Downwind faster than the wind

Nice! [post=2131828]rpenner's post[/post] in the other thread covered this well:
rpenner said:
The best test of such a sail car is that it should continue to move faster than the wind when the air is at rest. It is clear in the classic 1-D setup (parallel wind and movement, frictionless bearing as wheels) it is impossible for a classic sailcar to continue to accelerate faster than the wind.

The two most likely reasons for an honest experimenter giving such results are 1) measurement error or 2) poor experimental design such as the car is actually going down hill which would be difficult to demonstrate was not the case from video.

But this sail car is not a sail car. It's a propeller car. And the propeller is not being driven by the wind, but by the wheels. And this can work, because it is driven by the difference in speed between air and ground, not (as for a sail car) by the wind itself.

http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attempts_to_aton.php

And there will be a Myth-Busters (locally produced!) episode to cover it.

http://boingboing.net/2008/12/03/directly-downwind-fa.html
Click to expand...

The thing I find most mind-blowing about this is that it means that if this craft were placed on a giant moving conveyor belt in still air (yeah, just [thread=76507]like that[/thread]), it could use that conveyor belt energy to go in the opposite direction to the belt!
 
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Pete said:
Nice! [post=2131828]rpenner's post[/post] in the other thread covered this well:





The thing I find most mind-blowing about this is that it means that if this craft were placed on a giant moving conveyor belt in still air (yeah, just [thread=76507]like that[/thread], it could use that conveyor belt energy to go in the opposite direction to the belt!
Click to expand...

Ah. Thanks. But I note it is for sale on e-bay, nonetheless........
 
Read-Only said:
Yep, I can see that working - just as long as they don't use a fan to create the wind in the first place. ;) (You know what I mean, the cartoons that show an electric fan blowing into the sail of a boat.) :)
Click to expand...
Blowing your own sail works too. There is a Mythbusters episode about it.
 
Pete said:
The thing I find most mind-blowing about this is that it means that if this craft were placed on a giant moving conveyor belt in still air, it could use that conveyor belt energy to go in the opposite direction to the belt!
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Yes, this is actually the best controlled test, of what happens at windspeed.

[video=youtube;AVDLiNbCjN8]http://www.youtube.com/watch?v=AVDLiNbCjN8[/video]

[video=youtube;1BRvYZd81AQ]http://www.youtube.com/watch?v=1BRvYZd81AQ[/video]

[video=youtube;o9xnlbKY-qw]http://www.youtube.com/watch?v=o9xnlbKY-qw[/video]
 
Pete said:
Beautiful!
Click to expand...
They actually did that test with the big one too, on a dynometer:

[video=youtube;5JjXudThU88]http://www.youtube.com/watch?v=5JjXudThU88[/video]

When they saw that it pulls forward the dyno, they went to the dry lake. At about 3:00min it passes windspeed:

[video=youtube;EEuAqq8FINw]http://www.youtube.com/watch?v=EEuAqq8FINw[/video]
 
Originally Posted by exchemist
Since this seems to be some sort of powered machine, I don't see the relevance of wind speed at all. Or am I missing something?
A.T. said:
It is powered by the wind.
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To add some substance, the mechanism extracts energy from the wheels to spin the prop which pushes the cart faster so more energy can be extracted from the wheels which rotates the prop faster and push the card harder... and on and on. I suspect that the peak cart speed will be a function of the lift to drag ratio of the prop blades and the wind speed.
 
KitemanSA said:
Originally Posted by exchemist
Since this seems to be some sort of powered machine, I don't see the relevance of wind speed at all. Or am I missing something?

To add some substance, the mechanism extracts energy from the wheels to spin the prop which pushes the cart faster so more energy can be extracted from the wheels which rotates the prop faster and push the card harder... and on and on. I suspect that the peak cart speed will be a function of the lift to drag ratio of the prop blades and the wind speed.
Click to expand...

I don't get it, because I have not (yet) understood how this is NOT a perpetual motion machine.

Accelerating it requires work (f x d) to be done on the machine, since it is not weightless. From where does the energy for this come?
 
The wind.
Nobody seems to have a problem with the fact that boats can sail faster than the wind across the wind. This mechanism just turns that cross wind motion into usable thrust.
 
KitemanSA said:
The wind.
Nobody seems to have a problem with the fact that boats can sail faster than the wind across the wind. This mechanism just turns that cross wind motion into usable thrust.
Click to expand...

Ah, so that would mean, I presume, that the wheels cause the propellor to turn in a direction "opposing" the wind direction, i.e. blowing in the opposite direction, then? If so then I think I see.
 
exchemist said:
Ah, so that would mean, I presume, that the wheels cause the propellor to turn in a direction "opposing" the wind direction, i.e. blowing in the opposite direction, then? If so then I think I see.
Click to expand...
Yes, the propeller accelerates air backwards, against the true tailwind, thus slowing down the air relative to the ground and so extracting kinetic energy from it.
 
Nobody seems to have a problem with the fact that boats can sail faster than the wind across the wind.
Click to expand...
The only thing moving across the wind is the propeller blade, and nobody is doubting that the propeller blades can be driven faster in rotation than the wind across them. That requires a force preventing them from being blown downwind - similar to the keel on a boat.

At forward or downwind speeds greater than wind speed, the rotation force on the propeller shaft is counter to its driven direction - it begins to act as a rotation brake as well as a source of air resistance.
 
The guy who invented this has a BETTER invention.

The yellow first down line in football. That's way better than some windmill on wheels. He even got an Emmy award for it.

first-down-line.jpg


So now all the players know where they have to run.






Otenay: Atthay astlay inelay aymay avehay eenbay away okejay.
 
iceaura said:
At forward or downwind speeds greater than wind speed, the rotation force on the propeller shaft is counter to its driven direction - it begins to act as a rotation brake as well as a source of air resistance.
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During directly downwind travel:

The aerodynamic torque at the rotor is always against the rotation.
The aerodynamic force at the rotor is always downwind or forward.

Nothing changes about those facts when the cart passes windspeed.
 
A.T. said:
Yes, this is actually the best controlled test, of what happens at windspeed.

[video=youtube;AVDLiNbCjN8]http://www.youtube.com/watch?v=AVDLiNbCjN8[/video]
Click to expand...

Hmmm, it seems to be going slower than windspeed.
 
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