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Simpler sandbag testing?

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akrotodd

Member
Joined
Aug 5, 2007
Messages
5
Howdy,

I am new. I've built a couple plans-built and custom jobs. I've got the aero degree. But I realized pretty early that I don't know much from school.

My current project is a very heavily loaded wing. In designing the spar and trying to finish the stress analysis, I find that simple Beam Theory of just the spar, 3-spar box theory, and many others I've beat up on aren't giving me valid answers. I've purchased a FEA package that has allowed me to at least easily model all the components of this wing, and get results that are valid. But how accurate the data is remains an unknown.

SO, I AM BUILDING THE SPAR AND I'M GOING TO BREAK IT. Actually, to save materials and time I'm building a simpler, shorter, but still representative beam that I'll model in FEA the same way, predict the deflections and stresses, and take huge amounts of data while I load it up. This, I hope, will make me feel good about the FEA results of the real thing.

So here's the biggest problem: the spar needs to support a BIG load, and to break it I'll need to stack a quarry-worth of sandbags on top, if they're each about 75 lb. There simply won't be room to make a big enough pile without them shifting or falling off! And making a "table" on top of the spar means making something so strong, it will change the strength of the spar itself.

I've thought of suspending a flexible (hinged?) table under the spar with straps, so I can adjsut the point loads at each strap as I go outboard along the spar. But what I'd really like to do is strap the spar to a large steel I-beam at many points outboard from the center, and jack up the middle with a force gage that tells me what the load is.

SO, THE REAL QUESTION: How could I make sure that the straps that attach the spar to the "base beam" are transferring an elliptical load? If I were to simplify to a uniform lift distribrution, I could envision a series of pulleys that all take the same rope, at both the spar and the base beam. Each pulley is bolted to the base beam, and at the spar they're each held near the spar with straps around the spar (no holes!) When I jack up the middle, the tension at the pulleys will be the same. But the lift distribution is not uniform--it is the modified elliptical distribution due to Schrenk's, and from a Purist's standpoint this bothers me to simplify.

What have other people done when sand isn't dense enough? Where's the local spent uranium shop?

-akrotodd
 
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