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Thought experiment for science/aero nerds


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This one was very popular years back and generated a lot of interest and discussion. I figured it might be worth putting out here in case some people are getting cabin fever and want to argue about stuff.

 

Can a device that runs directly in line with the wind move faster than the wind, powered only by the wind?

 

Here's the video that started it all:

https://www.youtube.com/watch?v=aJpdWHFqHm0

 

Real or faked?

Edited by mender
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2 hours ago, wvumtnbkr said:

I thought the new Americas sailboats did that already.

Not directly downwind; they have to tack.

3 hours ago, enginerd said:

Great question. I read a great article about this in a magazine 10 years ago... can’t remember the name of it. Left me saying “what???” And “oooohhh” and “what?” A lot. 

I should be able to answer some of those if you have specifics...

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Directly downwind eh? Lots of stuff with low friction that can easily go faster by making its own apparent wind. But how does the downwinder cross over to make its own apparent wind? By giving it a push like in the video maybe? Or by driving down from a reach while maintaining speed? At 5-10 knots winds speed what's the difference in direction if you are doing 30 knots in ground speed? I'd say not

much. 

 

FWIW if you are kite skiing (or on skates?) on ice, you can dive the kite to make your own apparent wind. Then on a low wind day I suspect you could go whichever direction you'd like, no?

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2 hours ago, thewheelerZ said:

Directly downwind eh? Lots of stuff with low friction that can easily go faster by making its own apparent wind. But how does the downwinder cross over to make its own apparent wind? By giving it a push like in the video maybe? Or by driving down from a reach while maintaining speed? At 5-10 knots winds speed what's the difference in direction if you are doing 30 knots in ground speed? I'd say not

much. 

 

FWIW if you are kite skiing (or on skates?) on ice, you can dive the kite to make your own apparent wind. Then on a low wind day I suspect you could go whichever direction you'd like, no?

Are you saying that something can start by using the wind, then make its own apparent wind and continue on when the actual wind dies down to zero? I hope not, because that would be perpetual motion.

 

This is constant straight downwind, not coming from a reach or other tricks. It uses only the wind and moves faster than that wind.

 

The push in the video was just to get it rolling; from there it accelerated until the streamer which was off to the side started streaming backward to show that the cart was going faster than the wind powering it.

Edited by mender
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2 hours ago, thewheelerZ said:

Not perpetual motion but certainly a high conservation of energy right? If the wind dies to zero while you are headed downwind then your apparent wind actually goes up. 

 

So why is the cart in the video not perpetual motion? 

Let's look at your statement first.

 

If the wind dies to zero, what is powering the vehicle? Even low friction with low drag would slow down with no energy source to draw on. Does it keep going because of the apparent wind?

 

Next, the cart: if it is moving directly downwind, when it reaches the speed of the wind there is no apparent wind. And once it exceeds the speed of the wind, it now experiences a direct headwind as the apparent wind, yet it moved continuously faster than the wind.

 

Let's assume for the moment that it is doing exactly that, and also that it isn't a perpetual motion device. Where is it getting the energy to continue moving?

Edited by mender
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In the case of a kiter on skates.... I think between the apparent wind and the energy you put in with your legs COULD be enough. Definitely a maximize efficiency exercise though. But yeah, the apparent wind becomes significant. 

 

Not sure how something like an ice boat would work, they go so much faster than the wind they are basically going upwind all the time. 

 

As for the kart, I  don't see where the extra energy comes from. I don't see how that could be done without an external source. Maybe slightly downhill? I'm skeptical!

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44 minutes ago, thewheelerZ said:

1. In the case of a kiter on skates.... I think between the apparent wind and the energy you put in with your legs COULD be enough. Definitely a maximize efficiency exercise though. But yeah, the apparent wind becomes significant. 

 

2. Not sure how something like an ice boat would work, they go so much faster than the wind they are basically going upwind all the time. 

 

3. As for the kart, I  don't see where the extra energy comes from. I don't see how that could be done without an external source. Maybe slightly downhill? I'm skeptical!

1. That's an external energy source. No input allowed from the skater.

 

2. For the purpose of this thought exercise, the ice boat has to be aligned precisely with the direction of the wind. What happens at wind speed? What's the angle of the sail? What happens when the wind drops to zero?

 

3. No downhill, no internal motor or batteries, etc, just the wind. If the wind dies the cart stops; if the wind continues steady state, the cart continues steady state. It is powered by the wind - but how can it move directly downwind faster than the wind that is powering it?

 

Just to be clear: apparent wind requires an angle to the true wind and by that definition, if there is no true wind, there is no angle and therefore no way to have an apparent wind steady state. 

https://en.wikipedia.org/wiki/Apparent_wind

https://questeria.info/tw/

Edited by mender
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Ok. After reading a bit anout it, it makes a bit more sense. Has nothing to do with ice boats or litres on skates. If I understand correctly basically drive train losses being less than the difference of propeller force compared to wheel force using the difference in speed of kart vs "apparent" speed of prop. Prop Always has a little help. 

 

So now, I'm off to read and think about how these guys made one go upwind faster than the wind...

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8 hours ago, mender said:

 

Just to be clear: apparent wind requires an angle to the true wind and by that definition, if there is no true wind, there is no angle and therefore no way to have an apparent wind steady state. 

https://en.wikipedia.org/wiki/Apparent_wind

https://questeria.info/tw/

 

I disagree with this though. Why does it need an angle? The wind an object in motion sees is the apparent wind. The better definition from the wiki sorce is that it's the sum of the velocity and true wind vectors. W(ind) = 0, A=sqrt of Vsq 

 

IMG_8076.PNG.26e48c006a0bf897ddb15351229312e5.PNG

 

 

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42 minutes ago, thewheelerZ said:

So now, I'm off to read and think about how these guys made one go upwind faster than the wind...

Not too hard to do; as you said, it has to do with efficiency.

 

The big trick is to go directly downwind at 2.8X the speed of the wind. Going upwind essentially takes 1X away from that, leaving 1.8X upwind. Properly geared of course.

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