Downwind faster than the wind

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Re: Downwind faster than the wind

Postby Diadem » Tue Dec 16, 2008 8:02 pm UTC

spork33 wrote:
Diadem wrote:Oh and, no, I didn't solve the problem,that's too much honour. The guy that wrote the explanation ThinAirDesigns linked to did. Though he didn't look at energy/momentum balance. That part of the puzzle I did solve.


Actually, Tad wasn't the first to solve this. I posed this as a brainteaser three years ago on another forum. I posted solutions back then that included energy and momentum solutions (as well as a number of other proofs).


Apologies, I never meant to claim that either that link or my addition were the first ever. Just the first in this thread.

But you might expect folks to take it poorly when you attack our results, conclusions, integrity, and even intelligence


Interesting that you'd consider an attack on intelligence worse than an attack on integrity :)

But hmm, I certainly attacked results and conclusions. But I don't think I attacked anybody's integrity or intelligence. At least I never intended to do that.I was - and still am - highly annoyed by gmalivuk's discussion techniques, and i did express that. But that's not the same thing.

and then pronounce the problem "SOLVED" when you find your own error.


Isn't a problem solved then, once you've found and corrected the errors? Most of theoretical physics consists of finding and correcting errors. A lot of experimental physics as well.

Anyway if my earlier posts were so obviously in error, then why didn't anyone correct them? Why didn't anyone write a corrected version showing momentum conservation is not violated? Maybe it is a bit cheap to claim you've solved a problem when all you've done is correct your own error. But it's even cheaper to make fun of an incorrect argumentation when you haven't even attempted any proof yourself. (not pointing at you here, spork33, your posts have been quite useful).
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Re: Downwind faster than the wind

Postby Tass » Tue Dec 16, 2008 8:06 pm UTC

gmalivuk wrote:
Tass wrote:
gmalivuk wrote:Also, I'm glad Diadem finally solved this for us. It means I no longer have to keep "trolling" every time I don't buy overly simplified explanations that ignore the ground...

What do you mean Diadem solved it? The energy and momentum considerations? Is that not pretty much what you yourself has been trying to say all the time?

Well, yeah. That statement is what we in the business like to call "sarcasm". :-)


I was quite obviously not awake. :)

I was just still astonished that people cosidered it a "problem" in need of "solving", rather than simply explaining for those who didn't understand it yet.

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Re: Downwind faster than the wind

Postby gmalivuk » Tue Dec 16, 2008 8:31 pm UTC

Diadem wrote:Anyway if my earlier posts were so obviously in error, then why didn't anyone correct them? Why didn't anyone write a corrected version showing momentum conservation is not violated? Maybe it is a bit cheap to claim you've solved a problem when all you've done is correct your own error. But it's even cheaper to make fun of an incorrect argumentation when you haven't even attempted any proof yourself. (not pointing at you here, spork33, your posts have been quite useful).

I pointed out your error, numerous times, in the form of mentioning how your reasoning ignored the ground, and how it would also rule "impossible" the wheel/ruler device. I wasn't arguing that the wind cart worked as advertised, because I don't know enough about propulsion physics to make that argument from first principles, and I don't currently have the means or motivation to build such a device myself. (Not that you'd have believed it was anything but a hoax had I done that and posted another video, of course.)

What I was arguing was that your supposed "proof" of its impossibility was flawed, and I brought up examples of the things you were ignoring, and of the other things that would be ruled impossible if your argument had been correct. In turn, you insisted that I was a troll who wasn't reading your posts, instead of actually bothering to do the calculations in the first place.

Yes, science is about making mistakes and then learning from them. It is *not* about name-calling and insult-throwing from the person who is just so convinced he's correct, even though he later proves not to have been...
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Re: Downwind faster than the wind

Postby Mr. Beck » Tue Dec 16, 2008 8:49 pm UTC

Quiet, both of you.
You agree on the physics, so stop arguing about how you got to be in agreement about the physics.

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Re: Downwind faster than the wind

Postby Diadem » Tue Dec 16, 2008 9:43 pm UTC

gmalivuk wrote:I pointed out your error, numerous times, in the form of mentioning how your reasoning ignored the ground, and how it would also rule "impossible" the wheel/ruler device.


But that's just the thing. My reasoning never ignored the ground. And I never said anything about the wheel/ruler device. In fact I have been convinced it is possible from the very start. It just didn't seem very interesting, because I didn't (and still don't) see why it would be analogous to the DDWFTTW device. The ruler does work, the amount of energy stored in the ruler is infinite (you can push it as hard as you want). There's no problem with energy conservation violation.

Reading back the thread with the current understanding of how his machine works, I still do not understand most of your arguments against my reasoning. First you claimed I ignored the propeller, which I didn't. Then you said I ignored the ground, which I didn't. Etc. Though the point you made about having to look at the difference in speed between ground and air was very useful.

Maybe it is partly my fault. Maybe I'm just not very good at explaining myself. But my arguments were all about momentum conservation. You didn't even mention the term. I really do not see how you were trying to counter my reasoning.

(Not that you'd have believed it was anything but a hoax had I done that and posted another video, of course.)


Of course, but if you have to choose between a video - or a thousand video's - being faked and basic conservation laws being invalid, which do you choose? I'm a theoretical physicist. If theory says something is impossible, then you're going to need a lot of experiments to convince me otherwise.

Yes, science is about making mistakes and then learning from them. It is *not* about name-calling and insult-throwing from the person who is just so convinced he's correct, even though he later proves not to have been...


For the record: I did neither. At one point I did say that it seemed to me like you were trolling. I guess that remark was over the line, and I apologize for it. But it's still far away from name-calling and insult-throwing. My aim with that remark was not to insult you, but to express my exasperation at how you were consistently ignoring the main point of my posts.

Anyway, I guess this is enough fighting for one day. We agree about the physics. Let's not be enemies. I do sometimes get carried away a bit too much in arguments, and so are we all, I guess.
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Re: Downwind faster than the wind

Postby gmalivuk » Tue Dec 16, 2008 9:55 pm UTC

Diadem wrote:I never said anything about the wheel/ruler device. In fact I have been convinced it is possible from the very start. It just didn't seem very interesting, because I didn't (and still don't) see why it would be analogous to the DDWFTTW device. The ruler does work, the amount of energy stored in the ruler is infinite (you can push it as hard as you want). There's no problem with energy conservation violation.

The amount of energy stored in the wind is also infinite. And my point was that, whether you were specifically addressing the ruler thing or not, your argument looked to me like it would rule that equally impossible, considering that both the ruler and the ground have negative velocity relative to the car, and so the momentum and energy transfer should look just like it did with your reading of the propeller car.

Of course, but if you have to choose between a video - or a thousand video's - being faked and basic conservation laws being invalid, which do you choose? I'm a theoretical physicist. If theory says something is impossible, then you're going to need a lot of experiments to convince me otherwise.

Well, sure. But what you were missing was the third choice, which turned out to be the correct one, and which everyone arguing against you was saying: you were making mistakes yourself. No one was claiming this gave free energy, or free momentum (which would generally amount to the same thing). They were just saying that it gave extra speed to one thing, by putting more force into another thing than might intuitively seem doable, at first.

Anyway, I guess this is enough fighting for one day. We agree about the physics. Let's not be enemies. I do sometimes get carried away a bit too much in arguments, and so are we all, I guess.

Agreed. Sorry for my part in it.
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Re: Downwind faster than the wind

Postby phonon266737 » Wed Dec 17, 2008 12:03 am UTC

[ I just worked through this for myself and posted it. Feel free to read, comment, ignore, whatnot ]

Let's assume that there is no air drag. Lets assume that tire-ground friction is infinite, but rolling resistance is zero. Assume all gearbox losses are zero

Finally, let's asume that the speed of the car is slightly greater than the true wind. This means that it is experiencing a headwind [Faster than wind downwind=headwind. Got it]

So you have this car, moving into a headwind. There's a torque on the propeller that comes from the wheels. If we assume everything works perfectly, the amount of energy going IN to the wheels is equal to the amount coming OUT of the propellor. There is no posible way for that [Prop Thrust > Wheel Drag] . If everything is perfect, there's no air resistance, no rolling resistance, the thrust coming out of the prop is exactly equal to the drag onthe wheels. This is like throwing a metal ball in outer space - it keeps going, ideally.

So our only consolation is we still have the headwind. Our prop makes high pressure on the backside and low pressure on the front side, provided the wheels are going forward. Some people think that the headwind wil spin the prop, and then spin the wheels. If it was actually going UPWIND, this would be possible. But, don't think of it as a headwind, because it's not.

Apparent wind is simply put when you imagine an OBJECT moving into a mass of STATIONARY AIR. Claiming that you can get energy worth of acceleration by moving in stationary is a preptual motion machine stuff. The headwind that our car is gaining energy from is really just a result of its forward motion. Any energy gained by the prop, comes out of the cars inertia.

conclusion:
The car can't go DWFTTWEdit : I misunderstood my conclusion. I properly said that the cart could get to the point where there is a headwind. the fact that there would be a headwind means it IS going faster than the wind, by a little bit. If the true wind were to dissapear the apparent headwind would become huge and it would slow down immensely, but with the true wind there is less headwind than if the cart were "free" Thanks to the following post.

The car can go upwind. It can also go downwind as fast (plus a little) as the wind. It also follows the laws of inertia, and if the wind dies, it will be going downwind faster than the wind. But it can't accelerate to a speed greater than the wind, it can only accelerate up to that speed, and wait for the wind to slow down.
Last edited by phonon266737 on Wed Dec 17, 2008 1:17 am UTC, edited 1 time in total.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 1:10 am UTC

phonon266737 wrote:conclusion:
The car can't go DWFTTW
The car can go upwind. It can also go downwind as fast as the wind. It also follows the laws of inertia, and if the wind dies, it will be going downwind faster than the wind. But it can't accelerate to a speed greater than the wind, it can only accelerate up to that speed, and wait for the wind to slow down.


If I'm understanding your scenario correctly your conclusion is definitely faulty - because I have a cart that will definitely go DDWFTTW steady state. I can put smaller wheels on it, and it will then go directly UPwind steady state.

It seems to me that you're mixing definitions when you talk about wind. This cart exploits the energy of the "true" wind. In other words, it can extract energy by extracting energy of the wind relative to the ground, even when the cart itself is going downwind faster than that wind - and thus experiencing a "relative" headwind. We don't want to be confused into thinking that it's extracting energy from the headwind. What it is doing is leaving a wake of low energy (relative to the ground) wind, even though it adds energy to the wind relative to its own frame. Thus is the bizarre nature of energy.

By the way, it's not a perpetual motion device. It breaks no laws of physics, and it has no practical use.

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Re: Downwind faster than the wind

Postby Diadem » Wed Dec 17, 2008 1:53 am UTC

By the way, spork33, have you tried your machine outdoors? So on the road with real wind, instead of on a threadmill. How does it fare? And can you accelerate to faster-than-wind speeds? Or do you need to push it to faster-than-wind speeds first before it starts working?
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Re: Downwind faster than the wind

Postby Fallible » Wed Dec 17, 2008 2:06 am UTC

Hi all,

I'm a relatively new poster here and I thought I might be able something of value to this discussion, although it seems like most people are convinced by now that Dwftw is possible.

It seems as though most of the confusion arises when people try to use the cart as an reference frame.

Don't use the reference frame of the cart. We're trying to determine if the cart is accelerating or not, but accelerating reference frames are bad news. Other wise known as non-inertial reference frames, they tend to break some of the normal conservation laws. For example, Centrifugal Force and Coriolis Forces are examples of the corrections necessary when trying to use non-inertial frames. These would seemingly break 'every reaction has an equal and opposite reaction'.

Also, phonon, you seem to be mixing up Torque, power and force.
phonon266737 wrote:If we assume everything works perfectly, the amount of energy going IN to the wheels is equal to the amount coming OUT of the propellor. There is no posible way for that [Prop Thrust > Wheel Drag]

It seems you've used a statement about power to justify a statement about force.

There's nothing requiring the net power into the cart or net force on the cart to be zero. The cart can gain both energy and momentum from the wind.

I think the key difference between this and any other sail craft is what it does to the air molecules. A sail craft basically slows the air from wind speed down to the speed of the craft. Hence it can never go faster than the wind. However what this cart is doing is reflecting the wind. An individual air molecule might be traveling along at wind speed, but after the propeller hits it it would be traveling slower than the cart.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 2:24 am UTC

Diadem wrote:By the way, spork33, have you tried your machine outdoors? So on the road with real wind, instead of on a threadmill. How does it fare? And can you accelerate to faster-than-wind speeds? Or do you need to push it to faster-than-wind speeds first before it starts working?


We did our videos to address the many concerns the skeptics had about Goodman's outdoor test. The wind was not steady, he might be going downhill, perhaps it's being towed, it's not going "directly" downwind, etc. We decided to perform our tests under very controlled conditions (i.e. treadmill with speed control, spirit level in enclosed room, etc.). We were naive enough to think this would satisfy the critics. We couldn't possibly have been more wrong. We have done outdoor tests just to demonstrate that it self-starts (many believe it won't). If I had a day of very steady wind and a nice flat area where I could run directly downwind I'd put small smoke cannisters on board and post the video. If anyone has a very long plexiglass wind tunnel please contact me : )

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Re: Downwind faster than the wind

Postby simdude » Wed Dec 17, 2008 2:27 am UTC

I've been trying so hard to come to my own conclusion of this. Right now I think I'm stuck wanting to say that the long road wind test is a fake and the treadmill one is not.

I just for the life of me can't see where the energy is coming from on the long road. I'm still not seeing if the propeller is powering the wheels or if the wheels are powering the propeller. Is the secret behind the idea that it is both? If so I still fail to see grasp the idea. I would assume (ass out of me on this one) the two would have an equilibrium where they are both powering each other equally.

Now when talking about the treadmill I can fathom how it may work. The treadmill is powering the wheels which is powering the propeller past the equilibrium. Energy is being put into the system in the form of the energy powering the treadmill. Is this how other people are seeing it?

And I'm sorry but the wierd angles on the road video are so constrained to never looking ahead that my bull-o-meter just HAS to go up.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 2:30 am UTC

Fallible wrote:It seems as though most of the confusion arises when people try to use the cart as an reference frame.

Don't use the reference frame of the cart. We're trying to determine if the cart is accelerating or not, but accelerating reference frames are bad news.['quote]

Agreed that you should avoid non-inertial frames if at all possible. What you can do (and I have done) is to assume the cart is at a steady speed greater than true wind speed and analyse energy and momentum at that state. This is fairly straightforward, and at least valid for demonstrating that the cart can maintain this state (but not necessarily whether it can achieve the state without assistance).

I think the key difference between this and any other sail craft is what it does to the air molecules. A sail craft basically slows the air from wind speed down to the speed of the craft. Hence it can never go faster than the wind.


Many sailboats, kiteboards, windsurfers, ice boats, land yachts, etc. go faster than the wind regularly. The high performance ones can maintain a downwind velocity component faster than the wind. The blades of our prop are exactly analogous to the sail of an ice boat on a 45 degree downwind tack in the way they interact with the vessel and the air.
Last edited by spork33 on Wed Dec 17, 2008 2:39 am UTC, edited 1 time in total.

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Re: Downwind faster than the wind

Postby Fallible » Wed Dec 17, 2008 2:37 am UTC

spork33 wrote:
Fallible wrote: I think the key difference between this and any other sail craft is what it does to the air molecules. A sail craft basically slows the air from wind speed down to the speed of the craft. Hence it can never go faster than the wind.

Many sailboats, kiteboards, windsurfers, ice boats, land yachts, etc. go faster than the wind regularly. The high performance ones can maintain a downwind velocity component faster than the wind. The blades of our prop are exactly analogous to the sail of an ice boat on a 45 degree downwind tack in the way they interact with the vessel and the air.


My apologies, I meant 'Any other sailcraft traveling in the same direction as the wind'.

Wait, that's not true either. I think kite boarders can obtain extra speed by flying the kite side to side to generate lift. (Can anyone confirm)

I guess I meant a sail craft with a statically arranged sail traveling in the same direction as the wind.
Last edited by Fallible on Wed Dec 17, 2008 2:39 am UTC, edited 1 time in total.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 2:37 am UTC

simdude wrote:I've been trying so hard to come to my own conclusion of this. Right now I think I'm stuck wanting to say that the long road wind test is a fake and the treadmill one is not.


You only have to talk to Jack Goodman for 5 minutes to know it's not a fake - or understand the analysis to know it needn't be faked.

I just for the life of me can't see where the energy is coming from on the long road.


The cart exploits the energy available at the road/wind interface. It's a clever sort of "lever".

I'm still not seeing if the propeller is powering the wheels or if the wheels are powering the propeller.


Surprisingly enough that's actually a *very* tricky question (that we can get into if you like), but I can certainly say this much easily enough - the wheels provide the torque that turns the prop shaft which in turn turns the prop.

Is the secret behind the idea that it is both?


It can be one or the other or both. It simply comes down to the frame in which you do the analysis.

Now when talking about the treadmill I can fathom how it may work. The treadmill is powering the wheels which is powering the propeller past the equilibrium. Energy is being put into the system in the form of the energy powering the treadmill. Is this how other people are seeing it?


That's a perfectly valid way to look at it. But what keeps it from just rolling off the back of the treadmill? Also, if you can see how it works on the treadmill you already understand how it works on the road. A basic principle of physics tells us that a 10 mph wind over a stationary road is absolutely no different than a roadway that moves at 10 mph under still air.

And I'm sorry but the wierd angles on the road video are so constrained to never looking ahead that my bull-o-meter just HAS to go up.


You can give your bull-o-meter the day off. Jack Goodman is as sharp and honest as they come.

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Re: Downwind faster than the wind

Postby phonon266737 » Wed Dec 17, 2008 3:12 am UTC

Unless I am mistaken, torque, work, energy, are all interchangeable untis. I simple sside that, with no wind and no drag, the amount of work (newtons*meters) done by the road on the wheels is equivalent to the amount of work done by the prop on the air. Break it down into forces all you want, but with stationary air and a moving cart, the energy into the wheels equals the energy going out of the prop, minus losses.

This works at any speed. But there's that pesky apparent wind. Your prop foil will have some sort of lift drag ratio - perhaps integrate over the length of the prop it to get torque per drag unit. So you get X units of torque for every Y unit of drag. If your gearing is right, you should be able to generate more than y units of force at the wheels.

Lets say the foil has an L/D of 2. If you've got one blade that's 1 foot long, there's 2 lbs of lift generated, 1 ft-lb of torque to drive the wheels with, and 1 additional lb of drag.
With clever gearing and a good foil, I can see some net torque being applied to the wheels.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 3:29 am UTC

phonon266737 wrote:Unless I am mistaken, torque, work, energy, are all interchangeable untis.


Definitely not interchangable - but related. torque * angle_rotated = work or energy.

Work and energy are the same units.

Force times distance = work (or energy).
Force time time = mass * velocity = momentum

With clever gearing and a good foil, I can see some net torque being applied to the wheels.


For the downwind cart the wheels apply the torque that turns the prop. For the upwind cart the prop provides the torque that turns the wheels.

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Re: Downwind faster than the wind

Postby simdude » Wed Dec 17, 2008 3:30 am UTC

That's a perfectly valid way to look at it. But what keeps it from just rolling off the back of the treadmill? Also, if you can see how it works on the treadmill you already understand how it works on the road. A basic principle of physics tells us that a 10 mph wind over a stationary road is absolutely no different than a roadway that moves at 10 mph under still air.


That's right. I might accept that it works on a sort of proof by induction that I see how it may work on the treadmill and I will accept that the wind over the road is the same as the still air with the moving treadmill.

I suppose I'll go back to lurking until someone posts a visualization that just clicks correctly in my head.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 4:04 am UTC

simdude wrote:I suppose I'll go back to lurking until someone posts a visualization that just clicks correctly in my head.



Don't lurk - tell us what works for you - math, analogies, vector diagrams...?

Watch these two videos (in order). See if this makes something click.


http://www.youtube.com/watch?v=E7vcQcIaWSQ
http://www.youtube.com/watch?v=k-trDF8Yldc

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Re: Downwind faster than the wind

Postby mamalala » Wed Dec 17, 2008 4:08 am UTC

Hello simdude,

maybe the visualizations on this page will help you:

http://www.grogware.com/ddwfftw/

Especially the "Cart Video" there, using a thread, nut and wheels.

Greetings,

Chris

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Re: Downwind faster than the wind

Postby phonon266737 » Wed Dec 17, 2008 6:04 am UTC

Hey spork,
You're right on the angle necessary. Stupid me forgot about unitless radians! I even used them in my chickenscratch about airfoils :)

For the downwind cart the wheels apply the torque that turns the prop. For the upwind cart the prop provides the torque that turns the wheels.


I'm still not sure if I buy this. From a big-picture perspective, no matter what, this cart is gathering energy from the true wind. Net torque transfer needs to be from the prop, to the wheels. Any energy that you get out of the wheels necessarily involves putting drag on the cart, and claiming that the added drag forceis negated by the thrust from the prop seems to violate conservation laws.

I mean, it doesn't matter a whole lot. downwind, until you hit wind speed, the whole thing could be a chunk of wood on wheels. and once you surpass wind speed, you're technically "upwind" with the prop driving the wheels. So I think i'm thinking too hard about something where we agree!

I always believed about the dead-upwind with a prop. It's really not much harder to understand the same thing can work at high speed downwind.

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Re: Downwind faster than the wind

Postby spork33 » Wed Dec 17, 2008 6:42 am UTC

phonon266737 wrote:I'm still not sure if I buy this.


It's true. I never lie about torque.

From a big-picture perspective, no matter what, this cart is gathering energy from the true wind. Net torque transfer needs to be from the prop, to the wheels.


Not really. The cart is a bit like a watermellon seed being squeezed between your thumb and forefinger. When it shoots out - where did the energy come from - your thumb or your forefinger? Both of course. In any case energy it depends on the frame you choose for your analysis. The torque however is invariant. You can measure it with a strain gauge, and it'll always show the wheels are turning the prop in the downwind case. Doesn't matter if the cart is on the road or the treadmill.


Any energy that you get out of the wheels necessarily involves putting drag on the cart,


True.

and claiming that the added drag force is negated by the thrust from the prop seems to violate conservation laws.


It seems that way, but it ain't. That's what makes this a good brainteaser. Remember there's no such thing as conservation of force.

I mean, it doesn't matter a whole lot. downwind, until you hit wind speed, the whole thing could be a chunk of wood on wheels. and once you surpass wind speed, you're technically "upwind" with the prop driving the wheels.


When you pass the wind speed you're going upwind to the relative wind, but you're still going downwind in the true wind sense. In any case the wheels still produce the torque to turn the prop.

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Re: Downwind faster than the wind

Postby She » Wed Dec 17, 2008 9:42 pm UTC

(disclaimer in order to make this solution correct (no, I understand that's not how it works): I have physics knowledge up to 11th grade.)
Here's what made me understand it, is it even correct? If so, meybe it will help someone else.

I watched the Under the ruler, faster than the ruler video. Then thought about that picture where the ruler was made into a green arrow representing the wind, the top wheel into a shovel wheel and the rest of the cart simply made as little affected by the wind as possible. Everyone kept saying that the cart moved relative to the ground, and not moved relative to the wind. Thus it would, at best (without any friction) keep the same speed (I don't really know how to use velocity and momentum, so I hope I won't need the distinction), but most likely would start to slow down. The wind could not push anything forward, because everything stood still relative to it.

Then I realized that nt all the cart was standong still relative to the wind. The top of the shovel wheel was moving backwards (relative to the cart). Thus air could hit its shovels, and they could function like a sail. In this model, the point where it starts violating physical laws is the speed at wich the top of the top wheel - the part of the cart that moves slowest relative to the road - ia at wind speed. Then no energy could be had from the wind - but this would in theory not be a problem since you could just make the top wheel bigger. In theory.
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Re: Downwind faster than the wind

Postby Technical Ben » Wed Dec 17, 2008 10:19 pm UTC

The problem seemed to be in the misconception the device moves away from the wind [One I made :P]. It faces it and moves towards it. Hence it does not match the wind speed and effectivly become stationary. It moves into it, and increases it's effect. :)
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Re: Downwind faster than the wind

Postby gmalivuk » Thu Dec 18, 2008 2:22 am UTC

Well the original devices do move in the direction of the wind, though can be adapted to move against the wind, too.

I think the best simplistic explanation for at least why it's not impossible, if not for the actual workings, was the one spork33 made about it being a clever kind of "lever", in that it does counterintuitive things with forces. (Just like other simple machines. It also, for instance, seems counterintuitive that I could rig up some pulleys that would allow me to pull myself up in an elevator, by pulling up on a rope.)
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Re: Downwind faster than the wind

Postby Tass » Thu Dec 18, 2008 2:37 pm UTC

gmalivuk wrote:Well the original devices do move in the direction of the wind, though can be adapted to move against the wind, too.

I think the best simplistic explanation for at least why it's not impossible, if not for the actual workings, was the one spork33 made about it being a clever kind of "lever", in that it does counterintuitive things with forces. (Just like other simple machines. It also, for instance, seems counterintuitive that I could rig up some pulleys that would allow me to pull myself up in an elevator, by pulling up on a rope.)


Well thats not hard :) it can even be done with one pulley, though you'd have to be pretty strong :P
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Re: Downwind faster than the wind

Postby Goemon » Thu Dec 18, 2008 7:50 pm UTC

Ok, I totally get the carts moving downwind faster than the wind. Very cool.

Also, I get that sailboats can have a speed greater than wind speed when moving downwind at an angle - so long as the downwind component is less than windspeed.

But this I do NOT get (emphassis mine):

spork33 wrote:Many sailboats, kiteboards, windsurfers, ice boats, land yachts, etc. go faster than the wind regularly. The high performance ones can maintain a downwind velocity component faster than the wind.


You're saying that the component of velocity parallel to the wind can be greater than the wind speed, steady state? For a simple sail craft?
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Re: Downwind faster than the wind

Postby MotorToad » Thu Dec 18, 2008 8:12 pm UTC

Goemon wrote:Ok, I totally get the carts moving downwind faster than the wind. Very cool.

Also, I get that sailboats can have a speed greater than wind speed when moving downwind at an angle - so long as the downwind component is less than windspeed.

But this I do NOT get (emphassis mine):

spork33 wrote:Many sailboats, kiteboards, windsurfers, ice boats, land yachts, etc. go faster than the wind regularly. The high performance ones can maintain a downwind velocity component faster than the wind.


You're saying that the component of velocity parallel to the wind can be greater than the wind speed, steady state? For a simple sail craft?

They don't mean the boat can sail on a spinnaker faster than the wind. The way modern wing sails work, as they travel faster they actually make the apparent wind faster... which makes them travel faster, etc. They can have a downwind component faster than the down wind magnitude, but they can't travel directly downwind faster than the wind. (So far as I know.) If it can travel at 45º to the wind at a very high speed, though, it'll be faster than the wind in the wind's direction.
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Re: Downwind faster than the wind

Postby Tass » Thu Dec 18, 2008 8:31 pm UTC

I didn't think it could be done either. It does seem intuitively impossible.

However having given it a bit more thougth it does seem theoretically possible, but dang you need some great engineering to make this small effect overcome friction.

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Interestingly it seems the sail will have to be quite close to the center of the boat - it is as though the boat is tacking upwind! Actually this is completely analogous to the propeller cart, the sail works as the propeller blade affecting the wind with a backwards force, the keel gives the sideways velocity component nescesary to make this posible. Only difference it that the whole craft has to follow in this sideways component in this case. In the cart the blades' sideways component is taken care of by the rotation while the bulk cart moves directly downwind.

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Re: Downwind faster than the wind

Postby Goemon » Thu Dec 18, 2008 9:40 pm UTC

Ah, now I get it. Just put a "sailboat" on the treadmill with the sail at a 45 degree angle. Then moving sideways fast enough propels the sail forward.

Never knew this was possible...
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Re: Downwind faster than the wind

Postby ThinAirDesigns » Thu Dec 18, 2008 11:49 pm UTC

Goemon wrote:Ah, now I get it. Just put a "sailboat" on the treadmill with the sail at a 45 degree angle. Then moving sideways fast enough propels the sail forward.

Never knew this was possible...


Yes Goemon, if the treadmill was wide enough to allow for the 45degree track of the traditionally rigged land-yacht for instance, it would quickly climb to the front of the treadmill and fall off the front -- showing a VMG greater than 1.0.

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Re: Downwind faster than the wind

Postby spork33 » Fri Dec 19, 2008 3:59 am UTC

You're saying that the component of velocity parallel to the wind can be greater than the wind speed, steady state? For a simple sail craft?


Yup. Ice boats can get downwind at 3X to 4X the wind speed by tacking. The only limit to their downwind speed is given by real-world losses. In other words you could beat the wind by 100X or 1000X if you had an efficient enough vehicle (although I don't expect that will ever happen).


Tass wrote:Actually this is completely analogous to the propeller cart


In fact the blades of the prop cart are EXACTLY analagous to the sail of the ice boat on a downwind tack.

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Re: Downwind faster than the wind

Postby Tass » Fri Dec 19, 2008 6:29 am UTC

Spork33 wrote:
Tass wrote:Actually this is completely analogous to the propeller cart


In fact the blades of the prop cart are EXACTLY analagous to the sail of the ice boat on a downwind tack.


Well I am not a native english speaker, so i migth be wrong about the semantics, but how big is the difference between "completely" and "excactly"? :)

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Re: Downwind faster than the wind

Postby spork33 » Fri Dec 19, 2008 7:36 am UTC

Tass wrote:Well I am not a native english speaker, so i migth be wrong about the semantics, but how big is the difference between "completely" and "excactly"? :)


Fair enough - I'm an idiot. It's just that so often people tell me the two are only remotely analogous, or not analogous at all. So I suppose I'm just adding an exclamation point to your statement. By the way, I am a native english speaker, but I'm sure your english is better than mine.

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Re: Downwind faster than the wind

Postby spork33 » Fri Dec 19, 2008 2:59 pm UTC


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Re: Downwind faster than the wind

Postby dic_penderyn » Sat Dec 20, 2008 8:52 pm UTC

Spork, I am still confused so I have a few questions that I hope you'll be able to answer if you have the time.

1. I saw the road test video on youtube. Impressive as it was, how can you be certain that it was traveling faster than the wind?

2. To my mind, the cart accelerates downwind but at the point when it apparently reaches the same velocity as the wind, where do you think the energy comes from to further accelerate the cart to a velocity greater than the velocity of the wind?

3. At the point when the cart reaches the same velocity as the wind, there is NO wind in reference to the cart. Can you please do the treadmill experiment again but this time change it slightly so it reflects what you are claiming?

Let the cart get up to speed on the treadmill as you have done in the previous treadmill videos, at that point put a large perspex box over the cart. This will simulate the zero wind speed relative to the cart.

I think if you can do these things you would put the majority of doubters to bed.

Thanks.

Dic

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Re: Downwind faster than the wind

Postby Tass » Sat Dec 20, 2008 9:14 pm UTC

Well the answers are allready in the thread, but I am going to answer them again anyway.

dic_penderyn wrote:Spork, I am still confused so I have a few questions that I hope you'll be able to answer if you have the time.

1. I saw the road test video on youtube. Impressive as it was, how can you be certain that it was traveling faster than the wind?


They meassured the velocity of the wind, and the car, besides there was the streamer hanging backwards. It is still subject to error, but thats what the rollerband tests are for.

dic_penderyn wrote:2. To my mind, the cart accelerates downwind but at the point when it apparently reaches the same velocity as the wind, where do you think the energy comes from to further accelerate the cart to a velocity greater than the velocity of the wind?


The energy still comes from the wind, the velocity relative to the bulk of the car does not matter because it has a moving proppeller and wheels. What matters is that the speed of the wind is different from that of the ground, that means there are energy to be extracted. Se the rest of the thread or the various links provided for a breakdown of the forces involved.

dic_penderyn wrote:3. At the point when the cart reaches the same velocity as the wind, there is NO wind in reference to the cart. Can you please do the treadmill experiment again but this time change it slightly so it reflects what you are claiming?

Let the cart get up to speed on the treadmill as you have done in the previous treadmill videos, at that point put a large perspex box over the cart. This will simulate the zero wind speed relative to the cart.


It is done indoors, is that not enough? What would the box change?

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Re: Downwind faster than the wind

Postby dic_penderyn » Sun Dec 21, 2008 6:20 pm UTC

Thanks for the reply Tass.
Im having great difficulty with this. Its just so counter intuitive!


dic_penderyn wrote:
2. To my mind, the cart accelerates downwind but at the point when it apparently reaches the same velocity as the wind, where do you think the energy comes from to further accelerate the cart to a velocity greater than the velocity of the wind?

The energy still comes from the wind, the velocity relative to the bulk of the car does not matter because it has a moving proppeller and wheels. What matters is that the speed of the wind is different from that of the ground, that means there are energy to be extracted. Se the rest of the thread or the various links provided for a breakdown of the forces involved.


But when the cart is moving faster than the wind (into a head wind) wont the proppeller try to reverse direction and provide a braking torque on the driven wheels?

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Re: Downwind faster than the wind

Postby mamalala » Sun Dec 21, 2008 7:02 pm UTC

dic_penderyn,

no. The wheels drive the prop, that is, the prop does _not_ act as a turbine. So, when the cart is moving, the prop rotates against it's "bias" to work as a turbine. That means it already rotates the right direction _if_ the headwind (when the cart goes faster than the wind) _would_ try to spin the prop (which it doesn't anyway). For that reason it won't try to brake the wheel's rotation. This also means that the wheels have to supply the most energy to the prop while it is below windspeed, because it has to be driven to rotate against it's bias. Once it is at windspeed or above, there no longer is any bias that tries to rotate it "the other way".

Greetings,

Chris

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Re: Downwind faster than the wind

Postby Tass » Mon Dec 22, 2008 10:43 am UTC

mamalala wrote:dic_penderyn,

no. The wheels drive the prop, that is, the prop does _not_ act as a turbine. So, when the cart is moving, the prop rotates against it's "bias" to work as a turbine. That means it already rotates the right direction _if_ the headwind (when the cart goes faster than the wind) _would_ try to spin the prop (which it doesn't anyway). For that reason it won't try to brake the wheel's rotation. This also means that the wheels have to supply the most energy to the prop while it is below windspeed, because it has to be driven to rotate against it's bias. Once it is at windspeed or above, there no longer is any bias that tries to rotate it "the other way".

Greetings,

Chris


I couldn't really make sense of the above post. It is true that the propeller does not act as a turbine or windmill. The propeller is giving thrust. It does this at the cost of a braking action on the wheels, but because the "headwind"'s relative velocity is smaller in magnitude than the road speeding under the car, it can still generate a net forward force. If the wind stopped the car would feel a much stronger headwind and would lose its source of thrust.

Imagine the car standing still with wind from behind. Nothing is rotating. The wind on the propeller tries to make it turn clockwise, which would drive the car against the wind. But the wind also gives a forward push on the car, the propeller - still not rotating - also acts as a simple sail. Because of the gearing the second effect is more powerfull. This means that the car slowly begins moving forward. Because of the gearing the propeller turns slowly anticlockwise. As the car picks up speed the propeller turns faster an faster anticlockwise giving a little more push than it would if it was simply a sail. As the car moves at windspeed the propeller is still pushing back on the, now still, air. Aproaching a specific fraction above windspeed the gearing means that the propeller begins having troublcan't keeping up and the car reaches its terminal velocity. This terminal velocity could in theory be made as larges as one wants, but it will be harder and harder to overcome windspeed as the gearing ratio becomes closer to one. But 1.5 times windspeed is quite feasible.

Look at the second video in the original post. Imaging the ruler being able to slip across the top wheel only slowly pushing the car up to speed. Then the two systems are completely analogous. (By the way, the gearing in the "under the ruler" video is in not in the big wheel / small wheel, but in the ratios between the cottong reels' edge and waist diameter.)


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