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SkyBolt Structure Strength

Randy

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The following is from a posting made to the now defunct Hanger web site dated 9/02. I thought some of you may be interested. The poster was Dan Horton and the Jim is Jim Doyle.
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"Understand that a structure analysis is always based on certain assumptions. The analyst must accept things like published wood data, even though he knows all trees are not created equal. He may choose to ignore a strength or stiffness contribution from structure attached to the primary member he is checking. He makes judgement calls about certain things like fixity in buckling calculations. And so on. If the analysis is part of a project at an aerospace firm, he then hands they whole thing over to another analyst, who applies his own experience to a complete review. That's the rub when it comes to something like the SkyBolt. It's unlikely that anyone has ever done a complete analysis to aerospace standards. It's a hobby airplane. The good news is that a whole lot of them have been built and flown and they don't seem to fall apart.

I'll refer to the archives for Jim's words. Note that he was very careful to state that he does not consider his analysis to be perfectly accurate, or to be based on perfectly correct assumptions. He's a pro, and I wish he was still here. When last heard from he was deep in his new job as lead analyst for the forward fuselage of the Joint Strike fighter.

Anyway, the general conclusions were that at 9 g's and VNE, the upper wing was marginal at 2000 lbs. gross. The inboard antidrag wire should be upsized a little. The prime failure mode was predicted to be compression failure (buckling) of the front spar in the inboard bay. It was suggested that the spar could be stiffened by using additional nose ribs inside a bonded ply leading edge, thus forming a box structure, but no analysis was done to my knowledge. Jim did not much like the routed spar for 9 g's at 2000 lbs, which is not to say that the routed spar isn't perfectly fine at lower weights or shorter wing spans.

It's worth remembering that very few of us will reach 9 g's, although most builders wouldn't consider building for any less. My entirely unprofessional opinion is that if you build to the plans, you're quite unlikely to break it."
 

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