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marina
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Postby marina » Mon Jul 30, 2007 2:44 pm

hi guys,
Jesse's jump was totally legit. There are two videos: one that shows 17 seconds which we all saw at the awards dinner Sat night, and another which is being mailed by a spectator (I think!) who caught the whole jump, 22 seconds. .. The SF event was a first so the organizers I'm sure as well as Steve Kent, the bay area representative for Cabrinha will get something posted soon.

RE: Jesse's jump, I believe he sent it in convergent air, where two opposing air masses meet and create lift. I experienced this a few weeks ago at my local spot, which also is where we paraglide. It was only blowing 10-13 miles on the beach but I was getting 15-20 foot airs when I boosted. A guy on the beach was trying to launch and there was no wind while I was riding for about 40 minutes. Then my kite fell out of the convergence when I approached the beach and just completely fell out of the sky.

I believe Jesse's jump was a combination of skill and being in the right place at the right time. He said he 'felt something weird and just pulled the trigger'. ...

endorphin
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Postby endorphin » Mon Jul 30, 2007 3:22 pm

where are these ominous videos???

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bluestatekiter
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Postby bluestatekiter » Mon Jul 30, 2007 5:24 pm

Whoever has the video- please up load it to youtube- It only takes a few minutes-

Please paste the link on the forum-

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JS
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Postby JS » Mon Jul 30, 2007 6:46 pm

Peter_Frank wrote:...STAY IN THE LIFTZONE...


Peter has got it right.

1. The only way the rider could have stayed aloft that long is with the assistance of vertical air currents. The air current that Jesse was riding may have been a thermal bubble, rotor, or combination. Either way, the rising air mass would have been moving downwind within the prevailing air mass (wind), so looping "helicopter style" probably have kept him in that moving column of air. It completely makes sense, just like glider pilots making tight turns to stay in a rising column or bubble of air.

2. If he hadn't looped, his kite would have quickly flown upwind out of the vertical current as the current drifted downwind with the prevailing wind. His example is a good lesson. Most of us have experienced short bursts of crazy vertical currents (usually rotor) but those currents quickly pass. If we quickly initate a loop when we get hit by one of these updrafts, we might be able to stay with it and get some otherwise unimagineable hangtime.

3. Other than staying in a rising air current, there is no aerodynamic benefit to looping or turning a kite, other than possibly squeezing the last bit of energy out of the kite (potential energy) and rider (kinetic energy) just before touchdown. In fact, turning a kite will generally reduce the kite's aerodynamic efficiency.

4. My guess is that the sink rate (relative to still air) of a good, well trimmed kite with a kiteboarder hanging from it is about 500 feet per minute, or about 5 knots. So, if you can manage to stay in a column of air that is rising at 5 knots or more, you can probably stay aloft for a long time. Good gliders have a minimum sink rate of less than 200 fpm (2 kts), so they can gain altitude by finding a column of air that is rising faster than that, and circling in it like a bird. I have flown gliders in more than 2,000 fpm (20 kts) vertical air currents, which is like riding a high-speed elevator.

5. A common misconception is that looping a kite makes it fly faster, but that is wrong. The speed that a kite flies through the air during a jump is dependent on two things: 1) its lift and drag characteristics, and 2) the effective weight of the rider and equipment. It's that simple. During jumps, kites usually seem slow because they are flying against the wind. If the kite is flying at 20 kts into a 20 kt wind, it will appear to be hanging almost motionless, but if the kite is turned or looped downwind it will still be flying 20 kts through the air, but with a 20 kt tailwind, so it will be flying at 40 kts over the water. It will look really fast, but its airspeed will be unchanged, and so will the lift that it generates.

Cheers,
James

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ScottM
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Postby ScottM » Mon Jul 30, 2007 7:29 pm

The previous guy known for a hangtime record Adam Koch also had an unusual air current and he too used looping the kite to great effect. :D

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bluestatekiter
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Postby bluestatekiter » Mon Jul 30, 2007 7:37 pm

What about that guy who got lofted in Maui? It was caught on film as they were taping Top Hat on Kite Beach- he seemed to be up there for at least 20 plus seconds-

Isn't that the record?

Windguy
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Postby Windguy » Mon Jul 30, 2007 7:38 pm

dude didn't have a board. It was a record lofting.

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bluestatekiter
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Postby bluestatekiter » Mon Jul 30, 2007 7:49 pm

ahhh- good point

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spork
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Postby spork » Mon Jul 30, 2007 8:24 pm

JS wrote:1. The only way the rider could have stayed aloft that long is with the assistance of vertical air currents. The air current that Jesse was riding may have been a thermal bubble, rotor, or combination.


As I'm sure you're aware there are birds that stay aloft without flapping for many many miles with no vertical air currents. They do this using a version of dynamic soaring. All that's required is a healthy wind gradient (even with all air moving horizontally). I have a hard time believing he got much help from a thermal as I've never seen any evidence of significant thermal activity at 3rd Ave. I have on a couple of occassions seen a shear set up there. I don't know that he was riding the shear or using the wind gradient to remain aloft that long, but my guess is that those are the most likely guesses.

Most of us have experienced short bursts of crazy vertical currents (usually rotor) ...


Conditions vary greatly from one place to the next, but I always felt the boosts I got were typically from horizontal wind gusts. In my experience hang gliding, well formed thermals low to the ground are a rare thing - over water far more rare.


3. Other than staying in a rising air current, there is no aerodynamic benefit to looping or turning a kite,


I believe you could dynamically soar indefinitely in a strong enough steady wind gradient with proper kite control. Whether a loop would be the best answer I don't know.

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JS
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Postby JS » Mon Jul 30, 2007 9:06 pm

spork wrote:As I'm sure you're aware there are birds that stay aloft without flapping for many many miles with no vertical air currents. They do this using a version of dynamic soaring. All that's required is a healthy wind gradient (even with all air moving horizontally).

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. 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.

But soaring birds are vastly more efficient gliders than kiteboarders are, and can therefore take advantage of atmospheric nuances that would be insignificant to a kiter.

spork wrote:I have a hard time believing he got much help from a thermal as I've never seen any evidence of significant thermal activity at 3rd Ave. I have on a couple of occassions seen a shear set up there.

I referred to thermals as a possible source of vertical currents, but in reality, it was almost certainly a rotor or other similar turbulence. I agree that a significant thermal at low altitude, especially over water, wouldn't likely be relevant. And regarding windshear, it is related to a rotor effect anyway in some circumstances. I think you focus on windshear too much. 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.

spork wrote:Conditions vary greatly from one place to the next, but I always felt the boosts I got were typically from horizontal wind gusts.

I agree mostly, but 22 seconds of hangtime wasn't just caused by horizontal gusts. 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. Gusts at the surface usually tumble down from aloft, due either to a geographically induced rotor, or to vertical mixing caused by temperature induced density differences. Many sailors and kiters would do well to perceive wind and gusts as three-dimensional phenomenon, instead of the conventional two.

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. The energy that you would have to derive from a steady wind gradient to overcome these limitations is totally impractical, unless you could find a steady gradient that is many times the magnitude of any created in earth's atmosphere (without more significant vertical activity).

Cheers,
James


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