If you apply an upward load to a wing, the bending of the wing imposes a compression load on the top spar cap and a tension load on the bottom spar cap. If we place a web between the top and bottom spar caps, that difference in tension and compression becomes shear in the web, hence the name "shear web"... clever, no?


Tension loads can be resisted until the spar cap reaches its max strength and breaks.

Compression loads are different. Take a soda straw, and place it vertically on a table while you press down on it from the top. Everything's fine as long as the straw is dead straight... Give it a tap across the middle, and the straw will fail at a load far less that its maximum tension failure load..... Since the top spar cap is in compression, it behaves the same way, and being firmly attachted to the shear web keeps it from failing in compression until its ultimate load is imposed.... HTH

Cheers!

Jim

Good information there.

Ok, one last shot at why vertical is better (don't take this too seriously, ok?)

If the forces are at an angle, and we all agree that balsa failes more easily along the grain than cross-grain, then any failure in the web should show up at a grain-wise crack. If the web is veritcal, the crack has little effect, since both ends are still glued to the spars. If the web is horizontal, even a small crack disconnects the top and bottom spars

I'm only talking about cases where the sw is individual sheets between the ribs, not the one long sheet between the spars.

After all the opinions expressed on this thread (mine included ), I am convinced that the only solution is to box in the spar caps by installing a shear web on the front and aft sides of the spar - one horizontal grain and one vertical..

Better yet, make 'em out of .125 inch titanium with stainless steel rivets holding them together and we'll have no more failures... Of course, the airplane will be too heavy to fly, but hey - no failures!!

Can we get a quorum to declare this horse officially dead, or do we need to beat it some more?

Cheers!

Jim

Oh, it's long dead, I just like to debate sometimes .

[sm=thumbup.gif][sm=thumbup.gif]

Grain schmain his are plywood. BTW, what kit are you building?

Andy

You're right. When airpressure tries to lift the wing, it is "Held down" in the center by the weight of the Fuse. If all you had were two spars (pivoted in the center of the Fuse) and you lifted them, they would "Shear" against each other.

To save a few thousand words, refer to the pictures below:

From a structural engineering point of view I don't know which way is best, However, That being said I can speak from some 1st hand experience. I acquired a used Hobbico superstar (pre Monokote) which had loose sagging covering. Over the winter I decided to recover it. After removing the old covering I noticed that on both ends of the wing the shear webs were split on the inner 3 or 4 bays. You guessed it, the grain ran parallel to the spars. There was no obvious signs of a crash which required any repair so I assume that it had lived an easy life up to that point.

I have always placed the shear web grain perpendicular to the spars and will continue to do so.

Vertical makes more sense to me, and that's the way I'll always do it.

Badbart,

1/8" shear webbing is a bit on the large side. What are you building that need that thick of webbing? Is in only on a few inward bays?

MinnFlyer, You are correct for a simple shearing action typical in building structures but by far are NOT all the loads on an aircraft wing. An aircraft wing is much more dynamic than this. Your picture does explain why they're called shear webs but in no way accuratly shows all the forces in a wing. This could go on for pages and pages to explain it but the greatest amount of shearing happens closer to the mid way between the root and tip. It happens towards the tip as well but stop increasing again as you reach the tip. NO shearing happens (if the load is "perfectly" equal on each side) at the wing root. Even if the load is not prefectly equal there will be a point close to the center where shearing does not oocure.

I know you guys are smart and I don't mean to insult anybody but I don't believe there's any way to explain this in layman's terms. Aircraft wing forces are way to dynamic.

Mike, I realize that there is much more that comes into play here, but as you said, this is why they're called "Shear" webs.

Yes. Cool.

Maybe this is why vertical grain is used:

I think we all agree that the shear force is at 45 degrees to the wing span. However, this force can be broken down into a spanwise force and a vertical force. With the diagonal force broken into components you can see how the different grain orientations perform.

Lets look at vertical grain shear webs: Here the vertical force, tension or compression, is resisted by the vertical grain. All is OK. The spanwise force, T or C, is resisted across the grain. This is not an ideal condition. However the wood only spans from chord to chord so this isn't so much of an issue.

Now for spanwise shear webs: The spanwise component is resisted along the grain and all is good. The vertical componenet is resisted across the grain, bad things might happen here. The web spans from rib to rib, a much longer distance than spar to spar. Also, the rib grain is oriented in the same direction as the shearweb, so this doesn't really help hold things together either. I can see where the shear web could split or buckle under load easier with the grain running in this direction.

DLBirks