Comparing shear strain in a solid web and diagonal structural elements that replace a solid web of a bending beam is confusing for most of us.
A solid web of a bending beam supports combined forces of tension, shear and compression, each of which has a different distribution and magnitude. Shear forces created by the pure bending are zero at the spar and maximum at the middle or neutral line and their direction is longitudinal. Shear forces created by lift of the wing run perpendicular to the web and are more or less uniformly distributed.
The elements of a bending truss can only support forces of tension and compression.
As we all know, solid webs and Warren type trusses have been used for the structure of fuselages as much as for wings.
Please check the following links:

http://en.wikipedia.org/wiki/Shear_stress
http://en.wikipedia.org/wiki/Shear_strain
http://en.wikipedia.org/wiki/Truss_b...polar.29_truss

Warren truss
The Warren truss was patented in 1848 by its designers James Warren and Willoughby Theobald Monzani, and consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. Loads on the diagonals alternate between compression and tension (approaching the center), with no vertical elements, while elements near the center must support both tension and compression in response to live loads. This configuration combines strength with economy of materials and can therefore be relatively light. It is an improvement over the Neville truss which uses a spacing configuration of isosceles triangles.

Use in full scale airplanes:
http://www.americanflyers.net/aviati.../chapter_1.htm
http://nptel.iitm.ac.in/courses/Webc...20and%2029.htm

I, too, have mulled over this knotty question, doing a fair bit of inconclusive stress analysis. Several years ago, I decided that the only way to resolve it was to build 15 to 20 webbed specimen spars and test them to destruction. The test spars had hard balsa flanges (top and bottom longitudinal members), with various designs of balsa webs. Length was about 20 times depth.

The commonly used balsa shear webs with grain oriented vertically failed miserably in the webs, at about 25% of the calculated load. (Very distressing, but model design surely keeps one humble, if nothing else.) The load was applied in the middle of the spar, and each end supported, since the ideal uniformly distributed load would be much trickier to achieve experimentally.

Web failure always initiated near the ends of the spars, where the shear deflection was greatest, proceeding inward. With web grain at 45 degrees, the webs were about twice as strong for the same weight. The compression flanges of the spars were restrained from buckling by a continuous slot into which the flange was fitted.

Best arrangement proved to be ply balsa webs, with grain direction crisscrossed, as in normal plywood, and 45 degrees to the longitudinal axis of the spar. For the same weight, this was about four times as strong as single ply balsa webs with grain oriented vertically, but I will have to consult my notes for more definitive figures

The test fixture could be loaded uniformly with several uniformly spaced dixie cups. Similar to how Rutan tested his spars with several jugs of water.

In the case of the Warren Truss, the Wiki article did not address the vertical component that the rib provides. Granted, you guys are getting way over my head with the engineering. I'm just recalling my impression as the Tsunami wing construction was taking place. I have not done any controlled testing, and frankly, I'm surprised at how much interest there is in this issue. With that said, I did like the result of the truss style webbing but I'll continue to follow this discussion and learn what I can learn. Thanks for all the interesting discussion.

By the way, I did "crash" test the Tsunami this past weekend.... a flame-out at low altitude and low speed heading away from the runway. Tip stalled as I tried to turn back and fell 20-25 feet. Broke loose the wing hold down blocks and the servo rails and stress cracked the fuselage directly behind the wing and near the stabilizer, but absolutely no damage to the wing.

ChiefK

I'm still amazed at how so many people refuse to accept the results of many text books and studies on mechanical forces as taught in the physics and strength and materials books. Having spent years in the actual measurements and instrumentation field on real aircraft and missiles I have a hard time accepting the miss understanding of so many people on how and why cantilever beams are constructed for maximum strength with minimum weight. Do not take my word for it, check out a few physics books and strength of materials books and see what the engineering world uses. I wish I had a dollar for every strain gauge I've mounted and measurements I have made on just such applications.

I've been following this thread from the beginning. Here are some FEA models showing what is being discussed. These are very simple models covering a very complex situation. I have gone thru all this before on a number of models so it is not new. I now longer have wood for a material and I've just been too busy to dig it up again. For these models I just used 6061 aluminum. It's light and in some cases probably pretty close to what some guys build in wood. I just had to put that in haha.

these show one half of a wing. the spar is 48 inches long 2 inches tall at the fixed end and 3/4 inch tall at the outer end, 1/8 inch wall thickness all around and 1/2 inch wide. I show what happens when you put the load at the tip vs a more realistic evenly loaded spar. the load is 20 pounds so it represents a fairly heavy warbird or perhaps a light 3d plane in heavy maneuvers.

Again before you young engineers take the old engineer to task remember this is a very simple representation of a very complex situation.

Shown is the spar made of 6061 aluminum weight of .94 pounds
2 pictures show end loading stress and deflection
2 pictures show even loading stress and deflection

You will note a twisting of the spar
No account is made for ribs or skin. I would design for these being adders that don't count in the design. The spar needs to be able to take all the load as the skin may have various holes that make it not a load bearing surface.

Of particular interest is that end load shows .900 deflection while the even load shows .003.

Don't even bother to get carried away with actual numbers, this is done just for clarification of what some of the posters are talking about.

You can clearly see that picking a heavy model up by the wing tips is not a very good idea and does not represent flight loads at all.

I did another version out of 1/16 material. 1/2 the weight, a little more deflection and a little more stress. Other factors begin to enter seriously that being buckling. The ribs would help here as well as some other load bearing members. As you can see, reduce the weight...increase the complexity....square it, cube it, or power it you determing how light you want. oh yeah I forgot cost....generally "black hole" defines it. haha

Now for the text book guys I'm with you all the way. If you have a Roark's, a good calculator, a nice .7mm pencil full of lead and a couple hours you should be able to check the results. Just for reference it took me only 1/2 hour to build the model and run the FEA. Don't you just love CAD. I had to do this stuff with a GD slide rule in school.

....and now you need to calculate the bending loads with the forces distributed as they are in real life. They are not evenly-distributed along the span. You have more force at the root than at the tip, so the support structure must become stronger as you progress from tip to root. I don't know the exact distribution, but I think a plot would show something close to an elliptical curve.

....and to paraphrase Dick Hanson: "If it's light enough, any structure will do. If it's too heavy, structure is irrelevant". (heh, heh)

That's right, the forces going inboard are compounded by the leverage that the forces at the tips have.
To put it bluntly, all who think they have a "leg up" on the next guy in the engineering department should try AMA Fast Combat for a few years. Then those degrees in Engineering will have been qualified for what we're talking about here.
C'mon guys, let's see those 10 oz airframe designs that will hold up to 130 mph, 2 loops per second and an occassional meeting with the Earth.

Crate cruncher, I'm afraid it is you who fails to understand. Check out http://members.eaa.org/homebuilders/...orkBasic.,html
or Wood Design Worksheet-EAA-The Spirit of Aviation-Oshkosh,WI which shows by actual tests that the shear strength for wooden beams in nearly double for Spruce and Mahogany when the grain is at 45 Degrees to the span versus vertical or horizontal. If you still doubt that, just Google "Aircraft Shear Webbing grain orientation" and check out the some 1000 plus topics that will turn up substantiating this. The original question for this item in the forums was "What is the effect of horizontal versus vertical grain on shear webs". All valid discussions by the experts show that for maximum shear strength, the grain should be at a 45 degree angle to the span.

If we look at the way in which old-time "stick and tissue" open fuselages are made, we find a mix of vertical and "45-degree" braces...as the common structural girder.

If you remove the 45-degree braces, the structure becomes near useless for its purpose. Try it.

Seems to me that the typical model' wing comprises an infinite number of vertical braces. Now, if we applied that constructional method to a fuselage structure, we would have, in effect, a fully-sheeted fuselage side.

But, who would cut a sheet side with the grain running vertically?

Then again, the academic opinion would seem to say that the side should comprise two lamina, each with its grain oriented at 45 degrees.

I seem to recall seeing a cut-away of a (Wimpy?) wing which had its main spar braced by "45-degree" members. They used to say, "triangulate and add lightness".

Here is one more reference on how shear stresses are distributed. It is from the Massachusetts Inst. of Tech. so should be recognized as an authority by nearly all readers.
http://www.silentflight.net/images/s.../shearwebs.pdf
http://www.silentflight.net/index.ph...nt/view/74/67/