>When I plug these numbers and assume a 200lb loading
>and using piano wire (affordable) I came up with a
>0.32" piano wire joiner.
>
>a = 0.45 (gross interpolation of joiner at 0.33 span)
>F = 200 (load number frequently tossed about on RCSE)
>b = 100"
>sigma = 180000 (piano wire)
I seriously doubt you will see 200 lb pull on your winch.
>(I wanted to work back to the load given a
>0.25 radius wire, but I've forgotten how to rewrite an
>equation that is raised to a fractional exponent.)
200 lb * (0.25"/0.32")^3 = 95 lbs
Probably more realistic. You might still put a small bend
in the 0.25" wire on a very hard launch, but I doubt you'll
fold it completely.
>Further design questions - If I were to make the
>polyhedral by putting dihedral at each joint (except
>the root) would it be hard to bend the piano wire, or
>would it be better to put the dihedral outside the
>joiner area?
Why not just use a straight joiner wire and embed the
tubes at an angle in each panel? The 9/32" joiner tube
should fit angled like this in your 9" chord. A straight
rod is easy to check for a permanent set and easy to
replace.
>What is the rule-of-thumb for how long a joiner should be?
Depends on the quality of the shear bond between the joiner tube
and the sparcaps. The better the bond, the shorter the rod can be.
Embed the tube in plywood filler between the caps, and wrap 2-3 layers
of 2 oz glass at 45 degrees around the whole sparcap/ply/tube assembly.
A short 3" tube will be OK with this kind of joint (6" rod).
If you're not using pre-cured sparcaps, the structural sizing
of the joiner rod attachment system gets very uncertain.
Can't give advice in this case.
- Mark
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