OK, I've seen a lot of posts on shear web grain direction....on several websites. Virtually all of the model building sites say to orient the grain vertically, but what really have to be looked at are the loads applied to a wing spar in flight.

In the case of a channel beam spar (top and bottom spar connected by a shear web at either the front or back), during positive load factors (level flight, inside loops, etc), the top spar is under compression and the bottom spar is under tension. That is due to the bending moments acting on the wing. The top and bottom spars really don't move spanwise in relation to each other very much. In aerospace engineering, we usually assume that deflections of the wing are very small and neglect them. In the case of model airplanes, this is definitely the case.

When the shear stresses in the shear web are calculated, it is found that they are at 45-degree angles to the span of the wing (one direction for positive load factors, the other for negative).

Because of this, the optimum grain direction for a shear web would be BOTH 45-degree directions.

In full-scale (ie "real" airplanes that you build, get in, and fly) composite (fiberglass, carbon fiber, etc) aircraft, shear webs are built with the plies oriented at 45-degree angles to the span. In fact, they make fiberglass and carbon fiber cloth that is bi-directional (50% of the fibers go in one direction, 50% in a perpendicular direction). This is OK to use for relatively low-load shear webs, but main wing spars usually require many plies for strength, so usually several uni-directional plies are used, in perpendicular directions (still at 45 degrees), on top of one another.

But, balsa is unidirectional (all grain in one direction). So, which direction do you pick?

Guessing from the several posts that I've seen, it seems apparent that many people think that the primary loads on the shear web are those that would "pull" the top spar away from the bottom spar (or push them together), so I think this is the reason behind everyone wanting to put the grain in the vertical direction.

BUT, I'd ask you to cut 2 rectangular pieces of 1/16" balsa, maybe about 1" by about 4" each, one with the grain going in the direction of the short side, the other along the long side. Now, hold the one with the grain running along the short side with both hands, as if it would be attached to the spar, and try to bend it upward. The tensile stresses at the bottom edge pull apart the balsa between grains VERY easily.

Now, try it with the other piece of wood, with the grain running along the long edge and repeat the test. Not too easy to break, is it?

Besides bending, there is also another load applied to the spar....that of shear. Imagine the joint at the wing root....where the wing meets the fuselage. The wing lifts upward, but the fuselage is pushing downward (due to its own weight). So, at the wing root, there is a vertical shear similar to the load that a pair of scissors would have on paper (hence the name "shears"). This load actually exists all down the wing out to the tip, but it decreases in magnitude as it goes out (it is highest at the root of the wing).

Cut two more pieces of balsa exactly like the last two. Now, hold one up and try to pull up one end while pushing down the other. The one with the grain along the short side will shear apart along the grain very easily, whereas the other one takes a lot of effort.

My $0.02 is that if you have to use a unidirectional material (like balsa or ply), install the shear web with the grain running spanwise (horizontally, not vertically). Lite ply would be great if weight isn't an issue.

Now, to the other question about whether to glue it to the front or back of the spars....

If the spar is in front of the 1/4 chord point (1/4 of the distance from the leading edge to the training edge), then it would be best to glue it to the back of the spars. If it is behind the quarter chord point, it would be best to glue it to the front of the spars. If it is right on the quarter chord point, then it would be best to glue it in between the spars. The 1/4-chord point is where the resultant lift load acts. In other words, the vast majority of the lift load is being applied along a line running spanwise at about 1/4 chord. The shear center (the point through which a load on a beam produces no torsion (twisting) loads) is actually BEHIND a c-channel. That is, it is about like this + C (where the + is the shear center and the C is the channel beam (top and bottom spars with the shear web on the left)). Putting the shear web on the correct side will reduce and possibly eliminate any torsional loads on the main wing spar. There is a lot of structural interation in the entire wing frame....leading edges, trailing edges, sometimes a front and rear spar, etc....so this is not easy to calculate, but it kind of gives you an idea....in an engineering "geek speak" kind of way.

There are a lot of books on aircraft design (not usually model aircraft....but rather full-scale ones).

Anyway, chew away at this. I don't know what the backgrounds of all of the folks on here are, but mine is in aerospace engineering, with a major in aerospace structures, and I have a few textbooks on aircraft structural analysis that go into a lot more detail than this. That's not to brag, just showing where I'm coming from.

If anyone has another opinion, please include details of the loads you're applying to the wing spar and how the balsa grain works to counter them. Also, try the experiments above.

I looked into this on here because my GP Christen Eagle II lower wing had a vertical crack in the main spar's shear web, and the lower spar failed in tension (basically a postive-g bending failure). After examination of the spar, the shear web was found with the grain running vertical, and the crack in the web was right along the grain, from the bottom up. Fortunately I found it before a flight and can fix it, and I now will be adding a lite ply shear web with the grain running horizontally from the wing root to about the 2nd rib from the center (the crack was just outboard of the 1st rib).