Shear web theory ?

[FlyByMike]

I can't quite figure out what direction the stresses are coming from that a shear web are designed to counteract.
Anyone care to enlighten me ?
My best guess is it en easy way to build a cantilever without all the little stick built triangles. See attached pic.
-Mike

[Ollie]

The webs between the spar caps have two functions:
1. To resist the shearing forces between the spar caps.
2. To hold the spar cap on the compression side of the spar in column so that it doesn't buckle.

The best grain direction for resisting the shear forces is at 45 degrees to the length of the spar. The best grain direction for holding the compression side in column is at right angles to the length of the spar.

Most spars are not designed for the best strength to weight ratio but for ease of construction.

The most well designed spars I have seen are those of Dr. Drela. See the files section at:
http://groups.yahoo.com/group/Allegro-Lite/?yguid=108420033

[Viper Pilot]

I always thought they were to reinforce the spars.

Good question. Maybe some of our Aero. Eng. can explain.

Viper

Ollie beat me by seconds ! ! ! :cool: :cool:

[Sparky Paul]

Shear webs make the spars (plural) into a spar. Usually a C-section beam, but sometimes an I-beam. Tieing the spars together seriously stiffens the wing against bending loads.
For simplicity, we generally use the shear web with the grain vertical. Less of a hassle to cut and fit.
A "proper" spar with a diagonal shear web could probably use the web in one 45 degree direction (to the tip)on one side, and with the other web going 45 degees (to the root), as a fully enclosed beam.
Way too much work!
Bestest (in terms of ease of construction)is probably a vertically grained web fitted between the spars in an I-beam. With the web the spar width for about 1/4 the semi-span, stepping down in 1/32" or 1/16" widths out to the tip, between the ribs.
Fitted carefully with as little air-gap between the web and spar as you can do.
Depends on how much work you want to do.
Combined with a sheeted leading edge, the wing will be very stiff.
Add a similarly built-up trailing edge and the wing will be as stiff as it can possibly get.
But a spar-web-spar CAN buckle under load, with the upper spar taking a S-shape with the compressive load. Loaded too far, the ribs will shear along the chords of the ribs/grain. Then two pieces of wing flutter down, and the fuselage buries itself.
And as Ollie says, the most well designed (but labor intensive) spars are those of Dr. Drela...

[FlyByMike]

Well, I asked for it. And now I've gotta pay :D I'm goning to digest this, and chew it for a while and go see Dr. Drela.
Thanks guys.
-Mike
ps. Happy 4th !

[vintage1]

Sparky Paul is right as far as he goes.

If you want the real skinny, do a google serach on the 'Euler slender column' theory.

Basically its all about how stiff a slender column has to be before it fails through compression, rather than buckling. Obviously under tension itt just snaps anyway, no matter how floppy it is..

Or spars are all rather slender, and need to be prevented from failing by buckling: Shear webs with vertical grain do this. Then they can fail in the other plane, but leading edge sheeting stops this.

Which is why a full 'D' box is about as good as it gets in terms of stiffness AND strength. If that goes, its because the tensile or compressive strength of the wood has been exceeded, and there is no clever way to improve that oher than making the darn thing just BIGGER.

[Ollie]

If the airfoil has more than about two percent mean line camber it can generate more than twice as much lift upright as inverted. Also, wood is about twice as strong in tension as in compression. In such a case, the upper spar cap should have twice as much crossection as the lower spar cap. This will result in about a 25% better strength to weight ratio than a symmetrical spar. The bending loads are maximum at the centerline and decrease to zero at the tips along a third degree curve. By linearly tapering the spar caps in width and thickness, 2/3 of the weight of the spar caps can be saved while matching the strength more closely to the load along the span. Ramin can be substituted for spruce for an 80% increase in strength and only a 60% increase in weight. Thus, at the cost of some labor and inconvenience, the strength to weight ratio of wood spars can be increased by about three times. Much greater strength to weight ratio improvements are achievable with composite materials and rational design.

[markdrela]

Originally posted by FlyByMike
I can't quite figure out what direction the stresses are coming from that a shear web are designed to counteract.
Anyone care to enlighten me ?
My best guess is it en easy way to build a cantilever without all the little stick built triangles. See attached pic.
-Mike

You answered your own question. A shear web's main function is the same as the diagonal members in a truss. Both serve to keep the longerons, or sparcaps, from sliding past each other under load. The difference is that the diagonals restrain the sparcaps only at the joints, while a shear web restrains the sparcaps all along their length.

As Ollie mentioned, a secondary function of a continuous shear web is to keep the compression-side sparcap from buckling.

Here's what will likely happen if you remove the shear webs from a typical spar:
As you load the spar with wing lift, the now-empty rectangles between the sparcaps and the ribs would deform into parallelograms as the caps slide past each other. This parallelogram deformation will put large tearing loads on the sparcap/rib joints. Eventually these joints will crack, the sparcaps will pop free, and the whole structure will splinter. It's also possible that the top sparcap will buckle between the ribs before the sparcap/rib joints crack. A shear web will prevent either failure, and allow a much greater load to be reached.

[FlyByMike]

OK, I think I've got it. Mark's answer finally let me put it all together in my mind. Someday I hope to be good enough where I'll really depend on these fine points of construction. For now, I think I have a good feel for why and how a designer is taking care of these forces and hopefully a good feel for what a wing may withstand. :D
-Mike