Funny, while looking for a link to an explanation for this on the internet I uncovered the same exact thread topic on another R/C website that quickly devolved into a 6 way debate too! It seems everyone has heard about ONE or TWO of the stress modes and figures THAT is the only possible answer. End of story!

There is no simple answer to James question. There's a lot more going on inside a spar than most (including me) realized.

1). There is horizontal shear due to flight loads best illustrated by Minnflyer's graphic.
2). There is additional horizontal shear caused by wing twist, or torsion.
3). There is localized compression due to the flight load creating a bending moment near the root where the wing and fuselage connect. (Johnson's website described this mode only.) Note: Whenever there is compression on a beam there is also shear in both the vertical and horizontal direction and they are always equal. That is why the total shear due to compression is at a 45 degree angle. I think this is what Rodney is referring to.

In addition, there are other modes causing stress including: induced and parasitic drag loads forcing the wing backward relative to the fuselage, engine vibration in the wings natural frequency, localized landing gear shock loads, wind buffeting, and many other things that have to be considered in order to FULLY describe a complete picture of whats going on deep inside an infinitesimal section of a spar web.

Rather than spending a lot of time in FULLY describing a problem, design engineers often rely on their experience to ignore the smaller loads, calculate the worse case effect of the big ones, and then multiply the calculated minimum design strength requirement by some multiple called the Safety Factor (I call it the CYA factor).

The most important loads in an R/C sport plane's wing as best I can tell are the first three modes described above and possibly landing shock. The worse case magnitudes of these loads are the maximum g-loads sustainable which is related to maximum speed and aircraft weight. Safety factor for aircraft is low because of weight penalty, usually 3 or 4. A column in a building might be as high as 10!

I haven't actually calculated the loads and shear stresses involved in a "typical" .60 size sport plane spar but given the first three modes described above it seems to me that orienting the grain vertically benefits the structure in two important ways: it is stronger against localized compression at the junction with the fuselage or landing gear, AND presents the strongest direction (cross-grain) to horizontal shear which is much higher than the vertical shear.

Sorry for such a long boring post[8D]

CrateCruncher

P.S. James, I understand for a small park flyer a fully sheeted wing is probably overkill. But I think from now on I will be fully sheeting my .60 and bigger sport planes. Open-bay wing design is more work and results in a weaker wing.