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PL Kites auto-zenith?

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MrBonk
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PL Kites auto-zenith?

Postby MrBonk » Mon Apr 23, 2007 10:31 am

Just wondering if someone can tell me why/how the PL kites achieve the auto-zenith thing?

lemur
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Postby lemur » Mon Apr 23, 2007 4:22 pm

I think the how is through design with Peter Sr's experience designing and building MASSIVE single line show kites. These large kites demand extraordinary stability characteristics and through his experience he has learned a thing or two.

The why is partially due to his interest in designing kite boats and if you watch the short kite cat video on his site you will see the advantage of auto zenith while launching the boat.

I find this feature indispensable on snow/water and land based activities as it allows you to fiddle with skiis\snowboard\surfboard bindings etc...Also comes in very handy while taking a smoke\beverage break and is VERY nice when you wipe out and most often find kite patiently waiting at noon for you to get footgear sorted out

Rob

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Postby FredBGG » Mon Apr 23, 2007 5:50 pm

Auto zenith is a convenient feature, but at the same time the zenith is not a safe place to park your kite. That said the PL kites are very stable and you can fly them nice an low to the side of the wind window while going towards the water.

I think the main reason why they fly so well at auto zenith is because the actual lift of the kite comes from the very front of the leading edge.....
the front lines are attached at the very front of the tips and that hter is plenty of passive kite in the tips and trailing edge giving the kite a sort of flag effect.

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Postby OzBungy » Tue Apr 24, 2007 3:41 am

I don't really know (like everyone else who is not Peter Lynn) but I will theorise:

The kite has no structure at all without tension on the lines. The tips have autostable profiles (I do know this). The tips tend to come forward if they get too high an agle of attack (autostable profiles do this).

As the kite turns off the wind the tension gets uneven between the front lines so the kite turns. Each tip self corrects to get to its stable point.

The only time this effect of tips coming forward and stabilising is when you have even tension on both lines and that place is the top of the window. Therefore, auto-zenith.

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Postby munteruk » Tue Apr 24, 2007 3:44 am

This is a good question and one I haven't been able to work out yet. What is it about he twin-skin design that makes it auto-zenith?

The moderately unconvincing idea that I came up with was that the cross section of the wing tips somehow changes under the force of gravity depending on where the kite is in the window. The effect of this change in profile is to generate lift on horizontal surfaces regardless of whether it is the upper or lower wing tip (when viewed as a kite low but at the edge of the window)

This would be different to an LEI where the wing tips always have positive angles of attack and are therefore always pulling in opposite directions.

I'd be interested to hear some comments from people who know the kites well or have other theories.

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Postby jakemoore » Tue Apr 24, 2007 4:25 am

through design with Peter Sr's experience designing and building MASSIVE single line show kites.
Don't a lot of these use a smaller pilot kite?
The moderately unconvincing idea that I came up with was that the cross section of the wing tips somehow changes under the force of gravity depending on where the kite is in the window. The effect of this change in profile is to generate lift on horizontal surfaces regardless of whether it is the upper or lower wing tip (when viewed as a kite low but at the edge of the window)
I agree with this theroy and believe gravity is acting on the wingtip spars causing them to rotate around a relatively fixed taut front line. I think the autostable profile has more to do with keeping the kite from overflying.


Jake

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GreenBomba
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Postby GreenBomba » Tue Apr 24, 2007 5:13 am

As you can tell from my log in name I am the owner and fan of a 15m Bomba, green in color. I will tell you my theory on how it auto-zeniths. I have staked my kite down on the frozen lake many times and watched it from afar.

When the kite begins to head down toward the ground, the rear edge of the kite sinks closer to the ground due to the weight of the battens at the tips of the kite. They "flop" toward the ground on the rear lines because there's no tension on the rear lines, while there's a lot of tension on the front lines. This steers the kite back up toward the zenith, and it goes back up there.

It's a neat principle, and I'm glad PL patented it because it's a cool effect that I don't know of any other kite having.

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Postby munteruk » Tue Apr 24, 2007 8:36 am

GreenBomba wrote: When the kite begins to head down toward the ground, the rear edge of the kite sinks closer to the ground due to the weight of the battens at the tips of the kite. They "flop" toward the ground on the rear lines because there's no tension on the rear lines, while there's a lot of tension on the front lines. This steers the kite back up toward the zenith, and it goes back up there.
Thanks for your post - that sounds pretty valid to me. Now that you've pointed it out, an automatic change in angle of attack sounds much more likely than a change in the profile of the foil. I guess the large tip areas in twin-skins also give the kite more lifting area to get up to the zenith.

I guess you could test it by checking whether it still happens when the back lines are under enough tension to stop the natural tip droop.

Another test would be to add weight to the end of the tip batten and see if that increased the auto-zenith tendency.

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Postby rokap3 » Tue Apr 24, 2007 10:00 am

Hey green bomba,

I had the same line of thought a you for a while, but now Peter Lynn is on his 5th or 6th version of a kite called SL ARC (single line arc) which as far as I know does not have any spars. Maybe the trailing edge tips also contribute to auto zentih some how

I reckon it also has something to do with the tow points being so far forward, lay your kite on the ground deflated and it looks like a back to front bow kite. Also the wing profile and trailing end not being sharp adds to the stability.

There is more than one thing responsibile.

I may be wrong and misguided...I quite often am.

Cheers,
:o

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Postby lemur » Tue Apr 24, 2007 11:20 am

jakemooremd wrote: Don't a lot of these use a smaller pilot kite?
Yes some of the show kites require a piliot but they are mostly used to provide additional lift in gusty\lully conditions. Flying at a higher altitude the piliots generally get stabler wind.

Rob


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