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Shear Webs - always vertical grain? - 3/19/2008 11:45:42 PM   
JamesDL


 

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Joined: 2/1/2005
From: Coram, NY, USA
Status: offline 		I'm working on my Great Planes BLT and the instructions call for shear webs with the grain running horizontal. However Paul Johnson says on his site to ALWAYS have shear webs run vertically. Is this true? Should I make the shear webs on this plane have their grain vertical?

Here is Paul's article http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/rc_aircraft_design/shear_webs_in_model_aircraft_wings.htm

Thanks!
James
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RE: Shear Webs - always vertical grain? - 3/19/2008 11:49:28 PM   
daveopam



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Status: offline 		I would! The strength in the shear webing is up and down not side to side. Maybe the instruction have a missprint.

David

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RE: Shear Webs - always vertical grain? - 3/19/2008 11:52:27 PM   
Acs_guitars



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Status: offline 		No matter what the instructions say, ALWAYS run shear webs with the grain vertical. If the grain goes horizontal it will break along the grain when under stress and negate their purpose.

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RE: Shear Webs - always vertical grain? - 3/20/2008 1:24:48 AM   
gboulton



Posts: 1862
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From: La Vergne, TN, USA
Status: offline 		Seems to me that Paul's article directly addressed this very issue:

quote:


The grain of the web is vertical because that is the direction the loads come from. I have seen designs that had webs arranged with the grain running span-wise. That is absolutely wrong and if you ever see this in a kit or on a plan, then make the webs with the grain running vertically between spars.


(Emphasis mine)

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RE: Shear Webs - always vertical grain? - 3/20/2008 1:52:27 AM   
JamesDL


 

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From: Coram, NY, USA
Status: offline 		Great, thanks for your advices - I've never deviated from the instructions in any of the kits I've built except to occasionally add gussets or reinforcement, so I just wanted to double check

James

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RE: Shear Webs - always vertical grain? - 3/20/2008 1:56:17 AM   
Stickbuilder



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Status: online 		In addition to running the grain in a vertical orientation, I also cut a groove spanwise in the spars that will be shear webbed, and actually make the spar set an I beam. It takes a whild longer, but the strength gained is worth the effort.

Bill, AMA 4720
WACO Brotherhood #1

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RE: Shear Webs - always vertical grain? - 3/20/2008 2:43:11 AM   
mmattockx


 

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quote:

ORIGINAL: Stickbuilder

In addition to running the grain in a vertical orientation, I also cut a groove spanwise in the spars that will be shear webbed, and actually make the spar set an I beam. It takes a whild longer, but the strength gained is worth the effort.

Bill, AMA 4720
WACO Brotherhood #1


Bill,

Your craftsmanship is superb (your Waco's are incredible), but I have to tell you there is no significant strength gained by doing that. There is a very small amount gained because the I-beam shape is symmetrical and the C shape is not, and this can cause a bit of warping at large deflections. This is not an issue for our aircraft because they do not fail due to elastic buckling, they fail due to tensile/compressive overload of the material. IMO, the greater gluing area that comes from lapping the shear webs onto either of the spar faces is more important than the assymmetrical shape of the C. If you want really bombproof spars, use thinner shear webs and put one on both the front and rear spar faces to make it a box cross section. Less work than slotting, symmetrical section and improved properties all around with a tiny weight gain.

Mark

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RE: Shear Webs - always vertical grain? - 3/20/2008 4:22:02 PM   
CrateCruncher



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From: Austin, TX, USA
Status: offline 		Paul Johnson's website is truly a gift to the RC community and I have never had a problem with anything mentioned there. His discussion of web compression during extreme bending loads is right-on. The top and bottom spar do want to approach one-another in order to allow the wing to fold. However, that is an extreme case and unlikely except in the case of very thin wings during high-g aerobatics, glider launches, snap rolls, etc.

I'm surprised he didn't mention the greatest benefit of shear webs in a D-Wing design with open bays - torsional stiffness! If you grab a wing panel prior to web installation by its root and tip rib it is fairly easy to twist a few degrees. After the shear webs are in place the wing is much, much harder to twist. The reason is because in order for the wing to twist the upper and lower spar MUST move slightly relative to one another in the direction of their length. This is what engineers call "shear". This is why the webs are so important and the MAIN reason why web grain should ALWAYS be oriented vertically. Thin balsa sheet has virtually no shear strength in the direction of the grain.

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RE: Shear Webs - always vertical grain? - 3/20/2008 5:36:52 PM   
JamesDL


 

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From: Coram, NY, USA
Status: offline 		I thought I'd post some pics of my completed wing half with the shear webs. It came out great except for one of the ribs is not perfectly square

This is the first project I'm doing without the use of CA. I'm using Titebond II, and even though it slows the process down a little, it makes it much more difficult to get into trouble... except for that stupid rib... I swear it was square when I pinned it....

James

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RE: Shear Webs - always vertical grain? - 3/20/2008 6:00:14 PM   
alex7403


 

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Status: offline 		the reason for the vertical grain is to make an "I" beam. all the stress and the weight is centered on the main beam.
if you make it horizontal the main beam will be easier to squash.

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RE: Shear Webs - always vertical grain? - 3/20/2008 8:55:01 PM   
mmattockx


 

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quote:

ORIGINAL: alex7403

the reason for the vertical grain is to make an "I" beam. all the stress and the weight is centered on the main beam.
if you make it horizontal the main beam will be easier to squash.


I see several people make the comment that the spars want to move closer together and this is not correct. What the spars do when the wing is subject to bending loads is move longitudinally relative to one another. This is longitudinal shear. The shear webs prevent this motion, forcing the spars to work together as a unit, greatly increasing both strength and stiffness of the wing. As noted, thin balsa has almost no shear strength along the grain direction. Thus, if we orient the grain along the spars, the shear forces will quickly rupture the webs and allow the spars to move relative to each other.

Mark

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RE: Shear Webs - always vertical grain? - 3/20/2008 9:19:14 PM   
Rcpilot


 

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Status: offline 		I'm not an expert in airframe design OR balsa properties but,

I was told that the reason for vertical grain shear webs is because balsa has tremendous compressive strength when loaded along the vertical grain. The spars ARE trying to move closer together and collapse the wing. The compressive strength of the shear webs prevents this structural collapse/failure.

Without shear webs, the only thing holding the spars apart would be the horizontal grain of the ribs. Thats not much. Horizontal grain is really weak. Without the vertical grain shear webs to keep the spars separated, the horizontal grain ribs would collapse under minimal flight loads.

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RE: Shear Webs - always vertical grain? - 3/20/2008 9:32:34 PM   
mmattockx


 

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quote:

ORIGINAL: Rcpilot

I was told that the reason for vertical grain shear webs is because balsa has tremendous compressive strength when loaded along the vertical grain. The spars ARE trying to move closer together and collapse the wing. The compressive strength of the shear webs prevents this structural collapse/failure.


They are NOT moving together, trust me, I am a mechanical engineer and I do structural design for a living. It appears that way, but they are actually moving longitudinally relative to one another. No matter, the most important thing to know is that the grain must run vertical on your shear webs or you will suffer a catastrophic wing failure.

Mark

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RE: Shear Webs - always vertical grain? - 3/20/2008 10:16:10 PM   
alex7403


 

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From: New york, NY, USA
Status: offline 		red lines: direction of the stress

black lines: direction of the grain

orange blocks: main spar

black circles: direction of the grain in the spar towards you


if you put the shear web horizontal it will be easy for the red stress to cut the profiles because of the direction of the grain in the ribs.

when the grain in the shear web is vertical think of it as collection of fibers that hold the spars together.

Alex





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