I'm not sure I agree with Motorman about the TE web grain direction.

The reason you want vertical grain between the main spar halves is that under bending, the spars want to close the gap between them so that the wing can buckle.  Vertical grain is more resistant to that than horizontal grain would be (and then the webbing isn't really resisting shear, it's resisting buckling, but it's not called "anti-buckling webbing" -- go figure).

I'm not sure if there's a justification for having it one direction or another on the TE -- that's going to be under a lot less stress; maybe it's just easier?

Hmmmm.

It is called a shear web, because its primary function is to assist the two spar caps such that all three elements behave as a single structural piece. The webbing in fact, carries shear loads. The shear vector is parallel with the spar, ie. spanwise. The peak shear value is at the bending neutral axis which is approximately the vertical center, assuming that the spar has top/bottom symmetry. If this concept is not obvious, think of it this way: in a hard inside pull-up, the wing is loaded such that the upper spar cap is in compression, and the bottom cap is in tension. The web has to connect both of these, whose bending stresses go thru a change of sign. Hint: the web does it by carrying shear loads.

Tim is correct that if you think of two spar caps behaving independently (ie.  two separate beams) that adding a web allows both a greater load without buckling. But it is confusing to say that the only purpose of the shear web is to prevent buckling. That's not correct.

As everyone intuits, wood of any kind does not have isotropic properties. You want to shear balsa across the grain, hence, the vertical orientation. That orientation is also preferred for compressive stress between the caps. The same is true for a TE "beam" but I suspect that the section depth is small enough (1/4" to maybe 3/8" for most models?) and the wood used on a TE (generally 1/16") is not real robust, so using span-wise grain balsa probably helps stiffness, and doesn't hurt it. The issue there, though, is that at each rib there is a discontinuity, so even with good fits, you're not gaining as much as you might think. The discontinuity is that the rib grain orientation does not match the lateral web grain, and it would be subject to crushing. A vertical grain orientation would reduce this tendency.

You may not want a super rigid wing. If the plane is light, and you fly over grass, and you can let the structure flex to absorb the energy, it is more likely to rebound without damage. If you web it all up, and the structure cannot deflect without rupturing, then the structure explodes.

I would agree with Ken's statement if we always built the same size, same weight, same same kind of planes. Which we mostly do, so his rules of thumb probably work nearly always. But if you understand the load path, and where the stresses concentrate, and look at construction options, you actually do have choices. In a lot of cases we are overbuilt until it hits the ground. So use a construction technique that is easy for your building tools/skills/style.

A good resource to explain this if anyone is trying to understand this is a college Strength of Materials text such as Higdon.

Dave

Tim is correct that if you think of two spar caps behaving independently (ie.  two separate beams) that adding a web allows both a greater load without buckling. But it is confusing to say that the only purpose of the shear web is to prevent buckling. That's not correct.

Given how very wimpy balsa is in shear, I'm not sure that typical shear webbing would do much.  Ken's x-bracing probably provides more rigidity in shear than the typical up & down webbing (this would be an interesting test).

You may not want a super rigid wing. If the plane is light, and you fly over grass, and you can let the structure flex to absorb the energy, it is more likely to rebound without damage. If you web it all up, and the structure cannot deflect without rupturing, then the structure explodes.

That depends on whether you crash or not.  If your honorific is "Sir Crashalot" then you should use planes with a wing structure that doesn't have much stiffness, and be careful about balancing the covering strength with the airplane.  If you simple Do Not Crash (which, strangely, is how it's been for me for the last seven years and counting) then go with the D-tube wing or the fully sheeted foam.

I have flown both sailplanes and CL stunt at the highest levels and the stress on a sailplane in a zoom launch is astronomically higher than what we experience BUT the physics are the same, we just don't need as much of them.  I used to use 2 pieces of 1/32 balsa cross-grained for webbing before carbon fiber took over.

Ken

Ok, I'll bite. How are you using carbon instead of balsa as a shear web?

I flew when Zoom launches were relatively new.  The wings were too stiff with the webbing and would fold if you weren't careful.  We went to an D-Tube with a 1/2" strip of CF on the top of the spar and a 3/4" went on the bottom under the sheeting.  We stopped using webbing to allow the wings to flex more.  I have no idea what they use today, the last one I built was in the early 80's but looking at the pictures I doubt there is any balsa in those planes anymore.  Maybe someone that is still flying them, under 400' of course, could chime in.

Ken

   Two major differences are that the wings are 5x+ as thick (reducing the stress on the spars) and that in one case, flexing is acceptable, and for stunt, you want to reduce the flex to the very bare minimum, even if it costs you significant weight, to maintain control consistency.

     Brett

The wings weren't *that* thin but your point is correct.  Sailplanes did not want flex when hand launched, towed or high-started but when the winch zoom launch became the weapon of choice it became mandatory.  What surprised me was how weak the center sections are on the modern class of stunt ships.  I suppose that the thicker wings, the geodetics, and use of glass and CF have lessened the need for plywood spar reinforcement and double planking.

My sympathies go out to the Sailplane fliers betrayed by the AMA.  Zoom launches go as high as 600' and even an aggressive high start will go 400'.   Sorry, wrong thread.

Ken