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Old 10th November 2008, 05:10 PM   #41
Thabiguy
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Okay, I have now come up with a model of a device similar to the OP that is simple enough to analyze and looks like it should work. It doesn't use a propeller, it uses a blower.

You need two same-sized wheels, wheel 1 and wheel 2. Wheel 2 is geared to 1 so that it turns in the opposite direction and half the angular speed of 1. (Yes, half, not twice.)

Wheel 1 will be rolling on the ground, wheel 2 will be fitted with blades and its upper portion will be exposed to the wind (think water-mill-wheel). The rest will be made as aerodynamic as possible.

The wheels are so geared that the blades are always moving, with respect to the ground, at half the speed of the whole device. Subsequently, when the device is moving at the speed of the wind, the blades are still moving slower than the wind, and the drag of the air will push them forward, accelerating the device.

The ideal device would move at twice the air speed; practically, its speed will be lower because of the drag of the aerodynamic part, ground friction, etc. etc. - but if these are lowered enough, it should still move faster than the wind.

So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.

And no, it's not over-unity and energy does not come from nowhere.
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Old 10th November 2008, 05:52 PM   #42
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Originally Posted by Thabiguy View Post
Okay, I have now come up with a model of a device similar to the OP that is simple enough to analyze and looks like it should work. It doesn't use a propeller, it uses a blower.

You need two same-sized wheels, wheel 1 and wheel 2. Wheel 2 is geared to 1 so that it turns in the opposite direction and half the angular speed of 1. (Yes, half, not twice.)

Wheel 1 will be rolling on the ground, wheel 2 will be fitted with blades and its upper portion will be exposed to the wind (think water-mill-wheel). The rest will be made as aerodynamic as possible.

The wheels are so geared that the blades are always moving, with respect to the ground, at half the speed of the whole device. Subsequently, when the device is moving at the speed of the wind, the blades are still moving slower than the wind, and the drag of the air will push them forward, accelerating the device.

The ideal device would move at twice the air speed; practically, its speed will be lower because of the drag of the aerodynamic part, ground friction, etc. etc. - but if these are lowered enough, it should still move faster than the wind.

So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.

And no, it's not over-unity and energy does not come from nowhere.
But is is paradoxical, I think
If the propeller's speed is proportional to the 'wind', to increase that speed, the angular momentum of the 'propeller' must be increased as the square of that speed.

Last edited by humber; 10th November 2008 at 05:59 PM.
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Old 10th November 2008, 06:08 PM   #43
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Originally Posted by humber View Post
But is is paradoxical, I think
If the propeller's speed is proportional to the 'wind', to increase that speed, the angular momentum of the 'propeller' must be increased as the square of that speed.
Nope, there's no paradox. It's important to realize that the device will not accelerate indefinitely: there is an equilibrium and the system will tend towards it.

Ideally, the equilibrium would be (for this particular gear factor) at twice the wind speed. Practically, the equilibrium will be somewhere below that - at the point when the forward force of the wind pushing against the exposed blades matches the resistance of friction, unwanted drag etc.

When the equilibrium speed is reached, the device will no longer accelerate, and in fact will slow down from speeds higher than that.
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Old 10th November 2008, 06:10 PM   #44
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Originally Posted by Thabiguy View Post
So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.
Your design won't work because of the same reason why the propellor won't work. Drag.
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Old 10th November 2008, 06:16 PM   #45
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Originally Posted by technoextreme View Post
Your design won't work because of the same reason why the propellor won't work. Drag.
Drag between what and what?

(P.S.: I wish you stopped editing your post so much. )

Last edited by Thabiguy; 10th November 2008 at 06:17 PM.
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Old 10th November 2008, 06:27 PM   #46
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Drag is what makes the device work.

When the wind is blowing and the device is at rest, all of the drag of the wind on the device pushes the device forward.

When the device is at wind speed, the drag of the wind on the device is zero, except for the exposed blades, where the drag results in forward force. So the device will still accelerate.

In other words: drag of the air on the exposed blades is a forward force dropping towards zero (at twice the air speed), while drag of the air on the rest of the device is a forward force dropping towards zero at the air speed and then a rising backwards force. The equilibrium (ignoring other kinds of friction) is where the forward and backward forces become equal - somewhere between the air speed and twice the air speed. By making the exposed blades more "draggy" and the rest of the device more aerodynamic, you push this equilibrium closer towards twice the air speed.
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Old 10th November 2008, 06:29 PM   #47
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Originally Posted by Thabiguy View Post
Drag between what and what?

(P.S.: I wish you stopped editing your post so much. )
The propellor/blower/whatever and the air.
Quote:
When the wind is blowing and the device is at rest, all of the drag of the wind on the device pushes the device forward.
No. If that wheel starts turning then you have the drag from the wheel pushing to stop it from rotating.
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Old 10th November 2008, 06:33 PM   #48
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Originally Posted by technoextreme View Post
The propellor/blower/whatever and the air.
That drag will exert forward force on the device, anytime that its speed is below twice the wind speed. See above.
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Old 10th November 2008, 06:37 PM   #49
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Originally Posted by Thabiguy View Post
That drag will exert forward force on the device, anytime that its speed is below twice the wind speed. See above.
Yeah in the opposite direction you assume it to be.
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Old 10th November 2008, 06:38 PM   #50
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Originally Posted by technoextreme View Post
Yeah in the opposite direction you assume it to be.
What the heck are you talking about? If the wind is blowing at something that is moving slower than the wind, the wind is pushing it forward.
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Old 10th November 2008, 06:43 PM   #51
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Originally Posted by Thabiguy View Post
What the heck are you talking about? If the wind is blowing at something that is moving slower than the wind, the wind is pushing it forward.
Right but the air is also pushing it backwards.
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Old 10th November 2008, 06:46 PM   #52
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Originally Posted by Thabiguy View Post
I think it could work - going downwind faster than the wind is directly equivalent to sailing against the wind (air -> water, ground -> air), which we know can be done. Whether this particular device can do it, I can't tell, but I don't think it matters in principle - even if this device couldn't do it, you could definitely build another one which could.
Just connect two sailboats, iceboats, or land yachts with a long sliding bar with a seat in the middle, and let them tack back and forth in opposite directions. The seat and the center of mass of the system could move straight downwind faster than the wind, or move straight upwind.
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Old 10th November 2008, 06:53 PM   #53
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Originally Posted by Modified View Post
Just connect two sailboats, iceboats, or land yachts with a long sliding bar with a seat in the middle, and let them tack back and forth in opposite directions. The seat and the center of mass of the system could move straight downwind faster than the wind, or move straight upwind.
Good idea! Hadn't thought of that.

Originally Posted by technoextreme View Post
Right but the air is also pushing it backwards.
Are you talking about the internal drag and friction of the device? Sure, that is why the equilibrium will not be at twice the air speed, but lower. It won't, however, prevent the device from working. - You may ask yourself, what is the equilibrium speed when the wind is blowing on the device.
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Old 10th November 2008, 08:16 PM   #54
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A vehicle has a 1:1 wheel/fan ratio ,and travels approaching windspeed x. You say that if you gear it down by two, the speed may approach 2x. You are ignoring the fact that only the same amount work is available in each case. The fan must obey the principle that pressure is inversely proportional to velocity.
This is another way of saying what has already been said by others. One gain cancels out another, etc.

The mass of the fan is not important, but to illustrate that you need more energy than is available, even on an incremental basis.
The filmed model probably exploits the flywheel effect. The builder remarks that the wind is variable. A gust will accelerate the vehicle, and store energy in the mass of the fan. There's a lot of energy in that fan. If the wind falls again, the vehicle is powered by the wind and the stored energy. In this way, the velocity is averaged, perhaps close to the speed of the gusts, which are higher than the measured or perceived average. The sock is useless indicator in this regard.
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Old 10th November 2008, 09:01 PM   #55
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Originally Posted by my_wan View Post
For power you need a difference somewhere to take advantage of. This craft takes advantage of the difference between road speed and wind speed.

The wind power doesn't come from the difference between the wind speed and road speed, but from the wind speed relative to the device.

If the device is traveling faster than the wind, then it's effectively travelling upwind.

It's possible, in theory, to create a wind-propeller powered vehicle that travels upwind, but because of friction, it must be geared down to work. If you have a 5 km/h headwind, then the device must travel at less than 5 km/h in the opposite direction. If this wind speed is caused by the motion of the vehicle, it will slow down to a stop.

(If it slows down, there is less headwind, which makes it slow down more, which means less headwind... and so on.)
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Old 10th November 2008, 09:35 PM   #56
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Hey folks,

JB and I are the ones that posted the recent DWFTTW vehicle videos on YouTube. We've described and debated the theory on several forums. For about three years people on various forums called us charlatans and fools for thinking such a thing would ever be possible. Finally, JB insisted we build the darn thing and put the questions to rest. You can find the videos we've posted on youtube at "spork33".

There's no particular rhyme or reason to the vids. Some are just tests and demos. Others were made to answer specific questions people have posted on other forums (such as "can it self start with a tailwind?"). If you have a specific test you'd like to see just let us know. If you think we're faking it somehow, tell us how we're faking it. We'll do whatever we can to eliminate any possibility of fakery.

The thing is real, it's simple, it goes downwind faster than the wind, and it has no real purpose. And NO - it's not perpetual motion. If the wind stops, it stops.

Last edited by spork; 10th November 2008 at 09:37 PM.
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Old 10th November 2008, 09:53 PM   #57
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Okay, here's my final analysis of the device in the OP.

The key characteristics of the propelling assembly (be it a propeller or a blower) is the leverage factor f = vprop / vdev : if you forcefully pushed the device forward at speed vdev (with respect to the ground), the propelling assembly would attempt to move the air forward at speed vprop (with respect to the ground).

For the blower, f depends on the gear transmission; for the propeller, it depends also on the angle of the blades. f ranges from minus infinity to plus infinity and is independent on the speed of the device or the wind.

There are three main forces acting on the device:

1. The effective propelling force of the wind pushing the device forward, which equals Fwind * f, where Fwind is the forward force exerted by the wind on the blades. This force acts to make the device move at forward speed vwind / f (with respect to the ground).

2. Ground friction, internal friction and internal drag - this force acts to make the device move at forward speed 0 (with respect to the ground). The designer of the device will want to minimize it.

3. Force exerted by the wind on the non-propelling structure of the device - this force acts to make the device move at forward speed vwind (with respect to the ground). The designer of the device will want to minimize it.


Depending on the leverage factor, the following scenarios are possible (we will assume that the wheels of the device always have contact with the ground):

A. f = +infinity: The device will not move; its propelling assembly acts as a brake on the wheels.

B. f > 1: The device will slowly move forward at low gear; the equilibrium speed will be below the wind speed (minus losses), but the force driving it towards that speed will be greater than the force of the wind. (The device will more easily overcome obstacles, for example.)

C. f = 1: The propelling assembly acts as a simple sail. The equilibrium speed will be the wind speed minus losses.

D. 0 < f < 1: The device will move forward at high gear; the equilibrium speed will be vwind / f minus losses, making it possible for the device to move at higher than wind speed (with less "oomph", though). However, as f approaches zero (the gear is set too high), the force that drives the device towards vwind / f will get progressively lower, to the point that it will begin to lose to the friction and drag forces - and this will lower the equilibrium speed. So there is a sweet-spot f in this range that maximizes the equilibrium speed for a given wind speed. This depends on the specific construction of the device.

E. f = 0: "Infinite gear"; the effective force of the propelling assembly is zero and does not affect the device. The only forces acting on the device are friction, drag and air pushing against the non-propelling elements, placing the equilibrium speed somewhere between 0 and vwind.

F. -1 < f < 0: The device will move backwards, at high gear. Because of the friction and drag forces, it will only actually move backwards when the gear is not too high (i.e. when f is sufficiently below zero). The equilibrium speed will be vwind / f, minus losses. Note also that the losses will be greater than in the forward movement scenarios, because the force exerted by the wind on the non-propelling structure of the device now works against us even more. As in scenario D, there will be a sweet-spot f in this range, maximizing the backward equilibrium speed for a given wind speed.

G. f < -1: The device will move backwards, at low gear. Again, backward equilibrium speed will be less than the forward speed of the wind (minus losses), but the force driving the device towards it will be higher than the force of the wind. The losses will be greater than in scenario B.

H. f = -infinity: The device will not move; its propelling assembly acts as a brake on the wheels.

And that's it.
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Old 10th November 2008, 10:15 PM   #58
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I refuse to believe that's your FINAL analysis.
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Old 10th November 2008, 10:17 PM   #59
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Cousteau's "Alcyone" could sail directly upwind. Which I know is different from moving faster than the wind, but it would imply that it could do so at right angles to it, which I do not know to be the case.
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Old 10th November 2008, 10:19 PM   #60
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Originally Posted by spork View Post
I refuse to believe that's your FINAL analysis.
Why?

ETA: Unless I made a grave error somewhere, it does indeed conclude my analysis of the device. It confirms to my satisfaction that your device should theoretically work as advertised, and that there is no reason to assume that your videos are fake. If I may, I congratulate you on such a brilliant idea (I wouldn't have thought of that) and thank you for this very inspiring exercise in theoretical mechanics.

Last edited by Thabiguy; 10th November 2008 at 10:35 PM. Reason: ETA
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Old 10th November 2008, 11:04 PM   #61
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Originally Posted by Thabiguy View Post
Why?
Mostly just making a joke. But it just seems it's never quite over. I suspect you'll continue to fight the good fight against the naysayers, and develop more analytical ammo in the process. I know I have. I originally conceived of this thing a few years ago (and later learned I wasn't the first to have done so). I certainly felt I knew everything I needed to know about just exactly how it worked. But I have to admit, in answering peoples questions (or more often claims of me scamming) I think I've come to understand some of the subtleties better. Perhaps more importantly, I've come to understand some of the different but valid ways it can be explained (as opposed to the many invalid ways that seek to explain it).

Quote:
It confirms to my satisfaction that your device should theoretically work as advertised...
Interestingly, I had convinced myself back at the beginning with a simple vector analysis. It wasn't until my buddy JB insisted that we build it that we did so. And yes, it actually does work as advertised. I plan to post some simple plans and parts list so anyone can prove it to themselves. JB has even threatened to mail his to deniers (that seem trustworthy enough not to mistreat it).

Quote:
If I may, I congratulate you on such a brilliant idea (I wouldn't have thought of that) and thank you for this very inspiring exercise in theoretical mechanics.

Careful what you say - that kind of talk puts you in the extreme minority (but thanks).

Last edited by spork; 10th November 2008 at 11:08 PM.
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Old 10th November 2008, 11:21 PM   #62
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Originally Posted by technoextreme View Post
Ahhh finally found the explanation as to why it doesn't work that you can comprehend. What happens when you are going faster than the wind? You have a headwind. What does the headwind do? It pushes the propellor. In what direction does the headwind push the propellor? The wrong direction. What will happen? The cart will stop and start going the other direction.
Think again. The headwind at above wind speed turns the prop in the same direction it takes to increase the speed of that wind in the same headwind direction. At below wind speed that same direction increases wind speed in the opposite direction as the tailwind. These descriptions do miss the relative force vectors operating the craft because it only considers the absolute direction.

I will not respond to the second half of your response because you were responding to a quote that didn't belong to me but failed to note that. Though Careyp74's who actually said it made a good point.
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Old 10th November 2008, 11:53 PM   #63
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Originally Posted by Careyp74 View Post
so a couple of questions. First (thanks JWideman) how is this thing steering? Second, why does this thing slow down and stop so drastically in the end?

I think this is a fake. I think there is either a battery driven motor and steering servo on the car, or the car is being towed behind something.
Of course the craft had remote steering and breaks. It says so. I can't attest that he didn't power the wheels with a motor or drag line of any sort. I do say the craft can in principle work as filmed without doing so.

Originally Posted by Careyp74 View Post
If you bring up the fact that it takes a couple of tries, so that proves it is real, then you really haven't been around long. That is an old trick. Oh, and Grandma couldn't be in on the prank, right?
If you listen to the video it took another try because they forgot to release the break that you made an issue of above. Of course Grandma can be in on a prank. It could even be her idea. My point is why would you fake something that can work. It would be like faking floating steal when it is trivial to get steel to float.

Originally Posted by Careyp74 View Post
BTW, this operation has nothing to do with tacking the wind. The prop is not being blown by the wind, it is being driven by the wheels. As cool as the plane on a treadmill is, and what we can learn from it, this video here is nothing like it, and doesn't make sense.
This is the quote that technoextreme made appear as if it was my quote. Agreed that it's nothing like a plane on a treadmill. At below wind speeds the prop acts more like a progressively reactive sail than a prop. At ground wind speed the average wind speed relative to the craft is still greater than the craft speed due to the prop. Note that with the prop wind speed are quiet different at different points around the prop. In fact given the prop the craft will never exceed average wind speed relative to the craft itself. The only wind speed exceeded on average is the wind speed relative to the ground without the craft involved.
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Old 11th November 2008, 12:03 AM   #64
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Originally Posted by my_wan View Post
The only wind speed exceeded on average is the wind speed relative to the ground without the craft involved.
But isn't that enough?
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Old 11th November 2008, 12:32 AM   #65
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Originally Posted by spork View Post
Hey folks.
Hey spork. Good to see you here . I wish I could have continued the previous debate. Your claim that I debated is probably not as far off as you probably think I claimed. It simply lack a quantitative capacity that the ideas I was trying to instill in the debate could give. The craft dynamics do not have a singular mode of operation, they depend heavily on a number of variables that can change with varying speeds or be changed in the design itself. It all comes down to how a long series of vectors add up or cancel on a particular set of design choices and speeds. The analytical problems are due to force feedback in these vectors such that simple addition is not adequate. I'm quiet sure the basic idea can be massively improved and even made practical in some circumstances.
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Old 11th November 2008, 12:33 AM   #66
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Originally Posted by spork View Post
But isn't that enough?
Yes, if the only question is if it will work. I want efficiency and practical applications.
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Old 11th November 2008, 01:01 AM   #67
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Originally Posted by Thabiguy View Post
D. 0 < f < 1: The device will move forward at high gear; the equilibrium speed will be vwind / f minus losses, making it possible for the device to move at higher than wind speed (with less "oomph", though). However, as f approaches zero (the gear is set too high), the force that drives the device towards vwind / f will get progressively lower, to the point that it will begin to lose to the friction and drag forces - and this will lower the equilibrium speed. So there is a sweet-spot f in this range that maximizes the equilibrium speed for a given wind speed. This depends on the specific construction of the device.
Very good. I brought up the sweet spot case for the propellers "design point" on physicsforums. It does apply also to the gear ratio. If the force ratio between the prop and wheels is too high then efficiency drops exponentially. If they exactly equal prematurely you can lose potential acceleration. They should only equal when the craft is at the maximum speed allowed by the force and drag components. It really needs a transmission for best efficiency at all speeds but that adds more overall inefficiency.
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Old 11th November 2008, 01:06 AM   #68
humber
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George Sychrovsky does not suggest cheating. The device runs on the inertial energy of the propeller. He means that if the treadmill were long enough, and the device remain undisturbed, it would eventually stop when that energy is dissipated. The device is restrained by the hand until the propeller comes to speed. Every time the device is stopped in the manner that is shown, the propeller picks up some energy in the same manner as when started.
If you remove a spinning device from the treadmill and place it on the ground, it will move forward, so of course it will run up the belt until losses dissipate that stored energy.
This is not a demonstration of the claimed effect.
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Old 11th November 2008, 01:14 AM   #69
spork
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Originally Posted by humber View Post
The device runs on the inertial energy of the propeller.
That's ridiculous. We posted a video tonight to address that very claim.

Quote:
He means that if the treadmill were long enough, and the device remain undisturbed, it would eventually stop when that energy is dissipated.
It would stop when I failed to pay my electric bill or the wheels wear out. Watch the video. Try it for yourself. This thing is about as simple as can be.

Quote:
This is not a demonstration of the claimed effect.
Thanks for that considered and in-depth analysis.

Last edited by spork; 11th November 2008 at 01:17 AM.
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Old 11th November 2008, 01:25 AM   #70
humber
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You have not addressed the stored energy. That is why it works. No analysis is required until you can demonstrate that it will run continuously without being stopped by hand or other means.
Whilst spinning, it will run for sometime on the ground, won't it?
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Old 11th November 2008, 01:31 AM   #71
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Watch


Direct Downwind Faster Than The Wind #5 (DDWFTTW)

At spork33

Your concerns are addressed directly.
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Old 11th November 2008, 01:42 AM   #72
humber
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No, same problem. As your commentator remarked, you are stopping the device again. It picks up energy. Practical considerations prevent you from waiting the time that you allow for the propellrr to stop when removed from the treadmill, before stopping it again. Replace the propeller with a mass, and you will see the same effect.
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Old 11th November 2008, 01:47 AM   #73
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You're kidding - right?

When removed from the treadmill the prop starts to slow down immediately. as soon as it starts to slow down, the cart moves backward on the treadmill. Our moves forward indefinitely. It will push on whatever we put in front of it... indefinitely. If we increase the incline of the treadmill sufficiently, it will just "hover" in one place - indefinitely - without ever running out of that 1/2 ounce of kinetic energy stored in a 10 gram plastic prop.
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Old 11th November 2008, 02:07 AM   #74
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Of course. The propeller creates drag, which also restrains the device. If the device were capable of taking the energy derived from the wheels, and converting it to equivalent thrust, then it would remain in the same position, but that would require 100% efficiency. Under these conditions, no work would be done ( the device would not accelerate). The stored energy adds this force, to drive it forward. Therefore, the time it takes to slow to a stop is extended beyond that time of the free-air case. This would also apply to a flywheel, but it will certainly not be dragged backwards at the same speed as the belt, but it is that differential velocity that is the indicator of the stored energy. Work is being done.only by the stored energy.

This is not a really connected to the actual cart. For all intents and purposes, the device can take as much energy from the belt as it the situation demands. In the wind-drive case, Bernoulli's law limits the energy that can be taken from the wind. The cases are not the same.

Last edited by humber; 11th November 2008 at 02:12 AM.
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Old 11th November 2008, 02:21 AM   #75
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Originally Posted by humber View Post
Of course. The propeller creates drag, which also restrains the device. If the device were capable of taking the energy derived from the wheels, and converting it to equivalent thrust, then it would remain in the same position, but that would require 100% efficiency. Under these conditions, no work would be done ( the device would not accelerate). The stored energy adds this force, to drive it forward. Therefore, the time it takes to slow to a stop is extended beyond that time of the free-air case. This would also apply to a flywheel, but it will certainly not be dragged backwards at the same speed as the belt, but it is that differential velocity that is the indicator of the stored energy. Work is being done.only by the stored energy.
I WIN!!! I bet JB you would use exactly this argument. He laughed his butt off.

Quote:
This is not a really connected to the actual cart. For all intents and purposes, the device can take as much energy from the belt as it the situation demands. In the wind-drive case, Bernoulli's law limits the energy that can be taken from the wind. The cases are not the same.
So you've figured out that the equivalency of inertial frames isn't really true (when Galileo, Newton, and Einstein failed to ever find an exception). Impressive!
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Old 11th November 2008, 02:32 AM   #76
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It says nothing about frames of reference. When you tilt the belt to make the device hover, no work is being done. When you restrain it with your hand, no work is being done. No laws are broken, this is quite in keeping with what Newtonian mechanics would predict. All you are seeing in these cases is that the motive and impeding forces are in balance. Choose your load, (gravity, drag or hand) and the belt will provide the energy to balance it.
As I said, if you disagree, replace the propeller with a flywheel and observe that the device will not move backwards at the speed of the belt.
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Old 11th November 2008, 03:46 AM   #77
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When the device is static upon a belt that is at an angle, what happens when the belt is lowered a little? It moves forward up the belt, until the forces are again in balance. You are confusing force and work, which is force x distance.
Lowering the belt lowers the devices gravitational potential energy, driving the device forward.
For the device to move forward relative to the belt, i.e. Vehicle velocity - belt velocity > 0, requires a source of energy that is not available from the belt. When level, that supply of energy comes from the energy that is stored in the angular momentum of the propeller when you first spin it up. Nature 'never forgets' this energy, so it will remain until dissipated. Waiting is otherwise of no importance.
These demonstrations perhaps show what the real device needs to achieve, but not that they are realised. That situation is quite different
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Old 11th November 2008, 04:59 AM   #78
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Boy - you have some pretty unique ideas about physics. Perhaps I'll help straighten you out when I'm awake.
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Old 11th November 2008, 06:21 AM   #79
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Originally Posted by my_wan View Post
Not quiet. Once the vehicle reaches ground air speed the prop tied to the wheels then creates thrust that effectively increases the air speed relative to the craft. Of course this places more drag on the wheels but because ground speed and actual air speed still don't match the power is there to pay for this drag. The available power is the difference between air speed and ground speed regardless of the motion of the craft or extra air speed created by the prop.
Just taking the case of the vehicle reaching the air speed. Then with respect to the propellor, the air is motionless. How can it now get any energy to supply to the wheels? If the propellor is directly connected to the wheels, which seems the case in the first video, the propellor would now act to stop the wheels. If the vehicle somehow exceeded wind speed the propellor would now be forced to rotate in the opposite direction further slowing the vehicle.
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Old 11th November 2008, 06:32 AM   #80
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Humber, think of the treadmill as an engine that drives the propeller. The treadmill uses a little more power from the grid when the device is running on it. Since the vehicle is not mounted on the treadmills frame the only forces pushing it back are the friction between the wheels and the treadmill, air resistance against the vehicle, and friction between different components of the vehicle. Did I miss anything? In any case, the sum of forces acting against the vehicle are smaller than the force generated by the propeller (which is powered by the treadmill). That's why the vehicle starts to move forward.
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