Disclaimer. The following is for balsa webs. Im not about to start making
webs of light ply. This is also meant to think about a less than perfect built
wing which... all mine are.</p>

So, Ive been thinking about this a little and I'll go along with the Free Body
Diagrams that show that the shear web grain orientation, vertical or horizontal,
probably doesnt matter much if the webs are properly fitted and adheared to the
ribs. No gaps, air tight.</p>

However,
(#1) how many of us have built wings with the webs not even contacting the ribs?
The webs are only adheared to the spars. So you basically have, what was described
as, a shear crack on either side of a rib.
Id bet my lunch that in cases like that, the vertical grain orientation (normal to
the spar) is the only way to go. Id surmise that the compressive and tensile strength
of the webs become even more critical so that the web does not buckle under flight loads.</p>

With no rib in contact with the webs, a horizontal grain, has no glue joints to keep
the web from buckling under load.</p>

(#2) also a wing in flight under dynamic loads arent really comparable to a static truss.
The FBD's given dont consider the twist or torque imposed on a wing during flight (lift),
nor the torque imposed on a wing due to flap and aileron operation, nor...
the rotational force imposed on a wing just from ailerons and flaps would further the argument
for a vertical web grain orientation.</p>

The reason I had to add my two cents is that Id guess 90% of the planes Ive built and seen
built fall into the catorgory described above. I dont know many people that take the time
to build that "perfect on paper" wing.</p>

The grain goes normal to the spars.</p>

Cheers,
James</p>