It has been an interesting discussion but I still think that there is some confusion about grain direction and the loads actually being carried. You need to keep in mind that wood is much stronger along the grain than across the grain. When you orient the wood so that the grain runs along the diagonal (not necessary at 45 degrees), the "shear" load is actually being carried in tension along the diagonal. You could in principle replace the shear webbing with cables or string running along the diagonals and end up with the same result as adding a shear plate. However, the last time I checked, cables cannot carry shear loads. I have enclosed a simple sketch which hopefully ilustrates the principle. The other thing to keep in mind is that the ribs on a model airplane has very little compressive strength and that is why it is better to have the grain oriented vertically. The links to full scale airplane construction techniques show that the ribs typically have vertical compression members where they meet the spars. If this is the case, maybe a plywood shear web at 45 degrees would be best. because it is crossed grained. At any rate, I think I will leave it at that and would like to thank those who posted the links as they make for fun reading.

Teo