Shear Web Grain Orientation, Please help! - RCCanada - Canada Radio Controlled Hobby Forum
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Old 12-26-2005, 02:38 PM   #1
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Shear Web Grain Orientation, Please help!

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I'm sure I remember a few years ago when I built a couple of kit planes, the instructions called for the grains of the shear webs to be up & down. Maybe this is why they're called shear webs?
Anyway, I have trouble seeing this as the right way. I think the grain of the wood should be parallel with the wing, horizontal. A couple of other modellers I talked to, agree with me.
Are there any "engineering" people who can tell me the correct way, but more importantly, WHY?
Thanks for any replies.
no such thing as too fast!
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Old 12-26-2005, 04:27 PM   #2
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"Simplicate and add lightness"
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Old 12-26-2005, 04:40 PM   #3
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Grain goes vertical! Why? Because that's how all the kit manufactuers do it.


I'm not a engineer but as you stress the wing, or the wing tips try to touch each other, you would want everything to stay together. You are going to get the most benefit from shear webs with the grain oreinted vertically, to prevent the spars from separating and more importantly you are creating a I-beam spar which is the stongest.

I suppose Ideally shear webs with equal strength in both directions would be the best, but their would be a expense in weight added and to what benefit.

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Old 12-26-2005, 07:18 PM   #4
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Deffinitely vertical.

Take a 2" piece of 1/2" thick soft to medium balsa and lay it legth wise on your workbench, the grain is running parallel to the surface of the bench.

Now take the handle of a screw driver or even your thumb and press down realy hard on the balsa. It will compress and you will dent it.

Take another 2" piece and stand it on it's end, the grain is running at 90 degrees to the surface of the bench . Now take your thumb and try and press down. Bet you will dent your finger and the balsa will remain the same length it startedat. It didn't compress.

Same idea with sheer webs as well as what Mike said about keeping the spars aligned.

You wing will be way weaker if you apply the shear webs with the grain hotizontal.
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Old 12-26-2005, 09:09 PM   #5
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I agree with Jim.

Vertical grain is the best way to do it.

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Old 12-27-2005, 12:32 AM   #6
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Simple explanation....

Wing in bending (anything in bending actually....), say +G loading...

Top spar is in compression (trying to be shortened), bottom spar in tension (trying to be extended) web between spars is loaded in shear (hence the name) .....if grain is placed longitudinally with the spars then this is loading the web along its place the grain vertically to shear across the grain.

Anyways, its easier to draw but that is it in a really small nutshell.

If you do a search on google for bending moments, neutral axis, beam bending, simple bending you will probably run across some diagrams and such which kind of explain what happens to a structure in bending....from that you will get what I mean.
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Old 12-27-2005, 08:28 AM   #7
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I have no engineering knowledge on this subject. However the explanations offered do not do it for me.

If the top of the web is in compression and the bottom in tension then the top of the web is being squeezed and the bottom pulled apart. Would it not be much easier then for a split to run right up the vertical grain, starting at the bottom, than if the grain was spanwise?

Is it not much easier to squeeze together a piece of balsa at 90deg to the grain than squeezing on either end of the piece?

Is it not easier to split a piece of balsa by pulling it apart at 90deg to the grain than by pulling on either end?

Try it with a piece of 1/16 x 3" x 3".

Years ago I used to build wings with what was then called "Eggbox" construction. The spar was full depth and interlocked with the ribs. The spar grain always ran spanwise. We never made the spar from short pieces glued together to give us a vertical grain.

Ed S
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