Wing Load Test Failure Aluminum Dragon

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proppastie

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Better to fail on the ground than in the air.....and why we test. It looks like classic Euler buckling of the top cap strip......So after my last test where there was buckling of the skin and adding stiffeners I mounted the wing properly upside down and at 15 deg. .... I made shelves so the sand would sit flat on top. (sorry no pictures with sand). at 354 lb (shooting for 698 total) or 50% or 2.5 Gs the top cap buckled....I obviously did not do the Euler calculation for that section like I should have.....it is somewhat discouraging....however a major goal of this project was to learn "Aircraft Stress Analysis" ....One more lesson is that the wing was talking to me......it seemed weak through that section but I did not properly understand what was going on and thought it would be stabilized through the drag spar and boom.....I can fix it..Those large angles at the center carry through should have been extended into the wing.....bad design on my part...and the wing will be heavier yet some more.
 

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plncraze

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Check out Kitplanes "Stressing Structures" series. They cover bending, buckling, local crippling and MOI. When you try for the lightest tube possible it will get interesting.
 

plncraze

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Don't beat yourself too hard about weight. Maupin couldn't hit his original weight estimate and there aren't very many originals which still fly. It was a minimal design that has a tough time in the real world.
 

proppastie

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Don't beat yourself too hard about weight. Maupin couldn't hit his original weight estimate and there aren't very many originals which still fly. It was a minimal design that has a tough time in the real world.
I can not imagine I would be able to find a DER that would certify this if I bust 254 #.....and there are too many grumpy old men around to ignore that and fly anyway.
my thoughts on why there are not more like it around are that it is difficult to tow because of the low Never Exceed speed...The Sparrow Hawk could be towed at higher speeds but then the "glider culture" seems to be fixated on contest machines so it did not sell well either.
 
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Jay Kempf

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Is it just the root rib periphery that failed? Hard to tell from the pictures. Seems a full rib would solve the first rib bay I am seeing.

Never mind, what I thought was some parallax and reflections is your compression cap. UGH!

Is it contained to the first couple rib bays?
 

Matt G.

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Is anything restraining the spar cap laterally, i.e. the skin? Doesn't look like the skin around it has deformed with it. I am kind of in and out here these days and haven't followed every step of your build, so I apologize if these are ignorant questions. Looks like the spar cap is layers of strips of thin aluminum with no lateral restraint, so they can easily buckle by bending along the weak axis.
 

proppastie

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Is anything restraining the spar cap laterally, i.e. the skin? Doesn't look like the skin around it has deformed with it. I am kind of in and out here these days and haven't followed every step of your build, so I apologize if these are ignorant questions. Looks like the spar cap is layers of strips of thin aluminum with no lateral restraint, so they can easily buckle by bending along the weak axis.
You really helped me lots at the beginning no need to apologize.....
the skin from the first rib outboard held, there is only 1" of deformation outboard of the first rib at the first bend of the top cap.....the caps are stacked flat stock and at the WS 0 to WS 6 is laterally supported with a 1/4 x 3" angle added because when I mounted the wing the first time on the load fixture I easily saw it needed lateral support. Had I extended this angle into the leading edge this would not have happened....bad design on my part. The fix is to have a piece of angle out to WS 18....... about 1/2 the way between the first rib and second rib....with an appropriate added splice plate where I cut out the bent aluminum........
post 202 onward is pictures of the original problems and existing fixes
 
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