Shear Webs - always vertical grain? (Full Version)

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

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



daveopam -> RE: Shear Webs - always vertical grain? (3/19/2008 11:49:28 PM)

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

David



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

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.



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

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)



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

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



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

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



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


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



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

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.



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

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



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

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.



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


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



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

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.



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


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



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

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






Rcpilot -> RE: Shear Webs - always vertical grain? (3/20/2008 10:40:39 PM)


quote:

ORIGINAL: mmattockx


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


I'm not arguing with you.

But what you are saying goes against all conventional wisdom and in-depth discussions on airframe and wing design. Everyone that I've ever talked to about wings has used terms like "collapse" and "pushing together" when discussing wing/spar failures.

Nobody has ever mentioned "longitudinal movement" of the spars in my discussions about wing failures.

Could you provide a link or some professional documentation to back up your statements?



CrateCruncher -> RE: Shear Webs - always vertical grain? (3/21/2008 1:56:21 AM)

You guys are both right. Webs prevent both modes of failure. I'm a mechanical engineer too. The shear webs prevent the wing buckling though compression AND increase wing stiffness by preventing the spars from moving longitudinally in shear. Its easy to test this shear mode idea next time you build a D-Type wing. Make a mark on the upper and lower spar before adding the webs. Twist the wing and the marks no longer line up. After installing the webs the spars are unable to move longitudinally and the wing is much stiffer. This is the anti-shear aspect of the web's function and the reason they are called "shear webs".

What surprises me is that so many airplane designers rely on the D-Wing cross-section in the first place. Why? A fully sheeted wing is much stronger (even without spar webs) and builds faster. After adding glue, webs, cap strips theres no weight advantage. I don't get it. Why are we doing all this tedious spar reinforcement to compensate for a weak open-bay wing that takes longer to build? I read somewhere that 40+ years of conventional wisdom among designers is that most builders find the fully sheeted wing is more difficult to build... Any opinions out there from folks that have built both? (Scale doesn't count! We all do crazy things for scale.)



khodges -> RE: Shear Webs - always vertical grain? (3/21/2008 2:10:48 AM)

quote:

ORIGINAL: mmattockx


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


I have to go along with Bill on this issue (of centering the web). I am not an engineer, but the article on Paul's website specifically addresses why the centered web is stronger and he refutes the reasoning that more glue surface on the lapped webs increases strength. Also, if you look at a house built with composite beams (usually 2x4's with a ply web), the web is always centered in a groove routed in the face of each 2x4. It would be much more cost effective in building those beams to lap the web (C-beam) than to route a groove and assemble them with a centered web, but the gained strength is worth the extra millwork (and cost).

I prefer to over-engineer something when I build it because I don't have the knowledge to figure out how much I need, but can usually recognize when there is more than enough. My wing spars are always box-beams and my shear webs will be anything from balsa for the smaller spans to 1/32 ply for the big ones. The box beams give additional strength and rigidity in forward-aft loads as well as greater torsional rigidity (twist). This comes in handy when we ground loop, catch a wingtip or otherwise apply loads other than the primary design of the wing to support the plane in flight.

I found the link in the article (beams used in aircraft....) regarding the diagonal application of the face grain of ply used for shear webbing to be very useful, and will adapt my construction to utilize this.



CrateCruncher -> RE: Shear Webs - always vertical grain? (3/21/2008 2:27:33 AM)

Here's a link to horizontal shear stress discussion in the context of basic beam theory. I couldn't find anything free on the web that was model airplane wing spar specific:http://physics.uwstout.edu/StatStr/statics/Beams/bdsn49.htm#Shear%20Stress-%20Example%20IV

Note that this discussion is only about shear under compressive loads not torsion. The link explains that the vertical shear and horizontal shear are equal in compression resulting in a combined diagonal (45 degrees) shear plane. If this were the only reason for the webs, grain orientation wouldn't matter. In the real world, wings are subject to both torsional and compression loads which puts compressive stress in the vertical plane and much more shear stress in the horizontal plane. Thus balsa grain should be oriented vertically for maximum shear and compressive strength.



mmattockx -> RE: Shear Webs - always vertical grain? (3/21/2008 6:24:08 AM)


quote:

ORIGINAL: CrateCruncher

Here's a link to horizontal shear stress discussion in the context of basic beam theory. I couldn't find anything free on the web that was model airplane wing spar specific:http://physics.uwstout.edu/StatStr/statics/Beams/bdsn49.htm#Shear%20Stress-%20Example%20IV


Thanks, CC. I hadn't considered local buckling and the webs supporting that failure mode in the description of the spars "moving together".

khodges, I wasn't really saying that lapping the webs was stronger (or I didn't mean to, anyway), just that it is easier to get a good glue joint that way. I agree the web in the center is stronger, but not significantly so for our purposes. It takes a lot more work and is harder to get sound glue joints with. All reasons why it is perfectly acceptable to lap the webs on the front and/or rear faces of the spars. If you want to go to the trouble of grooving the spars and fitting webs, feel free. It shows good workmanship and attention to detail. I just don't have that many R/C hours in my life to bother, that's all.


Mark



MinnFlyer -> RE: Shear Webs - always vertical grain? (3/21/2008 2:40:36 PM)

Alex has the right idea, but the wrong direction.

As air pressure tries to lift the wing, the weight of the fuse holds it down in the center. So the tips will try to move upward.

This will create a horizontal shear force between the spars.

If the grain in the shear webs runs horizontally, they can split which would render them useless and probably lead to a catastrophic failure of the wing.



blw -> RE: Shear Webs - always vertical grain? (3/21/2008 3:47:36 PM)

This is also the reason for sheeting foam wings. The thin balsa has strength along the span to prevent shearing forces. It is also why fiberglassing the center section is necessary. The top layer prevents the shrinking movements, and the bottom patch prevents stretching.



CrateCruncher -> RE: Shear Webs - always vertical grain? (3/21/2008 4:46:44 PM)

As I asked in post#8, why are we doing all this fiddly rib webbing and cap stripping anyway? Unless its a very thin wing or for scale authenticity, open bay wings are weaker than a fully sheeted wing, more time consuming to build, and only marginally lighter. I recently assembled skins for a wing I'm building. I edge-glued 7 sheets of 3/32" and cut it at a diagonal - rinse and repeat and I have a fully sheeted wing.



Ed Cregger -> RE: Shear Webs - always vertical grain? (3/21/2008 5:32:31 PM)

Shear webs, in order to keep the two spars from trying to come together, must always be installed with the grain running vertical.

To prove this to yourself make a shear web with the grain running vertically and one with the grain running horizontally (thin balsa sheeting please) and then squeeze them between your fingers. The strength of the vertical grain then becomes easily apparent. Also place the shear web pieces BETWEEN the spars, not scabbed to the front or back of the spars.


Ed Cregger



mmattockx -> RE: Shear Webs - always vertical grain? (3/21/2008 5:50:16 PM)

Ed, you should read through the about 20 or so posts, we have covered it pretty well.

Mark



MinnFlyer -> RE: Shear Webs - always vertical grain? (3/21/2008 5:53:53 PM)

But you guys are missing the point...

Shear webs are not there to keep the spars from coming together, they are there to keep the spars from SHEARING - that is, sliding in opposite directions.



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