Steff let me mention that for thermal sailplanes (they have relatively LONG spars) the suggestion (and practice) is to use carbon fiber material of a given thickness on the BOTTOM of the spar system and use material of the same type but twice as thick on the TOP of the spar system to prevent spar damage when using 'zoom launches' which very much stresses a sailplane wing structure.

So besides this interesting debate about the best type of shear web construction (I have had interest in this topic for years) you might look into the reinforcement I mention here since you may find it of interest as I have.

On many occasions, when driving down the highway, I have convinced myself that I really need to make a number of different spar systems using the materials with which I had worked (Aliphatic resin, spruce and balsa) and attempt to break them under controlled conditions (including low temperature and high temperature) just to figure out what works best with these materials. I have yet to actually do it. 8^)

I have built with vertical balsa between the spruce spars, balsa in front of the spruce spars, balsa behind the spruce spars, balsa in front of and behind the spruce spars (both on one spar system) and balsa in between, in front of and behind the spruce spars (all on one spar system). And I placed ply where required by the wing connection systems). I have not broken a spar yet so I assume each was adequate for the stresses that the wings encountered. But I am still curious about what others find out about building different spar systems. With the new materials used in sailplane structures there has been vast improvements in the strength of sailplane spar systems.

In particular I have wondered if adhering the shear webbing to the sides of the spar actually did or did not provide a superior shear web due to the increased area of contact. That strength is derived from the quality and penetration of the adhesive into the balsa and the balsa material transfering the stress out away from the glue joint. If the shear webbing is placed in the center of the top and bottom spar the adhesive is not transfering the stress in the exact same way and I believe this method provides a stronger spar. But I will watch this thread to find out if anyone finds results of actual tests performed in the past.

Aother thing is that since I scratch build I cut out shear web material using a table saw so that each piece is a 'standard' length and each corner of each piece is 90 degrees. Then I place a rib on the bottom sheeting then a precut shear web, then another rib then another precut shear web, etc. The precut shear webs brace the ribs 'vertically' and I have a near perfect fit (tight) of the shear webs to the ribs because I install them one right after the other. This has saved quite a bit of time. In those areas where the wing tapers I trim the excess (above and or below the spars) after the 'rib/shear web' installation dries.

Interesting thread.