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Thread: Wood Spar question

  1. #1
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    Wood Spar question

    The airplane I'm planning on building uses two wood caps to form the spar. Instead of using single "planks" for the top and bottom, I was planning to use wood strips glued together to form the caps. Example if the cap is 6 inches wide and 1 inch thick, I would glue 6 1x1 inch strips to form a cap. Now when I was in A&P school, my teachers taught us that a laminated spar was better than using a solid piece of wood, for this reason that is why I want to use a laminated spar.

    Question, if the spar cap design calls out a fixed deminsion can I use a series of strips to accomendate the design requirements, as long as it is the same type of wood and the same physical dimensions?

    Next question, should the strips be (6 inch by 1 inch spar size) 6 1x1 inch strips or.....4 1/4 x 6 inch strips?

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    Ok the reason the laminated wood is stronger is that you are mixing grain structures, althou because aircraft grade wood has a very restrictive grain runoff spec, it still has grains running off the edge, the laminating process, from diferent boards, counterbalances the various grain runout or unseen wood defects.

    well, how about 6 1x1 with a clamp every 4 or so inches

    i like to cover my bench with a scrap piece of flooring to catch the glue squeezeout and apply the glue with a small foam paint roller.

    enjoy the build

    dust
    maker of foam, fiberglass and wood dust and shavings, one day a plane will pop out

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    I'm never one to claim that I'm an expert, but for what it's worth, I agree with Dust--6 1x1s is probably the way to go.

    Not only is it more practical to layup and easier (and cheaper) to find material for that configuration, but I think that vertical glue lines would be beneficial for a spar cap. Even though it will be built up into a beam (either an I or box, I assume), the caps will still be bending about their wide axis when everything is loaded up. So, imagine what the laminations would do if there was no glue between them: the wide, flat 1/4" laminations would slide relative to one another and would deflect with little effort. Thus, to gain the stiffness of a solid wood spar cap, all that relative sliding has to be restrained entirely by the adhesive. That is, the glue lines are directly loaded in shear as the laminated beam is loaded.

    That's not necessarily a terrible situation, especially with good modern adhesives, but if your mind's eye now considers the beam composed of 1x1s, again without any glue, you can probably see that now all the bending load can be taken directly by the wood itself; the glue just needs to hold the strips together side-to-side. Make no mistake, the glue line will still be loaded in various ways--a little stretching on the tension side of the beam, a little compression on the other side of the beam, and some shear load as the cap flanges try to buckle, but those are all secondary loadings. To me, this seems a more fail-safe way to orient the laminations. Especially in a box beam, where the edges of the cap are restrained by the shear web.

    Of course, this all my non-expert opinion. If I'm out to lunch, I'd love to be corrected.

    Regards

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    Super Moderator orion's Avatar
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    Ivan, you're right on!

    The laminations of each cap assembly need to be oriented in such a way that the glue-lines are oriented vertically (parallel to the shear web[s]).

    In making up such a spar cap, I would actually recommend making the laminate pieces smaller then 1" wide though as this will allow you to do a better job of controlling the quality of the runout characteristics. For the sake of this discussion, you may want to take a look at keeping the 1" height for the cap but make the strips only a half inch or a quarter inch wide.

    Then. as you assemble each cap, keep a close eye at the points in the wood where the grain runs out and orient the pieces in such a way that you minimize the runout of any combination of strips, in one location. This overlapping will create a much more superior spar cap than what you would see with just a solid piece of wood.

    From the wood spars I've seen, the wood is usually of such quality that the runout is usually in one general location, as opposed to along the entire length. If you do have this type of runout, position it as far outboard from the root as possible, to where the bending loads are lower.

    If your wood tends to have some level of runout grains along the entire length, then make sure to orient the strips in such a way so as to minimize the chance of making a weak point.

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    also, when doing this glue up i would go to 1/14 thick and then plane it smooth and straight to the 1", just my furniture hobby thinking.

    when you glue it up be extra careful to get it flat and straight because if you introduce a curve in the glue up you have what we call "laminate bending" and you can make it into a pretzel if you would like

    You can also glue up taperred pieces to end up with a taperred spar


    enjoy the build

    dust
    maker of foam, fiberglass and wood dust and shavings, one day a plane will pop out

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    orion said
    the glue-lines are oriented vertically (parallel to the shear web[s]).

    Why would you care, from a structural standpoint, a proper glue joint is stronger than the surounding wood.

    That is why i would go the 1 inch route, easier to control the quality of the joint during glue up and the 1x1's would be easier to control from a grain orientation standpoint, but from a glue standpoint what gives

    enjoy the build

    dust
    maker of foam, fiberglass and wood dust and shavings, one day a plane will pop out

  7. #7
    Super Moderator orion's Avatar
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    By gluing the layers horizontally, you depend on only the top piece of wood to provide you with the primary load carrying ability. Remember, in bending, the highest stressed part of the beam are the outer-most fiber(s). If the outermost piece of wood gets damaged or has an area of less than ideal properties, then it automatically becomes the weak link in the laminate. The layers of wood underneath it will not reinforce it since the damage will propegate from the outer fibers, inward.

    Furthermore, if for some reason it starts to fail in any way, the crack it may form will become a stress riser in the cap laminate and will substantially degrade the rest of the laminate.

    By placing the laminates vertically, you are spreading the load over many pieces of wood and thus the degredation of any one piece is not going to degrade the surrounding pieces.

    You are right in that the glue joint may be stronger than the surrounding wood, but in general, you don't want to be depending on the glue to keep you airborne. Also, depending on the glue you use, it may be stronger in shear but it may not be stronger in tension.

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    Super Moderator Midniteoyl's Avatar
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    Thus the reason load bearing beams are made with vertically oriented lumber..
    Jim

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    Express 2000FT (hopeful - Again)

  9. #9
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    "ger rev ma agut" thank you in irish gailek, all phoenitically spelled.

    I don't fly the furniture i design and i don't design the airplanes i build. he he he

    In furniture, except in laminations and edge glueups, i design all joinery to take the loads and the glue to just hold it in position. And i always look for a method of using column strength, where possible


    enjoy the build

    dust
    maker of foam, fiberglass and wood dust and shavings, one day a plane will pop out

  10. #10
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    Re: Wood Spar question

    i know this is an old topic but my question belongs here.

    As stated above the spar laminations need to be oriented with the glue lines vertical. Does this mean that the spar constructed for a kr2s (pictured) is incorrect?



    The only concern i would have with the vertical glue lines is the box spar splitting in half under a load (there is no continuous piece of wood from side to side)

    Would having three pieces of wood with the two vertical lined pieces on the out side with a piece of wood glued horizontally on the inboard side of the spar be a better design?

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    Re: Wood Spar question

    From that photo it looks like one of the four pieces is flat sawn and the other three are quarter sawn. As the moisture content of the wood varies those two pieces with the growth rings 90 degrees off will predominantly expand and contract in different directions. I don't know if that will cause a problem but I would be extra carefull in sealing the wood to minimize that effect.

    I'm not convinced that the horizontal glue line is a problem but I'm usually wrong. Do the KR2 plans specify grain orientation and how to orient the laminate? I think ANC-18 gives guidance on this if the plans don't.

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    Re: Wood Spar question

    i have not finshed reading anc-18, its sure a sleeper. haha

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    Re: Wood Spar question

    I just looked at ANC-19 and I think it indicates that for laminations with horizontal glue lines like you have in the photo that the pieces should be flat sawn. All four pieces should have the growth rings oriented horizontally like the piece at the left side of your photo. I don't know how important that would be as it doesn't offer an explaination.

    Just curious--did you make the spars yourself or did you take over someone elses project?

  14. #14
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    Re: Wood Spar question

    Quote Originally Posted by davidb View Post
    I just looked at ANC-19 and I think it indicates that for laminations with horizontal glue lines like you have in the photo that the pieces should be flat sawn. All four pieces should have the growth rings oriented horizontally like the piece at the left side of your photo. I don't know how important that would be as it doesn't offer an explaination.

    Just curious--did you make the spars yourself or did you take over someone elses project?
    Picture is off of Mark Langford's website, i believe there are his spars.

  15. #15
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    Re: Wood Spar question

    so are the horizontal glue lines ok?

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