the events then... What I said was correct.

Re:store and release energy? (1)

u17 (1730558) | more than 3 years ago | (#34149698)

IANAP but there may be a logical explanation in layman's terms. If there is no wind, the vehicle is stationary and its propeller is rotating, it will accelerate for a little while before the propeller slows down to zero and the vehicle gets stopped by friction and drag. Now consider what happens when the vehicle is stationary but the wind blows from behind. It will make the propeller rotate and also push the vehicle forward. Just the wind will be sufficient to push it up to the speed of the wind. When this happens, the speed of the wind relative to the vehicle is zero (as if there were no wind) and the propeller is rotating, just like in the first scenario. So, as long as wheel friction and internal friction are small enough, the vehicle will accelerate to go faster than the wind.

Re:store and release energy? (1, Insightful)

whoever57 (658626) | more than 3 years ago | (#34149894)

When this happens, the speed of the wind relative to the vehicle is zero (as if there were no wind) and the propeller is rotating, just like in the first scenario. So, as long as wheel friction and internal friction are small enough, the vehicle will accelerate to go faster than the wind.

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 with no wind is equivalent to traveling over a road with a wind from behind is fundamentally flawed.

To critique your explanation: why should the propeller continue to turn? There is no energy being gathered from the wind, so the only reason that it might turn is that the kinetic energy of the overall cart is converted into the turning the propeller. The propeller will not add forwards force equivalent to the rearwards force from the front wheels (as they extract kinetic energy), hence the cart slows down.

Re:store and release energy? (1)

emt377 (610337) | more than 3 years ago | (#34149926)

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? (3, Informative)

marcansoft (727665) | more than 3 years ago | (#34150136)

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.

Re:store and release energy? (0)

Anonymous Coward | more than 3 years ago | (#34150186)

Folks, if you have any kind of degree which required a physics course, you ought to have that degree taken from you. Please look up what "frame of reference" means and what physicists mean when they say "motion is relative".

This is gonna be good (0)

Anonymous Coward | more than 3 years ago | (#34149458)

It's always interesting to see how people look for explanations when somebody does something that they had decided to be impossible.

Re:This is gonna be good (0)

Anonymous Coward | more than 3 years ago | (#34149708)

This is easily explained by the scientific theory of Intelligent Blowing. The wind is the breath of a Blower (who may or may not be identified with the Christian God), and He is clearly capable of blowing harder in some places than others when He chooses to do so; so the wind behind the cart can be temporarily stronger than everywhere else, if the Blower favors the motivations of the driver.

I believe Texas schools will start teaching this next year.

Re:This is gonna be good (1)

Interoperable (1651953) | more than 3 years ago | (#34149766)

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, but I can't see an obvious bound on the maximum thrust for a given drag while considering forces.

The clear place to turn to show that the idea is a crackpot idea, is lack of energy conservation. Again, the situation isn't clear. The cart is gaining kinetic energy but the wind is loosing it. I can't think of a clear argument as why energy conservation would restrict the speed of cart to going the wind speed. The restriction is that the cart can't gain more energy than the wind is losing but that can be satisfied even if the cart is travelling faster than the wind.

I don't think there's any clear reason as to why the cart can't push the wind backwards such that the velocity of the air is reduced. In such a case, the wind loses kinetic energy and the cart gains some. Rather, the wind speed would place a maximum on the possible thrust, which would have to overcome the drag. Again, it comes down to the issue of thrust to drag, but that's no a problem that can be resolved by simple intuitive arguments.

Duh? (2, Interesting)

Annymouse Cowherd (1037080) | more than 3 years ago | (#34149462)

If sailboats can travel faster than the wind, of course wind-powered carts can.

Re:Duh? (4, Funny)

camperdave (969942) | more than 3 years ago | (#34149482)

It should be easier than in a sailboat. After all, all you need to do is find a steep enough hill.

Re:Duh? (0)

Anonymous Coward | more than 3 years ago | (#34149488)

The conventional wisdom is that sailboats can only move faster than windspeed if they're moving at an angle to the wind direction, not directly downwind.

Re:Duh? (2, Informative)

Lonedar (897073) | more than 3 years ago | (#34149502)

They can, but not directly downwind - which is what the article claims the cart can do.

Re:Duh? (4, Informative)

gotpaint32 (728082) | more than 3 years ago | (#34149554)

I think they are overcoming that particular limitation with the propellor which is technically approaching the wind indirectly.

Re:Duh? (2, Informative)

Lonedar (897073) | more than 3 years ago | (#34149582)

Right, but I just cannot see what keeps the propeller turning once the cart hits windspeed, as at that point the apparent wind would be 0.

Re:Duh? (0)

Anonymous Coward | more than 3 years ago | (#34149684)

Wouldn't shaping the front face of the prop differently from the back face create different drag characteristics? The back face collects air, the front diverts it, that kinda thing.

Re:Duh? (0)

Anonymous Coward | more than 3 years ago | (#34149712)

The speed difference between the ground and the cart turns the propeller (via gears between the wheels and the propeller). As long as the propeller turns, there is a force which accelerates the cart relative to the surrounding air. As long as the cart moves forward against the ground, the propeller turns.

Re:Duh? (1)

temporalillusion (688393) | more than 3 years ago | (#34149808)

You have it backwards, the wheels turn the propeller.

Re:Duh? (1)

Pinky's Brain (1158667) | more than 3 years ago | (#34149886)

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.

Re:Duh? (0)

Anonymous Coward | more than 3 years ago | (#34149512)

Well, it depends on the traffic, obviously.

Re:Duh? (2, Insightful)

Psychotria (953670) | more than 3 years ago | (#34149658)

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

Re:Duh? (4, Informative)

beaker8000 (1815376) | more than 3 years ago | (#34149590)

In a sailboat or iceboat, to travel faster than the wind you head about 45 degrees off of the direction from which the wind is coming (called 'reaching'). The sails then work as airfoils, creating lower pressure on the outside of the sails, which in conjunction with the keel propel you forward damn fast if you choose (iceboats sometime 4-5 times the speed of the wind). However, when you are 'running' (heading directly downwind) the sails are not working as airfoils, but function merely as a wall the wind hits that propels you forward. You don't go faster than the wind in this case. The article specifically mentions heading directly downwind.

Re:Duh? (1)

Bigjeff5 (1143585) | more than 3 years ago | (#34149736)

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).

Re:Duh? (0)

Anonymous Coward | more than 3 years ago | (#34149936)

Why do people keep insisting on the tacking analogy when very clearly that has nothing to with the principle? You could replace the propeller with a different propulsion system which "pushes" the cart off the surrounding air. That wouldn't change a thing, other than invalidating the tacking explanation. Take a look at the various Youtube videos which explain the principle with simple mechanics, completely avoiding fluid dynamics.

Re:Duh? (2, Informative)

Edmund Blackadder (559735) | more than 3 years ago | (#34149820)

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).

What happens if you remove the wind? (0)

Anonymous Coward | more than 3 years ago | (#34149468)

So... After this thing has spooled up and is running... What happens if the wind stops?

Does it keep going? Does it accelerate?

If the former is true, I don't see how that would be possible.

-AC

Re:What happens if you remove the wind? (1)

DarkKnightRadick (268025) | more than 3 years ago | (#34149730)

Momentum. It'll keep going until an outside force (friction) works on it. Newton knew this.

Of course (0)

Anonymous Coward | more than 3 years ago | (#34149470)

Sailors have been able to go faster than the wind speed for hundreds of years. Why do some people still think this is not possible?

Re:Of course (2, Informative)

dougmc (70836) | more than 3 years ago | (#34149504)

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 (3, Informative)

Brandano (1192819) | more than 3 years ago | (#34149562)

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. Consider this greatly simplified concept: Build an enormous wheel, and set it up so that it has large sails around its circumference, between the thread and the shaft. 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. The wind will push the sail, that will lever against the ground and cause the wheel to roll forward. Since the shaft is above the sail, it can travel faster than the wind even if the sail is slower,, and if the resistance of all the setup is small enough, you have something that travels faster than the wind, even if it's actually pushed by it

Re:Of course (5, Informative)

Anonymous Coward | more than 3 years ago | (#34149632)

For the more visual people: http://www.youtube.com/watch?v=k-trDF8Yldc [youtube.com]

Re:Of course (0)

Anonymous Coward | more than 3 years ago | (#34149776)

this is clearly sourcery.

Re:Of course (1)

emt377 (610337) | more than 3 years ago | (#34149810)

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 cable, power rail, or catenary, then the vehicle is not limited to wind speed. The latter derives energy from the difference between the wind and ground; the former from the difference between wind and vehicle. What you're suggesting requires a stationary windmill.

Re:Of course (1)

Brandano (1192819) | more than 3 years ago | (#34150088)

that's not what I described, and if you look at the vehicle you will see that the propeller is not spun by the wind. The direction the propeller spins is as if it were pushing against the wind. what happens here is that the wheels are geared to the propeller, and spin it in such a way that it effectively becomes a sail moving backward relative to the whole of the contraption. The wind pushes on the prop, that pushes on the car, dragging it forward. This spins the wheels, that gear down the speed to convert it in torque to spin the propeller in the opposite direction the wind would spin it. Essentially the wheels are levering the small torque they have to generate a stack of sails to climb on. This is not a perpetual motion machine, the 2d law is safe... it extracts energy by the difference in speed between the wind and the ground. If you like another interesting theoretical puzzle, can you get a Brennan torpedo to travel down a river faster than the water flow by attaching the control cables to the shore?

Re:Of course (1)

johnjaydk (584895) | more than 3 years ago | (#34149566)

If You use a propeller, to do the pushing, then Your airfoil is exactly at such an angle to the wind. The google sponsored design is such a setup.

Re:Of course (1)

dougmc (70836) | more than 3 years ago | (#34149946)

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 need any wind, and we've got a perpetual motion machine.

Ultimately, if this thing is going downwind faster than the wind yet powered by the wind -- then it's drag is going to make the wind even faster -- energy for nothing!

Re:Of course (1)

Brandano (1192819) | more than 3 years ago | (#34150116)

It's not a perpetual motion machine. The treadmill is the source of energy. The wheels spin the prop, that pushes AGAINST the relative wind

Mythbusters (1)

tomaasz (5800) | more than 3 years ago | (#34149486)

I wonder what Jamie and Adam could come up with...

Re:Mythbusters (4, Funny)

Anonymous Coward | more than 3 years ago | (#34149496)

Something almost scientific that ends with an explosion?

Re:Mythbusters (0)

Anonymous Coward | more than 3 years ago | (#34149616)

It's funny because it's true!

Re:Mythbusters (1)

WinstonWolfIT (1550079) | more than 3 years ago | (#34149636)

Not sure, but it would be hard to top the stupidity of thinking that frozen chickens should have the same impact as thawed ones when ploughing them through glass.

Couldnt you add to this design (1)

Rivalz (1431453) | more than 3 years ago | (#34149492)

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.

Re:Couldnt you add to this design (2, Informative)

Omnifarious (11933) | more than 3 years ago | (#34149620)

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 resisted the spring that was trying to fold it up.

whooosh! (1)

M8e (1008767) | more than 3 years ago | (#34149520)

Is that the sound of the wind or of the vehicle going DDWFTTW?

Here is how it works... (1, Informative)

Omnifarious (11933) | more than 3 years ago | (#34149568)

It extracts energy from the potential energy difference between kinetic energy of the atoms in the wind and the atoms on the ground. A sail does this too, but a sail has a lot of drag. In fact, it has so much drag that you will never end up going faster than the wind.

A propellor has very little drag. That's the whole point of a propellor. In fact, a propellor can provide negative drag (aka thrust). So the cart's speed stabilizes when the total drag of the cart exceeds the thrust on the cart from the wind and the propellor.

That's why the treadmill example works perfectly. The energy is no longer being extracted from the air, it's being extracted from the treadmill. If you were to measure the total work being done by the treadmill when the cart is moving forward on it, you would discover it was doing a lot more work when the cart was moving than when it wasn't. With a treadmill that has no extra power capacity this will result in the treadmill slowing down when the cart is moving forward.

Re:Here is how it works... (0)

Omnifarious (11933) | more than 3 years ago | (#34149650)

Oh, and a simple sail has another problem. It can only extract energy from the kinect energy potential difference of the atoms in the wind and the atoms in the thing the sail itself. There is no connection the ground, and so no way to extract energy from the kinetic energy potential difference between the wind and the ground.

Re:Here is how it works... (1, Insightful)

Anonymous Coward | more than 3 years ago | (#34149762)

You fill your posts with technical terms, but you don't know what the hell you're talking about.

A propellor has very little drag. That's the whole point of a propellor.

If propellers had such a magical property, they would hang propellers on boats instead of sails. The difference between the two is not "drag" but the fact that propellers are moving with respect to the wind.

There is no connection the ground

Not true. Sailboats have fins that effectively constrain motion is one direction, much like wheels do on a cart.

Re:Here is how it works... (1)

Omnifarious (11933) | more than 3 years ago | (#34149970)

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.

Re:Here is how it works... (0)

Anonymous Coward | more than 3 years ago | (#34149862)

In both cases it's a sail; here it's just rotating one, and with different way of extracting speed difference between two mediums. Probably less efficient, too - static sails can be much more streamlined.

The reason why it can't do the same is because of its spatial configuration, not lack of efficiency. (and how in boats the resistance of the surface is extracted by the keel... which, considering also the characteristics of its working medium, probably works better in most scenarios...except in the ranges roughly parallel to the wind)

Re:Here is how it works... (1)

MrEricSir (398214) | more than 3 years ago | (#34149924)

So what happens if we put the wind-powered cart on a treadmill?

Re:Here is how it works... (1)

Omnifarious (11933) | more than 3 years ago | (#34149980)

It extracts energy from the fact the ground is moving faster than the air.

In this atmosphere... (1, Funny)

Anonymous Coward | more than 3 years ago | (#34149580)

...we obey the laws of thermodynamics.

Re:In this atmosphere... (2, Funny)

maxwell demon (590494) | more than 3 years ago | (#34149754)

...we obey the laws of thermodynamics.

I don't! :-)

Momentum (1)

denshao2 (1515775) | more than 3 years ago | (#34149594)

Wind speeds vary. The wind can stop completely while a vehicle continues moving. That is technically traveling faster than the wind.

Re:Momentum (1)

Bigjeff5 (1143585) | more than 3 years ago | (#34150002)

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 it works is as follows:

Up until the cart reaches wind speed, the propeller acts primarily as a sail. The wind pushes against it, which pushes the cart forward. As the cart moves forward, the wheels turn the propeller. The faster the propeller turns, the more it pulls the cart forward, but until it hits windspeed it is still at least partially acting as a sail.

By the time the cart reaches windspeed (relative windspeed of 0) the propeller is turning fast enough pull the cart forward on its own. It is no longer catching any wind from behind.

The cart continues to accelerate until drag and the headwind are able to cancel out the pull generated by the propeller. The cart is at max speed at this point. For the cart in the story, that happened at 2.8 times windspeed.

It's important to note that if the tailwind increases, the cart will be able to go faster, and if it drops the cart will slow down. If the wind shifts the cart will stop pretty quickly if there isn't any clutch on the drive mechanism.

It sounds like free energy, but it isn't. It's just getting the absolute most out of the energy available.

The easy solution (-1)

Anonymous Coward | more than 3 years ago | (#34149630)

How to go faster than the wind in a wind-powered cart:

Step 1: Use a windmill to generate electricity

Step 2: Store up enough "wind-power" in batteries or what-not

Step 3: Wait for a low-wind day

Step 4: Go faster than the wind in a wind-powered cart!

upwind vs downwind (0)

Anonymous Coward | more than 3 years ago | (#34149644)

TFA's page seems confused about which direction is "upwind" and which direction is "downwind." TFA casually interchanges the terms, and it even occasionally uses clarifying phrases like "into the wind" as a contradictory explanation of "downwind."

AFAIK:

  • The laws of physics state that you cannot receive a positive force from the wind while traveling directly downwind (in the exact direction of the wind / with the wind at your back) faster than the wind speed.
  • It's trivially easy to make progress traveling upwind (with your face into the wind) if you're using a propeller to turn drive wheels faster than the wind speed.

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.

The reason that I don't believe it. (2, Insightful)

7-Vodka (195504) | more than 3 years ago | (#34149662)

The reason that I don't believe this claim, is because physical demonstrations can be rigged. I want to see the mathematics. Is it too much to ask? I mean, they build lots of models, including expensive ones, they wrote articles claiming they can do it, they posted numerous videos on youtube claiming they can do it.... Where is the fucking math? Why it so hard to post it?

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. Until then, what am I supposed to believe? My gut instinct or my lying eyes?

Re:The reason that I don't believe it. (2, Informative)

Whammy666 (589169) | more than 3 years ago | (#34149760)

The math is not too bad, but it does involve propeller theory which is where the magic happens. The goal is to make a propeller and cart that requires less energy than is provided by the wind pushing against the prop thrust. The energy supplied is:

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. (0)

Anonymous Coward | more than 3 years ago | (#34149824)

take a look here for some math:
http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attempts_to_aton.php

I don't think you understand science (4, Interesting)

SmallFurryCreature (593017) | more than 3 years ago | (#34149844)

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:The reason that I don't believe it. (1)

catbutt (469582) | more than 3 years ago | (#34149874)

This may not be so mathematical, but it explains it in a way so that you can wrap your head around why it can work. (as well as why people might be inclined to think it wouldn't work, even though it does) http://karmatics.com/dwfttw [karmatics.com]

Re:The reason that I don't believe it. (1, Insightful)

Anonymous Coward | more than 3 years ago | (#34149892)

You don't have to be an accomplished mathematician to build a working model of something. It's quite simple to build something and have no idea how to accurately describe the math behind it. Just because there's no formula's doesn't mean they are lying.

Re:The reason that I don't believe it. (2, Insightful)

John Hasler (414242) | more than 3 years ago | (#34150052)

How about if you post your math showing that it cannot work?

Re:The reason that I don't believe it. (0)

Anonymous Coward | more than 3 years ago | (#34150218)

http://www.youtube.com/watch?v=k-trDF8Yldc

There you go.

Re:The reason that I don't believe it. (2, Informative)

Bigjeff5 (1143585) | more than 3 years ago | (#34150228)

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 acts like a sail. The wind pushes against it, pushing the cart forward. As the cart moves forward the wheels turn the propeller. The cart continues to accelerate, which in turn spins the prop faster. Well before the cart reaches wind-speed, the propeller is providing a significant amount of pull, which continues to accelerate the cart, which continues to drive the wheels which continues to drive the propeller faster. The cart stops accelerating when drag and wind-speed cancel out the propeller's pull.

Here's the math for you: tail-wind = 10mph, cart speed = 28 mph. 28mph/10mph = 2.8 times wind-speed.

Better?

In case you are interested, the land sailing speed record was set the same day with 126mph in a 40mph cross-wind. That's 3.15 times wind-speed. I can do the math again for you, if you like.

The physics work just fine, and they have for a couple hundred years now. It's the whole reason triangular sails were invented for heaven's sake! The sailing folks have it rough though, so much drag means the world sailing speed record is only about 20% faster than wind speed.

This is just a new application of old, well known and well established physics.

Seriously (1)

TranceThrust (1391831) | more than 3 years ago | (#34149678)

how about editing summaries before putting them on? This reads like, and I am sorry to say, a story straight from elementary school.

dupe post? (0)

Anonymous Coward | more than 3 years ago | (#34149692)

http://tech.slashdot.org/story/10/06/06/0518216/Google-Backed-Wind-Powered-Car-Goes-Faster-Than-the-Wind

Windsurfers and kitesurfers (0)

Anonymous Coward | more than 3 years ago | (#34149700)

They have been going faster than windspeed for years.
In fact I go twice the windspeed on water, even faster on land with a sail.
Current kitesurfing record is over 55 knots, in less than 40-50 knots wind.
On wheels there is even less friction.
It would be poorly designed if you can't go faster.

erm.. (0)

zmollusc (763634) | more than 3 years ago | (#34149784)

Okay, so i build one of these doohickeys and set off downwind. The wind is blowing a steady 40 knots and soon I am whizzing along at maybe 44 knots, laughing and mooning at all the people who said it could not be done. Gradually, the wind blows more softly, but i am still outrunning the wind. Eventually the wind slows to 0 knots, but I sail merrily and perpetually on. Awesome.

Re:erm.. (1)

Tacvek (948259) | more than 3 years ago | (#34149990)

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 there is no friction, you would continue perpetually (if you lived in an infinite flat world anyway, and only Newtonian physics were at play). No physicists would dispute that. Of course a frictionless world is absurd, and in fact the vehicle relies on friction (of tires against pavement) to be able to go faster than the wind.

Re:erm.. (1)

John Hasler (414242) | more than 3 years ago | (#34150094)

The wind is blowing a steady 40 knots and soon I am whizzing along at maybe 44 knots

So you are doing 1.1 times wind speed.

Eventually the wind slows to 0 knots

What is 1.1 times 0?

A much more interesting question (0)

Anonymous Coward | more than 3 years ago | (#34149794)

What is the maximum speed that could be reached by using such a scheme?

Re:A much more interesting question (1)

Tacvek (948259) | more than 3 years ago | (#34150046)

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 currently known, since we are continuously discovering new materials with different properties.

old news (1)

Swampash (1131503) | more than 3 years ago | (#34149816)

I have no familiarity with land-based wind vehicles, but sailing vessels have been able to travel faster than the wind for a long time. This is hardly something new.

Sailing faster then the wind (1)

houghi (78078) | more than 3 years ago | (#34149822)

Can a wind cart travel faster than the wind?

If boats can go faster [unsw.edu.au] than [straightdope.com] the [sciencedaily.com] wind [wikipedia.org] , why not a wind cart?

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.

Did you see the Matthew Modine movie "Wind?" (0)

Anonymous Coward | more than 3 years ago | (#34149828)

It blows.

Moving faster than the wind is known at sea (0)

Anonymous Coward | more than 3 years ago | (#34149918)

According to this http://madmariner.com/vessels/story/WINGSAILS_060909_VX

So how fast can you go with a wingsail? The record for boats currently stands at 47.36 knots (in a wind speed of 22.6 knots) set by Vestas Sailrocketon December, 5 of 2008. In addition, a wingsail has propelled a sailboard at more than 50 knots. There is also a land record of 116.6 mph, and iceboats have been timed at 84 mph.

sail carts aren't practical transports (1)

rossdee (243626) | more than 3 years ago | (#34150022)

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't work on a highway.

Good grief, ancient history (-1, Redundant)

Ancient_Hacker (751168) | more than 3 years ago | (#34150152)

Good Grief, what happened to, like, research?

I think Chinese Junks have been exceeding the wind speed for like, 5,000 years.

You can also analyze it as a glider by turning the world 90 degrees-- now you have a plane wing, which can have a glide ratio of up to 20-- that is, moving across (down) one unit while moving 20 units downrange (forward).

I am very confused. (0, Offtopic)

paxcoder (1222556) | more than 3 years ago | (#34150202)

Very, very, VERY confused. o.O

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