>I am planning a 7/32 rod, and about 3 inch insetion length in each panel.
A 3" rod is OK provided the joiner tube is solidly embedded in
the web between the sparcaps. The shear load taken by this web
(and the cap/web bonds!) is equal to the sparcap load, which
is about 300 lb max for a typical 0.5" x 0.125" spruce cap.
A lite-ply web will suffice. If you add a 0.5" x 0.014" carbon
strip to it, the max load will be more like 1500 lb, in which case
using regular ply will be more prudent.
The web's shear load drops dramatically outside the joiner rod.
The drop factor is approximately
S_outside/S_joiner = 4 h / b
h = spacing between sparcaps
b = span
For a 100" wing with a 0.75" sparcap spacing, the factor is only
4*0.75/100 = 0.03
So if the joiner web sees a shear load of, say, 500 lb (sparcap load),
the web just outboard of the joiner sees a shear of only 500*0.03 = 15 lb.
There's no reason to continue the ply web outboard of the joiner tube.
Balsa will certainly suffice.
>I'm running out of thickness between the spar caps with the foam cores
>and I don't want to cut into the caps if I can help it.
You can safely taper the top sparcap over most of the length
of the joiner, down to near zero thickness at the root rib.
This will give the cap's thickness worth of additional depth
to work in.
This assumes that
a) The tapered sparcap is solidly bonded to the ply web which is
then solidly bonded to the joiner tube all along its length.
b) The joiner tube is strong enough to resist bursting at its end.
A K&S 0.015"-wall brass tube will be quite adequate.
A soft 0.015"-wall aluminum tube maybe not. Wrap the end with
Kevlar if in doubt.
Doing both a) and b) is sound practice in any case.
- Mark
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