Going Faster Than the Wind In a Wind-Powered Cart

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."

Comments

Two words

[Anonymous Coward]

TACO Bell

store and release energy?

[BBrown]

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?

[Anonymous Coward]

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?

[Whammy666]

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?

[u17]

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?

[maxwell demon]

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?

[icebike]

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?

[l00sr]

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:

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?

[cetitau]

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?

[whoever57]

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?

[icebike]

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

Re:store and release energy?

[emt377]

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?

[cetitau]

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?

[marcansoft]

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.

Re:store and release energy?

[marcansoft]

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?

[marcansoft]

there is no differential to be stored or used in any fashion

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 outrun the wind in every sense of the word. It's very counterintuitive, but once you understand the mechanics it makes perfect sense and doesn't violate any laws of physics.

Re:store and release energy?

[leehwtsohg]

You should read the discover article on the thing. I was saying exactly the same as you before I did. The gist of it is this: Imagine the car going exactly at the speed of the wind. In the car there is no wind, except that the ground is moving. The ground moving turns the wheels powers the propellor, which rotates and gives the car a force forward. Since wind speed is 0, there is no resistance, no force to counter the propellors force, the car will now accelerate, i.e. start going faster than the wind. Once you accept that it will then go faster than the wind, the only question is how much faster. (The ground acting on the wheels will exert an very small force on the car, which can be made infinitesimal by reducing the friction)

Re:store and release energy?

[marcansoft]

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.

Re:store and release energy?

[Short Circuit]

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.

Re:store and release energy?

[Bigjeff5]

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 run the test.

Re:store and release energy?

[KnightMB]

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:

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 reaches the maximum speed that the wind is able to push the vehicle. At the same time, the ground is supplying energy to the propeller to push the vehicle faster. So what is really happening is a waveform; the vehicle moves with the wind, then moves faster than the wind, then slows down, gains more energy, moves with the wind, faster, repeat, etc. Basically if you compared this vehicle side by side with another one with just sails for example, the vehicles would reach the same destination at the same time overall. So while the vehicle might be traveling faster than the wind in burst, it won't get you any place faster than the next wind powered vehicle.

Re:store and release energy?

[ByTor-2112]

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.

Re:store and release energy?

[Bigjeff5]

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

Re:store and release energy?

[temporalillusion]

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.

If there's no wind then it won't move. You are missing that it will accelerate to and beyond the average velocity of the volume of gas it sits in _with respect to the ground_. The ground doesn't move with the wind.

Re:store and release energy?

[bwcbwc]

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 workable, especially the greater resistance to motion through the water. Maybe a hydrofoil?

Re:store and release energy?

[marcansoft]

So while the vehicle might be traveling faster than the wind in burst, it won't get you any place faster than the next wind powered vehicle.

.
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 velocity Y for said X, where Y>X (for a properly designed vehicle using this technique). If the velocity momentarily exceeds Y, the friction losses of the wheels will be greater than the gain in push from the fan, and the car will slow down. If the velocity momentarily drops below Y, the friction losses of the wheels will be lower than the push from the fan, and the car will accelerate forward. It stabilizes at Y, faster than X. The feedback loop keeps it at that stable Y.

Re:store and release energy?

[Romancer]

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

Re:store and release energy?

[wagnerrp]

It will accelerate to some multiple of the average velocity of the volume of gas it sits in. Zero times any multiplier you want is still zero.

Re:store and release energy?

[marcansoft]

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 very small. On a boat, you need a large sail to extract a large amount of energy from the wind to overcome this friction and achieve a significant fraction of the speed of the wind, while a lightweight ground vehicle with a small sail can easily achieve a speed very close to that of the wind. On a boat, the friction with the water is hard to direct towards some kind of turbine and not towards the hull of the boat, while on a car you can easily use wheel motion almost exclusively to turn the fan (fully static friction between the wheel and ground), expending a relatively small amount of the energy as friction on the axle. I think this might very significantly undermine the ability to "port" this trick to a boat. I hope someone proves me wrong, though :)

Re:store and release energy?

[SashaM]

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 direction). Can you use wind2 to propel yourself in the direction of wind2?

As the grand-grand-parent says, however, it is very much possible to achieve the feat in the article by storing energy during the acceleration phase (you would end up accelerating slower than you would otherwise) and then once you reach wind speed, use that energy to move faster. Once that energy runs out, however, you will slow down to wind speed.

Re:store and release energy?

[dwye]

> So here's a question for everyone: could you make it work in a boat?

Yes, but that is so old-hat that it is beneath comment. Iceboats have done it since the 1890s, at least, and sailboats for decades. Google "moth sailboat" for an example with YouTube videos.

Re:store and release energy?

[KnightMB]

So while the vehicle might be traveling faster than the wind in burst, it won't get you any place faster than the next wind powered vehicle.

. 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 velocity Y for said X, where Y>X (for a properly designed vehicle using this technique). If the velocity momentarily exceeds Y, the friction losses of the wheels will be greater than the gain in push from the fan, and the car will slow down. If the velocity momentarily drops below Y, the friction losses of the wheels will be lower than the push from the fan, and the car will accelerate forward. It stabilizes at Y, faster than X. The feedback loop keeps it at that stable Y.

I don't believe it can, no matter how stable, energy in will never be greater than the energy out. A balloon wouldn't be a good example, another vehicle with just sails to catch the wind running side by side would be the best comparison. Basically, the sail vehicle would start off much quicker and be far ahead of this vehicle before the faster-than-wind vehicle got up to speed. Afterwards, it would just be an infinite slope towards the finish. I think what everyone misses here is the time part. It may go faster than the wind at some point, but it won't continue that way forever nor can it make up for the time. Basically the vehicle is just trading out time for speed. The mystery would lose a lot of luster if they did the same run with two side by side vehicles in a race (one straight sail, the other being the faster-than-wind vehicle). All I've seen for video footage is a single vehicle run, which takes the faster-than-wind part out of context and makes it appear to be perpetual energy, which it certainly is not. We all know that, but the single vehicle run is what gets everyone in an uproar over the laws of physics.

Re:store and release energy?

[robbak]

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.

Re:store and release energy?

[PopeRatzo]

The propeller is used to take energy from the wind, which is then used to drive the wheels and move the vehicle forward.

For some reason, this sounds like the "kink springs" from the great sci fi novel The Windup Girl by Paolo Bacigalupi.

If you haven't read it yet, get your hands on a copy.

Re:store and release energy?

[robbak]

No. It works exactly as described.

You have a tail-wind. That tail wind pushes the craft forward. Not very well, mind you, as you do not have a large sail area. This turns the wheels. The turning wheels are then used to turn the prop, which pushes the craft forward *relative to the wind*.
Do it efficiently enough, and you will be able to travel faster than the wind. In their case, 2.8 times faster. They are harvesting the difference between the crafts higher landspeed and its lower windspeed.
Those who have a trouble with the concept probably also have trouble with the plane-on-a-treadmill conundrum. Which we will not discuss here!

Re:store and release energy?

[PopeRatzo]

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.

So, it's not "perpetual motion", but it's free, which is maybe even better.

I admit I'm a little fucked up over here because my wife is out of town at a math conference at Notre Dame and I poured a few fingers of slivovitza after defrosting my dinner, but still, going faster than the wind with just the wind as a power source sounds pretty cool.

I wonder if you can do the same thing with water. If I'm shooting the rapids in a wave-powered boat, can the boat go faster than the water is moving?

Re:store and release energy?

[Sparr0]

At Vcar=Vwind, with arbitrarily low rolling friction, and without coupling the wheels to the propeller, there is no force acting on the car.

A 1000kg car moving at 100m/s with a tailwind of 100m/s (that is, 0m/s relative to the car) applies a braking force of 1000N against the ground. That braking produces power proportional to the difference in velocity of the car and the ground. That power is applied to the propeller (in this case, nearly perfectly, by a drive train, but if it helps you can pretend it was converted to electricity by a generator at the wheels then back to thrust by a motor on the propeller). At an airspeed of zero, how much thrust (accelerating force) will that of power applied to a propeller produce?

I think that in answering that question, you will discover why all of the little bits of friction along the way are important, but there is a wide gap between the lowest friction we can produce and the highest friction at which acceleration still happens. Once you accelerate to Vcar>Vwind then wind resistance becomes an issue, which also affects your top speed, but at Vcar=Vwind there is no wind resistance.

Re:store and release energy?

[jbengt]

The propeller is tacking even if the cart is moving strictly downwind.

Re:store and release energy?

[robbak]

               ----> Wind speed - 10
               <-- Net air speed (ground speed - wind) 5

       <-          )
Propeller         |-A
force 5           ( H
@ air speed         H
                O------------O < drag on wheel, 5, at ground speed.
------> Ground speed - 15

if input Ek 5*15 = output 5*5 - losses, vehicle will continue to travel.

Re:store and release energy?

[jellyfrog]

Conservation of energy doesn't prevent this from working. After all, with a turbine powering the wheels, a cart can travel faster than windspeed into the wind anyway.

In this case, a plain sailcart (with no propellor) reduces the velocity of the mass of air that is pushing it by a bit --> reduces the wind's kinetic energy. This enables the cart to move forward (the energy is transferred to the cart) until it's almost at windspeed, where it has a negligible effect on the velocity of the wind.

If a fan is then added at windspeed, it starts to push air back, reducing the air's kinetic energy again. This energy has to go somewhere, either as heat or the kinetic energy of the cart. Since most propulsion systems are not 0% efficient, I'd bet on the cart accelerating (to a speed faster than the wind).

So you see we're not getting free energy from anywhere. Kinetic energy is proportional to the mass involved, and there's a very large supply of moving air available.

Re:store and release energy?

[Beardydog]

Take a hand-powered drill, and replace the drill bit with a screwdriver bit. Fit the bit into a screw which has been screwed halfway through a board. Bolt the board and the drill to a platform with wheels on it so that the screw points backward, and gear the wheels of the platform to the hand-crank. When you push the platform, the wheels will turn the crank, the crank will turn the screw in the board, and the screw will slowly extend outward behind the whole contraption. Now, instead of pushing on the platform, push on the tip of the screw itself. The screw remains motionless relative to your pushing finger, but the rest ofthe contraption is cranked slowly away from your finger and the screw. The screw represents the wind, which the contraption is now moving faster than, as long as the screw drills through your finger more slowly than it drills through the board it's mounted in.

Re:store and release energy?

[whoever57]

Since wind speed is 0, there is no resistance, no force to counter the propellors force,

But how are the propellers powered? By the energy gathered at the wheels from the ground moving. This gathering of energy involves a backwards force applied to the car at the wheels. This force will be greater than any forwards force created by the propellers. Hence the car slows down (or in the altered frame of reference, it accelerates backwards).

Re:store and release energy?

[marcansoft]

You can only use it to change your velocity in the direction of the movement of that medium.

No, you can use it to change your velocity in any direction. It's easier to understand if you think about solid objects. Let's say you have a long open truck with a small vehicle on the back, and the truck is advancing on a highway. Now let's say you connect the wheels of the small vehicle, through an axle and some gears, to a wheel that you then lower onto the highway. The small vehicle will move forwards on the truck (assuming you got your gearing right), and therefore move faster than the truck, relative to the ground.

The same principle applies when the truck is the wind, except the system is less efficient.

Re:store and release energy?

[l00sr]

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.

The problem is that the experiment is finite. As such, one needs to be very careful about proving that any finite demonstration of faster-than-wind velocity necessarily entails that the vehicle must be able to maintain that state indefinitely.

Re:store and release energy?

[gardyloo]

There is no "pull", only push.

Re:store and release energy?

[l00sr]

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.

I believe you are mistaken on this point. According to the official report, the propeller is connected to the wheels via a fixed-gear transmission, so as long as the propeller spins, the wheels spin. However, the fixed-gear part of it does preclude the idea of harvesting the prop's momentum to accelerate the wheels. Most of my objection is that the rules were very vague and a little confused-sounding about all of this.

Re:store and release energy?

[marcansoft]

I wonder if you can do the same thing with water. If I'm shooting the rapids in a wave-powered boat, can the boat go faster than the water is moving?

Nope, because the boat cannot harness the power of the flowing water (to do anything but simply move with it) without attaching itself to some kind of ground reference (this is what the wheels in a DWFTTW vehicle accomplish). Well, except using the air (hopefully not moving at the same speed and direction as the water), but that's a little far-fetched. It would also work if you could somehow extend a wheel to solid ground or the bottom of the water body, but that's also not very practical.

It should, however, be theoretically possible to accomplish DWFTTW on a sailboat (unrelated to sailboats' ability to travel faster than the wind when not directly downwind), but it might not be very practical due to the different friction characteristics of a boat on water vs. a car on wheels.

Re:store and release energy?

[Mephistro]

I wonder if you can do the same thing with water.

Water? Instead of slivovitza? You must be joking! :D

Re:store and release energy?

[Iron Condor]

So here's a question for everyone: could you make it work in a boat?

Yes.

Re:store and release energy?

[znerk]

I think the part you're missing is that the propeller does not drive the wheels; as a matter of fact, there is a ratchet mechanism between the prop and the wheels that prevents this, specifically for the purpose of adhering to the NALSA rules about stored energy usage (the propeller could be considered a flywheel).

The wheels drive the propeller, not the other way 'round.

As far as I can tell (and I'm no physics wiz, so I may be completely wrong), the wind pushes against the prop, causing forward momentum. The wheels turn as the vehicle is pushed forward (essentially sailing, at this point), and the turning wheels turn the prop. The turning prop generates forward momentum by pulling air through itself, as well as creating a pressure front via its rotation.

In other words, it works just like the propeller on an airplane, except using the wheels' interaction with the ground as the motive force for the propeller, rather than an engine.

My big question is, how hard would it be to do the same thing against the wind, rather than running downwind? That is, can this same principle be used to go in an arbitrary direction, as long as there is wind to use for energy? I can think of many places where a "free" transportation service based on the ever-present wind would drastically increase quality of life.

Re:store and release energy?

[znerk]

You're missing the part where there are two surfaces (air and ground) being utilized for energy input.

The sail portion gets it moving, sure. The ground moving underneath the vehicle applies additional motive force, which is used via gearing to turn the propeller faster, which increases the motive force. It works just like an airplane's propeller, except using the ground to spin the wheels which turn the propeller, instead of a combustion engine.

As I understand it, the trick is using both surfaces - if it became airborne, it wouldn't work.

Re:store and release energy?

[znerk]

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

By "report", do you mean "summary"?
If so, I commend you for thinking a summary could have enough information to form an opinion with.

By "report", do you mean "article"?
If so, you didn't read it.

The wind pushes the craft forward, same as a sail-powered craft. The tricky part is using the ground's braking effect on the wheels to power a propeller that supplies additional forward motive force.

In effect, the craft is using the difference in speed between the air and the ground, and minimizing as much as possible the friction effects on the craft. In other words, it's not so much using the wind against itself for power, it is more using the difference between groundspeed and airspeed to propel itself.

Re:store and release energy?

[Zenin]

Your emphasis is misplaced. The point of the rule is about stored energy, which this kart never has (aside from it's normal momentum).

Energy is transferred from the wheels to the fan, but it is never stored. Any momentum from mass in the transmission system is negligible.

If there was an energy storage system the vehicle's speed would oscillate. It does not; Once at full speed it maintains consistent speed until either the wind changes or the kart is forcibly stopped (breaks).

Re:store and release energy?

[Nazlfrag]

Yes, you can easily attach a windmill to a cart and travel directly into the wind. Whether you can do this faster than the incoming wind, well my brain would melt trying to figure that one out.

Re:store and release energy?

[znerk]

And again, you're reversing the concept. Twice, actually.

A: The wind is coming from behind, and pushing the vehicle (ie, wind resistance), not coming from in front of it.

B: The propeller uses the wheels as a motive force. That is, the wheels' rotation causes the propeller to rotate.

Your example of driving a windmill-on-a-cart into the wind is completely and precisely perpendicular to the concept being described.

Re:store and release energy?

[znerk]

Gah, I'm an idiot. I just realized you were answering my closing question, not attempting to agree with my point in a braindead manner.

Sorry.

Re:store and release energy?

[Cecil]

That actually depends entirely on which frame of reference you choose to view the problem in. It is therefore both subjective and inaccurate.

Re:store and release energy?

[maxwell demon]

Nope, you've got it backwards, the GP got it right, and this is absolutely the key to understanding how this works.

Yes, on further thinking you're right. My explanation would have meant to accelerate the wind, which wouldn't make sense. Sorry for the confusion.

Re:store and release energy?

[Hognoxious]

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

Why would it be moving if it's on a treadmill? The whole point of a treadmill is to be able to go through the actions of moving while staying in one place.

Re:store and release energy?

[kiddygrinder]

it's not free, it's just an efficient way of gathering kinetic energy from the wind. i doubt you could gain more energy than attaching a battery to a dynamo on a fan the size of the one on this car. the car might be more or less efficient than converting it to electric energy tho (probably more from the sounds of it).

Re:store and release energy?

[drewhk]

It is interesting how people always come to conservation of energy, but they talk about speed at the end. They basically say "hey, if wind blows at v_1 relative to the ground and your cart goes at v_2 only gathering energy from wind, then v_2 = v_1 must hold (at least in the infinite), because otherwise we would have a perpetum mobile". Now the only problem is that speed is NOT energy, so relation of speed between an energy source and a worker depending on the source does not matter by itself alone.

Re:store and release energy?

[drewhk]

I wanted to write v_2 (lowerthannorequalto) v_1, but slashdot swallowed the relation sign.

Re:store and release energy?

[stenWolf]

Wish I had mod points to mod this up

Re:store and release energy?

[Fnkmaster]

If you look at Rick Cavallero's replies to posts here, 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.

Re:store and release energy?

[Nazlfrag]

No problem, this is an issue where brains are twisted regularly. The against the wind cart can be seen on youtube and such, it's fairly basic really, but this brainfuck seems to allow faster than incoming wind by using the delta of windspeed and groundspeed, ok my brain just melted halfway and I have to stop.

Re:store and release energy?

[PopeRatzo]

It should, however, be theoretically possible to accomplish DWFTTW on a sailboat (unrelated to sailboats' ability to travel faster than the wind when not directly downwind), but it might not be very practical due to the different friction characteristics of a boat on water vs. a car on wheels.

Thank you for your reply, marcansoft. Now that I'm awake and mostly sober, I can understand this "DWFTTW" a little better.

Re:store and release energy?

[PopeRatzo]

See, now that's funny.

Re:store and release energy?

[dmizer]

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:

First, the total weight of the vehicle is only 450 pounds (204 kg).

Secondly, the "windmills" are indeed sails which are (considering the total weight of the vehicle) fairly lightweight and have trivial kinetic mass (ie. not massive). Also relevant is that for a flywheel to be efficient, the mass should be as far from the axle as possible.

So in this case no, rotating sails =/= flywheels and the spinning mass of the sails is a "trivial force", especially considering the weight of the vehicle against the weight of a driver, since the driver would most likely counteract any intentionally or unintentionally placed flywheel of a mass relative to the total weight of the vehicle.

Re:store and release energy?

[camperdave]

use > for > or < for <

Re:store and release energy?

[Sethumme]

Pushing any air necessitates creating an area of lower air density where the air you are pushing used to be. This weak vacuum sucks in air from other directions, pulling air towards the fan which than then be pushed away in the opposite direction.

Re:store and release energy?

[gagol]

From the point of view of marketing, a push strategy would be more appropriate a than pull strategy.

Re:store and release energy?

[tjones]

Or better yet, something involving a horse and carriage?

Re:store and release energy?

[Alsee]

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 perfect balance. Imagine there's a 50 mph wind. That wind is slower than the car, but now the car only sees a 50 mph headwind instead of a 100 mph headwind. Everything else the car sees is identical. In effect we just added 50 mph to the propeller's thrust. The car is going to accelerate. The wind is slower than the car, but by breaking the balance between wheel and propeller that wind is effectively adding energy to the car.

Now of course we go back to reality where the drive system is less than 100% efficient. If the energy being effectively added by the wind is greater than efficiency losses then the car will speed up.

The wheels travel a large distance over the ground, but because of the wind the propeller sees itself traveling a shorter distance through the air. The key point is that energy = force * distance.

The wheels experience a small force * large distance.
The propeller experiences a large force * small distance.

small force * large distance = large force * small distance

Power at the wheels = power at the propeller.
Large propeller thrust - small wheel drag = net acceleration.

-

Re:store and release energy?

[yakovlev]

Thank you.

The "free velocity" description is the best explanation I've seen of how this works, and also explains why the treadmill test is a good test.

The idea is that, at windspeed in a traditional sail-type vehicle, there is zero force on the sail and the ground is moving at a constant speed below you.

In a downwind-faster-than-the-wind car, we use some of the "energy" in that moving ground to spin a propeller. So long as the resultant force exerted on the air by the propeller is greater than the frictional force from the ground used to drive the propeller, the system accelerates to faster than the wind.

Determining if this is theoretically possible becomes simply: Is it possible for a theoretical propeller to exert more force on the air than is used to drive the gear system of that propeller?

Based on these real-world tests, the answer to that question appears to be yes.

Re:store and release energy?

[mestar]

Tell me, how can you live in that world of yours where you see something done, you measure it with gps whatever, and you still think it is impossible. Must be fun.

Re:store and release energy?

[ThosLives]

Unfortunately, this is not physically accurate.

The car isn't using the propeller as a turbine as a source of energy to power the wheels.

This is correct, but not for the reason you think. The vehicle is not moving forward because of torque applied to the wheels but solely due to aerodynamic forces. In fact, the force at the wheels opposes vehicle motion.

That, indeed, would be impossible, because once you reach wind speed the force exerted on the propeller is zero.

If the vehicle is moving at wind speed the propeller is spinning so there are fairly significant aerodynamic forces acting on it. The force on the propeller is most certainly not 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.

All the energy in the system must come from the wind; there is no energy in the wheels that can be transferred to the fan. The propeller is not spinning because it is taking kinetic energy from the wheels. The propeller spins because that is the only possible kinematic state given non-slipping wheels and a forward motion of the vehicle center of mass. The aerodynamic forces on the propeller are indeed the forces that are moving the vehicle forward. You were correct, though, in stating the propeller is not "a source of energy for the wheels." The wheels, in fact, only contribute dissipation to the system. The wheels are simply there to provide necessary kinematic constraints through momentum exchange.

Re:store and release energy?

[marcansoft]

I must admit that I haven't performed the mathematical calculations to fully grasp the mechanics involved, but I don't think I was that far off with my explanation.

This is correct, but not for the reason you think. The vehicle is not moving forward because of torque applied to the wheels but solely due to aerodynamic forces. In fact, the force at the wheels opposes vehicle motion.

What you say is, of course, correct, and I made reference to that fact when I mentioned that you brake the vehicle (oppose its motion) by drawing energy from the wheels. That energy didn't come from nowhere, it came from the wind. At no point did I claim that the vehicle is powered by torque applied to the wheels (in fact, I said this is not the case).

If the vehicle is moving at wind speed the propeller is spinning so there are fairly significant aerodynamic forces acting on it. The force on the propeller is most certainly not zero.

That is correct for the propeller blades when the propeller is in motion. My reference was to the fact that the force applied to the propeller as an abstract static component is zero from the vehicle frame of reference, when moving at wind speed. Sure, the propeller can spin (and it must if the vehicle is moving and it is mechanically linked to the wheels), but it can only be an energy sink at that point, not an energy source, as there is no external air motion relative to its mounting point. In fact, with the hypothetical (incorrect) configuration with the propeller driving the wheels (which is what I was referring to when I made that statement), the propeller would be blowing air forwards, actively impeding vehicle motion.

All the energy in the system must come from the wind; there is no energy in the wheels that can be transferred to the fan.

There is no energy in the wheels, but there is energy being transferred through the wheels. Again, I was contemplating the portion of the system involving the propeller and the wheels and the mechanical link between them. The energy in this system is being transferred from the wheels, through the mechanical link, to the propeller. The energy being drawn through the wheels comes from the vehicle's kinetic energy (thus, opposing vehicle motion). This kinetic energy, in turn, was provided by the aerodynamic interaction of the propeller with the wind (and, to a lesser extent, by the body of the car acting as a sail, but only before achieving wind speed). It all boils down to energy from the wind, of course. The wind is what supplies the continuous flow of energy to keep the system in motion at a stable velocity.

In terms of force, which may be a bit more accurate, the wind exerts a force on the propeller. This force is transferred to the body of the vehicle, accelerating it. Through the action of the wheels, this force is also coupled into torque at the axle, which is transferred to the propeller. This force acts against the wind, increasing the overall force acting on the vehicle.

I see what you're saying, and I agree that an analysis of the kinematics of the system as a whole is the only way to truly explain its behavior with mathematical rigor, and how it achieves a steady state at a velocity exceeding that of the wind, while still drawing energy exclusively from the wind (as that is the only energy source). However, I think my broken down explanation is sufficient to make people understand the core of the idea to enough of an extent that they grasp how it is, in fact, physically possible, if counterintuitive.

Re:store and release energy?

[robbak]

Yes, that is one way it is being explained. It helps sailing types get the concept. But it is not quite accurate. After all, you could achieve the same thing, if you could make it efficient enough, using a flat paddle-wheel-type arrangement instead of a screw.

Re:store and release energy?

[ThosLives]

Seems like we are indeed saying the same things, just using slightly different language. While there may be some slight differences in the thought process, and we could probably argue about which detail is correct*, I don't think that would add much value to the discussion... so I'll leave it at that!

*For instance, I disagree that the propeller is an energy sink when the vehicle CG has the same velocity (with respect to ground) as the wind because the total force on the vehicle in that condition is still in the direction of motion (not opposed to it).

Duh?

[Annymouse Cowherd]

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

Re:Duh?

[camperdave]

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

Re:Duh?

[Lonedar]

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

Re:Duh?

[gotpaint32]

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

Re:Duh?

[Lonedar]

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?

[beaker8000]

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?

[Psychotria]

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?

[Bigjeff5]

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?

[temporalillusion]

You have it backwards, the wheels turn the propeller.

Re:Duh?

[Edmund Blackadder]

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).

Re:Duh?

[Pinky's Brain]

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?

[William_K_F]

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).

Probably more accurate to say sailboats cannot do this yet. Imagine the sailboat with a propeller doing the same thing as this land based craft but pushing off the water.

Re:Duh?

[Lazy Jones]

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?

[kermyt]

There is direct linkage between the wheels and the propeller. the motion of the cart turning the wheels is transferred into the propeller which when spun fast enough will in turn push back against the tailwind to push the cart steadily faster than the wind itself.

Re:Duh?

[turnedintoanewt]

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.

people come back to the explanation because they understand how a traditional sail boat works, and many other people don't. when discussing the issue detractors inevitably invoke the workings of traditional sail boats, which naturally makes this description enlightening. of course it is not required from a pure dynamics point of view, where you simply need to describe the end result of your propulsion system. it just so happens that the airfoil is the primary way REAL devices interact with gases, so one endeavors to understand how the specific case works, not just the general, free body diagram...

Re:Duh?

[im_thatoneguy]

Theoretically feasible but extremely difficult according to an MIT physicist who did the math.

Re:Duh?

[Kilrah_il]

It can only done in sailboat. Just find a tall enough waterfall.

Mythbusters

[tomaasz]

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

Re:Mythbusters

[Anonymous Coward]

Something almost scientific that ends with an explosion?

Re:Mythbusters

[WinstonWolfIT]

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

[Rivalz]

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

[Omnifarious]

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.

Re:Couldnt you add to this design

[fbjon]

That would be this guy's type of design then? Nope, still doesn't work, even 750 years later.

Re:Of course

[dougmc]

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.

whooosh!

[M8e]

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

Re:Of course

[Brandano]

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

[johnjaydk]

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.

Here is how it works...

[Omnifarious]

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...

[Anonymous Coward]

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...

[MrEricSir]

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

Re:Here is how it works...

[Omnifarious]

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...

[Omnifarious]

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

Re:Here is how it works...

[johanatan]

Got kinect on the brain, huh?

Re:Here is how it works...

[florescent_beige]

It extracts energy from the potential energy difference between kinetic energy of the atoms in the wind and the atoms on the ground.

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 kinetic energy from the fluid, thus slowing it down. Unavoidable. The change in momentum of the fluid caused by the energy removal produces a backwards force.

If you have a perfectly efficient energy extractor and a perfectly efficient drive train you can just maintain your speed but not accelerate. Otherwise you would be violating thermodynamics.

So what's the trick? Well, if the fluid flow is at an angle to your direction of travel (due to wind that you aren't headed directly in to or away from) then a component of the energy flux is available to you because when you extract it the resulting unavoidable force is at 90 degrees to your velocity and thus does no work. You get energy but do no work to get it. Magic.

How well you can play that game depends on the efficiency of your propeller (80% on a good day) and your drive train and rolling components. There's no natural law that says whether or not you can go faster than the wind speed.

In this atmosphere...

[Anonymous Coward]

...we obey the laws of thermodynamics.

Re:In this atmosphere...

[maxwell demon]

...we obey the laws of thermodynamics.

I don't! :-)

Momentum

[denshao2]

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

Re:Momentum

[Bigjeff5]

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.

Re:Of course

[Anonymous Coward]

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

Dupe story

[mister_playboy]

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

I don't see much in this article that is new.

Re:Dupe story

[Bigjeff5]

It happens.

The reason that I don't believe it.

[7-Vodka]

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.

[Whammy666]

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.

[Eharley]

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

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

[catbutt]

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

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

[Anonymous Coward]

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.

[John Hasler]

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

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

[Bigjeff5]

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.

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

[sznupi]

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 configuration of a tacking sailboat; it's dynamically identical.

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

[Jeremi]

. 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?

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

[IceFoot]

For Christ's sake, land sails already go 2-3 times faster than the wind using the exact same principles used in these carts.

No, they don't exactly the same principles.

Land sail, and sailboats too, cannot sail faster than the wind when they are aimed directly downwind.

But somehow, this cart can. And that's what makes it difficult to understand.

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

[Bigjeff5]

You must not understand how land sails and sail boats actually do it then.

They create an air-foil, which generates pull as the craft speeds up. This is basically a force multiplier on the wind pushing behind it. It is entirely dependent on the wind, but it can allow the craft to go faster than the wind itself is moving.

The reason land sails cannot go downwind is because you must have both the force from the wind behind the sail and the correct angle of attack on the leading edge of the airfoil to generate additional pull. Obviously if the top of the airfoil is hitting the headwind instead of the leading edge of the airfoil, there is no way to generate pull.

A propeller relies on the spinning of the blades to cause air to flow over the airfoil at the proper angle, and angle the airfoil hits the oncoming headwind is no longer a problem. Thus it will work in any tailwind situation, downwind, crosswind, whatever. Downwind gives it the most direct force, so it works best downwind.

Seriously

[TranceThrust]

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

Re:What happens if you remove the wind?

[DarkKnightRadick]

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

Re:This is gonna be good

[Interoperable]

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.

SImpler; just what sailboats do

[sznupi]

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

[Omnifarious]

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:SImpler; just what sailboats do

[sznupi]

The blades of the "propeller" (rotating sail) move sideways.

Re:SImpler; just what sailboats do

[sznupi]

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)

Re:SImpler; just what sailboats do

[sznupi]

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)

Re:SImpler; just what sailboats do

[robbak]

You'll have to publish and/or patent your design. I cannot see any way to build such a craft without some form of turbine.
No, just thought of it. A design using vertical blades that are moved backwards on a chain, and folding up for the return trip. But then the grandparents argument would still hold true: the vertical blades would have to have a negative wind speed.

Re:SImpler; just what sailboats do

[w0mprat]

Each turbine blade is a sailboat at a tack angle. You could also think of the Turbine blades tacking naturally, because they rotate around an axis pointing directly into the wind, thus only relative air speed is required.

I'm no engineer, have no other qualifications than a knack for gadgets and I've been staring at these things for a while, planning to build one myself.

Stored energy in the mechanical is only used to pass equilibrium, a DDWFTTW cart would be able to sustain faster than wind speed indefinatley (as numerous youtube videos attest to).

Here is the crucial component to how this works that everyone seems to miss:

Relativity as a general principal seems to apply here. Naysayers seem to be working from a absolute frame of reference. Simple fact if you reverse the flow of air over a turbine, it is still absolutely able to extract rotational work from the relative air speed flowing along it's axis. You just need the right gearing to extract torque to wheels to drive a vehicle. If it was impossible you'd have to demonstrate why a wind turbine suddenly is incapable of generating work if it is merely moving relative to the ground, rather than anchored to the spot. (Excluding equilibrium to the air mass itself where it will not turn, however if it is turning, it'll produce thrust as it dumps momentum).

If you also imagine the turbine cutting helical path through the air, where the helix actually appears stationary (a kind of 1:1 with ground speed) if the air is then moving relative to the helix, there is of course a torque created by the turbine.

There are a number of ways to think about it that give you an 'aha' mental model of how it works. This is so simple I really feel slightly ashamed and embarrassed for every credentialed expert the world over that said DDWFTW was not possible.

Lets say you have a 50kph wind. Cart is at 0, you give it a little push off, here's what happens:

To begin with the wind pushes the cart just by bluff body drag until it gains speed. Lift to drag ratio means the counter-torque produced by the turbine will always be less than the winds total force on the cart. It'll inevitably move off down wind. That's easy to understand.

The real trick is passing equilibrium. As the air becomes still relative to the cart, the stored energy of the turbine and driveline torques against the air, producing thrust, which pushes you past.

Once past equilibrium, the airflow reverses over the turbine, relative to the cart. As the apparent wind speeds up the turbine is able to generate torque from the relative air flow, this powers the wheels. At 100kph down wind, you have 50kph of aero drag, and 50kph apparent wind to generate power. That turbine is cutting that helix through the air relative to the ground and the air is moving relative to the ground this equals available wind energy at any speed or vector provided there is wind you just need the appropriate configuration to extract it.

Again how the real credentialed experts and self-credentialed internet experts in forums missed that I don't know.

You know what's really beautiful about this? A correctly designed (perhaps variable pitch and gear ratio) turbine powered cart, can travel faster than wind speed in any direction, including directly into wind where it may be indeed possible to go faster than the wind into the wind.

I'm off to my workshop...

Re:SImpler; just what sailboats do

[Omnifarious]

Why would a boat travelling downwind bring the air to a stop? It seems to me like it would only be able to slow it down to the same speed the boat was going.

The keel, on the other hand, allows the boat to stay stationary relative to the ocean with regards to the vector of the wind. This allows the boat to bring the air's speed down to the same speed as the water on the ocean. That's significantly more kinetic energy that can be extracted.

Re:SImpler; just what sailboats do

[sidyan]

The Blackbird craft has a ratchet in its drive system which categorically prevents the propellor from ever powering the wheels, whether by air interaction or by discharging built-up/stored rotational momentum. Its presence (and verification of functionality) was mandated by the organisation refereeing the record attempt (NALSA).

Re:SImpler; just what sailboats do

[sznupi]

NVM, we used different frames of reference there.

(though the second description is still not very precise - the sail doesn't influence the mass of the air so completely, and not only the mass it has "contact" with; just like with airfoil... (also described wrong even in school books) And BTW keel is another one, another "sail"; it's behavior more dynamic than just maintaining the same heading as movement)

Re:SImpler; just what sailboats do

[Augury]

Tacking is a maneuver and I doubt anyone will ever have a vessel moving faster than the wind while tacking:

http://en.wikipedia.org/wiki/Tacking_(sailing)

I think what you mean is reaching:

http://en.wikipedia.org/wiki/Points_of_sail#Reaching

Re:SImpler; just what sailboats do

[hairyfeet]

So what you are saying it is would work with say...a paddlewheel design, just not as efficiently. The paddle wheel couples the the car to the air, the air turns the wheel (which would NOT be in any way sideways on a paddlewheel) and a simple gear connects the wheel to the car.

Re:SImpler; just what sailboats do

[ThosLives]

The ratchet is actually irrelevant (and just adds inefficiency to the system), since energy* transfer between the wheels and the ground does not contribute to forward vehicle motion at all.

Recall that the ground actually performs at best zero work in the entire system (in real systems the wheel/ground interface dissipates energy). Pure tractive friction is a workless constraint, because there is no relative motion between the wheel and the ground at the contact point (assuming no slipping). Since the traction force is not working through a distance, it performs no work. This means that all the energy driving the vehicle must be coming from the wind. The entire process is aerodynamic; the coupling to the wheels only provides the necessary momentum exchange.

Consider the angle of attack of the prop blades with respect to the wind. Starting from a standstill, a tailwind has a massive positive angle of attack with respect to the blades, generating a lot of force in the "forward" direction. It also generates some torque on the blades that must be resisted by a traction force between the wheels and the ground (otherwise the prop would spin like a windmill facing into the tailwind); in this instance that traction force must actually be against the "forward" direction, because the prop is being forced in the other direction. Since the actual force depends on gearing it can be made less than the net thrust: the vehicle accelerates.

While accelerating, the prop starts to rotate so the local wind velocity starts shifting toward the front of the vehicle. This reduces the angle of attack on the blade, but the prop is still providing forward thrust. The traction force on the wheels is still in the direction opposite vehicle motion. The net aerodynamic force is still in the direction of motion.

When the vehicle speed reaches wind speed, the prop blades still have a positive angle of attack so are producing forward thrust; they are also still producing a torque which results in an adverse tractive force. Net force on the vehicle, though, is still positive. Only when the vehicle accelerates to a speed where the angle of attack on the propellers is reduced to a small enough value such that the thrust produced by the propeller no longer exceeds the tractive force necessary to keep the propeller spinning (because the tractive force is always adverse) will the vehicle speed stabilize.

There is also the effect of the vehicle body drag which helps the vehicle speed up to wind speed but provides adverse force when the vehicle exceeds wind speed. That does not, however, alter the analysis.

*The ground interface only plays a part in momentum transfer, not energy transfer (excepting dissipative work of course).

Re:Of course

[emt377]

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.

old news

[Swampash]

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.

Re:old news

[Prune]

Wrong: sailing boats require crosswind to do this and cannot outrun the wind directly downwind. This cart can, however. It goes faster than the wind _directly_ downwind, and that is completely new. It is possible because of the mechanical advantage of the gearing between the wheels and propeller (a sail is only 1:1).

Sailing faster then the wind

[houghi]

Can a wind cart travel faster than the wind?

If boats can go faster than the wind, 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.

Re:Sailing faster then the wind

[Man Eating Duck]

Directly downwind. No windsurfer/sailboat/whatever can do that.

Re:Sailing faster then the wind

[Man Eating Duck]

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.

Other ways to extract energy from the wind. 446mph

[lexical]

http://www.youtube.com/watch?v=ByOB4luuvy4 It's called Dynamic Soaring.

I don't think you understand science

[SmallFurryCreature]

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

[IceFoot]

I agree! These guys did something quite fantastic, and just because they can't clearly explain the mathematics to us doesn't detract from the fact that they did it!

Hopefully scientists will find time to develop the theory and publish it. Until then, I think it's just like living in 1904, when the Wright brothers first flew a powered aircraft.

Re:I don't think you understand science

[dcollins]

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/

Re:I don't think you understand science

[Nazlfrag]

The thing is they can clearly describe the mathematics, the problem is this doesn't make it any less counter-intuitive.

Re:Of course

[dougmc]

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:erm..

[Tacvek]

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.

sail carts aren't practical transports

[rossdee]

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.

Re:A much more interesting question

[Tacvek]

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.

Re:Of course

[Brandano]

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:erm..

[John Hasler]

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?

Re:Of course

[Brandano]

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

Re:Of course

[cynyr]

+10000000 internets for you, this is the best description of what is going on yet.

+5 informative as well mods...

L/D

[florescent_beige]

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

[Dunark]

I used four rubber stoppers and a six-inch metal rod. Two stoppers were #10's, and two were #5's. I pushed the #5's onto the rod first so they met in the middle of the rod with the big ends almost touching. I put the #10's on the ends of the rod with the big ends on the outside. The result was an axle with two big wheels at the ends and two smaller wheels near the middle. I set it on the table, then I slid a ruler under the small wheels and pressed upward lightly. The big wheels touch the table top, the small wheels touch the ruler. Move the ruler back and forth in the rolling direction, and presto: The contraption rolls in the direction of ruler motion, but at a faster speed. Having aerodynamics involved makes analysis much harder, but I'm beginning to think that the described wind car might really work.

Re:I just made a mechanical demonstration

[Vegemeister]

A yo-yo works fine too, and you might even already have one.

Re:Windsurfers and kitesurfers

[Man Eating Duck]

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.

Re:What happens if you remove the wind?

[marcansoft]

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, which is why it works. This is also why it doesn't accelerate to an infinite speed: the faster it goes, the more insignificant the wind's speed becomes relative to the vehicle's ground speed, and, as with no wind, the system (obviously) cannot generate energy out of nowhere.

One way to look at it is that the car always travels at a fixed multiple of the wind's speed. This isn't true in practice due to the varying efficiency of the system, but it would be true if you had perfectly static friction with the wind (the multiple would be a function of the gear ratio between the fan and the wheels). In reality, it always travels at a variable multiple of the wind's speed (but still a multiple). Wind speed = 0, car speed = 0.

Re:Of course

[Bigjeff5]

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

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.

Re:upwind vs downwind

[temporalillusion]

Yeah he duped the NALSA into using the wrong terminology... He's going down wind faster than the wind, when you understand the wheels drive the propeller and look at the propellers direction of rotation, it makes sense how it works.

Re:Windsurfers and kitesurfers

[gnapster]

What is the record for going directly downwind, on a run?

Very neat demonstration

[WinstonWolfIT]

Here's my question: Can the same principle be used to drive something similar upwind?

Re:Other ways to extract energy from the wind. 446

[gnapster]

What are we looking at there, exactly? With the camera moving back and forth, it seems like they are counting the number of YouTube viewers they can make ill from motion sickness.

Re:Of course

[Frnknstn]

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

I can almost see how it works.. almost

[Bitsy Boffin]

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:I can almost see how it works.. almost

[Dunark]

We all accept that this vehicle derives it's motion through the rotation of the propeller which drives the wheels.

No, the wheels drive the propeller. The wind still pushes against the propeller, and that's what pushes the car forward, but some of the energy is taken back from the wheels to turn the propeller and blow air backwards. The result is an addition to the wind speed. Alternatively, you can think of the propeller as part of the car that is going enough slower than the car that the wind can still push on it even if the car is going faster than the wind.

Re:I can almost see how it works.. almost

[Renevith]

"without reversing the blade pitch."

You've nailed it without realizing. The trick is that the blade pitch is already reversed from the direction most people expect when seeing pictures or diagrams of this sort of vehicle. The faster the car goes, the faster the blades spin "into" the tail wind, so they pick up a constant force from the wind.

Re:upwind vs downwind

[fbjon]

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 zero. And finally, this experiment is not hard to do on your own in a small scale test.

Re:Of course

[Kiltach]

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

Intresting concept. One huge logical flaw. speed at a point on the wheel that is in contact with the ground is 0. Speed at a point on the wheel that is at the top of the wheel is 2 times the velocity of the wheel. If you're trying to go DIRECTLY downwind the apparent wind of the sail will never be greater than ground speed.

Anecdotal Evidence

[kpwelch]

What you are forgetting about is apparent wind. A long time ago on a small lake in Massachusetts, at a Red Cross sailing camp, at which I was an instructor; I took a group out for an evening sail after dinner. We used a HobieCat, (I forget the actual size, about 14' -16' (30 something years does have an effect)), but it was rated for 4 passengers. I had four passengers, plus myself. The wind was (at the start) from astern, at about 8-10 knots. As we increased speed, the apparent wind moved forward. The further forward the wind moved, the more I trimmed in the sail. As the sail moved in, the less there was 'push' and more 'airfoil' effect became functional. As our hull speed passed the wind speed, we became “close hauled” which is a condition of sail trim where the wind is more forward than abeam. The actual wind was still behind. Our motion was creating our own 'wind'. Even though we were overweight, we actually passed the hull speed of the boat, around 22 knots. I know this as the windward (toward the wind) hull came out of the water, and the leeward (away from the wind) hull started to trip. This occurs when a non planing hull passes its “hull speed” (about 1.34 x the square-root of the water line length (see Wikipedia http://en.wikipedia.org/wiki/Hull_speed ). If you take a non planning hull past its rated speed the bow wake draws the bow down as the stern wake pushes the stern up and the hull will submarine (aka dive). I did not want to get wet, so I let the sail go, and we slowed down. The actual wind had not changed much and we had to beat into the wind to get back to the dock. Kevin

Re:Anecdotal Evidence

[ceoyoyo]

Assuming your story is true, you weren't going directly downwind. You were reaching the whole time. If you were going directly downwind, the apparent wind would never come from the side. You would feel maximum wind (from behind) when you were stationary, then the apparent wind would decrease as you moved faster, but always remaining in the same direction.

Re:erm..

[LordLimecat]

I imagine that if this contraption allows you to go 10% faster than the wind, the same will remain true @ 0 knots. 110% of 0 is 0, if my math serves me, so you would not have a perpetual motion machine.

Re:erm..

[jo_ham]

What is your current power (as applied to the prop via the wheels) multiplied by zero windspeed?

I'll give you some time. ;)

Your attempt at clever dig about how it's impossible because it's a perpetual motion machine falls flat, since the maths actually works. There's no energy from nowhere.

Gedankenexperiment

[polymath69]

Suppose we imagine the following: a large room with a level floor, with no open windows and still air all around. Put the proposed cart down somewhere. From the cart's point of view, there is a prevailing wind of zero no matter what direction you've placed it.

Now the claim is that the cart can go faster than the prevailing wind. So once you place the cart down, it should move on its own. Now just where did it get the energy to do that??!

Re:Gedankenexperiment

[Your.Master]

No, the claim is that it can go downwind faster than the prevailing wind, which is completely different in that it's not impossible. Nobody said it could go anywhere with no wind, or even essentially no wind -- it has to be decidedly downwind. Let's try this gedankenexperiment:

Even without any mechanism, a cart will tend to accelerate to almost the same velocity of the wind, accounting for wheel friction. At that point of steady-state, shift your frame of reference to an inertial frame where the cart is entirely still. It's still an inertial frame so Newton's laws still apply and we're all happy.

Now the wind is almost gone, the cart is still, but the wheels are spinning like mad again an Earth that's moving like crazy. Those wheels are your turbines, and this "free" rotation is where you pick up the energy to drive a propeller, in the same direction as soft remnants of the wind, to push yourself forward.

There's something of a similarity with the old riddle about airplanes taking off of a conveyer belt: the cart isn't driving itself forward by its wheels, so it doesn't matter a whole lot how fast they spin, they're just a low-friction point of contact.

Basically, you're going at the speed of the wind, and then you're skimming energy off of the difference between the speed of the wind and the speed of the earth to go faster. That's the same method stationary land-based windmills generate energy, except they are going at the speed of the earth instead of the speed of the wind to start with, and they don't generally apply their speed to propulsion.

In the windless scenario, there's no difference between airspeed and land speed, so there's no energy to skim.

Re:Gedankenexperiment

[jellyfrog]

No one claimed the cart could move in zero wind. "Faster than the wind" in this case means some multiple of the wind speed. So it just sits there in the still air, as expected.

Re:Gedankenexperiment

[Alsee]

It can go faster than the wind, but there does need to be sufficent wind in the first place. The wind moving relative to the ground is the energy source. The important point is that energy source exists, not matter what the cart might be doing. The neat trick here is that the cart can tap into that energy source even when the cart is moving at the same speed as the wind, and it can use that energy to continue accelerating above the wind speed. Since the energy source always exists it can be used to accelerate up to arbitrarily high speed, limited only by inefficiencies and friction losses.

-

Re:nonsense

[Sparr0]

Imagine that the device has a transmission that is disengaged during initial acceleration[1]. At the point you describe, where the vehicle is traveling at the same speed as the wind, and thus the relative windspeed is zero, you engage the wheels to the propeller with a gearing such that the wheels turning at [windspeed] cause the propeller to turn at [windspeed*110%]. The wheels will apply a braking effect to the vehicle of force X, and the propeller will apply an accelerating force Y. Y>X for a small but practical range of gearing and friction values.

[1] The transmission is not required because the device operates in a physical feedback loop. If you didn't care about the inertia of the prop driving the wheels in some cases you could even do without the one-way drive bracket that NALSA installed.

Logical issues with this.

[Kiltach]

1) i'm not sure why you point out that a sailboat cannot go faster than windspeed. It can, just not in the same direction as the wind. Look up how sailboats/airfoils/tacking actually works. 2)I hate it when people use treadmill arguments its virtually never the same thing as the "actual" setup. one thing people forget is that wheels are 0 velocity at their contact with the ground REGARDLESS of actual speed provided there is no slip (the highest point is 2 times speed at the highest point on the wheel) If there is no wind there is no way to beat the treadmill (no way to keep up if you don't ignore losses) The question is how is this energy supposedly being extracted. The treadmill has only 1 way to impart energy into the system and that is through the traction/friction of the wheels. Take a wheel on a perfectly efficient/0 friction bearing and no friction from the air. (not possible obviously) that is supported off he treadmill and allow the wheel to get up to speed. The wheel would maintain speed, there would be no force imparted to the bearing from the wheel because there would be no relative motion between the point of contact of the wheel and the treadmill. No energy is being imparted into this system at all. It's all well and good to talk about extracting energy, but this setup doesn't do it.

Its simple energy extraction

[toygeek]

Or maybe not so simple. You have wind. It took energy to create that wind, and energy can be extracted out of it. Look up Dynamic Soaring. 445MPH with a unpowered radio controlled glider. Not only is it possible, but 500mph may be possible with the right plane and conditions.

if this guys from MIT, we should all give up now..

[Kiltach]

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. He shows the apparent wind on the propeller causing it to propel itself in the opposite direction. Also includes the drag force on the turbine but not on the propeller. Ironically not that it matters at this point, but later he also assumes that a wheel functions identically to a water turbine, which is actually not the case in this scenario. So yeah, like 90% of sensationalist science, does not hold up to more than casual scrutiny.

An explanation that makes sense

[Ped Xing]

Here is a link to a post that provides the best explanation I've found so far about why this works.

http://callenish.blogspot.com/2010/11/directly-downwind-faster-than-wind.html

From a moving floor.

[robbak]

Read the other explanations. Watch the you-tube explanations. Even see my ascii-art above.

The item is moving. Energy is harvested from that movement to push the craft forward.

Re:All depends

[Goaway]

No, it does not.

Re:Of course

[OopsClunkThud]

It's not a logical flaw at all and the energy available has nothing to do with the speed of the vehicle. The delta between the wind and the ground is the same regardless of what the vehicle is doing, and that's the source of the energy. The actual speed of the vehicle is limited by power that can be extracted from that delta and applied to propelling the vehicle. When the power consumed in wind drag is equal to the power propelling the vehicle then you've hit your top speed.

I'm from Missouri - show me

[taskiss]

When the cart is going downwind and there's a flag flying with the training end consistently pointing into that wind from a standard attached to the cart, I'll believe it.

Re:I'm from Missouri - show me

[im_thatoneguy]

http://www.youtube.com/watch?v=XQRW1yuYLnc

Re:I'm from Missouri - show me

[taskiss]

yeah, I read more into this and found that there's a one way clutch on the drive wheels.

Re:Of course

[John Hasler]

One huge logical flaw. speed at a point on the wheel that is in contact with the ground is 0. Speed at a point on the wheel that is at the top of the wheel is 2 times the velocity of the wheel.

The sails are feathered when they are at the top and so pick up no wind. They are unfeathered at the bottom, where they are halfway between the ground and the axle and so move at half the speed of the axle with the wind pushing on them. It will continue to push on those sails until the wheel is moving fast enough to move them at wind speed and therefor the axle at twice wind speed.

old hat

[hherb]

Going faster than the wind is an old hat - has been done in sailboats for millenia, and I have been doing it in my youth (some 35 years ago) in small lightweight go-carts using a similar sail rig as nowadays windsurfers do.
The only problem is when you want to go against the wind which is not possible (other than zig-zagging), or with the wind straight from behind (when you will always be slightly slower than the wind) - and that's where smart engineering solutions set in.

Re:upwind vs downwind

[RingDev]

Say the wind is blowing due south at 10 mph. You begin moving due south at 8 mph. You are now traveling "down wind". At some point, you increase your speed to 12 mph, still moving due south. You are not traveling "up wind" as your inirtial reference frame shows a 2mph head wind.

So yeah, in TFA and videos, they talk about the vehicle going from down wind to up wind, because that is exactly what it is doing.

It isn't casually interchanging the terms, it is accurately describing the state change as the vehicle go from speeds below the wind speed, to speeds above.

-Rick

Re:Other ways to extract energy from the wind. 446

[lexical]

Watch it in 1080HD mode. It's an RC glider exploiting the energy difference in two air masses. There is a pocket of still air behind a ridgeline while fast moving air is moving over the top of it. By transitioning between the air masses the airspeed of the plane increases with each boundary crossing.

Re:All depends

[Gerzel]

True. Indeed tricks for getting a wind powered vessel to go faster than the wind have been around for centuries. It's a common part of sailing.

Re:All depends

[jc42]

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 wind. Is this physically possible? I'd think that you could build a perpetual-motion machine if you could do this.

Re:All depends

[wealthychef]

OMG, we are going to hear all the same arguments here as have been posted elsewhere. Clearly the machine is going directly downwind faster than the wind. Look at the flags on the cart -- the are blowing first forward then backward. There is no gearing trickery. It's really happening. There are many videos, and you can build one yourself. I have not heard anyone build one and say it does not work except Mr. Platt and the inventors claim he built it wrong.
What's interesting to me is that argument that at a nonzero angle, "of course" this is possible, but not directly downwind. I note that the blades of the propeller are angled at a nonzero angle -- is this a factor? I really don't understand that part.

As shown by sailing thousands of years ago...

[cenobyte40k]

I know this place is full of geeks that don't like to be outside on the water or anything but sailing vessels have been doing this for thousands of years. Did we really need a Google grant to prove that sailboats really work?

Re:As shown by sailing thousands of years ago...

[germansausage]

Nope. See any number of posts above. Sailboats can go faster than the wind when the wind is at an angle to the direction of motion. This gizmo goes _downwind_ (ie same direction as the wind) faster than the wind, which no sailboat has ever done ever.

Re:All depends

[mosb1000]

Of course, the blades on the propeller are at an angle to the wind. . .

Easiest way to understand this

[m.dillon]

The easiest way to understand this is to consider a boat or vehicle heading in one direction with the wind going crosswise to that direction (e.g. left to right while the vehicle is going straight). Then add a sail.

In a (forward-to-back) frictionless environment no matter how fast the vehicle is going there will still be wind going from left-to-right capable of driving the sail, powering the vehicle forwards even faster. In reality the faster the vehicle is going the more the apparent wind direction shifts forward, and ultimately friction combined with the cross wind effectively becoming a head wind limits how fast the vehicle can go.

I dunno whether this would work in space. The reason a vehicle can be powered forwards with a cross-wind is, of course, that the vehicle is anchored left-to-right. i.e. it can't slide left to right, it can only slide forwards, so the power generated by the sail all goes to driving the vehicle forwards. A sail-boat works in a similar fashion (though there is an added feature in that the keel of a sailboat actually forms a foil in the water which counters the pressure on the mast). A space vehicle has nothing to 'anchor' it per-say, though I think it would be possible to create a keel-like solar sail element to partially anchor the vehicle and drive it in a more desirable direction.

-Matt

Re:Easiest way to understand this

[Antique Geekmeister]

This is how solar sails work. The sail is telted to reflects the light and solar wind at an angle, to slow or speed the sail in the course of its orbit.

That only works well inside solar systems, but that's plenty for the immediate future of space travel.

Re:All depends

[Nazlfrag]

It's not perpetual motion, it's just efficient use of the avaliable energy. I have a hard time explaining it though as does everyone I think. The trick is that the wheels drive the prop, which pulls the cart, which drives the wheels, not the other way around. A prop works just fine in a headwind, so going at or faster than downwind presents no problems.

Re:nonsense

[Alsee]

A claim like this requires some explanation of how it could be done, and such an explanation is obviously missing from the article.

You are right that this article does not get into any explanation. It does however talk about an earlier article that *did* cover the explanation. This subject was posted on Slashdot some time ago with a link that did explain it. The method is extremely non-obvious and it took me some hard thought to wrap my brain around it, but once I understood the energy flow it's clear that it does work.

The first important point is that there is an energy source. The wind is moving relative to the ground. That energy source is always present even when the bike is moving at the same speed as the wind. That energy source exists when the bike is moving faster than the wind. The trick is finding a way to tap into that energy source while you are moving at or above wind speed.

The wheels are linked to the ground. We can spend energy in the wheels to accelerate, *or* we can do the opposite by putting a load on the wheels to extract energy.

The propeller is linked to the wind. We can spend energy in the prop to accelerate, *or* we can do the opposite by putting a load on the prop to extract energy.

It seems very strange at first, but what we are going to do is put a load on the wheels. This will tend to slow the bike down, but it allows us to extract energy. We then use that energy to drive the propeller. The propeller pushes against the air - in fact it is pushing back against the wind. It supplies a force pushing the bike even faster downwind.

At first it seems obvious that you are going to lose energy if you try to push one way with the wheels and push the opposite way with the fan. In this case "obvious" is mistaken. Energy = force * distance. If the bike is running at wind speed the wheels cover a large distance of ground. You can put a fairly small load on the wheels and that force * distance will extract a fairly large amount of energy. Now look at the prop. We put that energy into the prop pushing backwards to drive the bike faster. The energy going into the prop is going to push air. That energy will equal force * distance of the air we push through the prop. Since the bike is moving at wind speed the prop is seeing MOTIONLESS air. We can apply a relatively larger force, and that force is through a much smaller distance relative to the air. The wheels travel a much larger distance relative to the ground than the prop travels relative to the wind. That difference means the force on the wheels is smaller than the force on the prop. The prop speeds up the bike more than the wheels slow it down.

1 The wind is the energy source. (no free lunches here)
2 The wind effectively pushes forwards on the air coming out of our prop.
3 The air coming out of our prop pushes forwards against the prop.
4 The prop pushes forwards against the bike frame and wheels.
5 The wheels push forwards against the ground, and energy is extracted to drive the prop.

The arrangement of prop and wheels creates a very strange but successful linkage to pass the wind force down to the ground for energy extraction. The freaky part is step 2 while the bike is moving faster than the wind.

-

Re:All depends

[Hognoxious]

The trick is that the wheels drive the prop, which pulls the cart, which drives the wheels

So the wheels' movement is both a cause and a result? Seems fishy to me.

It also implies it could move in a flat calm.

Re:All depends

[Software Geek]

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.

Re:if this guys from MIT, we should all give up no

[Alsee]

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 wheels on a bike. We put a load on the wheels to extract energy.

Pt is the power that comes out f the turbine (or equivalently, the power we receive from putting a load on the wheels).

Pp is the power we supply to the prop. This is the same as the power we obtained from Pt, less some negligible percentage of loss.

Fp is the force CREATED by the power-driven prop.

The Fp pointing forwards is greater than the Ft pointing backwards, which indicates a net acceleration.

does not hold up to more than casual scrutiny.

It fails under "casual scrutiny" because the overall operation is extremely counter intuitive. However the math does work out once I managed to wrap my brain around the strange arrangement of forces and energy flow.

Your gut reaction is probably screaming that there MUST be a net energy loss in trying to extract energy from the turbine to drive the prop and that the prop's forward force MUST be less than the turbine's backwards drag. But you must remember that the wind is a source of energy relative to the water (or relative to the ground). That wind-water difference is an energy source, and that energy exists no matter how the vehicle might be moving. The trick is how to access that energy source while you're moving faster than the wind.

Note that force and power are not equivalent. Power is energy over time, and energy is force through distance. The turbine is moving through the water while the prop moves through the air. There is a speed difference (and an energy difference) between the water and the air. The turbine moves a large distance through the water. A large distance times a small force generates one unit of power. The prop is moving in the air, and even though the vehicle is moving faster than the wind the wind greatly DECREASES the apparent speed of the prop relative to the air. Because of the wind, the prop only moves a relatively small distance through the air. The prop generates a large force over a relatively small distance, which costs one unit of power.

Turbine extracting energy: small force * large distance = 1 energy extracted
Prop consuming energy: large force * small distance = 1 energy consumed

The equations balance. The large prop force accelerates the vehicle. The wind-water difference is the energy source. It's a very unintuitive arrangement, but it does successfully tap into the energy available in the wind-water difference, even when traveling faster than the wind. That energy source covers the inevitable inefficiencies in power transfer and it the pays the cost of accelerating the vehicle.

-

Re:All depends

[mykdavies]

I'd think that you could build a perpetual-motion machine if you could do this.

No, because it relies on the wind blowing: first to accelerate, and second to maintain whatever velocity it reaches. Once the wind stops blowing, it will slow down and stop. Here's a nice illustration of how this phenomenon works: http://www.youtube.com/watch?v=9Yt4zxYuPzI&feature=related

Re:This is gonna be good

[drewhk]

It is great to see that we can produce scientific puzzles in the 21th century that are based on simple mechanics (no quantum theory, no higher math), and confuses the hell out of a lot of knowledgeable people! Really intriguing!

Re:All depends

[sFurbo]

I think it utilises the difference in air and land speed relative to the cart, so it wouldn't work in a flat calm. But I don't understand the concept fully, and I don't have the theoretical insight to tell whether it is possible.

Re:All depends

[Nazlfrag]

The motion is a factor of windspeed, so a windspeed of 0 wound imply no motion. It's not free energy, just efficient use of the groundspeed/windspeed differential.

For those who have a hard time with this

[esandover]

There are many examples (levers, pulleys, gears, etc) where the output speed can be greater than the input speed and yet energy is conserved. It is about energy not speed. The thing to look at here is the total energy imparted to the sail by the wind, and how much speed can be obtained using that energy. Imagine two vehicles of equal weight, but one with a much larger sail than the first, and the first is able to go with the speed of the wind. The second vehicle, capturing moar energy, should be able to go faster than the first. Doing so then becomes a problem of mechanics.

"Can't work, because I don't understand how"

[MartinSchou]

Thankfully science and engineering moves forward at all times, in spite of how most of their work is beyond the understanding and comprehension of the average person.

If you don't believe me, try asking people how exactly a combustion engine works. Or for something a lot simpler - a flushing toilet.

Does anyone have a link to an animation or similar

[RichiH]

As my brain is starting to melt every time I try to understand what is going on....: Does anyone have a link to an animation which explains how this works?

mrtn

[wazerface]

A thought experiment for why you can travel UP wind in one of these: Simply face the cart up-wind and lock the wheels so that it doesn't get pushed back (via some ratchet or otherwise). Store energy from the wind turbine (lets say in a battery). Then after T seconds, drive the cart into the wind (note it will have significantly less drag now because we have a spinning prop). After we use up the stored energy. Lock up the wheels and repeat. Now take the limit as T->0. There is some math for everyone :).

Velocity difference between wind and GROUND

[OeLeWaPpErKe]

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

Re:All depends

[Lord Kano]

If you're traveling downwind faster than wind speed, your sail would act like a parachute and slow you down. But if one were to build up speeds at right angles to the wind, drop sail and turn in the direction of the wind, for a while you would be traveling downwind at a faster speed than the wind.

LK

Re:nonsense

[Hank the Lion]

A claim like this requires some explanation of how it could be done, and such an explanation is obviously missing from the article. You shouldn't try to make up explanations for them (although that is how this nonsense continues to thrive).

Or to put it another way, if this thing can go through the point where it is going at the speed of the wind to then go faster than the wind the, assuming that it isn't storing energy from some time in the past and that the wind doesn't slow down, then it could also sit in zero mph wind and start going forward all by itself, just powered by a zero mph (non-existent) wind. Perhaps you believe that. I don't.

Parent has been moderated "Troll" but I think he is sincere.
And, at first, I did not believe this to be possible either.
I now understand how it works.

The propellor is used as a sail.
As long as there is pressue on the back of the 'sail' it can accelerate the cart.
If the sail were fixed w.r.t. the cart, it would stop receiving back pressure as soon as you reach wind speed. When you pass wind speed, you would get wind pressure from the front, and you would slow down.
So, what do they do: they drive the propellor from the wheels, so the surface of the propellor gets a forward speed that is lower than the forward speed of the cart itself.
In that way, even when the cart itself has passed wind speed, the surface of the propellor hasn't, and the wind can keep pushing the propellor forward, and thus, the cart.
The cart itself will feel the wind coming from the front, but the surface of the propellor, because it is turning, will still feel the wind force from the back.

Then a last question remains: will this force on the back of the propellor be greater than the force in the opposite direction on the wheels (that causes the turning of the propellor)?
This depends on the gear ratio. If you choose the gear ratio 'wrong' you will create a cart that will propel itself against the wind instead of along with it.
Notice that this is something that has been done earlier many times.

If it would be stationary, it would get close to wind speed, but never be able to pass it.
But the propellor is being driven by the wheels.
This gives the surface of the propellor blades a speed forward (wrt the ground) that is lower than the wind speed, even when the cart itself is already above wind speed.
In this way, the wind can still exert a forward force on the blades, and thus propel the cart.

Re:Of course

[dougmc]

For the more scientific people --

http://kimballlivingston.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf

Drela's expertise in matters regarding aerodynamics is well known, and so if he supports the idea (and explains how it can work), that gives it far more credence than a video of a ruler being moved over a small model.

I haven't gone through the analysis step by step yet and I'm still pretty skeptical -- but if Drela says it's possible, it probably is. (Of course, there's no guarantee that this really is written by Drela, but even if it's not, the thing to do is still to verify the reasoning done in every step ...)

It runs on a differential boundary

[Anonymous Coward]

It really is simple. It's an engine that works on the principle that a differential boundary exists between the two media upon which it is in contact with. Since water wheels work (exact same principle) and the stirling cycle works (the thermal variation of the concept), there's no reason why this shouldn't. The fact that engineering and physics didn't consider the frame of reference problem is what's hilarious. (Or rather sad, depending on your perspective.)

In this case the principle is being exploited in the velocity differential between a fluid and a solid. (air & ground) It has also been proven to work with a fluid and fluid, as various boats have been built using the same concept. This concept could very easily be exploited for things that crawl along inside pipes, etc.

Now if somebody want's a real engineering challenge, try making an aircraft designed to work specifically at a wind shear boundary. THAT would be interesting.

Re:Does anyone have a link to an animation or simi

[fxmastermind]

Animations to show physics
http://www.youtube.com/watch?v=FqJOVHHf6mQ
http://www.youtube.com/watch?v=IMEerIkOVZ0
http://www.youtube.com/watch?v=vVMqa7Mft0k
http://www.youtube.com/watch?v=zPFzHoubQzg
Perhaps the best one of the bunch
http://www.youtube.com/watch?v=UGRFb8yNtBo

The original video of a cart that most people didn't believe
http://www.youtube.com/watch?v=aJpdWHFqHm0

Video made by spork and JB for MythBusters
http://www.youtube.com/watch?v=xHsXcHoJu-A

Mod parent Informative

[RichiH]

Thanks. And yes, http://www.youtube.com/watch?v=UGRFb8yNtBo is the best one by far. The only one I needed.

Re:if this guys from MIT, we should all give up no

[benthurston27]

You seem to understand this well, what if you tried to sail into the wind? Collecting wind energy with a propeller to turn a propeller in the water, could you make any forward progress directly into the wind?

Re:All depends

[yndrd1984]

What's interesting to me is that argument that at a nonzero angle, "of course" this is possible, but not directly downwind.

Er, because they use completely different techniques?

A boat with only a sail uses the difference between the speed of the wind and the vehicle as a source of power - as soon as you reach wind speed, you've removed your own power source.

I note that the blades of the propeller are angled at a nonzero angle -- is this a factor?

No - it makes no difference. As long as the vehicle is approaching wind speed in the direction of the wind, and the blades move forward with the vehicle, there's no speed difference for it to use.

But you can go upwind if you use the right technique (tacking a boat is an example) - but to do this you need a keel (or wheels) that let you push off against another medium. Now you can use the difference in the speed of the air and the speed of the water (or ground), rather than just the difference between your speed and the wind's.

And (here's the trick) it doesn't make a difference if you consider the ground or the air as the thing that's stationary. So when you're stationary relative to the air, you can use the ground rushing past you to "tack" into the onrushing flow of ground that's coming at you by pushing off the "motionless" air. It's tricky, but I hope that got the idea across.

Buthurt causes denial? Scientific unsound.

[Vernes]

Charles Platt: 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. Suddenly, I wondering why this guy's opinion matters?

Re:All depends

[Mikey48]

It's clearly a hoax, at least in the implication that the cart can sustain faster than wind travel.

The propeller is capturing wind energy while the cart is moving slower than the wind. Later that energy is used to temporarily propel the cart faster than the wind. Note that in the video they stop the cart immediately after they get to top speed.

Another thing to consider -- while the cart is moving near wind speed there may be other effects (air turbulence) that cause the flags to move opposite the direction of travel.

Obviously, if the cart were moving faster than the wind, then in it's reference frame the wind would be going against it. How can you get positive acceleration from a negative force?

Re:All depends

[wealthychef]

bviously, if the cart were moving faster than the wind, then in it's reference frame the wind would be going against it. How can you get positive acceleration from a negative force?

That sounds reasonable, but the cart is also mechanically connected to the ground, so it seems possible to me that it is able to in a sense use the relative velocity of the wind to the ground to propel itself even though the relative velocity of the wind to the cart is low or even a headwind. Again, I have not see the math involved. I was just convinced by the treadmill experiments.
If it's a hoax, it's quite a good one. I have not built one myself, has anyone else on this list?

Re:All depends

[Goaway]

It's clearly a hoax

A hoax, repeated by many people? Whose videos you can go look at, all over Youtube?

The only thing that is clear is that you are dismissing something out of hand because you have not actually put in the effort to understand what is going on.

Re:All depends

[jc42]

Hey, imagine how entertained I was to find that the thread I'd replied to had migrated to the top of the discussion. ;-) It's no longer preceded by all the misdirection about sailing, sails, sailboats, etc. Well, except for the message that I replied to that described the faster-than-wind trick as "a common part of sailing".

But now that the furor has died down, and the linked videos aren't slashdotted, it might be time to explain: No, it has little if anything to do with sailing. The gadgets under discussion don't have sails. If they did, when running downwind, the drag would be too strong to get up to wind speed; the sail would act as a big parachute to brake the motion.

Instead, what is shown is a simple mechanism to convert the wind-vs-ground differential to rotary wheel motion. It's fairly simple if you look at it. And its upper speed is basically limited by the friction in the mechanism (including friction with the surrounding air ;-). It's fairly obvious that the engineers who built these have managed to build the linkage with low enough friction that the mechanism as a whole can build up some pretty good speeds.

This really should be no more surprising than the fact that sailboats (there I go introducing them again ;-) can sail into the wind. An amusing historical factoid was that back in the 15th century, Europeans apparently didn't know how to do this, and their big square-rigged sailing monsters couldn't sail into the wind. They learned how to do it when they finally visited the Pacific and Indian oceans. There have been some pretty funny stories about the confusion among the crews when they approached Pacific islands, where the inhabitants came out to greet them in their puny, primitive sailing canoes - and sailed in circles around the big European ships! So much for advanced European technology. Of course, the European sailors quickly got over their surprise, took a few sailing lessons from the natives, and figured out how to build and use that sort of sail.

Actually, I once saw some hobbyists demo a small boat with a square sail that could tack into the wind. It had internal battens that maintained its curved shape, and was attached to the mast like polynesian sails, so it could be swung around without the shape change required by most sails. It wasn't quite as efficient as a big triangular sail, but it worked as an airfoil in the same way.

Anyway, it could be interesting to see if these guys can make their mechanism work in a boat pushing through water. The resistance might be too high there, though. OTOH, they might be able to doctor it into a mechanism that work for all headings relative to the wind. Sailboats can't sail within 35 degrees of the wind or so, depending on things like hull shape, but this mechanism probably could. Whether it would achieve better efficiency overall isn't obvious.

Of course, another idea might be to use the prop to drive a turbine. I wonder how the efficiency would work out in that case, over a wide range of speeds and directions.

Re:All depends

[jshazen]

That youtube link provides a great explanation!

Re:if this guys from MIT, we should all give up no

[Alsee]

Yes, all of the same physics apply to a boat and you can use it to go up wind as well. I don't have the link handy, but I was reading about someone who built a wind-powered boat that goes up wind no problem. I read he took his website down because he got sick of being attacked by people calling him a liar and fraud over it.

The wind is a power source. It's not much different than having a gas-engine on board. Direction and top speed are "mere" design issues. It's merely a question of efficiency and drag. The only fundamental difference for boats is that they obviously have bigger issues with drag :)

I was thinking that an efficient hydrofoil could probably reach at least double downwind speed. That would be double-awesome just for being a hydrofoil! :) A hydrofoil would also improve your upwind top speed potential, but I have a feeling that lifting into hydrofoil mode with an upwind design might present a nasty engineering challenge.

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