longeron grain orientation

Discussion in 'Wood Construction' started by ToddK, Jan 4, 2017.

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  1. Jan 4, 2017 #1

    ToddK

    ToddK

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    Question, it seems to me that longeron grain should run vertically (up and down). Is that correct?
    Thanks
     
  2. Jan 4, 2017 #2

    bmcj

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    Your grain should run parallel with the length of your longeron, otherwise you will have no strength.

    I may be misinterpreting your question. Can you give us some more detail or specifics?
     
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  3. Jan 4, 2017 #3

    max_burke

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    Vertically when looking at a cross section view, perhaps?
     
  4. Jan 4, 2017 #4

    TFF

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    For a longeron I would be turning it to where it is easiest to bend the biggest bend. But that is just me.
     
  5. Jan 4, 2017 #5

    Aerowerx

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  6. Jan 4, 2017 #6

    ToddK

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    Here is a drawing. P1040675.jpg
     
  7. Jan 4, 2017 #7

    Turd Ferguson

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    Absent specific instructions in the plans, it's not critical. The fuselage is a truss so it's not a simple bending load, which would be affected by grain direction.
     
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  8. Jan 4, 2017 #8

    ToddK

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    Thanks Turd!
     
  9. Jan 17, 2017 #9

    DangerZone

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    I'm somehow surprised by the lack of instructions in your plans for such a critical structural element.

    In Europe, the longeron grain orientation has to be at 45° (degrees). This allows both sides to have firm contact with all grains. The grains hold the joint, not the soft wood between the grains. If you would make a joint like in the picture you posted - the vertical wood brace would hold along ALL grains on the contact surface while the horizontal brace would hold only onto ONE grain along the whole contact surface. It would be wise to review your project or the plans you bought. If you would have some material epoxied to the fuselage sides (usually plywood or some weave) in a full vertical grain orientation, there would be high risks of plywood/weave delamination or wood cracks along the outermost vertical grain.

    Bear in mind that longerons and wingspars are two most important structural elements in a wooden fuselage. A longeron L joint with the grains like in your pic would only hold vertically with the designed strength. An easy way to test this is to make two L joints of one foot long spruce strips: make one L joint with two 45° spruce strips and one with the desired 90°/0° and see how both behave. You'll be able to snap the one which holds only the outermost single grain layer with only ONE single finger pressure on the foot long spruce strip. The one with the grains at 45° will need a firm full hand hold to break apart. Then ask yourself which one would you'd prefer to keep ypu safe in an aircraft when flying over 100 kts an hour.

    If your airplane would need full joint strength along the longerons, it might be smart to build with 45° grain orientation like professionals do.
     
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  10. Jan 25, 2017 #10

    skeeter_ca

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    I have never heard of this in the US. Does anyone know if we do that here?

     
  11. Jan 26, 2017 #11

    davidb

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    I think we use gussets or nuts and bolts. The simplified sketch left out the plywood gussets which are likely called for in the plans.
     
  12. Jan 26, 2017 #12

    TFF

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    Wood grain at 45 makes sense, but would require milling your own stock, which is rare in the US. We take what the supplier gives within reason and most want spar stock for everything. You would either have to have boat building experience or advanced furniture building to know that. Slab sided fuselages are pretty simple.
     
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  13. Jan 26, 2017 #13

    ToddK

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    Realistically, completely vertical grain like I drew is probably not going to happen. It’s all going to have a bit of a slope. My wood slopes, but with a clear vertical or horizontal bias. The grain is so tight with any airplane building wood that any slope at all is going to give us some lines of slide grain.
     
  14. Jan 26, 2017 #14

    mcrae0104

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    I hadn't either. I don't believe the theory that "the grains hold the joint" is true but I would be interested to hear why this is so or find some others sources to corroborate it. This study indicates that the grain angle does not affect the shear strength.
     
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  15. Jan 27, 2017 #15

    wanttobuild

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    Which way would you run the grain in the main spar, horizontal, vertical or on a 45?
     
  16. Jan 27, 2017 #16

    TFF

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    Horizontal.
     
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  17. Jan 27, 2017 #17

    Little Scrapper

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    Grain direction on longerons have little, if anything, to do with strength.

    I'm not an engineer but I highly doubt a designer designs for grain direction in longerons. How would he possibly do that? I suspect a designer knows the math behind the material itself and and just uses that alone.

    Longerons are connected together by smaller pieces creating triangles. Each leg of a triangle is pretty short. Then the entire structure is assembled by 3 other sides.......with more triangles.

    Again, I'm guessing but I'd need some pretty hard proof to see it another way.
     
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  18. Jan 27, 2017 #18

    davidb

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    Growth ring orientation for spars is indeed preferred horizontal because it makes the spar more dimensionally stable with moisture changes. I haven't seen any data that suggests growth ring orientation affects mechanical properties. Since longerons are typically square and relatively small in cross section, dimensional changes from varying moisture content isn't an issue.
     
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  19. Jan 31, 2017 #19

    Lendo

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    That sketch looks like layers of grain to me, where the gains runs the length of the aircraft (horizontally) - grain in the spar runs the same. The grains would run vertically in vertical members. No strength in it otherwise.
    George (down under)
     
  20. Feb 5, 2017 #20

    Pete Plumb

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    Ok, I have to chime in here. Little Scrapper is correct: the grain orientation in longerons has little to do with strength. All members in a properly designed fuselage truss are either going to be in compression or tension and not bending. I know of no empirical or theoretical data that indicates that "F-sub-c" or "F-sub-t" (compression fiber stress, tension fiber stress respectively) of wood is a different value in vertical grain than flat grain. I also have never heard of the 45 degree theory. I'd be screwed if the US regs change to that rule because I re-saw everything so my end-grain is either perfectly vertical or flat.

    Now members subject to bending is different. A member subjected to a load that bends it along its edge grain will likely fail before one subjected to bending along its vertical grain. This is why wing spars are always made so their vertical grains carry the bending loads. Edge grain will almost always fail spontaneously and cleanly along its slope of grain (angled from one edge to the other). Failures in vertical grain pieces generally show a definite compression wrinkle on top first followed by shearing between the growth rings followed immediately by the bottom fibers pulling apart. Try it in your shop with a small stick mounted in a fixture at one end. Grab the end and bend it until it fails - both in vertical and then in edge grain. If the slope of the grain (on the flat-sawn side) is very steep at all, it will break right down that slope.

    One last thing. Nails and staples will tend to propagate cracks along the edge-grain slope. Its not so bad in V.G. (although I have seen those big "ring-shank" nails Champion used send huge cracks out in the V.G. spars). Keep this in mind while gusseting. Just make sure when you are building your ribs that the nice tight, straight, vertical grains are all facing up at you and not the mottled edge-grained side. Hope this helps.
     
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