Wing Attach Brackets

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RJW

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I’ve been trying to figure out a good way to bolt a one-piece aluminum center wing section to a fuse. These are the brackets I’ve come up with. They are shaped to keep the wing attach bolts on the centroid of the spars and aligned along a common longitudinal axis. Does anyone have any concerns with this arrangement? Am I going overboard worrying about alignment? Might it just be simpler/lighter to attach the brackets closer to the tops of the spars and forget about alignment (ala Mustang 2)?

I haven’t stressed the brackets, bolts, or fuse bulkheads yet. Brackets will be made from 4130 and I’m sure will end up lighter than what is shown. Also, appropriate doublers and other structure will be added as dictated by analysis.

Thanks,
Rob

Brackets 1.jpgBrackets 2.jpg
 

Autodidact

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You could get a rough but ballpark idea of the amount that the upper spar cap contracts under compression by putting half of the ultimate wing load [gross weight times n(ultimate)] as a concentrated force at about 40% of the semi-span of one wing panel and then dviding that moment (the load times 0.4*semispan) by the distance between the spar cap centers for the length of spar between the attach bolts. That would give both the compressive force in the upper caps and the tension force in the lower caps. You'd only need the compression in the upper caps since your bolts would be next to them in that scenario. The modulus of elasticity is stress/strain, so, devide the force in the upper cap by the cross section area of that cap and you have the stress in that cap (for 2024-T3, size the cap to have 17,000 to 21,000 psi, maybe? Someone jump in here?). Then devide that stress by the cap materials modulus of elasticity and that will give you the strain. Strain is the amount of contraction or lengthening of a material devided by it's original length; multiply the strain times the length between the attach bolts and you get the contraction of the upper spar cap and the amount that the lower fuselage flexes inward.

P.S., I'm not trying to insult your intelligence by being so pedantic, but all kinds of people read these posts and on some of the forums I visit they aren't as clear as they could be.
 
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RJW

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Wisconsin and Kansas
Thanks AD. I came up with about .05 inches maximum displacement for a point on the extreme fibers of the main spar at yield. If I did this right then “normal” flexing should not lead to a displacement of more than about .01 inches. I think the fuse structure will handle that amount of flexing. I guess I’ll make the brackets attach as close to the bottom of the upper spar caps as possible and ignore longitudinal alignment. It will make construction simpler and save a pound or so.

Thanks again,
Rob
 

Autodidact

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I think the fuse structure will handle that amount of flexing.

Well it might, but I think you should design the flexed part of the fuselage so that it has less stress at full flex than the spar cap does, just to be conservative. It might not be possible since the lower longerons need a certain amount of stiffness (and the bulkhead too) and at that size the flexing might fatigue them too quickly. Locating the bolts at the neutral axis, if you find that needs to be done, you could link the two fittings by an inverted U-channel that would help prevent flexing of the upper longerons under high yawing loads. The U-channel might only be needed at the rear spar attach while you could make the front attach bulkhead to "arch" over the pilots knees to prevent it flexing into a prallelogram under yaw load. Just thinking out loud.

Bret
 
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