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

[A.T.]

Really? (...) Can a shopping cart go faster on level ground, than the person who is pushing it, in a continuous fashion?

Yes, if the person is pushing the cart via a gearing mechanism. This has been demonstrated by Carlos and his friends, and already referenced in this thread:
http://de.youtube.com/watch?v=9Yt4zxYuPzI&feature=channel
You just need a very long ruler, to keep contact with the cart.

 

[schroder]

I would be very happy to perform any test that you suggest as long as it makes some sense and is possible. This thread is moving very fast and I’m behind a heap of previous posts without even keeping up with the new ones. Rather than me looking through heaps of posts to find out what test you want could you please explain (again) exactly what test you want me to conduct and what the purpose of the test is and what will it is designed to prove.

Thanks! We were posting at the same time and went right past each other.

OK. In order to prove that this does not truly simulate a cart in a down wind situation: I would like you to place an electric fan so it blows only on the propeller, not on the crossbeam, wheel or anything else. Try to place the fan so it effects only the propeller, Please note which way the propeller turns from the wind on it. The fan must be blowing from the direction of the relative wind in your running experiment. The table is not running. Does the propeller turn in the same way as in your running test or the opposite way? Do the wheels turn, and if so which way, same as before or opposite? That is test number one. I have another for you but this one is of immediate importance. Thanks again.

 

[swerdna]

Thanks! We were posting at the same time and went right past each other.

OK. In order to prove that this does not truly simulate a cart in a down wind situation: I would like you to place an electric fan so it blows only on the propeller, not on the crossbeam, wheel or anything else. Try to place the fan so it effects only the propeller, Please note which way the propeller turns from the wind on it. The fan must be blowing from the direction of the relative wind in your running experiment. The table is not running. Does the propeller turn in the same way as in your running test or the opposite way? Do the wheels turn, and if so which way, same as before or opposite? That is test number one. I have another for you but this one is of immediate importance. Thanks again.

I think the answers to your questions are fairly obvious without doing any testing but I’m happy to do the test as you ask and will do so right away. While I’m doing it could you please explain the purpose of this test. What is being demonstrated/proven?

 

[zoobyshoe]

Really? So if a shopping cart is extracting power from someone pushing it, and that someone slows dow or stops pushing or just walk away, the cart can still extract power from the person walking away? Can a shopping cart go faster on level ground, than the person who is pushing it, in a continuous fashion? You might give it a mighty shove, but it will soon stop. Over any reasonable distance , it will never outrun you. And if you were not pushing the cart at all, you would cover the distance much faster. Same with the wind pushing that cart.

I think that a person doesn't constitute a surrounding medium. We're pretty much limited to air over land or air over water. The geared vehicle has to be able to take advantage of the difference in speed of the surrounding media over a large range for it to matter.

Fear not: I was not necessarily announcing a conversion, just that I finally understood the principle that Jeff Reid kept alluding to without formally stating it. It explained why he kept referring to friction as if it were a power source.

 

[schroder]

I think the answers to your questions are fairly obvious without doing any testing but I’m happy to do the test as you ask and will do so right away. While I’m doing it could you please explain the purpose of this test. What is being demonstrated/proven?

They are obvious to you and to me, but not to some others here so we would like to do this layer by layer, if you do not mind. If the prop turns the opposite way, and the wheels turn the opposite way, it shows that the cart is actually configured to go upwind, not down but it is being tested in a down wind jig. It will invalidate any conclusions being drawn on the basis of similar reference frames. But I am getting ahead of the story. Can we please peel this onion one layer at a time?

 

[swerdna]

No problem in conducting the test as requested.

Test 1 - When the motion of the turntable was held and the fan held close to the “wind side” of the prop virtually nothing happened (no movement of prop or wheel). For the prop to turn the way it “wanted” to the wheel would have turned and moved the prop and cart closer to the fan. The opposing forces of the fan and wheel through the prop and cable were pretty much equal and opposite. If the force of the fan had been greater than the force of the wheel the prop would have moved closer toward the fan.

Is this what you expected and what are your conclusions?

 

[schroder]

No problem in conducting the test as requested.

Test 1 - When the motion of the turntable was held and the fan held close to the “wind side” of the prop virtually nothing happened (no movement of prop or wheel). For the prop to turn the way it “wanted” to the wheel would have turned and moved the propand cart closer to the fan. The opposing forces of the fan and wheel through the prop and cable were pretty much equal and opposite. If the force of the fan had been greater than the force of the wheel the prop would have moved closer toward the fan.

Is this what you expected and what are your conclusions?

OK. Thank you. That is exactly what I expected. I think it would be safe to say that if we could place this cart out in the wind, with the wind blowing from the back, same direction as the fan and same direction as the apparent wind in your running test, that the cart would go nowhere. It would not go downwind, and it would certainly not go downwind faster than the wind. What we have shown here is how totally erroneous conclusions can be drawn by incorrectly matching reference frames. Do you agree?

If so we can proceed to the next test.

 

[swerdna]

OK. Thank you. That is exactly what I expected. I think it would be safe to say that if we could place this cart out in the wind, with the wind blowing from the back, same direction as the fan and same direction as the apparent wind in your running test, that the cart would go nowhere. It would not go downwind, and it would certainly not go downwind faster than the wind. What we have shown here is how totally erroneous conclusions can be drawn by incorrectly matching reference frames. Do you agree?

If so we can proceed to the next test.

I will think about it before commenting and I have some other things that I need to be doing right now. I will get back as soon as possible.

 

[schroder]

I will think about it before commenting and I have some other things that I need to be doing right now. I will get back as soon as possible.

OK Thanks for your help. You have already proven more than five or six threads of arguing. You may not achiueve fame for making a DDWFTTFW machine, but you will be the one who finally debunks this thing. That is something!

 

[zoobyshoe]

I will think about it before commenting and I have some other things that I need to be doing right now. I will get back as soon as possible.

I can't believe he wimped out after only 20 hours of posting.

 

[pallidin]

Yes, if the person is pushing the cart via a gearing mechanism. This has been demonstrated by Carlos and his friends, and already referenced in this thread:
http://de.youtube.com/watch?v=9Yt4zxYuPzI&feature=channel
You just need a very long ruler, to keep contact with the cart.

Exactly. I've seen that vid before.
Now, the question is how to sustain this reaction without an infinite linear ruler.

 

[pallidin]

I can't believe he wimped out after only 20 hours of posting.

:rofl: Yeah, this whole subject can fry a couple brain cells.

 

[A.T.]

Can a shopping cart go faster on level ground, than the person who is pushing it, in a continuous fashion?

Yes, if the person is pushing the cart via a gearing mechanism. This has been demonstrated by Carlos and his friends, and already referenced in this thread:
http://de.youtube.com/watch?v=9Yt4zxYuPzI&feature=channel
You just need a very long ruler, to keep contact with the cart.

Exactly. I've seen that vid before.
Now, the question is how to sustain this reaction without an infinite linear ruler.

The air is the infinite linear ruler in DDWFTTW machines.

 

[swerdna]

OK. Thank you. That is exactly what I expected. I think it would be safe to say that if we could place this cart out in the wind, with the wind blowing from the back, same direction as the fan and same direction as the apparent wind in your running test, that the cart would go nowhere. It would not go downwind, and it would certainly not go downwind faster than the wind. What we have shown here is how totally erroneous conclusions can be drawn by incorrectly matching reference frames. Do you agree?

If so we can proceed to the next test.

Sorry but I can’t agree with your conclusion or test. It’s not a valid test to have the wind only blowing on the prop. In “real life” the wind blows on the cart fame as well as the prop. This is important because in the first instance the cart and prop have to act as a sail to start the cart moving with the wind (the prop doest act as a prop in the very first instance). When I add a flat cardboard sail to the tether arm close to the prop the cart moves away from the fan, the wheel turns and the prop spins to thrust against the fan wind. After the cart begins to move over the surface the wheel spins the prop the wind doesn’t directly spin the prop. Surely you have seen video evidence (mine for example) of the cart moving downwind only powered by wind?

I like your step by step approach and we shouldn’t move to the next step until the current step is resolved. So no next test yet please. If you don’t agree with anything I’ve said please explain why.

 

[swerdna]

I can't believe he wimped out after only 20 hours of posting.

Am I right to take that as humour?

 

[schroder]

Sorry but I can’t agree with your conclusion or test. It’s not a valid test to have the wind only blowing on the prop. In “real life” the wind blows on the cart fame as well as the prop. This is important because in the first instance the cart and prop have to act as a sail to start the cart moving with the wind (the prop doest act as a prop in the very first instance). When I add a flat cardboard sail to the tether arm close to the prop the cart moves away from the fan, the wheel turns and the prop spins to thrust against the fan wind. After the cart begins to move over the surface the wheel spins the prop the wind doesn’t directly spin the prop. Surely you have seen video evidence (mine for example) of the cart moving downwind only powered by wind?

I like your step by step approach and we shouldn’t move to the next step until the current step is resolved. So no next test yet please. If you don’t agree with anything I’ve said please explain why.

I don’t agree because the carts do not have “sails” to catch the wind and force the cart to move downwind, turning the wheel, which then turns the propeller to act as a propeller instead of a turbine. Not only that, but they do not have a big crossarm on which you can mount such a sail. The carts are just skeletal frames, which offer very little blunt body wind resistance. The prop by far offers the most wind resistance and it will turn the opposite way, as we have just verified. If you were to put up such a sail do you still think you can achieve DDWFTTW? Would it not offer substantial wind resistance at wind velocity? Tell you what, this was not my second test, but we can try this instead: Put up your sail, it will need to be fairly rigid, and run your test again just as before. As long as the sail does not blow off, I would bet the cart never exceeds the table velocity because of the new drag. We either need to stick to the original configuration or keep testing in different configs over and over again. If the carts have no sails, it is a violation of the original reference frame to introduce one now. Up to you what you want to do next. Your call.

 

[schroder]

Am I right to take that as humour?

He wimped out also, so yes that was definitely humor! In fact, I can't stay with this much longer but would like to see one more test if possible. I do appreciate what you are doing. I never could have gotten this sort of cooperation from the thinair group!

 

[swerdna]

I don’t agree because the carts do not have “sails” to catch the wind and force the cart to move downwind, turning the wheel, which then turns the propeller to act as a propeller instead of a turbine. Not only that, but they do not have a big crossarm on which you can mount such a sail. The carts are just skeletal frames, which offer very little blunt body wind resistance. The prop by far offers the most wind resistance and it will turn the opposite way, as we have just verified. If you were to put up such a sail do you still think you can achieve DDWFTTW? Would it not offer substantial wind resistance at wind velocity? Tell you what, this was not my second test, but we can try this instead: Put up your sail, it will need to be fairly rigid, and run your test again just as before. As long as the sail does not blow off, I would bet the cart never exceeds the table velocity because of the new drag. We either need to stick to the original configuration or keep testing in different configs over and over again. If the carts have no sails, it is a violation of the original reference frame to introduce one now. Up to you what you want to do next. Your call.

We really should stick to one point at time but . . . (this is the last time I will do this ;-) - I have designed a sail concept where the sail stands vertical while the cart is in the wind then it falls down to the horizontal at wind speed and beyond. It could fall away from the cart altogether.

Back to the current point please (there’s actually two)

Point 1 - Do you agree that a valid test should have the wind blowing on the full cart surface?

Point 2 - Do you agree that however skeletal a frame is it is an increase in wind resistance (however slight) compared to it not being there?

Question - How do you explain the many videos where carts can be clearly seen moving forward in a wind from a stationary position? I can provide links if you haven‘t seen them.

 

[swerdna]

He wimped out also, so yes that was definitely humor! In fact, I can't stay with this much longer but would like to see one more test if possible. I do appreciate what you are doing. I never could have gotten this sort of cooperation from the thinair group!

I don't care how long it takes (it's not a DDWFTTW race ;-) and think it's really important to stick to one point at a time.

 

[schroder]

Surely you have seen video evidence (mine for example) of the cart moving downwind only powered by wind?

I have seen some downwind tests but they are very inconclusive. At first, the prop seems to swing one way, then swings the other way. I am sure in at least one video that it is rotating as a turbine, and yet the wheels are driving in the correct direction to go down wind (but not faster than the wind) I have yet to have anyone explain how the gear reversal was arranged to allow that to happen. There HAD to be a gear reversal because the prop was turning the opposite way as on the treadmill but the wheels were turning the same. A bit of foul play at work, had to be! I don't believe I have seen your outdoor test.

 

[atyy]

No, of course not. The windmill could almost be massless (made out of neutrinonium ?) and the car could go as fast as it goes (call it "lightbullet").

You could consider having two extremely light windmills, planting one down (connected with a rope to your car) that will generate electricity for a few seconds, then fold it up and take it in (with the rope, almost no effort as it is essentially massless), and plant at the same time the second windmill, having it produce electricity for a few seconds, fold it up and take it in while planting the first one again, etc...

A kind of "walking on windmills". Very clumsy, but as a proof of principle, I don't see what stops it.

Of course, there is conservation of momentum, and hence what must remain at the same velocity is the center of gravity of the air (consider a big, but limited amount) and the car, which should move at a velocity slightly smaller than the wind speed (as the car is initially at rest). So we have to "slow down" enough wind to compensate for the increase in speed of the car ; but as there is no limit as to the amount of wind we slow down (or even reverse direction), this doesn't put a hard limit on the speed of the car. This will come out of the energy balance of the whole thing I guess.

I guess I buy your original scenario in the case where the car experiences no drag, isolating all resistance to the windmill ie. I buy it even without walking windmills, since the car can be accelerated to any finite speed greater than wind speed, and after a sufficiently long time the average speed of the centre of mass of car and windmill will exceed the wind speed.

In the case where there is drag on the car also, it seems the scenario can also hold by making the windmill bigger to get more power from the wind.

 

[swerdna]

I have seen some downwind tests but they are very inconclusive. At first, the prop seems to swing one way, then swings the other way. I am sure in at least one video that it is rotating as a turbine, and yet the wheels are driving in the correct direction to go down wind (but not faster than the wind) I have yet to have anyone explain how the gear reversal was arranged to allow that to happen. There HAD to be a gear reversal because the prop was turning the opposite way as on the treadmill but the wheels were turning the same. A bit of foul play at work, had to be! I don't believe I have seen your outdoor test.

Yes, in the very first instance it’s a battle between what the wind wants the prop to do and what the wheel wants the prop to do, For the cart to move forward with the wind the wheel has to win and it does it by the prop and cart frame acting as a sail against the wind.

I haven’t done an outside test yet. Are you saying that you will only accept outside tests as being valid? If so why are we bothering to use my turntable to do your tests?

If you have time now or later could you please answer two “point” questions. A simple yes or no in each case would be fine - thanks.

 

[rcgldr]

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.

I've come up with a better explanation for why the cart will respond to a tailwind by moving downwind if the advance ratio is sufficiently < 1.

Assume zero loss factor in movement of air by the prop. For each revolution of prop and wheel, the prop moves the air 6" upwind, while the cart moves 10.5" downwind because the advance ratio (prop pitch / wheel circumference) is 6 /10.5. That means for each forward revolution of the prop and wheel, the air moves 4.5" downwind. The net air flow is in the direction of travel of the cart. The path of least resistance is to allow the air to flow downwind and this only occurs if the cart moves downwind in response to a tailwind.

The air does exert a countering torque on the propeller and wheels, which causes the wheels to exert a forward force on the ground at the contact patches of the wheels, and the ground will respond with an equal and opposite backwards force on the wheels, the Newton 3rd law pair of forces here. The air also exerts a forward force on the propeller, and this force will be greater than the opposing backwards force from the ground related to the windmill torque effect, if the advance ratio is sufficiently < 1. Since the forward force from the air is greater than the backwards force from the ground (related to windmill torque), the net force is forwards and the cart accelerates downwind.

Yes, in the very first instance it’s a battle between what the wind wants the prop to do and what the wheel wants the prop to do. To move forward the wheel has to win and it does it by the prop and cart frame acting as a sail against the wind.

For the reasons explained above, what the wind wants the prop to do and what the wheels want the prop to do results in equal and opposite torques, because the ground will only react to the torque related forward force from the contact patch of the wheel with an equal and opposing force, in compliance with Newtons 3rd law. The real battle is beteen the forward force of the air on prop and cart, versus the backward force from the ground. Even if the cart were drag free, the forward force of the air on the prop will be greater than the backwards force from the ground opposing the windmile torque on the prop.

In reality, any rolling resitance factors also create a countering torqe on the wheels, so the total backwards force from the ground also includes these loss related factors. Still, if the advance ratio is sufficiently < 1, then the forward force of the air will be greater than the backwards force from the ground and the cart accelerates forwards.

 

[schroder]

We really should stick to one point at time but . . . (this is the last time I will do this ;-) - I have designed a sail concept where the sail stands vertical while the cart is in the wind then it falls down to the horizontal at wind speed and beyond. It could fall away from the cart altogether.

Back to the current point please (there’s actually two)

Point 1 - Do you agree that a valid test should have the wind blowing on the full cart surface?

Point 2 - Do you agree that however skeletal a frame is it is an increase in wind resistance (however slight) compared to it not being there?

Question - How do you explain the many videos where carts can be clearly seen moving forward in a wind from a stationary position? I can provide links if you haven‘t seen them.

I have just answered your question three with another post (while you were posting)

1)OK, if you can manage to point the fan so that it covers teh prop and the cart body, that would be fine. I am concerned about that crossarm acting as a sail. I don't know how you can ensure that the crossarm will not act as a sail and move the cart. It should be only the body of the cart and the prop. I still believe the prop will spin the opposite way. And remember you should not allow the cart to get away from the fan it should follow right along with the cart, same as the natural wind would do. I cannot believe you will achiene any significant downwind speed under those conditions.

2) Yes there has to be some increase, but I have seen many of these carts and the body amounts to almost nothing. They are almost all propeller!

 

[schroder]

I haven’t done an outside test yet. Are you saying that you will only accept outside tests as being valid? If so why are we bothering to use my turntable to do your tests?

I definitely did not say that! I think what we are doing here is quite valid and it would be great if we could continue.

 

[swerdna]

I have just answered your question three with another post (while you were posting)

1)OK, if you can manage to point the fan so that it covers teh prop and the cart body, that would be fine. I am concerned about that crossarm acting as a sail. I don't know how you can ensure that the crossarm will not act as a sail and move the cart. It should be only the body of the cart and the prop. I still believe the prop will spin the opposite way. And remember you should not allow the cart to get away from the fan it should follow right along with the cart, same as the natural wind would do. I cannot believe you will achiene any significant downwind speed under those conditions.

2) Yes there has to be some increase, but I have seen many of these carts and the body amounts to almost nothing. They are almost all propeller!

Can I take it that your answers to the questions are - point 1 = yes, point 2 = yes, third question = inconclusive?

 

[schroder]

I've come up with a better explanation for why the cart will respond to a tailwind by moving downwind if the advance ratio is sufficiently < 1.

Assume zero loss factor in movement of air by the prop. For each revolution of prop and wheel, the prop moves the air 6" upwind, while the cart moves 10.5" downwind because the advance ratio (prop pitch / wheel circumference) is 6 /10.5. That means for each forward revolution of the prop and wheel, the air moves 4.5" downwind. The net air flow is in the direction of travel of the cart. The path of least resistance is to allow the air to flow downwind and this only occurs if the cart moves downwind in response to a tailwind.

The air does exert a countering torque on the propeller and wheels, which causes the wheels to exert a forward force on the ground at the contact patches of the wheels, and the ground will respond with an equal and opposite backwards force on the wheels, the Newton 3rd law pair of forces here. The air also exerts a forward force on the propeller, and this force will be greater than the opposing backwards force from the ground related to the windmill torque effect, if the advance ratio is sufficiently < 1. Since the forward force from the air is greater than the backwards force from the ground (related to windmill torque), the net force is forwards and the cart accelerates downwind.

Never the less, we just saw the prop turn the opposite way, as a turbinee

 

[Phrak]

Jeff, where the loss of energy due to friction occurs--the wheel or propeller--determines whether the wheel is driving the propeller, or visa versa. Additionally, from perturbation from wind or surface, if the cart in speeding up or slowing down to the equilibrium advancement ratio, the direction of power transfer changes (ignoring offsets from friction).

Dunno if you've said this or not. This has become a very long thread.

 

[schroder]

Can I take it that your answers to the questions are - point 1 = yes, point 2 = yes, third question = inconclusive?

yes with the caveat that you do not allow the fan to blow the crossarm or any kind of sail on the crossarm. Only the cart and the prop. Sorry for repeating myself, but this is a critical point.

 

[rcgldr]

Never the less, we just saw the prop turn the opposite way, as a turbine

This only occurred when the wheels were sliding, and the cart still ended up moving downwind. With an advance ratio < 1, the cart will not respond to a tailwind by moving upwind, it will always move downwind. If the tires don't slip, the prop will turn in the "forward" direction. If the tires slip, then the prop will be free to windmill with the tires rotating backwards while the cart moves forwards. This would easily happen on a low friction surface, such as a cart on ice.

If there is no slippage at the wheels, then as I mentioned before, the net airflow off the prop is always in the same direction as the cart, and the path of least resistance to the tail wind is for the cart to move downwind, with the air flow through a perfect prop being 4.5" downwind for each revolution of prop and wheel.

 

[schroder]

This only occurred when the wheels were sliding, and the cart still ended up moving downwind. With an advance ratio < 1, the cart will not respond to a tailwind by moving upwind, it will always move downwind. If the tires don't slip, the prop will turn in the "forward" direction. If the tires slip, then the prop will be free to windmill with the tires rotating backwards while the cart moves forwards. This would easily happen on a low friction surface, such as a cart on ice.

If there is no slippage at the wheels, then as I mentioned before, the net airflow off the prop is always in the same direction as the cart, and the path of least resistance to the tail wind is for the cart to move downwind, with the air flow through a perfect prop being 4.5" downwind for each revolution of prop and wheel.

No this just happened on swerdna's test jig with a fan blowing in place of the wind. The cart actually tried to move towrds the fan, not away. The wheels were not slipping, as far as I know. I'm taking a break guys, it is 6 am where I am at and I've been up all night. I will check back later. Thanks again swerdna and all for the great cooperation on this thread and no animosity, the way it should be!

 

[swerdna]

yes with the caveat that you do not allow the fan to blow the crossarm or any kind of sail on the crossarm. Only the cart and the prop. Sorry for repeating myself, but this is a critical point.

But the tether arm is part of the body of the cart. I’m at a loss to understand what you have against a cart having a surface area that acts as sail. How does having even a large sail area negate the DDWFTTW claim? It’s only using the immediate wind after all. It’s not storing energy. This really puzzles me and it would be really good if you could explain. As Iv’e said - If I was designing an outside cart I would definitely include a large sail area to help the cart get up to wind speed. The sail area would be removed at wind speed and beyond (why does that bring Buzz Lightyear to mind ;-)

 

[swerdna]

No this just happened on swerdna's test jig with a fan blowing in place of the wind. The cart actually tried to move towrds the fan, not away. The wheels were not slipping, as far as I know. I'm taking a break guys, it is 6 am where I am at and I've been up all night. I will check back later. Thanks again swerdna and all for the great cooperation on this thread and no animosity, the way it should be!

Sorry but the cart actually tried to move BOTH torward the fan and away from it. That's why it didn't move at all.

 

[swerdna]

No this just happened on swerdna's test jig with a fan blowing in place of the wind. The cart actually tried to move towrds the fan, not away. The wheels were not slipping, as far as I know. I'm taking a break guys, it is 6 am where I am at and I've been up all night. I will check back later. Thanks again swerdna and all for the great cooperation on this thread and no animosity, the way it should be!

WOW! - you're dedicated. Have a nice sleep. Thanks for your cooperation as well and hope it continues.

 

[rcgldr]

We've already seen these two videos of the cart self starting:

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

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

Don't forget that swerdna also made a self starting video:

To assimilate actual outside wind conditions as much as possible I have done a test where the cart is held to the turntable with a removable block so the cart initially has to move at the same speed as the turntable. The block is then removed (after about 3 - 4 seconds in video) and the wind then powers the thrust of the propeller to make the cart travel against the motion of the turntable.

Here’s the video - http://www.youtube.com/watch?v=k4owZkoeGAU&fmt=18

As mentioned before, with an advance ratio < 1, the cart will not tend to go upwind in response to a tailwind, because the net airlflow through the prop, even if a perfect prop, is in the same direction as the cart moves, just at a lower rate, 4.5" of air flow through the prop for each 10.5" of distance moved by the cart.

I suspect that static friction is the issue with swerdna's fan test. You just need a stronger fan, to duplicate sporks result with the fan blowing on his cart on a stationary treadmill.