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Author Topic: If You Want A Rigid Frame...  (Read 2753 times)
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chilese
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« on: November 06, 2010, 12:26 PM »

Bend the spars.

One of the most rigid kites I've ever flown is the Vapor. You think SUL, you think fragile and wimpy...noddly frame. Well the Vapor is an SUL and it uses 2PT G-Force spars and 6 of the 8 sticks are fairly stressed in normal configuration. And that frame stays rigid until well past the intended wind range.

The Vapor loads up quickly above 4 mph, but you'll not see the frame wiggle. Fortunately, I have always put the kite away before the failure mode was found. But that kite will pull you forward if the wind picks up during a gust. So what ever happened to the highly stressed design? Too curvy for the leading edges?

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« Last Edit: November 06, 2010, 11:36 PM by chilese » Logged

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RobB
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« Reply #1 on: November 06, 2010, 12:57 PM »

Hey John...
I guess this means that the rainbow Vapor recently seen available found its way to your bag ? Congratulations on adding another sweet rainbow Prism to your collection.
And yeah, I've never broken more than a nock off of a curved LE Prism (Illusion, I2k, Elixir...). Maybe there's something to your theory of preloading the sticks...
~Rob.
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chilese
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« Reply #2 on: November 06, 2010, 02:35 PM »

Not into my bag.  Sad

Just on loan for some photos.

It's already back with the new owner who needs to get off his butt and ship the trade kite. Although I'm sure he's good for it.  Smiley
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Michel
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« Reply #3 on: November 06, 2010, 03:00 PM »

One of the most rigid kites I've ever flown is the Vapor. You think SUL, you think fragile and wimpy...noddly frame. Well the Vapor is an SUL and it uses 2PT spars and 6 of the 8 sticks are fairly stressed in normal configuration. And that frame stays rigid until well past the intended wind range.

Is that the original frame John ?

My Vapor is framed with G-Force Skinny UL.
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« Reply #4 on: November 06, 2010, 03:08 PM »

Not into my bag.  Sad

Just on loan for some photos.

It's already back with the new owner who needs to get off his butt and ship the trade kite. Although I'm sure he's good for it.  Smiley
Even so, just to see it with your own eyes must be a treat. I have only seen a Vapor rolled up in it's plastic tube, the day was too windy to even take it out.
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chilese
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« Reply #5 on: November 06, 2010, 04:18 PM »

My mistake on the frame. I said 2PT, but G-Force is probably correct. I didn't even check. Just a fault in the old noggin.  Roll Eyes
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Allen Carter
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« Reply #6 on: November 06, 2010, 05:05 PM »

There are sail designs that are easy on sticks and ones that seem to be jinxed. The curvature itself doesn't seem to be a determining factor. Nor does the overall stiffness in flight

The Illusion was generally thought of as a stick eating kite, though this was partly to to framing materials.

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JimB
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« Reply #7 on: November 06, 2010, 10:36 PM »

Classic Illusions and Vapors are stick eating kites, but to be fair, so were many high end kites of the era. Kites are generally sturdier these days.

And the failure mode? You don't want to know. Think along the lines of an implosion. No warning whatsoever.
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mikenchico
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« Reply #8 on: November 06, 2010, 11:00 PM »

I would have to see some data proving a bent or loaded spar was stiffer then an unloaded one. Or does carbon react differently, I guess it is concievable that it is more resistive to further bending when already bent but it would also be closer to it's failure point. My guess is the Vapors sail is very tight and well fitted thus becoming part of the structure, just like a monocoque or stressed skin structure, the bent spars provide the load to pull the skin tight. The Eyesight by Current Chasing Kites (Can't recall the designers name) used a similar method running a tight line from the Center T up to the Upper Spreader Connectors, across and back down providing a triangulation to the frame that stiffened it while adding negligible weight.

Mike (Zippy) will have to figure out a way to test that in the next go round when he has too much time on his hands.

« Last Edit: November 06, 2010, 11:34 PM by mikenchico » Logged

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zippy8
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« Reply #9 on: November 06, 2010, 11:46 PM »

I would have to see some data proving a bent or loaded spar was stiffer then an unloaded one.
Pre-tensioning is quite a common method. If you need another example from the kiting world then consider the Synergy Decas.

An issue with apply a lot of curve~pre-load to a spar is that there is a limited amount of deflection~bend that it can take before it fails. If you start off with some of that already taken up then you're closer to failure. In practice stiffer spars that start off with a curve are actually closer to that mode of failure than a more flexible one.

The Prism designs of the Vapor-Illusion-Fanatic style had an unpleasant tendency to simply blow out their LEs just below the LS connector. Tip stabs or even energetic Snap Stalls (now with added SNAP !) could easily take care of this.

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Mike (Zippy) will have to figure out a way to test that in the next go round when he has too much time on his hands.
Easy:- arrow nock each end, adjustable line to set preload~curve, then load test as with a straight spar. One problem would be that you'd get two results - one "with" the curve, and one "against" it.

Mike.
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mikenchico
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« Reply #10 on: November 07, 2010, 12:41 AM »

Pre-tensioning is quite a common method. If you need another example from the kiting world then consider the Synergy Decas.

Mike.

The Synergy spar isn't necessarily bent to make the spar stiffer, that bent spar is used to provide the tension necessary to support the sail and other rods since nothing is connected together except by lines. It's acting as a spring to provide tension. It could be considered an example of a stressed skin structure though. What did they call it Tensionegrity?

I could conceive that some materials may require increased force as they bend, a carbon matrix may be one such material. Hard to test using the methodology you were using but then you were testing for a completely different property, return from deflection, to add to our deflection figures. It is easy to test though, use the standard two point support testing and pre-weight the spar and set the dial indicator to zero at that point then add the measuring weight and measure the deflection and compare to the standard test. But as you stated that would only test the one direction, the spar would be less stiff in the return to straight deflection so the average deflection would be very close to the standard tests deflection. Thus no appreciable gain in stiffness.
« Last Edit: November 07, 2010, 12:57 AM by mikenchico » Logged

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Allen Carter
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« Reply #11 on: November 07, 2010, 10:44 AM »

We're not talking about a spar getting stiffer, it's a frame made up of a bunch of spars. The arrangement, connection and tension of the spars does influence the integrity of the frame. The sail is a component in the structure, of course, and is often the single largest influence on the percieved rigidity of the kite.
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chilese
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« Reply #12 on: November 07, 2010, 11:07 AM »

Exactly Allen.

A spar which is bent by the sail and other spars is much like a non-compound bow (of bow and arrow Robin Hood style). Pulling on the string of the bow (as in drawing an arrow for flight) would be the equivalent of bending a spar more. Pushing against the bow in an effort to "straighten" it (maybe with the tips of the bow on the floor) while tensioned is almost impossible. Either the bowstring will break, or the bow will shatter.
« Last Edit: November 07, 2010, 11:09 AM by chilese » Logged

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« Reply #13 on: November 07, 2010, 12:52 PM »

Geeks   Cheesy
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Doug
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Allen Carter
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« Reply #14 on: November 07, 2010, 03:27 PM »

Hey, better than talking about how much weight Lam stuffs in the tail!
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