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Left alone, I sometimes experiment 
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Joined: Mon Sep 29, 2003 7:43 pm
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Location: Vihtavuori, Finland
Post Left alone, I sometimes experiment
So... how long does it really take for a spar to stop wobbling once deflected ? One of the claimed benefits of the very spendy Aerostuff spars (and to a lesser extent Icones and Skyshark's Nitros) is that they return to normal/straight more quickly than other, more common spars. I know this to be true because I've made this very claim myself. But is it actually true ?

This is a bit of a kitchen sink experiment and, rest assured, I'll go back and rework it :wink: but here's a short video of the results for what happens to Skyshark's 3PT and 5PT, Avia's Skinny SUL and (regular) SUL and Aerostuff's Gold spars. It's 19.7Mb and 2:30 long.

I find these preliminary results interesting but then I'm odd that way. :P

Mike.


Thu Aug 05, 2010 9:04 am
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Post Re: Left alone, I sometimes experiment
Thanks thats very interesting.

I guess what we feel isn't so much how long it continues to vibrate for but how fast the first return from bent to straight happens (period of the first cycle?). How long it vibrates for has to be a big indocator though

In winds strong enough to flex lower spreaders I'd agree you can certainly feel an AS snap back faster than a 5PT.


Thu Aug 05, 2010 10:11 am
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Post Re: Left alone, I sometimes experiment
Thanks for the video.
Would be nice if you could plot the actual movement on a graph then you could overlay them and compare them.


Ian Newham wrote:
In winds strong enough to flex lower spreaders I'd agree you can certainly feel an AS snap back faster than a 5PT.


Ahh but would you prefer that it didn't bend at all in the first place in stronger winds? :roll:


Thu Aug 05, 2010 7:38 pm
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Post Re: Left alone, I sometimes experiment
Sub wrote:
Ahh but would you prefer that it didn't bend at all in the first place in stronger winds? :roll:


No, some flex can be good but thats another story for another thread


Thu Aug 05, 2010 10:28 pm
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Post Re: Left alone, I sometimes experiment
This test might yield more useful results if the spars were under a load.

Denny

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Fri Aug 06, 2010 9:22 pm
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Post Re: Left alone, I sometimes experiment
Interesting, Mike.

Quote:
In winds strong enough to flex lower spreaders I'd agree you can certainly feel an AS snap back faster than a 5PT.


And some will testify that the assertion is equally valid if you omit the word "back"! :-)

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Sat Aug 07, 2010 10:47 am
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Post Re: Left alone, I sometimes experiment
Just to share:-

  • my camera only shoots 10 second bursts at 240fps which wasn't enough to catch all of the 3PTs and Skinny SULs vibrations - they went on a bit longer
  • most spars got a "second wind" of subsequent wobbles after the first wave seemed to die down
  • the Avia SUL stopped fastest

TBH I just wanted to see what happened and I have a camera with a gimmicky slo-mo mode. :)

I've gathered a few more types to try and next time I think I'll support the spar at both ends. A rig has been set up :wink: I'll use a standard distance of deflection rather than a standard load again. I think we're likely to get "interesting" results rather than "definitive". I'm not sure that trying to replicate the installation on a kite is worth bothering with.

Mike.


Sat Aug 07, 2010 12:23 pm
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Post Re: Left alone, I sometimes experiment
Zippy8 wrote:
Just to share:-
[*]most spars got a "second wind" of subsequent wobbles after the first wave seemed to die down


Pluck a guitar string and you'll see that, you'll sometimes see the vibrations appear to move up and down the string, I think its harmonics but you'd expect a tapered spar to mess that up?

Quote:
[*]the Avia SUL stopped fastest [/list]


Less inertia from a light spar? Could it that the duration of wobble is some sort of product of weight & stiffness?


Sat Aug 07, 2010 3:22 pm
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Post Re: Left alone, I sometimes experiment
Ian Newham wrote:
I think its harmonics but you'd expect a tapered spar to mess that up?

I've done some work on drill string harmonics (several kilometre long metal tubes, spinning around inside another tube.... #-o ) and I was expecting something but I was surprised at how strong the second events were.

Quote:
Quote:
the Avia SUL stopped fastest

Less inertia from a light spar? Could it that the duration of wobble is some sort of product of weight & stiffness?

That's the full sized SUL, not the Skinny SUL. This spar is marginally lighter than the Skyshark 5PT and heavier than the Gold but stiffer than both (according to this). I know Patrick Somelet swore by Avias over Skyshark for this very reason so I wonder if we've overlooked something here :-k

Factors involved ? Stiffness certainly but I suspect geometry (overall diameter, wall thickness, degree of taper) must be in there somewhere too. When a moment opens up I'll see if I can't gather a bit more info.

Mike.


Sat Aug 07, 2010 3:35 pm
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Post Re: Left alone, I sometimes experiment
I was told (by whom I don't remember) SUL from Avia means Strong ultra light, not super ultra light. So maybe it's less light than you think.

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Sat Aug 07, 2010 4:33 pm
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Post Re: Left alone, I sometimes experiment
Vee wrote:
I was told (by whom I don't remember) SUL from Avia means Strong ultra light, not super ultra light. So maybe it's less light than you think.

It does indeed mean Super (above UL but below STD in the G-Force range - similarly for the Skinnies; UL is the lightest, with SUL heavier/stiffer) but having weighed the spar I know precisely how light it is :wink:

Mike.


Sat Aug 07, 2010 6:51 pm
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Post Re: Left alone, I sometimes experiment
So although I was misinformed it still sits mid range, not at the bottom end of the range, have I got that right now?

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Sat Aug 07, 2010 8:52 pm
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Post Re: Left alone, I sometimes experiment
weight and/or mass vs stiffness refrenced to time and then factored by length; thats what we all really want. Good luck with with that!
then there are things like lot variance...

"Facts by themselves can often feed the flame of madness, because sanity is a spirit"- G.K. CHESTERTON


Sat Aug 07, 2010 10:49 pm
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Post Re: Left alone, I sometimes experiment
Well I've had a bit more of a play and, in all honesty, it's a bit tricky to dream up a more appropriate experimental setup that actually does anything different to what I first did. :(

I tried fixing the spar at both ends then pulling the centre down - I'd need more than 240fps to see anything as even the wobbly 3PT stops in under two seconds.
I tried loosely damping one end - pretty much as above.
I even tried adding an end weight - definitely not. :oops:

I did come up with an improved way to record the experiment but all in all it's the same result. If the perceived advantage of Aerostuff spars does indeed come from superior vibration damping then it's not by a huge amount and the only marginally heavier yet far cheaper (under a tenner each) Avia G-Force SULs should be an even better bet for a standard kite.

Not sure where this leaves us. :-k

Mike.


Wed Aug 11, 2010 10:01 am
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Post Re: Left alone, I sometimes experiment
Zippy8 wrote:
Well I've had a bit more of a play and, in all honesty, it's a bit tricky to dream up a more appropriate experimental setup that actually does anything different to what I first did. :(


All the same, thanks for trying. We've seen lots of spar bending tests over the years but this is the first attempt to see how fast they recover =D>

Quote:
Avia G-Force SULs should be an even better bet for a standard kite.


They seem to be unpopular these days but I've always liked GF-ULs and never noticed them boing back[TM] the way 3/5PTs do. FWIW Ron Graziano uses them in his UL Machine as a light alternative to AS in the std. He certainly puts them through their paces.


Wed Aug 11, 2010 10:44 am
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