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Rule of thumb for shear webs

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Wild Bill

Well-Known Member
Joined
Dec 11, 2013
Messages
111
Location
Vidalia, GA
I've studied this pretty well, and the more numbers I crunch the more muddy the water seems to get.
I havent seen a general rule of thumb for shear webs for box spars. (especially for those with a foam core)
The design I'm working on is a spar for a electric powered ultralight. Design weight goal is around 300 lbs gross.
This is a single strut braced, D tube, box spar type construction.
Spar caps are to be lamination's of either fir or pine 1/4" X 1.5"
Spar will be lightly loaded but this is also a relatively thin wing. Spar length will be 152" and will taper from 6.5" to 2.375" The wing section is 10% somewhat laminar flow, low pitching moment.

Given that I will have a D tube the torsional stiffness of the spar should not be too much of a factor.
My delimma is in whether or not to use a foam core material between the spar caps and use a thinner shear web (say 1/16 or 1.5mm) or leave it hollow with some vertical stiffeners and use a more suitable web material like 2 or 2.5mm finnish birch.
I like the idea of using 4 or 5 ply finnish birch and simply running the face grain vertical. I just wonder if this is overkill in 2 or 2.5mm.
If I were to use a thinner web I would want to use a foam core material to keep the web material from buckling. And with 3 plies I would definitely want to cut from a 4 x 8 sheet on a 45 degree so that the face grain is running at a 45 degree.
 
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