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Thread: Grain orientation on wooden box spars

  1. #11
    Grain orientation in box spars is, and has been, a hot-button topic since Orville and Wilbur. Many years ago, the plans vendor for Piel aircraft designs in North America, Gene Littner (an aircraft engineer and designer specializing in wood airframes) maintained that spar caps need to be built from flat grain rather than vertical grain laminations. Here is an excerpt from his Emeraude newsletter . . .



    This statement sparked a controversy which lasted for months, until Gene finally refused to discuss it further. I built mine with VG orientation because that's the way my rough spruce was sawn.

  2. #12
    Mike Switzer's Avatar
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    Ok, how about if I throw another log on the fire. It has been a while since I did anything with the THEORY of wooden structures, because in the real world we use span & load tables for 99% of all wood structural design, and if that doesn't cover it I get a structural engineer to do the job.

    I got to thinking, all #1 Southern yellow pine (pretty much the standard for structural work) is cut with more of an edge grain/vertical grain than anything else. I am attaching a pic of a piece left over from construction of a large header. Granted this is not cut to the same standards as aircraft wood, but you can see the grain is mostly vertical.

    I am also attaching the first section of the Wood Structural Design Data manual. Read & discuss.
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    Last edited by Mike Switzer; 05-17-2013 at 05:57 PM.

  3. #13

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    Vertical grain is liable to split easier than flat grain. So I agree with Clark's drawing # 2, the VG box spar could split lengthwise from a torsion force.
    For these reasons, a laminated cap of three layers to prevent splitting, should be excellent.

    P.S. Mike mentioned end grain. End grain is only use for balsa core, as far as I know.

  4. #14
    Mike Switzer's Avatar
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    Quote Originally Posted by Bill Berson View Post
    P.S. Mike mentioned end grain. End grain is only use for balsa core, as far as I know.
    OOps. Meant edge grain. Editing..

  5. #15
    Mike Switzer's Avatar
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    I suspect that this may be a case where the correct answer depends entirely on how the spar is loaded - do you need the greater strength in the direction of thrust, or the direction of lift?

  6. #16

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    If you make your spar caps or spars out of single planks, then you worry about movement with changes in humidity and things like splitting. Now I will point out that the widths of materials used in aircraft actually are narrow enough so those issues do not exist for practical purposes. Cabinetmakers worry about wood movement because we make wide panels or table tops. Movement and stress is a factor of width. A 1" wide spar cap is unlikely to move enough with humidity changes to cause glue joint problems.

    Now you solve a whole bunch of problems when you build up your part using 1/4" thick laminations. I have a wood spar hanging on my hangar wall that is massive, built up from 1/4 laminations. I believe that it is 40 years old and the original glue shows no signs of distress anywhere.

    I would not worry about these issues in a wood spar of the dimensions that we use in Pitts, Skybolts, etc. A 1" x 1" box spar cap is not large enough in cross section for the vertical or flat orientation of the grain to make a practical difference in strength. Theoretically there is a difference, but not really in the real world. The NTSB reports are not full of Piels that fell out of the sky.

    Now a box spar is designed to try to load the spar caps in pure tension or compression. So the stress and strain is intended to be along the length of the spar caps. So vertical or flat, the internal stresses of the spar caps should be lower than what is required to cause catastrophic shearing along the grain.

    On the topic of Southern Yellow Pine, I installed an doubled LVL header when I turned a doorway between my kitchen and dining room into an 8' wide opening at the request of my wonderful wife. Interestingly, LVLs are composed of a large number of vertical laminations. Very strong. Very hard to drive a nail through, even with the air pressure turned all of the way up on my framing nailer. Go to your lumber yard and get a brochure that includes the span tables for the things. Compare those span tables to the building code span tables for dimensional lumber. You'll be a laminated beam convert.

    Best of luck,

    Wes
    N78PS

  7. #17
    Mike Switzer's Avatar
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    Wes - over a certain length it gets to the point where LVLs are more cost effective (or just plain easier to use, especially if you don't have a lot of room). I have the tables around somewhere but when that sort of work comes up I'm usually working with an architect that does the structural work.

    Personally, I prefer steel I beams, but then i have a lot of time doing industrial work. I am planning on making my spars out of welded steel tube (the inner sections, anyway) so I hadn't given this discussion a lot of thought.

  8. #18
    Hiperbiper's Avatar
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    AC 43.13b has a great section on wood aircraft components, wood substituions and grain direction, grain pitch and general woodworking procedures. No need to re-invent the wheel! Most of the info has been around since the Spam-Can fleet was made of wood (Spam Crate?).

    Once you get past all the FAA-speak (numbering, etc.) that one book has an amazing amount of useful info even if your bird is an Experimental.

    Chris
    Last edited by Hiperbiper; 05-17-2013 at 09:01 PM.
    You Tube only proves that more airplanes have crashed due to Video Camaras than any other single reason...

  9. #19

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    Mike,
    My spars are welded steel tube also, light, strong, cheap and simple. Don't use wood much.
    But that ANC-19 ( link above) was interesting. The part about making wood props was useful. I saved the entire 336 page book on my iPad.

  10. #20

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    Well, nothing in aviation is flawless and that includes welded steel wing spars. Go look at how Beechcraft fared with the -18 wing spars. Read the AD that resulted in most of the Beech 18 fleet getting spar straps. Periodic X-ray inspections anyone?

    Pick your poison and study to fully understand your chosen subject. In the acro world, wood beefed up with the judicious use of carbon fiber looks really good. I am not aware of anyone, no matter how hard they have abused the airplane, breaking one of Jon Staudacher's wings. And I believe that the Edge's have the similar construction. I know pilots who think pulling 10G's is an easy practice flight. If you are still working on the drawing board, it might be worth looking at.

    On the topic of residential construction, the use of wood vs steel appears to be regional. When I lived in upstate NY, it seemed like everyone had a steel carrying beam under their house. In New England all of the houses have a big wood beam. I can only guess that availability of of steel beams was less (distance to mills?) and around here wood is plentiful. We laugh at city folks who think that trees are endangered. More recently, residential construction is moving to engineered beams for greater stiffness over longer spans. But the fire departments hate I joists because the come apart really quickly in a fire making it very dangerous to enter a burning house. Dimensional lumber structure keeps its integrity a lot longer in a fire. Commercial construction building codes drive the use of steel in office buildings and factories these days, but the residential code still does not weigh in favor of steel studs and the like.

    Best of luck,

    Wes
    N78PS
    Last edited by WLIU; 05-18-2013 at 09:55 AM.

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