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Thread: laminating spruce for box spars

  1. #1

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    laminating spruce for box spars

    Plans for the taylor monoplane I am building call for solid spruce upper and lower members boxed in with ply to make the center spars... they no longer carry spruce thick enough anywhere, so Aircraft spruce is asking if I want to laminate two pieces together to make each member. Is 2 boards correct, why not 3 or 4? How do they need to orient the grain, and more importantly the actual lamination? If you are looking at the center spar as if it were installed, end on, then should you see two pieces on top of each other or besides each other (|| or =). Attached is a picture of the center spar that I am having to recreate, spruce dimensions are 1"+11/16" by 2.5" Thank you.
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  2. #2
    Thomas Stute's Avatar
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    The answer to your question is within the function of the spar and which part of the spar carries which load. The task of the spar is to carry bending loads and, in case of a box-spar, torsional loads. In upright flight (positive g) the bottom member is under tension whereas the bottom flange is under compression (in inverted flight its the other wa round). Thus the wood fiber/grain orientation shall be in longitudinal direction, along the spar. The cross section size of the spar member is defined by the maximum allowable stress (for wood compression strength is the dimensioning case). When properly manufactured it doesn't matter if the member is composed of more than one piece. The plywood side walls of the box spar carry shear loads, hence a fiber/grain orientation of +/- 45 is ideal.
    I hope this was of help.

  3. #3

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    The folks at Steen Aero are making laminated spar stock for the Pitts and the Skybolt. They report that their laminated spars are stronger than plain board stock spars. I recall that their web site has some photos of tests to destruction.

    The answer is that building the component using multiple plies with the joints staggered allows any imperfect grain orientations and any other common wood imperfections to be cancelled out. Since we no longer see perfect boards with grain that is absolutely straight for many feet, and with the development of the great glues that we have today, laminated spars, meaning 4, 5, 6, etc plies, is a better solution. Used to be more labor, but today the cost of a perfect spar blank far outweighs the labor cost of a multi-ply assembly.

    Talk to the folks at Steen Aero Lab.

    Best of luck,

    Wes
    N78PS

  4. #4

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    Thanks. Good stuff. But we are talking about a 34 inch long center section... should I create joints to stagger them (the boards I order should be plenty long so will have to be cut)? Isn't the solid member going to be stronger then all those scarfs, if not technically then theoretically?
    How many laminations is best per member? And how should they be cut to laminate (when looking at the picture above should it be || or =)?

    Thanks

  5. #5

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    A laminated spar will be stronger. Solid wood has changes in grain direction and enclosed defects by its nature. Cutting it into laminations on a band saw, randomizing the plies, scarfing and gluing them back together, makes an overall stronger assembly.

    You should look up the USDA Forest Products Lab's reports or get one of Prof. Hoadley's books on wood. Like it or not, you are signing up to be a woodworker and you need to study the subject.

    Oh, The wood wing hanging on the wall of my hangar has the main spar caps constructed out of 1/4" laminations.

    Best of luck,

    Wes
    Last edited by WLIU; 04-16-2012 at 05:48 AM.

  6. #6
    Clarke Tate's Avatar
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    The odds of finding a decent 2 1/2" piece of aircraft grade wood are close to zero. I had quite a time finding a 2" thickness that was nearly 3" deep and I cut the first 7' center section spar only to find a defect on the last cut requiring it to be discarded. I was fortunate to have an exceptional 2" plus piece stored.

    That 34" length center section should not be a problem laminating and having no scarfs. Cut 1/4 inch laminations oriented with the grain parallel to the chord or alternating angles with a max deviation of 45 degrees. This will allow verification that no defects exist on such a thick spar. You're fortunate to have such a short center section spar. No scarfing is necessary!

  7. #7

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    So in the image above (which shows the main spruce members to be on the left and right, which would correspond to the upper and lower member) should the laminations be ll or = and how many? The gentleman at steen aero lab says = and four is best. It just seems odd to orient the cuts that way.

  8. #8

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    Quote Originally Posted by idamon View Post
    So in the image above (which shows the main spruce members to be on the left and right, which would correspond to the upper and lower member) should the laminations be ll or = and how many? The gentleman at steen aero lab says = and four is best. It just seems odd to orient the cuts that way.
    An old FPL report that says laminations can be horizontal or vertical with no significant change in strength. I can see some would perfer and recommend a horizontal (= ) glue line.....less chance of a void in the glue joint, less glue, easier to jig and clamp.

  9. #9

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    Horizontal laminations are easier to inspect at annual time for glue joint deterioration. Also, if your spars have a bend in them like mine, horizontal laminations are just bent into place and clamped until the glue dries. Vertical laminations would be much more of a pain to cut to shape and scarf in staggered locations. The wood wing assembly that I have is one piece, 20' from tip to tip, with beefy laminated upper and lower spar caps. The wing is curved into a gull shape. The thickness of the laminations is partly chosen to make the bending easier. If you are making straight assemblies, you can choose a different thickness and achieve the same strength.

    So the folks at Steen know what they are talking about. The technology is old and not close to rocket science.

    As a side commentary, those folks who think that you have to have a formal engineering education and access to lots of CAD tools to design an airplane should look up the story of Neal Loving. His little wooden airplane was designed without the benefit of any of that went 200mph at the Cleveland Air Races. He flew it to Detroit to Jamaica and back. His airplane is in the EAA museum. Mr. Loving is also an example of the inclusiveness of the EAA. He became the first minority member in 1955 or so.

    Best of luck,

    Wes
    N78PS

  10. #10
    highflyer's Avatar
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    Hi Wes,

    The Loving's Love is a delightful airplane. One of the first homebuilts I helped work on, back in the late fifties, was George Rattray's Loving's Love. We laminated the spar with horizontal laminations. The laminations were planed thin enough to follow the bends. We built a table from 3/4 inch plywood and bolted blocks to it to form the curve to laminate against. Gluing and squeezing all those laminations within the pot life of the glue was a lot of messy fun!

    From all of my friends with any considerable experience with aircraft woodwork, a laminated spar is preferred to a solid wood one. On a long lamination scarf joints do not either help or hurt the spar strength. Just don't stack them up one over the other. Don't use junk glue and watch your open time for working on the laminations. Be prepared ahead of time with all the clamps you will need. I like to laminate over a well waxed base with lots of blocks to hold the laminations in place. Put blocks some distance away on the open side of the lamination. Then use precut and preformed cauls after the strips are in place and quickly drive a pair of wedges at each backup block, point to point. This is the quickest and least expensive clamp to put the desired pressure on your lamination. Use enough clamps and caul and enough pressure for the glue you are using. For example, Resorcinol resin glues require higher clamping pressure than T88.

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