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DieCommie

Registered: 12/11/03
Posts: 29,258
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Wind powered car - travels faster than wind speed?
#14834771 - 07/27/11 07:05 PM (12 years, 9 months ago) |
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I was skeptical at first, but it is possible and has been built - http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/
Quote:
Rick Cavallaro and his friends have built a wind-powered vehicle that travels downwind faster than the wind, solving a riddle that can start fights.
The unusual wind-powered car hit a top speed 2.86 times faster than the wind during one recent run, a feat that — depending upon your perspective — is either the result of hard work or the same voodoo responsible for Ryan Seacrest’s hair.
Check it out and try to get a feel for how it works. Its able to run by using the wind speed relative to the car rather than the wind speed relative to the ground.
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The wheels turn the prop, which turns the vehicle’s wheels, which turn the prop, which turns the vehicle’s wheels.
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Seuss
Error: divide byzero



Registered: 04/27/01
Posts: 23,480
Loc: Caribbean
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Re: Wind powered car - travels faster than wind speed? [Re: DieCommie]
#14835027 - 07/27/11 07:50 PM (12 years, 9 months ago) |
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Interesting... something fun to lose sleep over thinking about. On a similar note, sailboats can sail faster than the wind because the sail is an airfoil. The forward motion of the sailboat comes from wind blowing across the sail, creating a low pressure in front of the sail, rather than the wind pushing the sail from behind.
-------------------- Just another spore in the wind.
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ChuangTzu
starvingphysicist



Registered: 09/04/02
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Re: Wind powered car - travels faster than wind speed? [Re: Seuss]
#14835153 - 07/27/11 08:16 PM (12 years, 9 months ago) |
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Quote:
Seuss said: On a similar note, sailboats can sail faster than the wind because the sail is an airfoil. The forward motion of the sailboat comes from wind blowing across the sail, creating a low pressure in front of the sail, rather than the wind pushing the sail from behind.
That's right. Interestingly, this also means that the fastest direction for a sailboat to sail is not directly downwind. In the downwind direction, the maximum speed of the boat is the wind speed. The fastest direction for a sailboat is almost perpendicular to the wind but slightly downwind. In this direction, the sail is efficiently extracting energy from the nearly opposing forces of the wind and water and will continue to accelerate until the force on the ship equals the drag force on the keel. The boat is acting kind of like a piece of ice being squeezed between 2 fingers...
Another interesting thing is that this ends up meaning that the max speed of an average sailing vessel is limited predominantly by the length of the ship (since the wave drag on the ship's hull increases more slowly with respect to velocity as hull length increases...).
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ChuangTzu
starvingphysicist



Registered: 09/04/02
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Re: Wind powered car - travels faster than wind speed? [Re: DieCommie]
#14835237 - 07/27/11 08:29 PM (12 years, 9 months ago) |
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Quote:
DieCommie said:
Quote:
The wheels turn the prop, which turns the vehicle’s wheels, which turn the prop, which turns the vehicle’s wheels.

I'm still not sure I get it...
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ChuangTzu
starvingphysicist



Registered: 09/04/02
Posts: 3,060
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Re: Wind powered car - travels faster than wind speed? [Re: ChuangTzu]
#14835981 - 07/27/11 10:58 PM (12 years, 9 months ago) |
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Update: I think I mostly got my head wrapped around the details now...
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Seuss
Error: divide byzero



Registered: 04/27/01
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Re: Wind powered car - travels faster than wind speed? [Re: ChuangTzu]
#14837317 - 07/28/11 05:58 AM (12 years, 9 months ago) |
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> The fastest direction for a sailboat is almost perpendicular to the wind but slightly downwind.
In nautical terms, when the wind is perpendicular to the vessel, it is called a "beam reach". If the wind is slightly behind, but mostly perpendicular it is called "broad reach". The sailing vessel that I work/race on from time to time is fastest when the wind is two points abaft the beam (which means the wind comes almost perpendicular to the direction of forward motion, but a bit behind (minus 22.5 degrees)).
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trendal
J♠



Registered: 04/17/01
Posts: 20,815
Loc: Ontario, Canada
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Re: Wind powered car - travels faster than wind speed? [Re: DieCommie]
#14837430 - 07/28/11 06:57 AM (12 years, 9 months ago) |
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This reminds me of an idea I once had, for a sailplane that was powered by the polar jet stream. If you had a very long cable, would it be possible to use the difference is speed between the jet stream and the air beside it to keep you up indefinitely?
Imagine something like a sailplane attached to a parachute by a very long (and light) cable. If you could keep the parachute inside the jet stream, then you could ride the sailplane just below the jet stream.
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Once, men turned their thinking over to machines in the hope that this would set them free. But that only permitted other men with machines to enslave them.
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ChuangTzu
starvingphysicist



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Re: Wind powered car - travels faster than wind speed? [Re: Seuss]
#14838238 - 07/28/11 11:20 AM (12 years, 9 months ago) |
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Quote:
Seuss said: The sailing vessel that I work/race on from time to time is fastest when the wind is two points abaft the beam (which means the wind comes almost perpendicular to the direction of forward motion, but a bit behind (minus 22.5 degrees)).
That would be in between a beam and a broad reach, correct?
I also do some sailing but have never raced. It's neat how the sailing world is full of physicists and engineers...
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ChuangTzu
starvingphysicist



Registered: 09/04/02
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Re: Wind powered car - travels faster than wind speed? [Re: trendal]
#14838244 - 07/28/11 11:20 AM (12 years, 9 months ago) |
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Quote:
trendal said: Imagine something like a sailplane attached to a parachute by a very long (and light) cable. If you could keep the parachute inside the jet stream, then you could ride the sailplane just below the jet stream.
That would work as long as materials with the right properties existed.
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veggie

Registered: 07/25/04
Posts: 17,538
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Re: Wind powered car - travels faster than wind speed? [Re: DieCommie]
#14838414 - 07/28/11 11:52 AM (12 years, 9 months ago) |
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Here is a video of the car in action.
(Neat idea but must be a bitch to parallel park)
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Seuss
Error: divide byzero



Registered: 04/27/01
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Loc: Caribbean
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Re: Wind powered car - travels faster than wind speed? [Re: ChuangTzu]
#14840262 - 07/28/11 06:03 PM (12 years, 9 months ago) |
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> That would be in between a beam and a broad reach, correct?
Correct. 
> I also do some sailing but have never raced. It's neat how the sailing world is full of physicists and engineers...
If you have the chance, racing is an absolute blast. We have beer class races here once every few weeks. Nothing too formal... just a bunch of day charter captains that get together once a week (or so) and race after work. An absolute blast! Winning boat crew gets a free round of beer at the bar, after the race, paid for by the losing boats.
-------------------- Just another spore in the wind.
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