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What kind of spar is best for a fabric wing?

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HumanPoweredDesigner

Well-Known Member
Joined
Sep 6, 2009
Messages
1,030
Location
Arizona
I have a spar design, but have not seen it used on airplanes, just cranes and bridges, and am really starting to wonder if there is a good reason it is not used in planes. What type of spar is most common in non-structural skin wings?

I know that spars don't need to be nearly as strong in structural skin wings, but I'm still suprised that so many wings, especially those on human powered airplanes, use tubes for their spars. It would seem far more efficient if they used 3 dimensional trusses similar to what you see on cranes or bridges.


I've read about the second moment of inertia, length divided by radius, and the shearing, kneeling, and euler's ranges, and take those into account when using the compression strength and cross sectional areas. From the pictures, it looked like at the border of the kneeling and eulers range, a column breaks at 1/2 the load of the theoretical compression strength. Is that accurate, or is that just how the picture looked? I've been designing my structures so they are inside the kneeling range, and have been using the 1/2 figure for my calculations.
 
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