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 Post subject: Wind Physics?
PostPosted: Mon Sep 10, 2012 9:00 am 
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Location: CLEARWATER, MN
I have sailed for many years and always believed that I understood how wind drives a boat via the sails. I have never been able to sail directly upwind because the wind tries to push the boat downwind. While sailing downwind, my boats generally cannot travel faster than the wind because if they do...the boat is going faster downwind than the wind, and thus the sails have to push the air out of the way. Going on other headings, the geometry of travel and wind causes an apparent wind which causes a flow over the sails faster than the true wind speed so the boat can sail faster than the true wind.
I have heard of people trying to connect a prop on a mast to a prop in the hull to power a boat. Popular Mechanics Mag has a short article about a tricycle car which has a large prop on a mast connected to the drive wheels. The article states that the car can travel 2.8 times as fast as the wind, going downwind! Is this possible...wouldn't moving downwind faster than the wind stall the prop and keep the car from going faster. In addition, the article says that the car can go 2.1 times the speed of the wind...directly upwind! Wouldn't the wind reverse spin the prop and thus drive the car downwind?
And no...it is not an April Fools edition!

A 'MythBusters' episode needed?

If the article is correct...why use sails...just a prop on a mast and you can sail in any direction as long as there is a wind!

Comments please from 'experts' on the physics of wind and sailing!


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 Post subject: Re: Wind Physics?
PostPosted: Mon Sep 10, 2012 11:19 am 
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This video does a good job of showing the physics of REGULAR sailing. But the prop on the sail is interesting. I'm not sure that I follow how that would work. Gearing could help you move faster, but that still doesn't explain how you can get the prop to spin when traveling faster than the wind.


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 Post subject: Re: Wind Physics?
PostPosted: Mon Sep 10, 2012 11:36 am 
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I reckon busted on that one! To achieve a speed like twice the true wind, going directly downwind, the propeller would have to be pulling >in< air, which would require power rather than generate it. Even allowing for some hypothetically incredible friction-free drive mechanism, this is clearly impossible. I don't think Mythbusters would waste their time on this one....

Now if they had been talking about going downwind at oblique angles, well we all know how fast iceboats can go in these circumstances due to creating far higher >apparent< winds, but remember, they have absolute minimum moving parts, certainly not some cumbersome giant propeller and driving shafts etc etc

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 Post subject: Re: Wind Physics?
PostPosted: Tue Sep 11, 2012 10:23 am 
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Here is more on the physics of "down wind faster than the wind" sailing:
http://www.fasterthanthewind.org/

Seems equally credible and unbelievable!!


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 Post subject: Re: Wind Physics?
PostPosted: Tue Sep 11, 2012 11:52 am 
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Location: Highland Beach, Fl
"Now if they had been talking about going downwind at oblique angles, well we all know how fast iceboats can go in these circumstances due to creating far higher >apparent< winds, but remember, they have absolute minimum moving parts, certainly not some cumbersome giant propeller and driving shafts etc etc"

Are the propeller blades going downwind at oblique angles? Exactly!


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 Post subject: Re: Wind Physics?
PostPosted: Tue Sep 11, 2012 2:05 pm 
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Location: Kailua 96734
I'll second that.

Here's the wind chart from earlier test runs. Top speed 40mph in 25mph "apparent wind"?. The car really takes off once it passes the dead downwind speed.

They claim they can go 3x downwind now. And are working on a direct upwind model.

Image

Blue = relative WS from vehicle
Red = GPS speed output

1: towing Blackbird upwind to N end of runway.
2: positioning Blackbird for run
3: waiting through low wind cycle
4: pushing by hand.
5: vehicle at WS
6: peak speed
7: reload at S end of runway and then repeat


It's either a brilliant hoax or the patent of the century.


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 Post subject: Re: Wind Physics?
PostPosted: Tue Sep 11, 2012 3:29 pm 
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Well I am utterly gobsmacked. My take on it is that once they reach windspeed, the propeller is effectively turning in zero apparent wind. At that point, any existing inertia in the spinning blade will provide some forward force due to the pitch of them , and I guess if this forward force exceeds frictional resistance in the rest of the mechanism, the cart can accelerate until wind and frictional resistance exceed forward drive.

Interesting head-scratcher eh?

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 Post subject: Re: Wind Physics?
PostPosted: Thu Sep 13, 2012 1:49 pm 
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There is an old thread on this allready:
viewtopic.php?p=142426#p142426

Some explanations





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 Post subject: Re: Wind Physics?
PostPosted: Thu Sep 13, 2012 2:29 pm 
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Loved Video #1. Thanks.


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 Post subject: Re: Wind Physics?
PostPosted: Fri Sep 14, 2012 12:55 am 
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tonystott wrote:
Well I am utterly gobsmacked. My take on it is that once they reach windspeed, the propeller is effectively turning in zero apparent wind. At that point, any existing inertia in the spinning blade will provide some forward force due to the pitch of them ...

The inertia of the rotating parts is irrelevant. The vehicle is not allowed to use internally stored energy for propulsion (NALSA rules for sail craft).

Check out the observer reports on their website:
http://www.nalsa.org/DownWind.html

NALSA observers wrote:
S2: We found no evidence of any energy storage devices. There has been some discussion on www discussion groups that the rotating propeller constitutes a form of stored energy that might be converted to propulsion by slowing it down or changing its pitch. Since the propeller is connected to the wheels with a constant ratio connection slowing the propeller to harvest some of its rotational energy also slows the craft. The fact that the craft is required to accelerate during the measurement means, for a fixed gear ratio craft, that rotational energy is being added to the propeller to speed it up during the measurement period rather than the other way around.
From:
http://www.nalsa.org/BlackBirdDDWSR/Obs ... 032010.pdf

As long there is true wind, the car can stay DDW above windspeed continuously, just like a ice-boat that can keep a downwind VMG greater than windspeed continuously. See "Sail to Propeller" video above.


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 Post subject: Re: Wind Physics?
PostPosted: Fri Sep 14, 2012 1:09 am 
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NOHUHU wrote:
Loved Video #1. Thanks.

Yes, it shows nicely the transition from a tacking sail to the rotating propeller blade. The same of course works in the upwind direction.

On a more abstract level the mechanism works without the motion across the wind. Here the moving airmass is represented by a strip of paper. The wheels on the moving paper represent the propeller (for downwind) and turbine (for upwind):



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 Post subject: Re: Wind Physics?
PostPosted: Fri Sep 14, 2012 1:16 am 
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Sorry reksio, I had not seen the first of those explanatory videos, which makes it quite clear how it works.

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