DDWFTTW Turntable Test: 5 Min Video - Is It Conclusive?

[zoobyshoe]

Except the propeller isn't stationary, it's rotating, generating an upwind thrust component. Say the wind is 10 mph, the cart is moving at 12 mph, and the propeller is generating -5 mph of thrust. Then the air flow at the prop is 7 mph, 3 mph slower than the wind speed of 10 mph. There is a limit, but the limit is faster than the wind speed.

You speak of the prop rotating as if it is now rotating on its own, "generating" a thrust, as if it's a power source. It is being rotated by the wind which is the only thing adding power to the cart, and which loses its ability to do so exponentially as the cart's speed increases.

 

[rcgldr]

True, but the faster you go over the floor, and the more energy you can pump out of that. Do you agree with me that if you'd have an infinitely big sail, then you would be static wrt the wind, move at windspeed, and be able to extract a lot of energy from the wheels on the floor?

Of course not. Energy is conserved. However large your sail, once you start using part of the wind's force to charge batteries your speed will decrease.

But the speed will decrease only very slightly as the sail is very large. The force from a huge volume of air decelerated even slight amount (less than 1 mph) is still large, and this force times ground speed represents the power input to charge batteries or spin up a flywheel. As long as the power to deploy and retract the sail is small, and the drag is sufficiently low when the sail is retracted, there's virtually no limit to how fast the average speed such a craft could go.

 

[zoobyshoe]

But the speed will decrease only very slightly as the sail is very large. The force from a huge volume of air decelerated even slight amount (less than 1 mph) is still large, and this force times ground speed represents the power input to charge batteries or spin up a flywheel. As long as the power to deploy and retract the sail is small, and the drag is sufficiently low when the sail is retracted, there's virtually no limit to how fast the average speed such a craft could go.

If you had an infinitely large sail, or even a 1000 square mile sail, the cart and mast would have to be correspondingly massive, and I think you'd lose a lot of your imagined "no limit" speed.

 

[rcgldr]

Except the propeller isn't stationary, it's rotating, generating an upwind thrust component. Say the wind is 10 mph, the cart is moving at 12 mph, and the propeller is generating -5 mph of thrust. Then the air flow at the prop is 7 mph, 3 mph slower than the wind speed of 10 mph. There is a limit, but the limit is faster than the wind speed.

You speak of the prop rotating as if it is now rotating on its own, "generating" a thrust, as if it's a power source. It is being rotated by the wind which is the only thing adding power to the cart, and which loses its ability to do so exponentially as the cart's speed increases.

No, when traveling at wind speed, from the cart's frame of reference the apparent wind is zero, and the induced wind from the propeller is much slower than the ground speed. It's the ground and it's speed relative to the cart that is providing a power source. Power = force times speed. Part of the thrust from the prop is opposed by a backwards force from the ground at the contact patch of the driven wheel. This provides a power source equal to that ground force times the ground speed. Through effective gearing, (advance ratio) the force is increased and the speed decreased, at the prop + air interface. There are losses in the conversion process, but the prop doesn't need to generate much power, just a higher amount of thrust at a much lower speed, taking advantage of the large speed differential between ground and apparent wind.

 

[schroder]

By Jove, I think I’ve got it! All I need to do is build a small axel at the rear of my car, and mount the spare tire there. Now I run a cable from the hub of the tire up to a propeller mounted on the roof of the car. Voila! As the wheel turns, it turns the flexishaft which powers the propeller and makes my car go faster. Of course, the trick is to make sure that the additional drag of the wheel is more than compensated for by the additional thrust of the propeller. But wait! Isn’t that one of those over unity devices, where the output energy is more than the input? Isn’t that another word for perpetual motion machine? And isn’t that EXACTLY the same thing these DDWFTTFW are talking about? Oh gee, did I hurt a nerve?

 

[rcgldr]

If you had an infinitely large sail, or even a 1000 square mile sail, the cart and mast would have to be correspondingly massive, and I think you'd lose a lot of your imagined "no limit" speed.

OK, that was a theoretical extreme. How about an iceboat downwind component during a downwind tack? Although more efficient than a DDWFTTW cart, perhaps it will demonstrate the possibility. A link to a .pdf file from an ice boat web site:

http://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

There's are a couple of diagrams from a real iceboat run. In the second one, "downwind angles: Skeeter", the wind speed is 18 mph, and the ice boat's heading is 30 degrees offset from true downwind. The apparent crosswind speed is 18 mph x sin(30) = 9 mph, regardless of the iceboats speed. In this case, the iceboat can achieve an apparent headwind speed of 54.4 mph with an apparent crosswind of 9 mph. This tranlates into a ground speed of 70 mph for the ice boat, and an apparent total wind of 55.15 mph (shown as 55 mph in the diagram, I included the .15 so the heading angle offset was 30 degrees). The net downwind speed is 70 mph x cos(30) = 60.6 mph, over 3 times the speed of the wind. Using my numbers (55.15 mph), and a 30 degree heading, I calculate a Beta of 9.4 degress (atan(9/54.4)) about 6:1 as opposed to the 8 (about 7:1) degress shown on the diagram, I'm not sure if this was a mistake or due to rounding errors.

 

[vanesch]

Of course not. Energy is conserved. However large your sail, once you start using part of the wind's force to charge batteries your speed will decrease.

Sure, but I can make that fraction as small as I want. Let's take an example: my wind car weights 10 kg. The wind blows 10 m/s. I can run at 99.9% of the windspeed, and extract, say 100 KW from the wheels (you should then have accordingly a certain sailing surface, which, we assume, weight, say, 10 grams - neutrinonium sails, ok - let us say, 20 000 m^2 or so - we're talking principles here). With those 100 KW, I charge a battery for 10 minutes, so I have stored now 60 MJ in my batteries. The next second, I take down my sails (that takes about 30 seconds, and, say, 1 MJ - there's just 10 grams to move). Next, I use my 59 MJ remaining to power my 30 KW motor (~40 horsepower on a 10 kg car!) and I accelerate with my now very airodynamic car to about 100 m/s (with 40 hp on a 10 kg car, that must be possible) in 30 seconds, and then remain at that speed. I can do that for 2000 seconds, or more than half an hour. Then I start all over.

Of course, the above is a gedanken experiment. Those materials don't exist. But they are not impossible in principle. So the fact that this gedanken experiment is possible in principle shows us that there is no violation of any conservation law or that there is any fundamental prohibition.

Quoi? It's just conservation of energy.

No, because you can tap into a very large amount of available energy (the wind versus the ground) to use in fact only a small bit of it (moving a light car only a bit faster than the wind).

 

[vanesch]

By Jove, I think I’ve got it! All I need to do is build a small axel at the rear of my car, and mount the spare tire there. Now I run a cable from the hub of the tire up to a propeller mounted on the roof of the car. Voila! As the wheel turns, it turns the flexishaft which powers the propeller and makes my car go faster. Of course, the trick is to make sure that the additional drag of the wheel is more than compensated for by the additional thrust of the propeller. But wait! Isn’t that one of those over unity devices, where the output energy is more than the input? Isn’t that another word for perpetual motion machine? And isn’t that EXACTLY the same thing these DDWFTTFW are talking about? Oh gee, did I hurt a nerve?

No, not at all. Why are you thinking that a sailing cart going downwind is an "over unity device", but why is a sailing boat going upwind faster than the wind not an over unity device ?

Again, look at the two purely mechanical examples I gave in an earlier post (with the rope pulling, and with the gears between a static and a rotating turntable) which are equivalent. You won't call those "over unity devices" would you ?

 

[schroder]

Sure, but I can make that fraction as small as I want. Let's take an example: my wind car weights 10 kg. The wind blows 10 m/s. I can run at 99.9% of the windspeed, and extract, say 100 KW from the wheels (you should then have accordingly a certain sailing surface, which, we assume, weight, say, 10 grams - neutrinonium sails, ok - let us say, 20 000 m^2 or so - we're talking principles here). With those 100 KW, I charge a battery for 10 minutes, so I have stored now 60 MJ in my batteries. The next second, I take down my sails (that takes about 30 seconds, and, say, 1 MJ - there's just 10 grams to move). Next, I use my 59 MJ remaining to power my 30 KW motor (~40 horsepower on a 10 kg car!) and I accelerate with my now very airodynamic car to about 100 m/s (with 40 hp on a 10 kg car, that must be possible) in 30 seconds, and then remain at that speed. I can do that for 2000 seconds, or more than half an hour. Then I start all over.

Of course, the above is a gedanken experiment. Those materials don't exist. But they are not impossible in principle. So the fact that this gedanken experiment is possible in principle shows us that there is no violation of any conservation law or that there is any fundamental prohibition.




No, because you can tap into a very large amount of available energy (the wind versus the ground) to use in fact only a small bit of it (moving a light car only a bit faster than the wind).

I sense desperation here. I suppose if aliens landed and had entirely new materials and even a new law of physics, they could demonstrate DDWFTLight! But we were talking about a wheel driving a propeller on a turntable or treadmill. A purely mechanical system which must obey the conservation of energy, conservation of momentum and Newton's Laws of motion. No such device has ever been demonstrated to be able to go directly downwind, faster than the wind which is pushing it. The treadmill and turntable demonstrations do not prove this, as they are essentially upwind-configured devices. You place that same cart on a stationary table with a wind at its back, it goes nowhere! With that, I unsuscribe from this thread, which is also going nowhere! I will only add, that we have a real chance to prove this with the new device, because it has unlimited length of track to run on. If the original poster who is the owner and maker of this, would like to conduct a few more experiments, which I have already mentioned here, I will be glad to come back and discuss the results. Until then, see you around the campus!

 

[rcgldr]

As the wheel turns, it turns the flexishaft which powers the propeller and makes my car go faster. Of course, the trick is to make sure that the additional drag of the wheel is more than compensated for by the additional thrust of the propeller. But wait! Isn’t that one of those over unity devices?

No because of the difference in ground speed versus wind speed.

The power input equals drag force at the contact patch of the wheel times the forward speed of the wheel. To get horsepower, the formula is speed (mph) times force (lbs) / 375 (conversion factor).

The power output equals thrust force at the prop times the induced wash speed of the prop. With a proper advance ratio, the prop pitch is a fraction of the wheel circumference. The distance advance per revolution at the prop is a fraction of the distance advanced per revolution at the wheel. The advance ratio acts as a lever, multiplying the force but decreasing the distance (per unit time), so the prop produces more thrust than the drag force at the contact patch, but at a slower still speed, so that power output is significantly less than power input.

Since the prop speed is a fraction of the ground speed, a tailwind (or the ground moving backwards) is required so that the apparent wind is small compared to the ground speed for the prop to generate sufficient thrust.

Do you have a better explanation for what is going on with these DDWFTTW cart videos?

 

[vanesch]

If you had an infinitely large sail, or even a 1000 square mile sail, the cart and mast would have to be correspondingly massive, and I think you'd lose a lot of your imagined "no limit" speed.

There is no physical law that tells me that I need a certain amount of mass to have a certain sail. I don't break any conservation of energy or momentum by having a 10 gram sail and mast which is strong enough to do so. I won't find any actual material that does so in the current state of technology, but I don't break any laws of classical mechanics by having a very high material strength to mass ratio.

What people here are claiming is that downwind faster than the wind is breaking some fundamental laws of classical mechanics. It doesn't. In order to show that it doesn't, I'm allowed to use any imaginary system that respects classical mechanics.

I'm trying to disprove a theorem. The theorem is: *classical mechanics* forbids in principle to go DWFTTW. Well, if I can find ONE counter example, that is, something that respects entirely all the laws of classical mechanics, and nevertheless goes DWFTTW, then I have disproven the theorem.

That's what I did, already a few times.

The theorem "DWFTTW is prohibited by classical mechanics" is an erroneous theorem.

 

[rcgldr]

You place that same cart on a stationary table with a wind at its back, it goes nowhere!

Examples of cart self starting with a wind at it's back:

http://www.youtube.com/watch?v=QTAd891IpRs&fmt=18

http://www.youtube.com/watch?v=kWSan2CMgos&fmt=18

setup for upwind

In the second run of the second video, a wind gust over powers the prop, and initially drives it the wrong way, spinning the driven wheels backwards, until the cart recovers. The wheels eventually gain traction and spin the prop the correct direction for a cart setup to go DDWFTTW.

In the second segement of this video, the treadmill is tilted up enough so that gravity opposed the excess thrust of the DDWFTTW cart, allowing for a relatively long period on it's own on the treadmill:

http://www.youtube.com/watch?v=7xL8gRJ5F6k&fmt=18

 

[vanesch]

I sense desperation here. I suppose if aliens landed and had entirely new materials and even a new law of physics, they could demonstrate DDWFTLight! But we were talking about a wheel driving a propeller on a turntable or treadmill.

A purely mechanical system which must obey the conservation of energy, conservation of momentum and Newton's Laws of motion. No such device has ever been demonstrated to be able to go directly downwind, faster than the wind which is pushing it.

I would like to see your derivation from Newton's laws that DWFTTW is impossible. And then you have to find where my imaginary devices break Newton's laws, because if it is a theorem you are claiming (namely, DWFTTW is impossible in classical mechanics) then clearly, my examples violate Newtonian mechanics as they violate a theorem in classical mechanics. An over-unity device would be such a violation.

So how do you derive the theorem DWFTTW is forbidden by Newton's laws ?

The treadmill and turntable demonstrations do not prove this, as they are essentially upwind-configured devices. You place that same cart on a stationary table with a wind at its back, it goes nowhere!

What's the difference with the turntable experiment ? If you put yourself in the reference frame of the cart, in what way is this different ? There is a difference, which is the fact that this reference frame is not strictly inertial, as it is rotating. But the bigger the turntable, the smaller this effect is. Do you think that the device wouldn't work as well on a much bigger turntable ? If you go to a very very big turntable, say, 1 km diameter, what would then be your objection from seeing this from the reference frame of the cart ?
After all, Newton's laws are valid in any reference frame. Imagine I put your windtunnel experiment on a train that goes at exactly the speed (but in the opposite direction) of the wind in the wind tunnel. Would it work or not ? Now, given that the wind inside the windtunnel on the train is going at the same speed as the train, the air mass inside the windtunnel is in fact stationary wrt the outside prairies. So if we remove the windtunnel, and have the experiment in the open on the train, that wouldn't change anything, right ? If the train would run on a 1 km radius circular track, that would still be the same, right ?

Now, replace the train with the (big) turntable. What's the difference ?

With that, I unsuscribe from this thread, which is also going nowhere! I will only add, that we have a real chance to prove this with the new device, because it has unlimited length of track to run on. If the original poster who is the owner and maker of this, would like to conduct a few more experiments, which I have already mentioned here, I will be glad to come back and discuss the results. Until then, see you around the campus!

You haven't yet stated why you are claiming a fundamental objection to DWFTTW. It is not violating any principle of classical mechanics. If it does, then show me the derivation, and show me where my different gedanken and real experiments violate classical mechanics.

 

[schroder]

Do you have a better explanation for what is going on with these DDWFTTW cart videos?

Yes, I do! But it seems no one either undersatnds it or is interested in knowing. Briefly, the prop is pitched to act as a true propeller, driven by the wheels which get their power from the turntable. If you were to disconnect the flexishaft and then run the table, you will find that the apparent wind turns the prop the other direction as before, as a turbine. That is what would happen if you place this device in a true wind, stationary table. The prop turning the other way, tirns the wheels the other way. The device tries to go upwind, not down. We can prove this easily with the turntable setup. So what you are doing is taking an up-wind configured device and driving it with a turntable, NOT wind, and claiming you have a DDWFTTW device! This is obviously not what you have since this device cannot even move downwind in an actual wind! If you want to test it as a ddw device, move the cable from one side of the wheel to the other, this is the same as reversing the gearing. Now run the test! The prop will spin up as a turbine and will not drive the cart at all until a true sufficient bacwind is built up. At that time it will spin the opposite way as before, but because of the gear reversal, it will attempt to drive against the turntable. However, it cannot drive faster than the table and will continue to move backwards, but sdlowly. This proves that DDWFTTW is NOT possible. It is a TRUE test because it does represent an equivocal reference fram with the same cart being pushed by a tail wind. The original test does not represent a true test, because it will try to move upwind, not down. This is what I meant by mixing and matching reference frames. Time for me to eat...I am a Vietnam vet who is in Thailand and my wife is calling me to dinner.

 

[rcgldr]

The prop is pitched to act as a true propeller, driven by the wheels which get their power from the turntable. If you were to disconnect the flexishaft and then run the table, you will find that the apparent wind turns the prop the other direction as before, as a turbine.

agreed.

That is what would happen if you place this device in a true wind, stationary table. The prop turning the other way, tirns the wheels the other way. The device tries to go upwind, not down.

I just posted two links to videos showing cart start up in a true wind.

http://www.youtube.com/watch?v=QTAd891IpRs&fmt=18

http://www.youtube.com/watch?v=kWSan2CMgos&fmt=18

In the second segement of the second video a strong gust does initially turn the the prop as a turbine, causing the wheels to be driven backwards and sliding as the cart moves forward, but the wheels regain traction and starting turning the prop in the correct direction.

I see what your concern is, that the prop could act as a turbine at sub wind speeds, but these DDWFTTW carts are using props with a low pitch factor. The geometric advance of the prop per revolution is small, much smaller than the circumference of the driven / driving wheels. The prop in this case has a 12" diameter and a 6" per revolution pitch. That means the prop tips travel 37.7" perpendicular to the wind for every 6" inches the prop moves forward. That's the equivilant of an AOA of around 9 degrees at the tips, so the turbine effect from the tailwind at sub-wind speeds is small. In this case the advance ratio is 6" / 10.5" (prop pitch / wheel circumference), providing the wheels with enough "leverage" to prevent a turbine reaction from the prop, unless the wheels slide as in the case of the gusting wind in the second video. The startup videos are evidence of how the carts behave in a true wind.

 

[vanesch]

I posted this already but it was an edit of a previous post, so maybe people didn't see it. Consider the "mechanical" equivalent, with a turning turntable at the bottom, and a stationary turntable at the top (the mechanical equivalent of a static air mass to which one can couple).

Now, assume a cart between the two disks of the turntable, with two wheels: one which rolls on the top disk, and one on the lower disk.

Now, consider that the two wheels are linked with a gearbox which specifies that the turning rate of the top wheel is p times the turning rate of the the lower wheel (both wheels have same diameter). p can be any number between -inf and +inf (given by the gearbox ratio). - means that the wheels run in the same direction (meaning that the outer velocities are opposite) ; + means that the wheels run in opposite directions (meaning that the outer velocities are in the same direction).

The top disk has a velocity (wrt the lab) of 0, and the lower (turntable) disk has a velocity v_A (wrt the lab). The cart has a velocity v_B. We want to find v_B as a function of v_A and the ratio p.

We move to the cart reference frame (which has velocity v_B wrt to the lab frame). Now the top disk has velocity -v_B and the lower disk has velocity v_A - v_B.

As the wheels are running smoothly on those disks, we have to have that:

(- v_B) / (v_A - v_B) = p.

Indeed, that's the only way to not have any slipping wheels, which we assumed, they wouldn't.

From that, we derive: (p - 1) v_B = p v_A, or:

v_B = p/(p - 1) x v_A

If p = 0 (the top wheel never turns, whatever the lower wheel does), we find v_B = 0
(the cart doesn't move: it is fixed to the top wheel).

If p = -1, then v_B = (-1 / -2) x v_A: the cart goes in the same direction, and at half the speed, of the bottom turntable. That's like in a ball bearing. The top wheel goes as fast, and in the same direction, as the bottom wheel.

If p = +1 there is no finite solution: the thing is blocked. Indeed, at no point, from the cart's PoV, the top and bottom disks can have the same velocity.

If p = + 1/2, then v_B = - v_A. The cart goes as fast, but in the opposite direction, as the turntable.

If p = +2/3, then v_B = - 2 v_A. The cart goes twice as fast backward, as the turntable is going forward.

This is the kind of demonstration that has been given, but instead of having a massive top disk and a wheel, we have an airmass and a propeller.

(for a picture, see the second attachment in post https://www.physicsforums.com/showpost.php?p=2031426&postcount=96

 

[vanesch]

So what you are doing is taking an up-wind configured device and driving it with a turntable, NOT wind, and claiming you have a DDWFTTW device! This is obviously not what you have since this device cannot even move downwind in an actual wind!

But what if you change reference frames ? What if you look upon this from the reference frame of the turntable ? In what way is this different ?

 

[zoobyshoe]

No, when traveling at wind speed, from the cart's frame of reference the apparent wind is zero, and the induced wind from the propeller is much slower than the ground speed. It's the ground and it's speed relative to the cart that is providing a power source. Power = force times speed. Part of the thrust from the prop is opposed by a backwards force from the ground at the contact patch of the driven wheel. This provides a power source equal to that ground force times the ground speed. Through effective gearing, (advance ratio) the force is increased and the speed decreased, at the prop + air interface. There are losses in the conversion process, but the prop doesn't need to generate much power, just a higher amount of thrust at a much lower speed, taking advantage of the large speed differential between ground and apparent wind.

But I don't believe the cart will ever get to wind speed. The faster it goes the faster the wind loses it's power to accelerate the cart. In any working model there will be some minimum wind speed beneath which the thing will just not operate no matter how many nudges you give it. Once the cart is accelerated to the point the apparent wind falls below that minimum it will cease to accelerate regardless of any gearing.

 

[rcgldr]

So what you are doing is taking an up-wind configured device and driving it with a turntable, not wind, and claiming you have a DDWFTTW device!

In what way is this different ?

I think his concern is that a tailwind could cause the prop to act as a turbine and turn the wheels the wrong way, making it act as an upwind device, which is the case if the advance ratio is much greater than 1, or that somewhere during acceleration at sub-wind speed, the tail wind against the prop would result in counter torque at the driven (driving) wheels to stop acceleration before wind speed was reached.

I just added a post explaining that if the advance ratio (prop pitch / wheel circumference) is sufficiently small, << 1, than the wheels have sufficient leverage over the prop to keep it turning against the tailwind as long as the wheels don't slip.

In the second video from my previous post, a gust of wind does cause the prop to turbine, and the wheels to spin backwards as the cart slides forwards. However I don't think this can happen unless the wheels lose traction as in the video (as long as the advance ratio is sufficiently less than 1).

 

[rcgldr]

But I don't believe the cart will ever get to wind speed. The faster it goes the faster the wind loses it's power to accelerate the cart.

True, but this doesn't mean the limit is the speed of the wind. The air flow through the propeller is accelerated upwind, and as long as that air flow from the propeller results in a net slowing of the wind, then the cart can achieve a faster than downwind speed.

As stated before, my guestimate is that the limit is probably about 1.5 times the speed of the wind. Compare this to an ice boat where it's downwind component of speed can exceed 3 times the speed of the wind when tacking (at an angle to the wind) downwind. Note that the iceboat can only achieve this speed if the air flow off the sail results in a net slowing down of the wind. In both cases the apparent wind has to be diverted or accelerated so that there is a net slowing of the true wind.

 

[zoobyshoe]

OK, that was a theoretical extreme. How about an iceboat downwind component during a downwind tack? Although more efficient than a DDWFTTW cart, perhaps it will demonstrate the possibility. A link to a .pdf file from an ice boat web site:

http://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

There's are a couple of diagrams from a real iceboat run. In the second one, "downwind angles: Skeeter", the wind speed is 18 mph, and the ice boat's heading is 30 degrees offset from true downwind. The apparent crosswind speed is 18 mph x sin(30) = 9 mph, regardless of the iceboats speed. In this case, the iceboat can achieve an apparent headwind speed of 54.4 mph with an apparent crosswind of 9 mph. This tranlates into a ground speed of 70 mph for the ice boat, and an apparent total wind of 55.15 mph (shown as 55 mph in the diagram, I included the .15 so the heading angle offset was 30 degrees). The net downwind speed is 70 mph x cos(30) = 60.6 mph, over 3 times the speed of the wind. Using my numbers (55.15 mph), and a 30 degree heading, I calculate a Beta of 9.4 degress (atan(9/54.4)) about 6:1 as opposed to the 8 (about 7:1) degress shown on the diagram, I'm not sure if this was a mistake or due to rounding errors.

This is dazzling at first, and can easily lure someone into thinking there's a way to translate it to ddwfttw, but the skeeter, itself cannot do it, which should give you pause. The fact we can sail 45 degrees off the wind if we exchange square sails for sloop sails suggests that an even cleverer sail might be able to go directly into the wind. There's that thing where if you stand a yardstick up vertically and let it fall the tip will have accelerated faster than g when it hits the deck, which is popularly known as "Freefall Faster Than g". It might suggest that there's also probably a way to freefall straight down faster than g.

The most convincing argument will be a proof: make an actual cart and send it down wind outdoors. Clock it and clock the windspeed. It should be easier to make an actual cart than the whole turntable thing swerda made.

 

[zoobyshoe]

There is no physical law that tells me that I need a certain amount of mass to have a certain sail. I don't break any conservation of energy or momentum by having a 10 gram sail and mast which is strong enough to do so. I won't find any actual material that does so in the current state of technology, but I don't break any laws of classical mechanics by having a very high material strength to mass ratio.

What people here are claiming is that downwind faster than the wind is breaking some fundamental laws of classical mechanics. It doesn't. In order to show that it doesn't, I'm allowed to use any imaginary system that respects classical mechanics.

I'm trying to disprove a theorem. The theorem is: *classical mechanics* forbids in principle to go DWFTTW. Well, if I can find ONE counter example, that is, something that respects entirely all the laws of classical mechanics, and nevertheless goes DWFTTW, then I have disproven the theorem.

That's what I did, already a few times.

The theorem "DWFTTW is prohibited by classical mechanics" is an erroneous theorem.

I'm not proposing any theorems. I have a big problem with the DW etc. notion because of "Power available from the wind is proportional to V^3" .

The OP presents an actual physical demonstration (of something) so I assumed the idea was that this had to be proven to be practically possible. Likewise, Jeff Reid seems to be getting most of his inspiration from actual ice boats, which supports my assumption. To the best of my knowledge you are the only one who is perfectly satisfied with a gedanken version and nothing more concrete.

 

[schroder]

agreed.

I just posted two links to videos showing cart start up in a true wind.

http://www.youtube.com/watch?v=QTAd891IpRs&fmt=18

http://www.youtube.com/watch?v=kWSan2CMgos&fmt=18

In the second segement of the second video a strong gust does initially turn the the prop as a turbine, causing the wheels to be driven backwards and sliding as the cart moves forward, but the wheels regain traction and starting turning the prop in the correct direction.

I see what your concern is, that the prop could act as a turbine at sub wind speeds, but these DDWFTTW carts are using props with a low pitch factor. The geometric advance of the prop per revolution is small, much smaller than the circumference of the driven / driving wheels. The prop in this case has a 12" diameter and a 6" per revolution pitch. That means the prop tips travel 37.7" perpendicular to the wind for every 6" inches the prop moves forward. That's the equivilant of an AOA of around 9 degrees at the tips, so the turbine effect from the tailwind at sub-wind speeds is small. In this case the advance ratio is 6" / 10.5" (prop pitch / wheel circumference), providing the wheels with enough "leverage" to prevent a turbine reaction from the prop, unless the wheels slide as in the case of the gusting wind in the second video. The startup videos are evidence of how the carts behave in a true wind.

I have looked at the videos, more than once! In the second (outdoor video) you posted, it is clear to see that the first initial gust of wind, at 0:08 seconds, does in fact turn the prop CCW as a turbine, and the wheels are dragging on the ground. (Because of wrong gearing) At 0:13 seconds, we are treated to a prop suddenly reversing in direction, driving the wheels the right way so the cart can in fact move down wind. Is it moving faster than the wind? I rather doubt that, as at 0:14 seconds, a piece of scrap paper, being blown by the wind, absolutely trashes the cart, as it blows past at a much higher velocity. To top it off, a man who was standing at the start line, takes off at a casual jog after the cart has a considerable head start, and easily catches up to it. Not very impressive! But getting back to your point that the force on the wheels is greater than the force on the prop, so the wheels overcome the initial direction of the prop and cause it to reverse direction. I don’t buy that for one second. The cart is quite aerodynamic, it offers very little in the way of blunt object wind resistance. The propeller is by far and away the biggest area presented to the wind. Once the wind starts to turn the propeller in accordance with its fixed pitch, and the wheels start dragging on the ground, there is only one way I can see which will allow the prop to reverse and the wheels start running the right way: that is a differential gear, which senses that forward motion is impossible and so slips the prop rotation into mode 2, which is to change the direction of gearing. If that is what is happening, this amounts to a hoax, nothing more, nothing less! I do not intend to spend the rest of my life busting this hoax, but I think you should realize that I am just an engineer (civils and electrical) and a member of New York Academy of Science. There are other people far more qualified than myself who are now expressing an interest in this, and in those forums which are promoting this. My advice is to let swerdna run some more tests, as I think he is genuinely interested in knowing the truth, and he is not trying to hoax anyone, and let the chips fall where they may. Is that acceptable to everybody?

 

[zoobyshoe]

True, but this doesn't mean the limit is the speed of the wind. The air flow through the propeller is accelerated upwind...

I don't think so. The airflow goes downwind through the prop transferring energy to the cart, but only so long as its speed relative to the cart affords it the power to do so. The more it succeeds the less it is able to build on its success: diminishing returns. Friction never sleeps.

 

[zoobyshoe]

Hey Jeff, here's your boat!

 

[schroder]

Hey Jeff, here's your boat!

I like that, although I would be happier with a pair of oars myself. Hopefully, no one is claiming that can go faster than the wind! But it can be useful as a fun vehicle.

 

[vanesch]

I'm not proposing any theorems. I have a big problem with the DW etc. notion because of "Power available from the wind is proportional to V^3" .

The OP presents an actual physical demonstration (of something) so I assumed the idea was that this had to be proven to be practically possible. Likewise, Jeff Reid seems to be getting most of his inspiration from actual ice boats, which supports my assumption. To the best of my knowledge you are the only one who is perfectly satisfied with a gedanken version and nothing more concrete.

Why is this demonstration so vehemently attacked ? After all, it does prove a DWFTTW case, with only a few minor points of discussion: the fact that it is on a disk with a radius which is not so much larger than the size of the installation and so on might make one think that locally, the reference frame attached to the turntable is not entirely an inertial frame (because of the rotation). This is the ONLY problem with the OP's experiment. If the reference frame of the disk can be seen as an inertial frame, then you put yourself in that frame, the turntable is at rest, the air in the room is flowing in at the opposite speed of the turntable (speed in the lab frame, because in our reference frame, it is at rest), and the cart is running backward faster than the wind it receives. So to me, that's DWFTTW in the reference frame attached to the turntable. It is not a perfect inertial frame because of the smallness of the radius of the disk. So would it work too on a bigger turntable ? I would think so but that's the only question.

Now, why is this simple experiment with a clear conclusion put in doubt ? Because people here think it is IN PRINCIPLE impossible to go DWFTTW. Some say it are "over unity devices", others say it is against conservation of momentum and all that. If that were true, then it would be a simple THEOREM in classical mechanics that there is no possibility of having a DWFTTW device. It is on this supposed theorem that people attack the rather obvious demonstration. If they would not be a priori convinced that DWFTTW were impossible, this would be just a fun demonstration of something that might not be intuitively evident at first sight, but no more surprising than to find that you can wind up a wire around a wheel by pulling on the wire (see my post with two pictures attached to it).
So the real problem here is that people ASSUME that there is some theorem in classical mechanics that forbids DWFTTW. Well, it is then sufficient to disprove this so-called theorem, and for that, a gedanken experiment is sufficient (it is also sufficient to see no proof emerge by those claiming nevertheless that DWFTTW is impossible...).

A theorem for which no proof is advanced, and for which there are "gedanken experiments" that serve as counter example is no theorem. In other words, NOTHING in classical mechanics forbids IN PRINCIPLE DWFTTW. And once you're there, the original objections to the demonstration are moot.

Because if the demonstration is genuine, then it is hard to motivate why this is NOT a practical demonstration of DWFTTW, unless we're not allowed anymore to assume that physics is independent of the choice of inertial frame.

So, in conclusion: we have a nice demonstration of an effect in the lab (true, with the small caveat that the radius of the turntable is maybe not big enough), and a disproof of any theorem that might forbid such an effect.

So where are the grounds to claim that the demonstration is not possible and that the thing "would not work" in a "real wind situation" ?

 

[vanesch]

I don't think so. The airflow goes downwind through the prop transferring energy to the cart, but only so long as its speed relative to the cart affords it the power to do so. The more it succeeds the less it is able to build on its success: diminishing returns. Friction never sleeps.

But at wind speed, you have power available on the axle of the wheels ! You can use that in principle to make a prop turn and get you faster. The energy goes from the wheels to the prop. The slightest bit of power that you can extract from the axle and put into the air (which is now "standing still") will get you move faster than the wind.

But this is not an over-unity device. The reason is that taking a small amount of power from the wheels will make them tend to spin less slow by a smaller amount than the increase in velocity you can get by the wind. The reason for that is that the kinetic energy goes as v^2.

As such, wrt the ground, at a velocity v_wind, if you take away energy E, you would tend to go from 1/2 m (v_wind)^2 to 1/2 m (v_wind)^2 - E. You would hence tend to slow down from v_wind to sqrt(v_wind^2 - 2 E / m), which is approximately E/ v_wind m.

However, wrt to the air (which is stationary), you go from 0 to E. So here you tend to increase your velocity by sqrt(2 E / m). For small E, this is a strong increase (the derivative of sqrt(x) at the origin is infinite). So for small enough E, this is always bigger than E/v_wind m. So taking some axle energy and putting it into any air acceleration will always win you some velocity for small enough losses.

The reason why this is not an over-unity device is because you have the air at 0 speed while the ground is NOT at zero speed, and it is this difference which makes that energy borrowed from the ground "pays off" to the air.
A self-propelled "over unity" device doesn't work, because there is no speed difference, so both energy functions start off at the same slope. There can only be losses. There's nothing to be gained from taking energy from the axle to put it in the air. Both have the same kinetic energy formula with the same origin.

 

[schroder]

Why is this demonstration so vehemently attacked ? After all, it does prove a DWFTTW case, with only a few minor points of discussion: the fact that it is on a disk with a radius which is not so much larger than the size of the installation and so on might make one think that locally, the reference frame attached to the turntable is not entirely an inertial frame (because of the rotation). This is the ONLY problem with the OP's experiment. If the reference frame of the disk can be seen as an inertial frame, then you put yourself in that frame, the turntable is at rest, the air in the room is flowing in at the opposite speed of the turntable (speed in the lab frame, because in our reference frame, it is at rest), and the cart is running backward faster than the wind it receives. So to me, that's DWFTTW in the reference frame attached to the turntable. It is not a perfect inertial frame because of the smallness of the radius of the disk. So would it work too on a bigger turntable ? I would think so but that's the only question.



So where are the grounds to claim that the demonstration is not possible and that the thing "would not work" in a "real wind situation" ?

I have already presented my detailed reason why this is NOT a valid inertial reference frame. Either you were missed it or simply chose to ignore it. Let me begin by an example: Place a sailboat in a frame in which the water is calm but the wind is blowing. The sail is up, and the skiff is sailing downwind. Now place the exact same sailboat in an equivalent reference frame, the air is calm, but the water has a current which is flowing in the opposite direction the wind was blowing in reference A. This causes a relative wind to blow which is indistinguishable in frame B from the wind in frame A. The sailboat sails downwind exactly as in frame A. The two frames are equivocal. I go through this exercise not to insult your intelligence, but just to be sure we are on the same page here.
OK. Now we take this cart, just a wheel with a propeller and hook it up as in the video and run it exactly as we have seen. The turntable spins CW, the cart gets dragged backwards for a short while due to inertia, static resistance but soon enough the apparent wind working against that wide yardarm, as well as the cart and propeller, slows the movement of the cart relative to the movement of the turntable. This slowing causes a relative difference in velocity as seen by the drive wheel, which spins up and via the cable also spins the propeller. The direction of spin and pitch of the propeller is such so that it acts as a true propeller, not a mill, and it propels the cart to move counter to the spin of the table. The cart can be seen advancing against the table. Now, your interpretation is that this is an equivalent reference frame to a stationary table and a true wind blowing on the back of the cart. You also interpret this as the cart going faster than the wind, since it is advancing against the table. Here is where you are wrong: This would ONLY be an equivalent reference frame if the cart were to move in exactly the same direction and at the same velocity, if it were driven NOT by the moving table, but instead by an actual wind blowing on the cart as it stands on a stationary table! I have already shown, and Jeff agrees, that if a wind were to blow on the propeller, with the cart stationary on the table, the propeller will act as a turbine, turning the opposite way, turning the wheels the opposite way, and the cart will try to work AGAINST the wind, not go down wind! Do you still consider the two frames to be equivalent? Under the same relative environmental conditions, the cart will behave two different ways! That is NOT an equivalent reference frame by any means! As I have said, countless times, you cannot test a cart which is configured for upwind motion and claim that it proves down wind performance! I have showed you how to set up the cart so that it is a true down wind cart; simply remove the drive cable from one side of the wheel and attach it on the other side that is all that is required. Now, when you run the test, it will be a TRUE equivocal reference frame with a cart in the wind. What you will find is that the cart on the turntable will run backwards with the table until the wind resistance turns the mill as a turbine, which turns the wheels, and it will indeed go down wind, but it will NEVER exceed the table velocity. This will PROVE the DDWFTTFW is impossible. You can take that same cart, same configuration, and place a fan behind it and it will go downwind, something the original configuration could NOT do! Can you now understand the importance of not mixing and matching reference frames? Test the DW cart on the turntable configured as it would be to go DW. Test the UP wind cart configured on the turn table in the same configuration it will be when going up wind. Please do not mix and match! Is there no one here who understands this?

 

[zoobyshoe]

Why is this demonstration so vehemently attacked ?

I haven't attacked it, vehemently or otherwise. I have, pretty calmly, expressed skepticism about it.

After all, it does prove a DWFTTW case, with only a few minor points of discussion: the fact that it is on a disk with a radius which is not so much larger than the size of the installation and so on might make one think that locally, the reference frame attached to the turntable is not entirely an inertial frame (because of the rotation). This is the ONLY problem with the OP's experiment. If the reference frame of the disk can be seen as an inertial frame, then you put yourself in that frame, the turntable is at rest, the air in the room is flowing in at the opposite speed of the turntable (speed in the lab frame, because in our reference frame, it is at rest), and the cart is running backward faster than the wind it receives. So to me, that's DWFTTW in the reference frame attached to the turntable. It is not a perfect inertial frame because of the smallness of the radius of the disk. So would it work too on a bigger turntable ? I would think so but that's the only question.

Now, why is this simple experiment with a clear conclusion put in doubt ? Because people here think it is IN PRINCIPLE impossible to go DWFTTW. Some say it are "over unity devices", others say it is against conservation of momentum and all that. If that were true, then it would be a simple THEOREM in classical mechanics that there is no possibility of having a DWFTTW device. It is on this supposed theorem that people attack the rather obvious demonstration. If they would not be a priori convinced that DWFTTW were impossible, this would be just a fun demonstration of something that might not be intuitively evident at first sight, but no more surprising than to find that you can wind up a wire around a wheel by pulling on the wire (see my post with two pictures attached to it).
So the real problem here is that people ASSUME that there is some theorem in classical mechanics that forbids DWFTTW. Well, it is then sufficient to disprove this so-called theorem, and for that, a gedanken experiment is sufficient (it is also sufficient to see no proof emerge by those claiming nevertheless that DWFTTW is impossible...).

A theorem for which no proof is advanced, and for which there are "gedanken experiments" that serve as counter example is no theorem. In other words, NOTHING in classical mechanics forbids IN PRINCIPLE DWFTTW. And once you're there, the original objections to the demonstration are moot.

Because if the demonstration is genuine, then it is hard to motivate why this is NOT a practical demonstration of DWFTTW, unless we're not allowed anymore to assume that physics is independent of the choice of inertial frame.

So, in conclusion: we have a nice demonstration of an effect in the lab (true, with the small caveat that the radius of the turntable is maybe not big enough), and a disproof of any theorem that might forbid such an effect.

So where are the grounds to claim that the demonstration is not possible and that the thing "would not work" in a "real wind situation" ?

I am not claiming it is not possible, but I admit to having a "real problem" with it, which I have described already.

Jeff has linked to an outdoor demonstration in real wind in which, I'll admit, the cart looked pretty zippy, so if it bothers you that I don't share your confidence in it, then just look forward to the possibility of saying "I TOLD you so!"