Tell me which spar arrangement is the strongest.

Check Out The Diagrams.

Spar 1 : 3/8 X 3/8 Hard Balsa, 1/8 vertical grain shear webbing.

Spar 2 : 3/8 X 3/8 Hard Balsa, 1/16 vertical grain shear webbing.


Keep The Faith: Johnny

I myself use the two 1/6 inch thick shear webs instead of the single 1/8 inch setup because you have more glue area, and the force is spread out over both sides of the spar. Now the 1/8 inch shear web is probably lighter than the two 1/16 inch setup but the force is spread out more with the two shear webs.
So as a guess, my call is the 2 1/16 setup....
And it will resist twisting more.

You are sort of correct on this one.

Actually, both have the same bending strength based on mechanics of materials theory. But the 2x1/16" webs is much easier to build and has better geometry for the glue. It is also the more STABLE arrangement (less likely to buckle elastically) and is indeed better torsionally as well. I use the 2x1/16" version myself simply because it is so much easier to build.


Mark

I agree with Mark. I personally like to use 1/4'' balsa cut to 3/8'' to 1/2'' side strips (one of my fav tools is a cheap plastic balsa stripper) to use as spars. Then use 1/16'' vert grain balsa sheer webs on both sides.