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PostPosted: Mon Jul 30, 2007 9:35 pm 
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Video. Please?


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PostPosted: Mon Jul 30, 2007 9:37 pm 
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JS wrote:
Birds are pretty good at grabbing any opportunity that they can get, but I don't think you could ever isolate miles of horizontal windshear without any vertical displacement or mixing.


Agreed. I think you're talking about two separate points I was trying to make. I've seen and soared miles of windshear at Funston many times. The shear that sets up there (as anywhere) has a very definite vertical component. It creates an "artificial ridge" of sorts. When this phenomenon occurs we can get at least a couple thousand feet of altitude, whereas we normally only achieve under 1000' in steady on-shore winds at the Funston ridge. The wind shear I'm talking about occurs when two air masses meet and one passes over the other at the shear-line. The shear line will sometimes advance toward the cliff or retreat, but generally moves far more slowly than the wind itself.

When I speak of getting lift from strictly horizontally moving air I'm talking about wind speed gradient in proximity to the ground (or water). If one could have an indefinite gradient (all horizontally moving air) one could continue rising indefinitely. However I don't think this condition ever occurs.


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And in real life, the windshear will never be a perfectly even gradient across great horizontal and vertical expanses, even in the middle of an ocean. If a bird flies headlong into increased headwind (or reduced tailwind - same thing, relatively) he can use the newfound kinetic energy to soar higher, and he'll probably be pretty intuitive about changing course to take advantage of the next horizontal gust or gradient.


Again I think we're talking about apples and oranges. There's no question that a bird (or glider with sufficient performance) can remain aloft indefinitely in a consistent gradient (not talking shear here).

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In a gusty environment (usually near geographical obstructions), any windshear near the surface is better characterized as turbulence, and in wide open, steady surface winds, evenly distributed windshear isn't significant enough to turn a kiteboarder into a soarer.


I think you're refering to a different phenomenon than I am. I'm not talking about a transient thing like a gust. I'm talking about a steady state shear that sets up and remains more on less in place. I've seen this happen at 3rd Ave a couple of times. The only turbulence to speak of in that case is at the shear boundary.

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I agree mostly, but 22 seconds of hangtime wasn't just caused by horizontal gusts.


I agree completely.


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And there is almost no such thing as a purely horizontal gust with no vertical displacement involved. You can't have a horizontal gust blasting along the surface without coming from somewhere, going somewhere, and displacing the slow moving air in its way.


Again, I'm with you 100%


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Many sailors and kiters would do well to perceive wind and gusts as three-dimensional phenomenon, instead of the conventional two.


Absolutely.


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spork wrote:
I believe you could dynamically soar indefinitely in a strong enough steady wind gradient with proper kite control.

Not when you are doing about 20 kts at a 6:1 glide ratio, sinking at 500 fpm.


I'm not sure I'm ready to buy your 500 fpm sink rate. In my 14.0 sq-meter glider I sink at about 185 fpm. I'm sure I sink faster on my kite but I would guess it's more in the 300 fpm range. I don't claim I could dynamically soar my kite in horizontal gradient, but I have no question it could be done given a sufficiently strong gradient (although I haven't worked out how strong that gradient would have to be or whether such a gradient would ever occur).


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PostPosted: Mon Jul 30, 2007 11:10 pm 
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spork wrote:
Etc...

I'm down with all of that.

spork wrote:
I'm not sure I'm ready to buy your 500 fpm sink rate. In my 14.0 sq-meter glider I sink at about 185 fpm. I'm sure I sink faster on my kite but I would guess it's more in the 300 fpm range.

I was only guessing. Maybe 300 fpm is possible with a good, big kite. I do believe that 6:1 or maybe 7:1 is the best L/D we can expect from any current LEI kites, lines and/or bridles.

What is the L/D ratio of your 14.0 sq m glider at 185 fpm?


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PostPosted: Mon Jul 30, 2007 11:24 pm 
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JS wrote:
I do believe that 6:1 or maybe 7:1 is the best L/D we can expect from any current LEI kites, lines and/or bridles.


That may be true. I keep meaning to get someone to sight the lines and the horizon with a protractor. I've tried eye-balling it myself, but that would never be accurate enough.

Interestingly enough I just this moment took a guess at the angle I think I see when I try and eye-ball it. I figured about 80 degrees - which comes out to a bit under 6:1 (of course 85 degrees comes out to over 11:1 and I'm sure I don't guess that well).

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What is the L/D ratio of your 14.0 sq m glider at 185 fpm?


We're talking about an OLD glider. It's supposed to be in the 12:1 range with a really clean pod and a pilot in an ideal hang position. My rigid wing "hang glider" got a little over 17:1 unfaired and was said to be 20:1 or better with fairing.


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PostPosted: Mon Jul 30, 2007 11:46 pm 
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spork wrote:
Interestingly enough I just this moment took a guess at the angle I think I see when I try and eye-ball it. I figured about 80 degrees - which comes out to a bit under 6:1 (of course 85 degrees comes out to over 11:1 and I'm sure I don't guess that well).


Don't forget to account for the weight of the kite.

For example, if the kite flies 10 degrees aft of vertical with optimal trim, that looks like about 5.7:1. But if the kite and lines weigh 10 lbs, and the vertical component of tension at the bottom of the lines is 40 lbs, then you'd have to add 10/40, or 1/4, to your figure of 5.7:1, resulting in 7.1:1.

(At a small angle like 10 degrees, the vertical component of 40 lbs ain't much different than 40 lbs, so you can skip the geometry on that part.)

Here is a more detailed post that I wrote, covering this matter:
http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=54

Cheers,
James


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PostPosted: Tue Jul 31, 2007 12:18 am 
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JS wrote:
Don't forget to account for the weight of the kite....


Interesting point. I'm talking about the agregate L/D of the kite and line "system" to the rider. That's why I take the angle of the center-line at the rider. But I realize even that's not completely fair. The kite would nominally not be at 12:00 when you're trying to ride upwind for example. On the other hand, I'm also assuming the weight of the kite and lines is small compared to the pull of the kite. I weigh 190 lbs (plus gear) so in a jump the kite is pulling 200 lbs+ on average.

In the final analysis it's important to bear in mind that my 80 degree estimate is little better than a wild-ass-guess.


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PostPosted: Tue Jul 31, 2007 1:50 am 
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Here are my 17 seconds. Sorry I missed the beginning ... I'd been concentrating on the Ramp of Carnage, waiting to see who would hit it first :) ...

http://www.youtube.com/watch?v=-bcy44aIr9k


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PostPosted: Tue Jul 31, 2007 3:05 am 
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stacey wrote:
Here are my 17 seconds. Sorry I missed the beginning ... I'd been concentrating on the Ramp of Carnage, waiting to see who would hit it first :) ...

http://www.youtube.com/watch?v=-bcy44aIr9k


Awesome. Thanks for posting that. I was only able to catch the the very end of it live as I was chasing my kids down the beach.


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PostPosted: Tue Jul 31, 2007 3:42 am 
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it looks like he touches the water and then goes up briefly one more time at the very end.


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PostPosted: Tue Jul 31, 2007 8:53 am 
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It looks a bit lame


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