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Merged DDWFTTW in a “Real” Wind
- Thread starter ynot
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There is no contradiction.
If the wind stops, then, in a frame moving at the speed of the wind, the ground isn't moving either. Therefore, the ground can't power the cart.
I clearly defined the frame my description and questions were based on . . .
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ynot, how would you explain the operation of a regular fan?
You plug it in, turn it on, it spins around, and it blows air at you. No part of the blade moves forward. Each point on the blade just moves around in a circle, in a flat plane. Yet the spinning blade as a whole ends up blowing air forward. How does that work?
You plug it in, turn it on, it spins around, and it blows air at you. No part of the blade moves forward. Each point on the blade just moves around in a circle, in a flat plane. Yet the spinning blade as a whole ends up blowing air forward. How does that work?
I don't see what this question has to do with what is being discussed. Please explain.
Thus proving my point.
If a fan can blow air, so can the propeller of a DDWFTTW cart.
A fan gets pushed forward when it blows air backward. But there's no air moving forward, pushing the fan forward. So why do we have to explain the forward force on a DDWFTTW cart in terms of air moving forward, pushing on it, or in some complicated way pushing on air that it has blown backward? It blows air backward; therefore, it gets pushed forward.
I agree that this is not a full explanation. For example, looking at it this way, it's not obvious to me why a cart moving in still air wouldn't also be pushed forward by its propeller, which of course it wouldn't be. The devil is in the details, I suppose.
However, I do believe that it's a mistake to focus solely on the air. What's important is that the air and the ground are moving relative to each other, and the cart is "attached" to both, so it can harness this relative motion. The ground definitely exerts a force on the wheels, which causes the propeller to turn. If not for this force, the cart wouldn't work.
I said - “I don't see what this question has to do with what is being discussed” not because I don’t understand frames or disagree with anything that’s been said regarding frames. I said it because I also said - “I clearly defined the frame my description and questions were based on”. Any discussion concerning frames is moot.
That’s what I’ve been saying all along. Don’t know why you might think I haven’t.
The cart is in still air but the cart and the air aren't still relative to the ground. The air is still relative to the propeller but as the air from the propeller (propeller wind) is moving backwards (and therefore slower that the propeller and cart) the air isn’t still relative to the propeller wind. When the speed of the propeller wind and the driving wind combine the cart travels faster then the driving wind.
Seems to me we are saying pretty much the same thing with different words
I don’t disagree.
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When I said "still air", I meant still relative to the ground.
This is what makes it a brain-teaser. The reason it works in wind but not in still air is because the cart is designed to exploit the energy available at the ground/air interface. With no true wind, there is no energy to exploit.
The reason it can work is that the wheels are operating against the ground, while the prop is operating in a medium that's moving relative to the ground. Thus when the wheels roll 30' over the ground, the propeller disk has only moved 15' through the air mass. Thus the same force the wheels apply to the ground can be produced at the prop for only half as much energy. If there were no wind, this would not be true - and then any real-world losses would quickly bring the operation to an end.
It looks like you beat me to it.
Actually, you did not. You describe a scenario. You never described the inertial frame of reference.
I'll show you why...
This is not true. It indicates a clear misunderstanding of the mechanism.
Michael C
Graduate Poster
Here’s some recent videos from I Ratant. Just click on his user name there to see the others. I’m pretty sure he posted what the gear ratio was on that cart but I don’t recall it. Anyone? Thought the foam popcorn was an interesting way to show ws. Too bad we didn’t get to see something similar at El Mirage. http://www.youtube.com/watch?v=zqW6BFCypoA
Wait…what!? Perhaps I’m just having another senior moment lapse in the synapse, but I could have sworn quite a few here were saying Humber’s claim to have built a cart that could go up a treadmill without a prop would re-write the laws of physics. Have to look at the TR archives again. Still want to find the picture I’m positive is there of Humber’s cart which all the regulars here repeatedly promised would never exist. Not you of course, Michael. You’ve sort of become the patron saint of DDWFTTW.
The point of humber's cart is not that it doesn't have a prop, but rather that it has nothing that could do the same job as the prop. In my post to Quarky I don't just give the examples of non-prop ddwfttw vehicles, I explain why they could work. For a ddwfttw cart to work, it must have a mechanism that takes advantage of the difference in velocity between air and ground. Humber's proposed cart has no such mechanism.
The simple answer to that question is "that's the way BB is designed to work": it's built so that it reaches maximum velocity when running directly downwind.
I could gave a more complete answer, but in order for that to be useful we would first need to enter into a discussion about some basic physical principles.
Yes and I’m using the term in relation to the air moving relative to the ground and only still relative to the cart.
It’s not just a matter of - “It blows air backward; therefore, it gets pushed forward”. If it was just a matter of that then it would work when the air was still to the ground, and that would be against the law. In my opinion it’s more - “It blows air backwards in to an opposing wind and it gets pushed forward by a combination of both winds”.
Yes, that's too simple, I agree.
But, relative to the cart when it's moving faster than the wind, the surrounding air is moving backward. There is no forward motion of air. The air is simply moving backward more slowly than if there were no wind. So the question now becomes, why is it easier to spin the propeller, to produce a given amount of forward thrust, in a slow-moving backward wind than in a fast-moving backward wind?
I don't know, but apparently it is.
I can see that we'd have to spin the propeller faster in a fast-moving backward wind than in a slow-moving backwards wind, to get any thrust out of it. But I'm not sure why it should require more torque to do so. After all, if it could spin freely, the fast wind would spin it faster than the slow wind would. So why is it harder to spin it somewhat faster than it would spin anyway by itself, just because the wind is faster?
Perhaps it's the power that's important, not the torque. And power increases with rotational speed even if torque doesn't. I'll have to think about this some more.
Consider the work done. Work is force x distance. In a given amount of time the cart moves a much smaller distance through the air than it does over the ground. Thus, the same amount of force acting on that parcel of air represents less work.
Consider trying to push a car at 1 mph vs. trying to push that same car at 10 mph. The latter will take 10 times the energy.
But as you've correctly observed, there is no case of converging air masses or air cushions. It would be a simple matter to show the flow in this case is identical to that of a prop powered plane pulling itself through still air (or any steadily moving air for that matter).
It's like trying to push a load while running on a treadmill.
Michael C
Graduate Poster
You said that way back in 2008: I replied that the idea was a "step on the way to understanding what is going on": http://www.internationalskeptics.com/forums/showpost.php?p=4236568&postcount=1311
Yes but the wind created by the propeller is still going slower than the driving wind. If the Cart went faster than the driving wind at the same speed as the propeller wind then the propeller wind would be still relative to the driving wind and the cart would have reached maximum speed.
Opposing winds aren't like opposing light beams that don't interact with each other. There is a region behind the propeller where the opposing winds interact with each other and create an area of pressure. I believe this area is quite close to the propeller blades. For the energy of the wind to transfer to the cart there has to be a point of transfer. I believe this point of transfer is this area of pressure. If anyone thinks the point of transfer is in a different place please explain exactly where it is.
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tobermoryphil
Thinker
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I'm not sure about videos; you have in the past said they can be faked! I would prefer to have two agreed impartial observers, in which case the offer stands.
Humber on props "the axis and centre of mass of the prop move at the speed of the cart" Indeed they do, old chap, but the blades do not, and on every prop I've met, it's the blades that provide the thrust!
[FONT=Verdana, sans-serif]So I did. Doesn't time fly. Given the “experts” say it's all a load of crap do you think the point where the energy is transferred to the cart when the cart is travelling faster than the driving wind is where the propeller wind interacts with the driving wind? If not where is this point?[/FONT]
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