Down wind faster than the wind

[sol invictus]

Have I been wasting my time reading this thread about something that's nothing more than a poorly conceived perpetual-motion machine?

This thing, if it works, is certainly not a perpetual motion machine. Neither is a sailboat, and for the same reason: it uses the wind for power, or more precisely it exploits the difference in velocity between the wind and the sea, so it has an energy input.

For those that think the treadmill is not equivalent to moving at wind speed, pause, take three deep breaths, and think again. As many have pointed out, relativity guarantees they are identical.

For those that think that moving downwind faster than the wind is impossible for a wind powered device, you've obviously never been on a sailboat or a windsurfer. With a bit of jibing it's very easy to do.

Finally, for those that agree with all of the above but aren't sure if this particular device will work, I agree. It's not at all obvious. But the video looks pretty convincing, and no principles of physics are being violated - so why not?

It may help to realize the following: a device at rest with respect to the ground but with the air in motion can use the wind to propel itself either up- or down-wind. A device at rest with respect to the air, but with the ground in motion, can use the motion of the ground to propel itself up- or down-ground. There is a perfect symmetry (apart from the obvious physical differences between ground and air). So if you can convince yourself that a device at rest on the ground/sea can move upwind, you're done. And of course it can - sailboats do it all the time, as can many other devices.

 

[spork]

Wait... I originally thought this device was supposed to be powered by the wind turning the propeller, like a windmill.

It's powered by the wind pushing on the propeller, as the wind pushes on the sail of an ice boat - but it the wind does not turn the prop as a windmill.

But from what's been written since, it's the propeller pushing the air backwards which supplies the forward momentum which turns the wheels which powers the propeller which pushes the air backwards...

Am I understanding this correctly?

Not quite - but not terribly far off. The wind does push on the prop (actually it provides lift over the airfoil of the prop. But it is the kinematic constraint of the wheels and transmission that causes the prop to turn - and this is clearly a key element. It's really far more easy to envision it when laid out flat (i.e. the ice boat on a downwind tack) rather than through the blades of the prop rotating about an axis and geared to the wheels. That just makes the problem more fun (and a little harder to believe). My vector diagram explains it through the ice boat analogy.

Have I been wasting my time reading this thread about something that's nothing more than a poorly conceived perpetual-motion machine?

We think its a very nicely conceived perpetual-motion machine

 

[JWideman]

I'm going to simply call you on this one.

You've just stated that you've "done the experiment" that proves the treadmill and the street are not the same.

I've stated no such thing.

 

[humber]

You have got to be kidding me.

It's NOT A FRAME OF REFERENCE it's a MODEL

The TREADMILL is the MODEL of the wind
The PROPELLER must be in the wind, so in the TREADMILL

There's a second magical wind. Please, tell me, without saying you have tried it, do you explain where the second wind comes from?

 

[my_wan]

No. The fact that it is not the same difference is essential to the function of the device. This is what you have not understood yet.

If there is a difference there is usable power, period. Even if the difference isn't fully maintained.

No, not at all. The difference is directly dependent on the speed of the vehicle. It has nothing to do with whether the craft is built properly or shoddily. It is maintained, but not constant.

Again you have admitted the difference is maintained whether constant or not. If the is a difference there is usable power.

Also, I'm not saying that it doesn't work. It works, but not the way you think it works.

Do you have any idea how many different ways you can describe the same thing and they all be equally valid? There are at least 3 different formulation of thermodynamics alone that are equally as valid.

Which is why I said to be careful with signs and reference frames. I said that for a reason. Listen well: when the propeller pushes the air backward with respect to the device, the air is moving forward with respect to the ground. Try to read again what I have written, and this time pay extra attention to signs and reference frames.

Ok, that's actually a perfectly valid statement but in what way does it relate to the argument I made? You didn't even relate it to any specific claim.

With respect to the device, yes. With respect to the ground, no. The air pushed back is moving in the opposite direction of the air wrt device, and in the same direction as the air wrt ground.

This was in response to me saying:
"When the craft first takes off the prop is moving air in the opposite direction of the wind."
When the craft first takes off wrt the ground and wrt the device is the same thing. Remember this, the speed of the craft is almost but not quiet zero here. Look at the diagram:

<-- Craft moves to the left
You say:
1) The air pushed back is moving in the opposite direction of the air wrt device.
2) The air pushed back is the same direction as the air wrt ground.
Number 2 is dead wrong. It will be right once the craft exceeds the ground air speed but the single sentence you quoted and responded to specifically labeled it, "when the craft first takes off".

If you don't realize this, then you don't understand how the device works.

You do of course now realize that the statement you want me to realize as the truth is dead wrong. If not reread my response one quote up with the diagram.

You can't see something very crucial in the video: the speed at which the propeller moves the air backwards, wrt device. This is less than the speed of the vehicle wrt ground. Which is what makes the air moving through the propeller move forward wrt ground.

Uugh....

At this moment, I'm afraid, you're missing the correct understanding of how the device works. This needs to be remedied in order to proceed. I would suggest that you put the propeller version aside for now, and concentrate on the much simpler version of the device, the illustration of which was kindly provided by Spork here.

Trying to say that is how the original craft works is like drawing a paddleboat and saying that is how a sailboat works. If these things are equivalent then wouldn't you say a propeller and a windmill are equivalent? Why the do propellers often get over 90% efficiency when a windmill is limited to less than 59% efficiency?

Answer these questions: if there were no friction and no drag (except air pushing on the blades), would the device accelerate? If yes, would the device accelerate indefinitely? If no, what would the top speed be? And why?

That's not even a question. Can air push on both sides of the blade? Apparently not sense there is no drag. Except you seem to assume the blade has drag because you are baiting me with indefinite acceleration. Do you realize that the speed of air has little to do with the speed of air molecules except statistically? No friction? Never mind, too messy. Didn't you mean to start this paragraph with, "Riddle me this"? This makes as much sense as asking what would happen if an unstoppable object hit an immovable object. No matter whether I get it right or wrong in your imagination you would then claim it means something real. Get real!!!

After you understand how the simple version works, we can move back to the propeller version.

So I guess I need to figure out what really would happen when an unstoppable object hit an immovable object. Then I could be as smart as you...

Don't debate with me anymore..

 

[Brian-M]

As I read it, and I could be wrong...

The propeller is a sail. The wind pushes the sail and the cart moves along. Because the wheels are linked to the propeller the propeller starts to turn. This is in the opposite direction than if the rotation was being caused by the wind. The rotating propeller is then meant to act like a sail on a tack (this is the terminology? I don't do sailing) and you can then go faster than the wind.

This is I think what is claimed to happen.

I hadn't thought of thinking of the propeller as being used as a sail while below wind-speed, but that doesn't affect what happens when it goes faster than wind-speed.

Originally, I thought he meant the headwind was supposed to turn the propeller, which would then turn the turn the wheels, moving it forwards. The problem with this is, the force of the headwind pushing the craft backwards would always be greater than than the rotational force imparted to the propeller pushing the craft forwards (by the connection to it's wheels).

For traveling upwind, this can be compensated for through gearing by sacrificing speed. However, since it's traveling downwind, reducing speed reduces headwind, which would then reduce speed even further, which would reduce headwind further... and so on, making it physically impossible to maintain the faster-than-wind speed.

From later posts, it looks like he means the spinning of the propeller would push the air backwards, providing the craft with a forward thrust which turns the wheels faster, which would then turn the propeller faster, which would provide more forward thrust... and so on.

If so, he's forgetting that the energy imparted to the propeller to produce this forward thrust is taken from forwards momentum of the craft itself, so he's subtracting forward momentum in order to spin a propeller to provide forward momentum. Somehow he thinks he can get more power out of the propeller than the propeller takes from the craft via the wheels.

 

[sol invictus]

You have got to be kidding me.

It's NOT A FRAME OF REFERENCE it's a MODEL

The TREADMILL is the MODEL of the wind
The PROPELLER must be in the wind, so in the TREADMILL

There's a second magical wind. Please, tell me, without saying you have tried it, do you explain where the second wind comes from?

I assume you're talking to me? You're asking what the analogue of the propeller is for the ground?

It's the wheels, of course. The motion of the ground relative to the device makes the wheels spin, just as the motion of the air makes the propeller spin.

Maybe it would be easier to think about if you imagine a boat with two propellers, one in the air and one in the water, connected so that they must rotate together. Or if you don't like that, a zeppelin floating in the air right at an interface: the air above it is moving with respect to the air below it. In all these cases, there is a nearly infinite supply of energy available in the relative motion of these two fluids, and one simply needs to tap it somehow.

 

[ThinAirDesigns]

I've stated no such thing.

JW, there are strange things afoot here

I attempted to explain that there's an ironclad principle of physics which states that there is NO experiment which can discern the difference between this device on the treadmill and this device on the street.

You responded:

It's not that I don't understand the physics. It's that I've done the experiment, ...

I have no idea what to say so I will say only this -- I can only go by the words you write.

JB

 

[humber]

Nevermind

 

[mhaze]

There are no sails. If the treadmill is the wind, how do you disconnect the propeller?

I don't know how the two are connected, but the treadmill tests could have nothing in common with real world claims of faster than wind travel.

The treadmill eliminates the issue of wind at a slant angle to the prop. In that case, I think we could all agree for a large variety of wind impact angles, the prop could pick up more energy than what is required to overcome forward drag for movement "forward".

However, if the prop is exactly head on to the wind, then as the vehicle increases speed, the relative wind increases and with this to a first approximation, lift increases as the square of the speed while drag as the cube. Effective thrust decreases, instead of increases. Result: As the vehicle accelerates against the wind, it must slow down and slow down until it is going slower than the wind.

That is not even taking into account frictional losses in the drivetrain. There is a bit more here related to prop bite/camber/airfoil but these will not affect the conclusion.

By the way, one of the tests on the treadmill was invalid - this is the one where the machine starts about 18" from the wall and moves forward away from the wall. This provides a semblance at start up of what is called "ground effect" for aircraft.

 

[JWideman]

JB, when I said "I've done the experiment", I was referring to the craft spork posted the plans for. Basically your craft is a prop coupled to a wheel, no weird gearing, no flywheel, just very simple. Had one of those as a kid, couldn't understand why it wouldn't run forever. After all, the wind turns the prop, providing energy to the wheel; the wheel turning provides energy to the prop. That's how I learned about entropy.
But if that's not what is supposedly happening, what is?

 

[Brian-M]

Lets make a simple example of a vehicle that can travel downwind faster than the wind, that we can all agree on. Assume an iceboat with zero drag and blade friction in the forward direction with the sail down. We know iceboats can go several times the speed of the wind at an angle, even if you don't believe the downwind component can be faster than the wind. Get the boat up to speed, then drop the sail and turn downwind, and with zero friction and air resistance it will continue downwind faster than the wind indefinitely. After some time the average speed in the downwind direction will be faster than the wind. With very little friction and drag, the boat will eventually slow down, but there are some non-zero values of friction and drag below which it is possible to average faster than the wind in the downwind direction, by re-accelerating occasionally. So I think we can all agree that in theory traveling downwind faster than the wind is possible.

Of course traveling downwind faster than the wind is possible. I do this all the time in my car, using stored energy from fossil fuels. I could do the same thing in an electric car using stored energy generated from the wind. Your iceboat example uses stored energy to do this too (in this case, it's stored as kinetic energy perpendicular to the direction of the wind). With zero friction, any idealized hypothetical vehicle can maintain this high speed indefinitely.

There's no problem with using stored energy to travel downwind faster than the wind.

The problem is that the craft in question is not using stored energy, but generating the energy to maintain it's faster than-the-wind velocity from it's own headwind.

It's velocity creates a headwind that is used to generate power to maintain and increase it's velocity.

That's like running a light bulb from an array of solar panels powered by the light produced by the same light bulb.

ETA: Generating energy from the headwind (or the wheels) reduces velocity. Basically, you're harnessing the kinetic energy of the craft itself. Using energy extracted from it's own kinetic energy to increase it's kinetic energy never results in more kinetic energy than you began with. (In real life, it results in less kinetic energy than you started with.)

 

[CyCrow]

I believe it works, and the key is the lift-to-drag ratio of the rotating airfoils (i.e. propeller). There is no doubt that energy is available in the differential motion of the ground and the air. Another key is the gearing and pitch speed of the propeller, which must match the desired top speed, which is related to wind speed.

So the top speed will be limited by the wind speed, the lift-to-drag ratio of the prop, and the friction and drag forces. There is a lot of similarity to ice boats in this respect. The prop is in effect a continuously downdind-tacking airfoil.

As an aside, another unintuitive example of extracting energy from relative motion is dynamic soaring: http://en.wikipedia.org/wiki/Dynamic_soaring The record is 365 mph, the wind speed was naturally much lower.

// CyCrow

 

[ThinAirDesigns]

By the way, one of the tests on the treadmill was invalid - this is the one where the machine starts about 18" from the wall and moves forward away from the wall. This provides a semblance at start up of what is called "ground effect" for aircraft.

Very observant mhaze.

For the record, that video was not done by us, but rather one of our biggest critic who after seeing our first cart decided he would try it for himself.

He was surprised and we were delighted.

JB

 

[Acleron]

*sigh*
Sometimes I wonder what is sadder - if it's woos who claim they invented machines that violate the law of conservation, or if it's people who miscomprehend a perfectly valid mechanism to the point of claiming it violates the law of conservation.

I think you have a little more chance of finding out that you're wrong than the woos. Good luck.

This should be a lesson to all skeptics, including myself: never be so full of yourself to think that you cannot be dead wrong about something.

Then answer the question I posed. If the vehicle is at the wind speed, what forces are acting on it?

 

[Thabiguy]

Do you have any idea how many different ways you can describe the same thing and they all be equally valid? There are at least 3 different formulation of thermodynamics alone that are equally as valid.

It's not a question of describing the equivalent thing differently. We are saying different things. You are saying that the device can, in theory, accelerate indefinitely (for a given construction). I'm saying that it can't, even in theory.

You say:
1) The air pushed back is moving in the opposite direction of the air wrt device.
2) The air pushed back is the same direction as the air wrt ground.
Number 2 is dead wrong. It will be right once the craft exceeds the ground air speed but the single sentence you quoted and responded to specifically labeled it, "when the craft first takes off".

Number 2 is right, all the time. Particularly when the craft first takes off. I know what I'm saying.

You do of course now realize that the statement you want me to realize as the truth is dead wrong. If not reread my response one quote up with the diagram.

No. The statement that I want you to realize is actually true. The problem is that you think it is false, and it is not.

Trying to say that is how the original craft works is like drawing a paddleboat and saying that is how a sailboat works. If these things are equivalent then wouldn't you say a propeller and a windmill are equivalent? Why the do propellers often get over 90% efficiency when a windmill is limited to less than 59% efficiency?

*sigh*

That's not even a question. Can air push on both sides of the blade? Apparently not sense there is no drag.

What I was asking was to ignore practical losses, practical inefficiency. To take it as an ideal model.

Except you seem to assume the blade has drag because you are baiting me with indefinite acceleration. Do you realize that the speed of air has little to do with the speed of air molecules except statistically? No friction? Never mind, too messy.

I have absolutely no idea what you're talking about here. What I asked you was a perfectly valid question. There is no "bait". It's all about understanding the physics of the model.

Didn't you mean to start this paragraph with, "Riddle me this"? This makes as much sense as asking what would happen if an unstoppable object hit an immovable object. No matter whether I get it right or wrong in your imagination you would then claim it means something real. Get real!!!

Again, I have absolutely no idea what you're talking about. I can only conclude that you must have misunderstood what I was asking you. I asked you to consider it as an ideal model.

So I guess I need to figure out what really would happen when an unstoppable object hit an immovable object. Then I could be as smart as you...

This has no relevance to what I was saying.

Don't debate with me anymore..

I apologize for assuming that you were here to discuss.

 

[humber]

MHaze,
Well, what I was saying is that is not an inertial frame of reference. This platform is not notional in any way. It's rolling-road model. It's not the wheels that are the ground it's the treadmill belt. Everything is literal. The wheels are wheels, the air is the air, the propeller is the propeller. Just like on Mythbusters.

So, only the labels are changed. But, the body of the treadmill is connected to real ground, so the belt is also real ground. Call the air the ground if you like. No matter, as long as you realise what that means.

There is little impetus. The impetus between an operator, and the cart is the same that would be felt in the real world. Because, it is the real world.
Friction aside, the bigger one is as easy to move around as the little one. The only impetus lies in the difference between the forces controlling the cart itself, and how they are coupled to the belt by friction through the wheels. Moreover, the impetus is the momentum stored in the propeller.

I am not saying that a windblown model cannot be, of course not, but that this model is not an indicator of the potential for greater then wind-speed vehicles, other than flywheel driven, of course.

Brian-M, Mhaze. At least 3 of us understand.

 

[Thabiguy]

Then answer the question I posed. If the vehicle is at the wind speed, what forces are acting on it?

The important force is the force of the air pushing the propeller forward. Other than that, it's ground friction, internal friction, lateral drag on the propeller as it rotates... You could list more forces, like gravity pulling the vehicle down, force exerted by the ground on the vehicle as it doesn't let it fall down, forces between individual components of the vehicle... It's difficult to answer your question when I don't know where you're going and what is therefore of interest to you.

The force that is key to accelerating above wind speed is the force of the air pushing the propeller forward.

Again, it's more apparent if you consider the simplified model, posted here. When the device moves at wind speed, the key force is the force of the air pushing the blades forward (as the blades are moving at half the wind speed).

 

[Thabiguy]

Spork, JB,

if you're reading this: can you please tell me whether I was right in what I was trying to explain to my_wan?

If I'm mistaken in some way, I'd rather find out.

 

[ThinAirDesigns]

humber:

Moreover, the impetus is the momentum stored in the propeller.

Anyone can come up with theories humber -- coming up with one that can be *tested* and falsified is the basis of scientific research.

I'll ask again for you to provide me with a test which can be executed which will prove your theory right and our theory wrong.

Please?

JB