Check out the discussion of aircraft spars for home-builders here http://www.auf.asn.au/scratchbuilder/beams.html

It talks about laminations of DIAGONAL grain ply as optimal for shear webs. It also show the shearing forces as diagonal across the web, which makes sense as I think about it. The spars are both compressing and sliding, resulting in a net diagonal force.

Does anyone use diagonal grain webs in model aircraft?

James

EDIT - I just read Rodney's post after I posted this... Seems diagonal ply is used in real planes, maybe the ideal solution is to make balsa ply from thin balsa sheets then cut diagonal grain webs from it. If you don't want to go to the trouble of laminating sheets of balsa, then the single vertical grain web seems like the best compromise.

Also, it is not only a question of how strong the spar needs to be, but how stiff. Strength will prevent it from breaking, but stiffness will make the wing perform better.

James