What do you mean by the second sentence?
Are you saying that the wheels push the cart forward, like the wheels of a car do? I don't think that's right.
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Merged DDWFTTW in a “Real” Wind
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What do you mean by the second sentence?
Are you saying that the wheels push the cart forward, like the wheels of a car do? I don't think that's right.
I agree. For that to happen the propeller would have to act as a turbine that drives the wheels. Don’t see how a wind could drive a turbine that is going at or faster than the wind. Also the wheel would drive the cart backwards not forwards.
Was my answer to your post #12 of any help?
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I'm not sure. I have to think about it some more.
The two winds pushing against each other doesn't sound right, somehow. It reminds me of the argument that rockets can't work in space because they have no air to push against.
My general way of thinking about the problem is something like this, although I haven't worked out the details: Suppose the cart is moving at the same speed as the air. Consider all the forces on the cart. The air exerts no force, directly (apart from the spinning propeller, which we'll get to). The ground exerts a backward force at the wheels. The ground also causes the wheels to turn, which spins the propeller. The spinning propeller exerts a backward force on the air, and thus a forward force on the cart. So, in order for the cart to speed up, the forward force from the spinning propeller needs to be greater than the backward force from the ground on the wheels. The magnitude of that backward force depends on how hard the wheels are to turn, which depends on how hard the propeller is to turn. If the propeller is relatively easy to turn, yet gives a relatively large forward thrust, the cart will speed up, and end up going faster than the wind.
Your pal has made some very amusing unforced errors recently.
Humber is basically correct here. What he means about the load line is that all this force I've described earlier has to be transmitted through the skin of the sphere. If the actual vessel gets just a little bit away from the perfectly spherical shape, the force will cause the vessel to be relying upon material strength other than pure pressure (shear, etc.) Most materials that would be considered for resisting pressure would perform better under pure compression than they do in shear conditions. So the material in an imperfect sphere configuration is much closer to massive failure if the forces change from perfect compression to a shearing condition caused by this fundamental asymmetry of the vessel.
Or to put it all in simple language, a slightly out of round sphere is much easier to collapse than a perfect one.
I hope this helps.
WindGrins
Applies if perfectly round. The central point being that there is no force path normal to the surface, over any part of the surface.
For any sequence to continue there has to be something associated with the cart that is going slower than the wind for the wind to push against when the cart is going faster than the wind. Only the wind provides the energy.
Instead of a propeller imagine there’s a conveyer belt going from end to end of the cart with paddles extending from it at right angles. Only the paddles on the top of the belt are in contact with the wind and they move from the front of the cart to the back. When the cart is moving at wind speed the paddles are going slower than the wind as they move backwards relative to the cart and wind and the wind pushes on them.
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Think of how a jet boat works. A jet of water coming out the back of the boat pushes on water to move the boat forward. A fluid pushing on a fluid. If the boat is going downstream it will go faster than if it would be in still water because the jet of water is pushing against water that is also pushing against it. I know there's also less forward friction on the boat when it travels downstream so if that’s an issue for you imagine a water jet powered hovercraft instead of a boat. Going upstream if the water was flowing at the same speed as the jet of water the hovercraft wouldn't travel upstream at all.
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Don't worry, no one could ever challenge your record.
I wouldn't be surprised to find that your errors outnumber everyone else's combined on all the DDW forums. In fact, I'd bet on it.
Correct.
In a perfect world, a cart could stay at any speed that it was pushed or towed to in zero wind. That would be perpetual motion. Some people think that it should speed up. That would be over-unity.
A simple way of looking at the cart is that it slows down the air to speed itself up. That's how every wind powered vehicle works. The only trick is doing that when traveling faster than the wind. The propeller does exactly that; it slows the speed of the air traveling over the ground (normally called wind) by thrusting against that air mass.
The "Lightbulb" concept for me is simply that the crafts airspeed is always less than it's groundspeed, even when both are faster than the wind.
Thus the speed of the ground under the wheels, and the energy that makes available, is more than the speed of the air, and the energy the prop can add to the system.
Thus the speed of the ground under the wheels, and the energy that makes available, is more than the speed of the air, and the energy the prop can add to the system.
Well, at least Windgrins had enough decency to say that in the recent examples I was "half right". The difference in the historical cases, is lack of any decency at all. NALSA data says so.
From their point of view, it's a marred top-speed record, but from the point of view of the original claim, an unabashed fraud.
Just like to point out that a DDWFTTW cart doesn’t have to driven by a propeller. Especially if the W stands for water. It simply needs to have part of the cart travelling slower than the fluid (air or water) pushing it when the rest of the cart travels faster than that fluid.
Sorry but that makes absolutely no sense to me. Can you explain in detail how the speed of the ground can be more than the speed of the wind when the speed of the wind is directly relative to the speed of the ground.
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Well, I don't really see the benefit of this as a separate thread, but if there is one, then surely referring people back to the original thread, and in general terms at that, not a link to a specific post, is totally pointless?
This isn’t a debate about the validity of DDWFTTW. It’s an attempt for those that know (or think they know) how it works to explain in non-technical, simple, plain English how it works for the benifit of those that don't know how it works. This thread assumes DDWFTTW works.
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I was thinking the same thing, Mac.
This sounds pretty good to me.
...and easily understood
So I just hope it is correct, otherwise I'm gonna be confused again.
Actually I just thought of a reason that this cannot be an adequate explanation:
If it was an adequate explanation, then any old cart&propeller would travel faster downwind than the wind speed. In fact, some specifications have to apply in order for the cart&propeller to be able to travel faster downwind than the wind speed.
I think 69dodge posts makes that point admirably:
Still, ynot's simple explanation is a good starting point in understanding the whole thing.
If it was an adequate explanation, then any old cart&propeller would travel faster downwind than the wind speed. In fact, some specifications have to apply in order for the cart&propeller to be able to travel faster downwind than the wind speed.
I think 69dodge posts makes that point admirably:
Still, ynot's simple explanation is a good starting point in understanding the whole thing.
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I would rewrite (5) to something like this . . .
(5) As the propeller wind blows backwards from the cart it is moving slower than the cart in the direction of the wind. When the cart reaches and exceeds the speed of the driving wind the driving wind continues to blow against the propeller wind as it hasn't reached or exceeded the speed of the driving wind.
(5) As the propeller wind blows backwards from the cart it is moving slower than the cart in the direction of the wind. When the cart reaches and exceeds the speed of the driving wind the driving wind continues to blow against the propeller wind as it hasn't reached or exceeded the speed of the driving wind.
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Humber is still right about the Vacuum sphere with his point.
Actually in regards to your claim about the vacuum sphere, I said Humber was "basically right".
The basically was added as a qualifier because it was jargon that did not add a lot of explanatory insight to his answer which I think is technically correct.
I also was pleasantly surprised, that even though it was a little difficult to read and extract the intended meaning, it was on the questioned topic, cogent, and technically correct and not self contradictory within the same sentence or paragraph.
A new high for the forum.
Congratulations Humber!
Wind Grins
Actually in regards to your claim about the vacuum sphere, I said Humber was "basically right".
The basically was added as a qualifier because it was jargon that did not add a lot of explanatory insight to his answer which I think is technically correct.
I also was pleasantly surprised, that even though it was a little difficult to read and extract the intended meaning, it was on the questioned topic, cogent, and technically correct and not self contradictory within the same sentence or paragraph.
A new high for the forum.
Congratulations Humber!
Wind Grins
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