Thread: Discussion Skinny Wing Spin Off
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Old May 06, 2012, 11:16 AM
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United States, CA, Marina
Joined Sep 2006
3,244 Posts
Spar design and testing

I've been designing and testing a variety of spars for this plane. Since I have loads of 50K carbon tow on hand, the design revolves around laid up tow for the spar caps.

An excellent shear web for carbon tow is solid vertical grain balsa, but it is toooooo much work to sand the many many 3" pieces needed for a full spar. So I'm trying a simple long grain balsa shear strip, which shapes quickly, then wrapped in glass cloth. The 5/8" wide spar has shear that is built up from 2 -1/4" wide balsa strips, each wrapped in glass cloth.

Currently the design is as follows:
complete spar is 5/8" deep x 5/8" wide, approx.
top cap = 400K carbon tow
shear wrap at each balsa strip = 4 wraps 4 oz glass at 90/0
bottom cap = 200K

This spar held 140 ft/lbs and failed at 150 ft/lbs. With multiple safety factors for construction errors and other loads, I come up with a total flight load of 42 Gs. That's a little over kill. Spar testing procedure was similar to this.

The photos are from a variety of test samples and are intended to show general procedure.

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Last edited by Knoll53; May 06, 2012 at 12:53 PM. Reason: REVISED CALCS.
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