Cut the shearwebs and use 'em to space the ribs... no gaps...
-- Philip Rawson
Cut the shearwebs and use 'em to space the ribs... no gaps...
-- Philip Rawson
A spar may bend and break, but buckle?
Shear webs increase resistance to longitudinal bending. They also increase resistance to torsional bending.
Dr.1 Driver "There's a Hun in the sun!"
Yes, but I'll bet your YS .91 that they buckled, they didn't shear. My point is that the failure mode of "shear webs" (as they're typically implemented in model airplane construction) is actually buckling, because they're so thin. if the webs were thicker, then the most shear resistance would be afforded by running the grain of the wood perpendicular to the shear force. Running the grain perpendicular to that of the spars reduces the tendency of thin material to buckle, since the spars stiffen the sheet to prevent that.
No, A shear web has but one purpose: To keep the top and bottom spar the same distance apart.
Lets assume that we are pulling up hard. A wing can fail in two ways under load. 1) The bottom spar can stretch and snap. 2) The top spar can buckle inward (its actually being compressed), allow the wing to fold up on itself.
The second kind of failure is what shear webs prevent. It keeps the spar that is under compression from buckling. The first type of failure is cured by capping the spars with carbon or using bigger spars.
If you want torsional strength, you sheet.
I've built like that. Makes the ribs tricky to get square tho, and weak before covering. Good for fully sheeted structures tho.
"Paul McIntosh" wrote in message news:...
Exactly. They're job includes more than keeping the spar caps a fixed distance apart. Imagine the spar caps being attached to the airplane with a couple of bolts, one on each cap. Without a shear web, lift loads would force the spar upward, and their relative positions would change. The upper one would appear longer at the tip than the lower one, as the assembly would behave in the manner of a parallelogram. The shear web prevents this, and grain should be vertical to prevent shear forces splitting it along the grain. In full-scale wooden airplanes, the web grain is always vertical, unless the machine is designed for maximum strength in the positive G mode; in that case, the grain may run 45° to the vertical, with the grain sloping upward and outward toward the tips. This puts the tensile forces in the web right along the grain. My Jodel (full-scale) has a box spar. It's about eight inches deep and 12 inches front-to-back. It consists of four capstrips, one in each corner, the whole thing is covered in birch ply, and there are also diaphragms, or bulkheads, inside it to maintain the rectangular cross-section. The spar runs from tip to tip and is the only spar in the wing, as it can take all lifting, drag and torsional forces. Washout is built into it, and the landing gear mounts to it. Building the spar is about a third of the work in the entire project. The design is known to be extremely strong: a fellow once crashed one in bad weather into a stand of smaller spruce trees, and the spar broke off trees four and five inches in diameter. He rebuilt the airplane using the same spar. Note that in built-up spars, the top capstrips are usually larger, unless the design is aerobatic. The wood is weaker in compression than in tension, and a larger top member is necessary to match the strength of the lower one. If weight is an issue, a modeler should pay attention to such details and modify his dimensions to eliminate unnecessary wood.
Dan
I will disagree with that based on the fact that I built a demonstrator for shear webs. Small up and down movements of the spars at the tip translate into far greater shear movements between the spars than compressive movements.
I think we're differing in terminology here. If the shear webs you're talking about had grain running spanwise (as your said previously), and they failed because they split with the grain, then they failed in buckling. Buckling is when the deformation of the material out of the normal plane (due to stress) induces failure. Take a thin sheet of balsa, and pinch it between fingers placed on the edges parallel to the grain. When it cracks, that's buckling, not shear failure. Karate chopping a wood block is the closest example to shear failure that I can think of for a material with grain. (The grain of wood tends to convert shear forces into longitudinal/bending forces, whereas a material without grain (like metal) will truly fail in shear if the force applied perpendicularly is sufficient.) If your shear webs had fractured top-to-bottom, not end-to-end, then that would be shear failure.
I agree with John Alt, that the primary benefit of shear webs is to keep the spars in their proper configuration. Doing that involves shear forces incidentally, but mainly the force that they have to deal with is compression between the spars, and resisting the tendency of the spar in compression to get shorter, and the spar in tension to get longer.
Whatever. All I know is that when the top spar is pulling one direction and the bottom is pulling the other, you want your grain perpendicular to those forces.
Nicely put :) Vertical grain both resists the tendency of the spar in compression to buckle, and the shearforce that tends to make one spar slide past the oher, as it were.
With ribs, the forces are far more multi-directional, and arguably ply is teh best material :)
No, compression forces actually.
Compression forces are minimal in the shear web. It's subject to shear forces as the spar caps try to shift their relative positions. In any canitlevered beam, the upper and lower flanges are in tension and compression, depending on loads, and the shear web makes sure they are held in place to take those loads; tension versus compression creates a SHEARING action. The only compressive or tensile loads on the web are those created when the flange in compression tries to buckle into or out of the beam.
Dan
Which it does all teh time.
I am not saying that there is no shear force, but the majority stress is near the cantilever mounting, and its tenlsie/compressive, to resist the buckling. In terms of actual failure anyway. I grant the shearing action is needed to stiffen the beam in bending. But its the compressive strength that prevents collapse of the commpressed spar..which is otherwise the weakest link..
depends if you want stiff, or strong. Eggs are stiff, car springs are strong..
RELATIVELY??????
Thank you. This is what I've been trying to say, perhaps not very well. What are probably mis-named shear webs in modeling are doing a lot of complex stuff. Shear stress is involved, but only in a secondary way. The webs are taking the place of diagonals in a truss, meaning that when the "beam" flexes, there's a compressive stress from one corner to the one opposite diagonally, and a tensile stress between the remaining two corners. And the most likely failure mode is for the web, at least as used in modeling, to fail by buckling.
If you take a sheet of 1/16" or 3/32" thick balsa, grab onto opposite sides, and try to shear it with the grain, or across the grain, it's pretty strong (though weaker with the grain) as long as you don't bend it out of plane; while if you put it between your fingers and bow it, it'll buckle and break with very little pressure.
Well, I've had time to review the subject a little since my first post on it. What I said was that the shear webs, as they are used in our application, are mainly there to prevent buckling.
I got this idea from a book on construction of model airplanes about 20 years ago. It stated that the structures we use, with relatively thick ribs and sheeting and large dihedral braces, negate the normal purpose of the webbing. Indeed, many models where built without them not long ago. In a thick sport wing with D tube construction, the shear component in the web is minimal. The main advantage of the shear webs is crippling failure of the compressed spar in this type structure.
However, on a long thin wing, or one without sheeting or with thin ribs, the sheer force is dominant. Such as a glider, or many of the new electric designs. So, I stand corrected.
I was a little zealous in trying to correct the misconception that they are for torsion resistance. But whatever you are using them for, the grain needs to run vertical.
Charles,
Those diagonals you speak of are there only for the shear forces. They can do nothing for buckling of the top ot bottom rails. They prevent relative motion of the rails. That is exactly what happens with shear webs in a wing. Without them, there would be virtually nothing to prevent relative motion of the top and bottom spars. D-tube sheeting also performs a similar function but also absorbes more of the buckling forces.
well I was merely trying to find example of stiff things that are weak, and bendy things that don't break :-)
Its dominant down the span, to stiffen, but near the center section, is where the wing will fail due to buckling.
Yes.
The real point is they do two different things, and in both cases the grain needs to be vertical.
They stiffen the outer sectons mainly, where failure is unlikley, useing shear resistance, and the stregtthen the center section, where failure is likley, using compression resistance and tensile strength, as well.
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