Load Test Question

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BBerson

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In the very first post it is the second from last picture. It's looks like the diagonal piece of the rib is pulling down the skin.
I see that pulling down, but post 1 said the serious buckling is on the lower surface that I can't see. Where is the serious buckling? Near the leading edge? Or near the spar cap? Buckling at the cap would be expected first. So I suspect the pulling down issue is lack of joining the two wings as in flight. Are the two wings connected at the centerline?
The extreme beam loads don't carry into the fuselage. The shear is canceled by the other wing.
 

BBerson

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I suspect there is a problem with the support structure. It appears the support may not be rigid in torsion to the D-cell torsion structure. But I can't really see enough to figure it out. The root rib needs to be beefed up more than the other ribs to transfer torsion into the tailboom. I don't see that rigid structure yet.
 

proppastie

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I see that pulling down, but post 1 said the serious buckling is on the lower surface that I can't see. Where is the serious buckling?
The root rib needs to be beefed up more t
Where the ruler is at the flange near the outboard attach of the drag spar....those buckling deformations extend forward on a 45 degree slant to about the plane of the end of the nose ribs. As stated in a former post the oil canning (buckling) is ellipsoidal in shape with the small ends at the flange and Leading edge. The bare aluminum and reflections made it hard to see and photograph so the best I have are at post one. Even if this oil canning is not permanent deformation the resulting airfoil is garbage....certainly not something one would want on a glider.

I do agree the root rib needs to be significantly beefed up or another one perhaps added with perhaps an aluminum skin between the two. Those ideas are all bouncing around in my head along with the added bracing shown in a previous post 92
 

BBerson

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The 45° would indicate a torsion load , I think.
Maybe you could twist it by hand and see what happens to see if it is torsion or beam bending is the cause.
Need to know the cause.
I don't know if a thin 11" diameter tube can be expected to not buckle on such a long span.
My recent wing load had considerable deflection at the root at 5g. I don't have a D-cell skin so no problem. My Ercoupe type wing is fabric covered so it may or may not wrinkle the fabric, but no concern.
I think the 2-33 has considerable skin oil canning at around 2-3G. That's why they switched to sandwich composites for high performance at high load.
 
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proppastie

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where did you get "edge restraint condition unloaded edge e=20" and where is it used
Figure C 5.15 refers to "edge restraint" on the RH side of the graph but I do not see it used as an input to obtain Kb of 43
I do not see how you came up with 43...I do see the red lines though
Also mentioned wave length but do not see where that is used as an input.
1594523692070.png

Edit I see where 20 goes to 43 looks like a/b=2 is not on the scale. I still do not see how E=20 is picked . The kit planes article following post has different scales....and says use 2 if no compression ...maybe the 20 is like the 2...use if no compression?
 
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BBerson

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I just had a thought that torsion causes wrinkles in both top and bottom together. So I twisted my RC wing and yes it gets wrinkles on top and bottom, more on the cambered top, less on the flat bottom. So if you don't have both sides buckling I think you have pure compression buckling.
Also, consider that Paul Schweizer said he overbuilt the 2-32 spar so that the cap stress was only 4500psi, I think. That was for fatigue life and it also limited bending deflection.
Overbuilding an ultralight isn't an option, unfortunately.
 
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