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budyerly(at)msn.com
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 Posted: Tue Jan 25, 2011 7:34 pm Post subject: Main wing bushings |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Duncan and Graham,
As for the wing failure point, the idea was to have the actual wing break before the attachments. Duncan is right, the failure occurs outboard of the root rib.
I asked why not an articulated forward socket and a proper attach point with a pin, however a pivoting pinned fitting would then imply that the forward pin if articulated without a pip pin would slide (not good), as the pin slides under bending and the forces are not translated due to the slipping pin I guess. The articulated forward attach was dropped. I guess since the forward root pin is in shear only, and as long as the tolerances during wing construction are met, the pin stays fairly straight, and I am sure under extreme loads, the front pins are pushed into the socket and with the stiffener added, flexing and failures are non existent.
The carbon fiber MG spar flexed under load and the more brittle carbon shear web cracked near the outboard wing pin. So the idea Dave and I talked about was simple, there is a bit too much taper in the spar tang and the height of the spar needed to be increased or said another way, the spar now has less taper. It is now massively strong.
I prefer the common way of distributing that stress concentration at the rib where the upper skin can collapses under the rib bending, and that is put in a span wise beam or spar to distribute the point load on the rib. The attach point is a horizontal bolt or pin in a proper spar. Also I prefer the beam to go through the fuselage caring the load. Of course it is that much more stuff to build.
That said, I am amazed this little airplane has soldiered on without that forward socket ever failing, or the pins. I guess it all works together without the fancy computer programs or stress analysis diagnostic programs or even my original skepticism. The combined attachment points keep things intact extremely well. I was impressed by the latest glider wing tests done by the factory. On the first test the rear pin broke because they grabbed a plain steel one used in shop alignment work and it snapped like a twig. The rear root pin was replaced and one of the guys in haste put a common bolt in the main wing and it sheared like butter. Once all the proper pins were correctly installed (and much egg removed from faces, I'm sure}, the wing with the spar mod went to nearly 9 G's, finally failing in the leading edge D tube area when the upper skin flexed until it collapsed. Not a bit of damage or problem with any of the sockets or the proper wing pins or bushings.
So, it works brilliantly and is extremely tough at current load limits imposed. I like it.
Bud
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budyerly(at)msn.com
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| Posted: Tue Jan 25, 2011 7:46 pm Post subject: Main wing bushings |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Already being done sir.
bud
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