shear web grain orientation

Discussion in 'Wood Construction' started by horchoha, Sep 28, 2007.

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  1. Sep 28, 2007 #1

    horchoha

    horchoha

    horchoha

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    I am about to start construction on "U" type or "C" type spars. The top of the spar is 1 1/4" x 3/4" Sitka. The bottom of the spar is 3/4" x 3/4" Sitka. I've been reading on the web that the strongest web is 45 degree grain to the spar. I plan on using 1/8" baltic birch and cutting the 5' x 5' sheet diagnally to acheive the 45 degree grain and scarffing the ply to get the length I need. Now the question - grain orientation, should the grain run from the bottom of the spar upwards toward the root (fuselage) to the top of the spar?
    This is a high wing basic ultralight 2 seat side by side aircraft with front and rear spars and front and rear struts.
    Any input will be appreciated.
     
  2. Sep 28, 2007 #2

    orion

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    I'm not sure that it matters. If you're using a ply, and assuming the ply laminate is balanced (same number of plies oriented one way as the other), then if you place the orientation on the 45, you'll have roughly half of the plies oriented as you say and the other half opposite.
     
  3. Sep 29, 2007 #3

    horchoha

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    Thanks for the reply. I have decided that I will be using a Finnish 5 ply plywood.
    Perry
     
  4. Sep 29, 2007 #4

    George Sychrovsky

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    The plies are not balanced in the typical aircraft plywood the number of plies is odd and the faces run the same direction making it a lot stiffer in one direction then the other , most people use it in the sheer web with the grain vertical because the buckling resistance is most likely the critical aspect rather then the sheer force but If you use it at 45 it should go inboard low to outboard high.
    George
     
  5. Sep 29, 2007 #5

    PTAirco

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    On the cantilever portion of the wing that is correct. But if you have a strut braced wing the direction should reverse on the inboard side of the spar and reverse again halfway between the strut and root. Too much bother really, maybe vertical is a simpler solution?
     
  6. Sep 30, 2007 #6

    horchoha

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    I've taken a piece of ply cut at 45 degrees and have played with it in both ply orientations while pushing upwards on the bootom of the ply. With the grain going from inboard low to outboard high it buckles easyly when pushed up from the bottom. When the grain orientation is outboard low to inboard high, the ply seems a lot more rigid when pushed up from the bottom.
    There is no mention of ply shear web orientation that I can find in AC 43 - 13 1B.
    Has anyone found any references in any books or manuals? There sure doesn't seem to be much concrete info on this.
    Perry
     
  7. Oct 4, 2007 #7

    dgeronimos

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    The shear plates on my revolution II/spacewalker II are vertical. The plans call for 1/8" finnish birch on the mains and 1/16" on the rear spars.
     
  8. Oct 4, 2007 #8

    orion

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    I'm quite surprised that the ply orientations aren't 0-90; that they're all along the primary "0" direction. As such, like the last poster indicated, I'd probably suggest orienting the ply vertically since the highest shear strength will be perpendicular to the grain direction.

    You can orient the webs on the 45 bias but for that configuration to work you'll have to design for the type of beam you're looking to build. Basically there are two types: The Shear Resistant Beam and the Semi-Tension Field Beam. The first resists the shear load in a rigid form through the simple shear strength of the web material, where no distortion of the web is allowed. This usually requires a slightly thicker web and/or more vertical stiffners dividing up the web into smaller sections.

    The second configuration allows for the web buckling, using the tensile strength of the material to resist the shearing motion. Although the latter does have application in certain conditions, it must be understood that the tensile field creates secondary loads that must be accounted for in the web to cap joints and attachments. Furthermore, the tension load also imposes a secondary displacement and bending load into the spar cap itself. Now you have to check for the added stress and on the compression side, also for columnar stability.

    As such, for the simple wooden structure I'd probably tend toward the shear resistant beam where the web is designed in such a way that it does not distort or buckle throughout the loading spectrum, and where its use results in a minimal amount of secondary considerations.
     
  9. Oct 5, 2007 #9

    horchoha

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    The spars will have verticle intercostals every 24", so the shear web will be divided into smaller sections.
    dgeronimos - did you cut the birch across the grain and scarff the pieces together for the shear web?
    Thanks
    Perry
     
  10. Oct 8, 2007 #10

    dgeronimos

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    Yes, I cut across the grain (4 ft sections) and scarfed with a 15 degree angle. I made a special mitre for my bench sander to get the right angle.

    Orion: I'll look at some plywood scraps and let you know what the orientations are on the plys. They do change orientation, but there is a definate difference in the strength/flexibility.
     
  11. Oct 10, 2007 #11

    horchoha

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    I've sourced some GL-2 3mm Finnsh 6 ply plywood from a local hardwood dealer. It's 50" x 50" sheets. I will be cutting it at 45 degrees, seeing that it's balanced laminate. I've made a scarffer from a router with a 2" long straight carbide bit with a bearing on the end. I've made a mount that I can screw to the side of my table at correct angle. I've scarffed my 5/64" leading edge plywood. I needed 25" wide LE sheets so I cut a 4' x 8' lengthwise, 1 piece 25" wide and the other 23" wide, I then scarffed a 3" x 8' long peice to the 23" wide peice. The scarf is 1" wide. With the router it take all of 90 seconds to scarf an 8' long peice and its perfect. The LE is 15' long so I'm using an 8' and a 7' peice. The ends of these are also scarffed.
    Anyone else building wood cap spars with shearweb? Sure would like to hear your comments on what your plans say for shear web orientation.
    Perry
     
  12. Nov 12, 2007 #12

    Tom Kay

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    Horchoha;

    I am interested in this subject as well. I wonder if you've been reading what I found on the web. Here's the kink:

    http://www.auf.asn.au/scratchbuilder/beams.html

    The picture shows the shear web being oriented at 45 degrees off vertical, and this makes it 90 degrees to the shear buckling "bumps." So you'd call it from the bottom, inward toward the root.

    I want to make sure i'm interpreting this drawing correctly. I assume the root of the wing spar is on the left, and the letter "s" is a force upward. So far, so good??? Tell me if I've inhaled that wrong, please somebody.

    This is based on plywood with an odd number of laminations.

    Did you read a different website? I'm trying to find all I can on this subject.

    Cheers, Tom.
     

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  13. Nov 12, 2007 #13

    orion

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    As far as I can see, yes, you are interpreting the picture correctly. The 45 deg orientation that is pictured works well since the greater number of plies will be working against the positive G case and the fewer for the negative. Since you'll generally see much lower negative Gs, having the fewer plies oriented in this way is the optimal configuration for the given load case.
     
  14. Nov 12, 2007 #14

    George Sychrovsky

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    Your interpretation of the picture is correct but what the picture shows is that some people got backwards, the little buckling distortions follow the orientation of the stress that’s why the grain should follow it also. If you put the grain the other way you will delay or eliminate those buckles however the plywood will fail sooner under the sheer load . Unless your spar cap buckles first anyway in which case you have another problem. Because what may seem like a sheer web failure could really be a cap buckling failure
    Clear as mud ???
    Or maybe I’m the one who got it wrong in which case you better make some samples and test it yourself .
    I still say forget about the whole thing and just install it vertical because stabilizing the spar cap against buckling is more important
    George
     
    Last edited: Nov 12, 2007
  15. Nov 12, 2007 #15

    Tom Kay

    Tom Kay

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    Hi George;

    I'd like to go through something, an analogy. This is a technique us simpletons use to grasp a concept when it may not be as clear as hoped. And please don't take offense, I really AM referring to me.

    I think you may still want to have the face plies (of plywood with an odd number of plies) oriented from the bottom spar cap up and inward toward the root of the wing, at 45 degrees, or maybe less if the shear bumps aren't at 45 degrees. I think you mentioned that the plies would fail sooner under the shear load in this orientation. I don't see this. As long as you don't overload the wing for a given design, wouldn't this orientation help the shear web not fail? I mean, it would take more force to fail than if you had the majority of the plies rotated 45 (now vertical) or especially 90 degrees facing tipward at the top spar cap?

    Here's the anaolgy part. Let's think of the plywood, say a 3-ply wood, as either uni-direction carbon fiber fabric, or even as a bamboo mat. All the cords (or grains) are lined up lying next to each other, and bonded together with something as simple as resin, glue, or a zillion threads for our bamboo mat. Rolling up the bamboo mat 90 degrees to the cords is no problem, in fact, it was made to be rolled up after you've finished your power-yoga class. No strength in that direction at all. Now, try rolling up the bamboo in the short direction. You can't, because you can't overcome the strength of each bamboo cord (unless your power-yoga class was more effective than anyone would have guessed).

    If you load a wing with positive g's, it bends upward at the tip, and tries to create those little buckling bumps, due to shear forces in the web. If you don't want that to happen, wouldn't it make sense to put all the strength you can working against the little buckling bumps being formed by shear force in the first place? If the bumps are 4 inches long, by an inch wide, I'd orient the majority of the plywood grains (in an odd-plied plywood) 90 degrees to the longitudinal axis of each bump.

    I must confess I did not think of this. I simply found it on a website from which I grabbed the picture in post #12 (above). I would have thought that it would be a ton of extra work to cut/scarf/glue all the short pieces to make one long shear web. I also might have thought that these pieces would be weaker as many short pieces than the original single long piece with grains facing spanwise or vertically. (maybe spanwise would be close to the worst possible orientation??) But that's why it's good to read as much as possible, I suppose.

    The nagging question that remains for me: Is there enough benefit, in terms of added anti-shearweb buckling strength to warrant this work? As you mentioned George, maybe a few tests would be good. Couldn't hurt!

    So, reactions y'all? I'm in the learning phase too !

    Tom.
     
  16. Nov 12, 2007 #16

    Tom Kay

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    A little addition to my last reply;

    I am building a mockup of a 75% Spitfire fuselage, a Mk XIV lowback, in hopes that I can gain enough confidence and answer enough questions to go ahead and build one to fly. I've learned a surprising amount already. Man, I didn't know much !

    Anyhoo, I bought cheap wood at Home Depot to build the fuse frames and longerons. One of the supplies I bought was 1/4 inch 3-ply wood. I don't know exactly what type it is, but 2 plies run in the long direction, and one ply runs 90 degrees to that. It is very surprising to see the strength difference from one direction compared to the other direction. If it were a full sheet, it feels like I could almost roll it up in the 4 foot direction to make a tube, while being unable to bend it much at all in the 8 foot direction. And this is only 3 plies.

    So, if a shear web were made out of similar material, I can now see that it could matter a lot whether you made the effort to orient the grain to purposely resist shear buckling. Again, it would be good to make spar models and torture them with loads to see how it acts.

    Tom.
     
  17. Nov 12, 2007 #17

    PTAirco

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    oops, double post
     
    Last edited: Nov 12, 2007
  18. Nov 12, 2007 #18

    PTAirco

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    I am looking at some grainy (pardon the pun) pictures of the main spars of an Airspeed Envoy here: The grain of the ply webs runs 45 degrees outwards and upwards from the wing root.

    If it's good enough for Airspeed and Mr Shute, it's ok by me. They knew what they were doing back then.

    I visualise it as if the web was a Wagner beam and the ply formed a tension field - you would want the grain to run with the lines of tension which run 45 degrees out and up.
     
  19. Nov 12, 2007 #19

    George Sychrovsky

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    I have plans for a German wooden glider ULF-1 it has most of the sheer web single plywood grain vertical except the root portion where it is doubled (both sides of the spar cap) it runs 45 degrees outwards and upwards from the wing root. We now have evidence of two real airplanes being made this way which would tend to support PTAirco and my analogy.
    Opinions of others may still be different. Definitely an interesting subject.
    George
     
  20. Nov 12, 2007 #20

    Tom Kay

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    Hmmm;

    I'm being ganged up on. I hate that when you use knowledge and evidence to combat my guesswork.

    It would be nice to have the best answer on this, from a strength/longevity/safety point of view.

    Thanks for the info guys. I'll keep snooping on this one.

    Tom.
     
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