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Old 01-09-2009, 05:40 PM   #81
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Probably a good thing the forum was down as I contemplated a response

Hmmm, this is a lot more interesting than the flat tire question!

As for the second cart with the cute animal toys (that's what our flat tire story was missing!), I just thought of a very simple analogy. Picture a 4 foot long stick, with an axle at the 1 foot point, so it is just a 3:1 lever. Now, holding it vertically by the axle, you could push on the short end, and the long end would move in the opposite direction but 3x faster.

That would not make a very compelling video, because it is so simple and transparent. But I think that that is all he has really accomplished with the wheels and cart, right? The different sized wheels just give more speed to the small wheels compared to the big wheels, and change the direction of motion. But it is very clever and well done - I couldn't really predict what would happen.

I'll ponder that propeller cart some more.... but I'm thinking it may be similar - with the propeller versus still air sort of forming a fulcrum point that the cart can move around, but maybe not....

Something tells me that the two Sierra Nevada Pale Ales (cloned homebrews) I have chilling are *not* going to help me figure this out

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Old 01-09-2009, 10:19 PM   #82
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I have still not gotten my mind around the cart with the propeller being driven faster than the wind. I know about gearing and I know about prop pitch, but I cannot get past the fact that once the car is exceeding the speed of the wind, the wind can't be turning the prop in the same direction and driving the wheels. But maybe it can.:confused:
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Old 01-09-2009, 10:32 PM   #83
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Originally Posted by samclem View Post
It demonstrates that a cart ... can be designed that will move downwind faster than the wind. It is counterintutitive, but true. This cart, powered only by the wind, can outrace a balloon floating along in that same wind.
This works only if you inflate the cart tires with milk and rub bacon on the sidewalls.
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Old 01-09-2009, 10:34 PM   #84
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I'm not sure I have it yet either - but they did mention somewhere that the *wheels* are driving the prop, not the wind. Does that help?

So I'm guessing at this point, that it is the same kind of thing as the guy with the stuffed animals and the thread spools ( cotton reels?) - the wheels drive the propeller *faster* so the car can move faster. Of course, there is no perpetual motion involved, it's basically like gear ratios, changing a slower speed to a higher speed.

ummm, I think. :confused:

Remember, at the point that the car is moving at the same speed as the tail wind, it is essentially in still air. It is moving with the wind. That is what they are trying to mimic with the treadmill. The air is still, the ground moves underneath. Moving slightly faster is moving into a slight headwind (very slight on the treadmill). I skimmed over the 'build the cart' videos, and they were very careful to use very light weight (carbon and aluminum tubes) stuff, and good bearings, and tweak the gears. So it seems like you can get just barely faster with careful engineering and construction.

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Old 01-09-2009, 10:35 PM   #85
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This works only if you inflate the cart tires with milk and rub bacon on the sidewalls.
Good one - I was thinking about working milk in there....

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Old 01-09-2009, 11:05 PM   #86
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I'm not sure how well a treadmill replicates the real conditions--it muddies the situation (for me) because I see the moving tread as supplying energy to the wheel on the cart that drives the prop. With an aggressive prop pitch and high gearing, I can see how the car could drive itself forward on the belt.

But in the real world, (off the treadmill) how would the wheels drive the prop (aside from the small push at the beginning)? And if the prop is driving the wheels, the energy supplied by the prop would decrease as the cart picked up speed and the relative wind decreased, finally stopping once the wind was in the driver's face.

Hmm. . .
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Old 01-10-2009, 01:43 AM   #87
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Sailboats can go faster than the wind. If you just hang a sail so the wind hits it squarely, you will not any faster than the wind. If you angle the sail so it gets 'inflated' into an airfoil shape, the wind will also blow over the airfoil and the aerodynamic force ('lift' of the airfoil) can move the boat faster than the wind. Propellers have an airfoil cross-section; same principle.
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Old 01-10-2009, 11:23 AM   #88
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Sailboats can go faster than the wind. If you just hang a sail so the wind hits it squarely, you will not any faster than the wind. If you angle the sail so it gets 'inflated' into an airfoil shape, the wind will also blow over the airfoil and the aerodynamic force ('lift' of the airfoil) can move the boat faster than the wind. Propellers have an airfoil cross-section; same principle.
I think this is the key. They do mention that sailboats can go faster than the wind when done at at angle (as you mention), and they make the point that this thing is going right with the wind, not an angle.

But as you point out, the propeller's airfoil shape makes it essentially at an angle to the wind, so I guess you could view this as angling to the wind, without *looking* like you are angling to the wind.


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