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Thread: Grain orientation on wooden box spars

  1. #1

    Grain orientation on wooden box spars

    Hi All,
    I am building a wing for a Serenity (formerly Warner) "Spacewalker," which uses a boxed (partial) I-beam spar on the outer wings. My question involves the selection of lumber for the built-up (three layers) spar caps at the top and bottom of the outer spars. The spar is a boxed I-beam (with three laminations of spruce for top caps and three for bottom caps) for the first two feet, and then transitions to no "I" web (but still boxed) for the remainder, with the center cap being 7 ft long and the outer caps being 10 ft long. Like so:
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    There are stabilizing blocks in the outer sections beyond the "I" web. My question is about the necessity for vertical grain in each of the spar caps. Is it alright if one of these (3/4" thick top to bottom x 3-1/4" wide) is not "vertical" (less than 45 degrees)? To me it is hard to comprehend the benefit of "vertical grain" in each laminated spar cap, when the plywood (outer) shear plates help bear the bending loads and shrinkage is somewhat better controlled by the lamination process. I've looked in the FAA Advisory Circular and in Bingelis' book for the answer but no joy. Does anyone know of an authoritive document on selecting and orienting lumber for built-up (laminated) spars of different types?

  2. #2

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    Stop. Do not pass go. Do not collect $200.

    Grain orientation is VERY important. The spar caps carry the wing's bending loads. The plywood shear webs hold the spar caps apart.

    The grain of the layers of the spar caps must be as close to the axis of the spar as possible. Every bit of angle away from the axis of the spar is reduced strength. Aviation grade spar blanks can have the grain slope a small amount and the strength values used by designers accounts for this.

    If you really really want to understand the topic, get the USDA Forest Product Laboratory Wood Handbook, or read it online.

    And don't go to Home Depot for wood for your wing.

    Best of luck,

    Wes
    N78PS

  3. #3

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    Grain direction matters in the usual Champ type spar. But a lamination is a different matter.
    I think if good wood is used, and the grain directions are alternated, a three ply lamination should be very good.
    Others may have a different view.

    ANC 18 is probably the book you need. Might be online.

  4. #4
    OK I'm on board with what BOTH of you are saying, but still need some clarification:
    Wes,
    What is the "axis of the spar?" up and down or front-to-back?
    Bill,
    I know that lamination is a different matter, but it's hard to find good info on this. I did review ANC 18 and there is a lot of good info there, but am still having trouble seeing the exact answer I am looking for (is vertical grain of the laminations critical, or tolerant as you suggest?). On the other hand I do always alternate grain directions (seems the right thing to do from both a shrinkage and strength perspective).

    By the way, I use "aircraft grade" spruce (not home depot, as indicated by the price!) but even when buying from a well known supplier (e.g. Wicks, McCormick) the grain still runs toward 45 degrees from vertical sometimes, especially as the width of the piece is increased (i.e. might be considerably less than 45 degrees near one edge but close to (or even more than) 45 degrees at the other edge). This is the issue I'm addressing at present.
    Thanks for your comments!
    John

  5. #5

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    So it sounds like the question that is asked is based in not understanding how wood grows, and how trees are cut into boards. You are likely seeing CAM 18 talk about growth rings and grain slope but have yet learned how the manner in which boards are cut from a log gets you the characteristics that you want.

    Not sure that all of that can be explained in a forum post. Prof Hoadley at the University of Massachusettes published Understanding Wood that explains more than you want to know.

    The short version is that you should look at the end of the board for the growth rings. Aircraft wood is cut from a log "quarter sawn", which means that the end of the board should show the growth rings near vertically across the thickness of the board assuming that your board is wider than it is thick. We like to harvest older trees that have a wide girth and that gives us boards where the growth rings look almost flat (long radius from a wide tree trunk). Now looking at the edge of the board, we want the grain to run straight the length of the board. Since we don't see exactly straight in nature, we accept some slope to the grain and design around it. The face of the board is another matter. If the tree trunk that the board was cut from did not grow perfectly straight up, you can see grain patterns that look like elongated ovals. This is due to the tree growing one way for a while and then back the other way but the sawyer cutting a straight board across that growth. I'll stop now...

    So read the references. Compare your boards to the specs, and cut out the parts that you need according to the plans and build instructions.

    If you are building a wood airplane, I suggest that first you build a small, all wood not plywood, cabinet for your spouse or for storing tools. Stop by a Woodcraft or Rockler store. Wood working is its own hobby. And if you want to learn enough to build a wood airplane you might discover that building some cabinetry for your spouse and your workshop, will make the project go more smoothly.

    Best of luck,

    Wes
    N78PS
    Last edited by WLIU; 05-15-2013 at 08:18 AM.

  6. #6
    This question comes out periodically , the answer is it does not matter which way the grain goes
    Disclaimer ; opinions of others will vary depending on what they’re selling.

  7. #7

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    Quote Originally Posted by bruesch38 View Post
    By the way, I use "aircraft grade" spruce (not home depot, as indicated by the price!) but even when buying from a well known supplier (e.g. Wicks, McCormick) the grain still runs toward 45 degrees from vertical sometimes, especially as the width of the piece is increased (i.e. might be considerably less than 45 degrees near one edge but close to (or even more than) 45 degrees at the other edge). This is the issue I'm addressing at present.
    Thanks for your comments!
    John, 45 degrees is still considered vertical grain. With the wide side of the board on the bench and measuring from horizontal or flat side, anything 45 degrees or greater is vertical grain. 44 degrees or less is flat grain. On a solid spar, only 2/3rds of the spar height has to be vertical grain. So for example, for a 5 inch high spar only ~3 3/8" of vertical grain is required to be acceptable as a solid spar in a certfied aircraft. Based on what you have said, I would imagine your wood is acceptable for solid spar material and should be acceptable for a box spar/laminated spar cap as well unless otherwise specified by the designer.

    I would use the vertical gain wood you have as long as it otherwise meets aircraft standards with regard to annular rings, grain runout, knots, compression wood, decay, etc.
    Last edited by martymayes; 05-15-2013 at 01:29 PM.

  8. #8
    Hi Wes, George and Marty,
    Thank you all for your helpful replies. I think we're getting somewhere!
    Wes, no problem with my skills (furniture maker since 1968, aircraft woodworker since 1996). My problem is knowledge of the design guidelines for LAMINATED spars (vs. solid) and (to a lesser extent), the terminology used when describing grain characteristics (and especially how these relate to a laminated spar instead of a solid one).
    George, thanks for your direct answer - I'll combine it with Marty's and shoot for vertical grain wherever possible.
    Thanks guys!

  9. #9

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    Good to hear that you have a handle on the topic. Communicating via the internet often is incomplete. I don't think that one aspect of building laminated wood structure has been mentioned. Cut the blank into laminations on your band saw then swap the laminations end for end when gluing up parts. That greatly increases strength by taking the defects or sub-optimal areas of wood, segmenting them, and moving those segments into locations surrounded by better wood. The Steene folks make spars that way. If you dig on the internet you can find photos and descriptions of tests of solid wood and laminated beams in testing machines, demonstrating the improved strength of the laminated structure.

    Best of luck,

    Wes
    N78PS

  10. #10

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    Hi,

    a major consideration in choosing the orientation (in any wood construction) is the way wood changes shape with changes in moisture content. Most of the dimensional change is around the circumference of the annual rings - they lengthen and shorten and effectively the ring radius changes. If the rings are tangential to the long face of a board then it will cup in one direction or the other as the moisture content changes and the ring radius changes. That also changes the board width by a significant amount. If the rings are tangential to the short edge then the long edge stays flat and the thickness varies but only by a small amount (the same % of course).

    In a box spar, if the top and bottom boom's depth were to be changing in relation to the spar web depth then they are stressing the glue bond between their edges and the ply. If the rings are perpendicular to the glue joint then there is insignificant movement in the glue line. The aim therefore is to have the rings at 90 degrees to the ply face, but of course we have to be realistic and use the best wood we have which will usually be a bit of a compromise.

    Grain orientation does affect bending stiffness, try bending a square section and you will see that it bends easier in one direction.

    I would recommend that you look up ANC-18 (Design of Wood Aircraft Structures) and ANC-19 (Wood Aircraft Inspection and Fabrication) both may be downloaded from here: http://www.lonesomebuzzards.com/cgi-...?m-1352247687/ They contain very clear explanations about the requirements for (and advantages of) laminating spars.

    regards,

    Colin

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