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

[zoobyshoe]

This is absolutely not perpetual motion, not any more than the little car between a static and a turning wheel is (see my previous post).
What you have is that there is a mechanical binding between the cart and the turntable on one hand (given by the wheel on the turntable), and there's another "binding" between the propeller and the air mass, and there is a mechanical link between both. For a certain RPM of the wheel on the table, a certain RPM of the propeller is set, and this, in turn, gives a certain force on the cart. There will be a balance between the power taken or given by the wheel on the table, and the power taken or given by the propeller. The cart will settle in that motion when both are in equilibrium, and that equilibrium is given by the equivalent of a gearing ratio (including, in this case, the efficiency of the propeller).

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you. If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

 

[schroder]

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you. If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

Exactly! But the claim is that a wheel rolling on the ground, which must overcome friction in order to roll, and must be powered by the cart, will now somehow add torque and thrust to that propeller and make the cart go even faster. I guess if I lower an anchor from my 18 ft. skiff, it should make it go faster, by the same reasoning.

 

[vanesch]

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you.

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 ? Now, imagine you charge a battery with that for 10 minutes (while you are going exactly at downwind speed). After 10 minutes, you take down your sail, and you switch on your electrical motor and drive 10 times faster than the wind. After, say, 30 minutes, your batteries are discharged, so you put up your sail again, go at downwind speed again, and recharge your batteries. 10 minutes later, again, you switch to you motor.

Your average speed will be (1 x 10 + 10 x 30 )/40 = 7.75 times the downwind speed.

If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

This is like Zeno's paradox :-)

Look at my attachment in one of the previous postings, with the gearboxed two-wheel cart between a fixed disk and a turntable, which can go at any speed. Think of the air as the fixed disk. Or the other example of the winding wire: disk moving faster than the wire downwire.

 

[vanesch]

Exactly! But the claim is that a wheel rolling on the ground, which must overcome friction in order to roll, and must be powered by the cart, will now somehow add torque and thrust to that propeller and make the cart go even faster. I guess if I lower an anchor from my 18 ft. skiff, it should make it go faster, by the same reasoning.

A wheel must not "overcome friction in order to roll". If it overcame friction, it would slide! The wheel serves simply as a mechanical binding, from which one can extract more mechanical energy from the difference in velocity between the air and the turntable. There is no problem in energy conservation here, because there is no strict limit on how much energy is extracted from the turntable.

 

[schroder]

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 ? Now, imagine you charge a battery with that for 10 minutes (while you are going exactly at downwind speed). After 10 minutes, you take down your sail, and you switch on your electrical motor and drive 10 times faster than the wind. After, say, 30 minutes, your batteries are discharged, so you put up your sail again, go at downwind speed again, and recharge your batteries. 10 minutes later, again, you switch to you motor.

Your average speed will be (1 x 10 + 10 x 30 )/40 = 7.75 times the downwind speed.



This is like Zeno's paradox :-)




Look at my attachment in one of the previous postings, with the gearboxed two-wheel cart between a fixed disk and a turntable, which can go at any speed. Think of the air as the fixed disk.

Sure, an infinitely big and massless sail. In other words, a free source of boundless energy! Very nice but that is not what we are talking about here at all.

 

[schroder]

A wheel must not "overcome friction in order to roll". If it overcame friction, it would slide!

It must overcome the static friction and only have kinetic and rolling friction. I thought that was understood but thank you for the correction.

 

[atyy]

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. If I push an object so that it attains an initial velocity above that of the wind, then it will continue downwind faster than the wind (assume no air resistance). Now, can I use the wind to get it to that point rather than me pushing it? I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels into translational energy (am I violating angular momentum conservation?). From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

 

[rcgldr]

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind.

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.

The same goes for a sailcraft when it's heading is not parallel to the wind. The sailcraft experiences an apparent crosswind = true wind speed x sin(angle between wind and sailcraft heading), independent of the sailcrafts forward speed (since apparent crosswind is perpendicular to sailcraft's forward speed). If the ground drag factors are very low, such as an ice boat, then the limiting factor is how much apparent headwind can be achieved for a given apparent crosswind. The iceboat downwind component of speed can be more than double the true wind speed, and it's faster to tack at an angle and back than it is to go directly downwind. Again, in order for this to work, the sail has to divert the apparent wind upwind so that the net air flow off the sail is slower than the true wind.

So for both the DDWFTTW cart and the sailcraft, the wind is slowed down in order to obtain power from the wind, but the wind is able to be slowed down even though these craft are moving downwind faster than the wind, due to the upwind thrust created by the craft.

 

[schroder]

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. If I push an object so that it attains an initial velocity above that of the wind, then it will continue downwind faster than the wind (assume no air resistance). Now, can I use the wind to get it to that point rather than me pushing it? I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels into translational energy (am I violating angular momentum conservation?). From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

Just how do you assume no air resistance? If there is no air resistance, there is no wind! The wind is a medium that the cart is within. It is in the front of the cart and in the back of the cart. Once the sail pushes you up to wind speed (if it can) then you would be pushing against the wind in front of you and all that is pushing you is the wind behind you! If you were stuck in molasses and it was flowing in one direction, do you suppose the molasses in the back can push you faster than the molasses in the front. What if you dragged your foot, or a wheel on the ground. Would that speed you up or slow you down?

 

[rcgldr]

So you put up your sail again, go at downwind speed again, and recharge your batteries.

Shh ... you weren't supposed to give away the secret of those DDWFTTW carts shown in the videos. No one was supposed to realize that the gear boxes on those treadmill carts were really hiding a clever generator/motor/capacitor setup.

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels.

Well instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

 

[atyy]

Just how do you assume no air resistance? If there is no air resistance, there is no wind! The wind is a medium that the cart is within. It is in the front of the cart and in the back of the cart. Once the sail pushes you up to wind speed (if it can) then you would be pushing against the wind in front of you and all that is pushing you is the wind behind you! If you were stuck in molasses and it was flowing in one direction, do you suppose the molasses in the back can push you faster than the molasses in the front. What if you dragged your foot, or a wheel on the ground. Would that speed you up or slow you down?

The sail is withdrawn at the moment we convert the rotational energy in the wheels into translational energy.

 

[zoobyshoe]

True, but the faster you go over the floor, and the more energy you can pump out of that.

No, Power is proportional to V^3 means you don't go faster over the floor: the wind loses its ability to accelerate you till you reach a terminal velocity which must be < wind speed. Your gearing won't help because the wind is simply losing its power to turn the gear train.

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.

This is like Zeno's paradox :-)

Quoi? It's just conservation of energy.

 

[atyy]

Well instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

Yes, that's equivalent to what I was thinking, but why doesn't this violate angular momentum conservation?

 

[atyy]

From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

Am I allowed to have the thing sprout wings at the moment I withdraw sails and wheels, so that it doesn't touch the ground from that point on? It would be much more convenient without gravity, but I can't get rolling without slipping without gravity, since I'm using it to produce the friction.

 

[rcgldr]

Instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

Yes, that's equivalent to what I was thinking, but why doesn't this violate angular momentum conservation?

Because it's not a closed system. You have an external power source, the wind versus ground speed. If you want to make it a closed system, then you'd have to include the atmosphere and the earth, then angular momentum would be conserved (ok, maybe you have to include the moon, sun, tidal effects, ...).

 

[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.