Orion's "Composite Think" in Plywood???

Discussion in 'Composites' started by Aerowerx, Sep 13, 2012.

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  1. Sep 13, 2012 #1

    Aerowerx

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    Orion, would there be any advantage to doing this in plywood? That is, comnpared to built up wood ribs, plywood skin, and built up wood spar?

    [​IMG]
     
  2. Sep 13, 2012 #2

    SVSUSteve

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    I would think (without doing any calculations) that it would be heavier and probably less strong than it's composite counterpart. It also would probably be more difficult to produce especially if you tried to produce the "beam" structure. Why would you want to do it in plywood out of curiosity?
     
  3. Sep 13, 2012 #3

    orion

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    Ply is generally a relatively weak material so in a structure like this, the load carrying capacity of the material would most likely be far below what you need, unless of course you seriously bump up the gauges. But then you have a massive weight issue. True, I have not done any studies in wood for this type of structural arrangement so the above is only my gut feeling.
     
  4. Sep 13, 2012 #4

    Aerowerx

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    Which is exactly why I asked the question---out of curiosity.

    Yes, it would be heavier than composites, but I was asking for a comparison to built up wood ribs and built up wood spar, skinned in plywood.

    I almost posted my question in the 'lowest parts count' thread because the method would have fewer number of parts in the wing, but it had already been hijacked a couple of times.
     
  5. Sep 13, 2012 #5

    SVSUSteve

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    Ah....I love theoretical "what if" discussions.

    I have no clue either despite having done calculations using wood (balsa cored mahogany plies) for my own designs but there was nothing akin to this structure in it. Then again, I also "over-designed" the structures I decided upon just to make up for my own insecurity regarding such calculations (the "factor of safety/stupidity")
     
  6. Sep 13, 2012 #6

    Aerowerx

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    So, in other words, by the time you beefed it up enough to meet the strength requirements you would be better off using the 'built up' method?
     
  7. Sep 13, 2012 #7

    BBerson

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    Plywood is excellent for shear (see Strognic books)
    The corrogations (shear core ) could be molded ply. The skin could be something different perhaps.
     
  8. Sep 13, 2012 #8

    Jay Kempf

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    If you used a method like engineered trusses? That would emulate the internal trapzoidal structure in light weight materials that are easy to make on a table saw, jig and glue. I see the problem as how to make your own plywood skin that will hold the airfoil shape. I guess you could laminate some very thin plywood into a thicker plywood but that would require a mold and vacuum. Could be done I suppose. But in the end what are you saving over what Orion designed?

    Orion, what sort of molds did you use to do these parts? To assemble?
     
  9. Sep 13, 2012 #9

    RJW

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    I built a walnut guitar a long time ago. The side skins were solid walnut about 3/32 thick and 5 inches wide. They were shaped by clamping them to a heated mold. I could barely believe my eyeballs when I started shaping the sides. The normally very stiff and brittle wood became plastic when heated. It was very easy to pull it along the mold and it held its shape perfectly once it cooled.

    The curves on the guitar sides were simple and along the grain. I wonder if you could make compound curves in a sheet of wood using the same technique? Could you mold plywood using this technique? Or would the glue be damaged by heat? Maybe you’d have to mold the solid sheets first and then glue the plies together? Seems a huge amount of work. It’s pretty obvious I know nothing about wood.

    Don’t really know the point of my post. I still have the mold but not the guitar. :cry:

    Rob
     
  10. Sep 13, 2012 #10

    Jay Kempf

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    People steam all manner of woods to prep them for bending in jigs. No reason you couldn't do the same with plywood as long as you researched the adhesive and temps it can withstand and then of course test samples to make sure you did it right.

    Orion, how did you do the spar attachments in that wing. Multiple bonded and bolted root fittings?
     
  11. Sep 13, 2012 #11

    WonderousMountain

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    Molded plywood is made by heating and steaming at different places. Constant curvature is obtainable, but custom shapes can be made too. The plywood has to be held while the glue dries as far as I know. It's not a cheap process, and the glues are often toxic, but with enough research it could probably be done well.

    Does anyone actually know the specific strength of any wood?, or is it just more fun to give opinions than share facts.

    Wonderous Mountain
     
  12. Sep 13, 2012 #12

    orion

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    For the horizontal tail I made my own skin tool off of a machined foam plug. The mold itself was made from a combination of glass and graphite arranged in a sandwich pattern where the glass acted like the core. Total thickness of the tool was just shy of 1/4".

    Parts were made from a Toray graphite BID prepreg (tool was built with Vinylester so tolerated oven quite well).

    But this turned out to be a lot of work so the more cost effective way it turned out was to have the CNC milled tools be made directly as a female cavity.
     
  13. Sep 13, 2012 #13

    orion

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    The horizontal (pictured) is a one piece stressed skin structure - no spar.

    The original design for the wing was similar to the tail but we couldn't figure out how we were going to handle the huge parts so a left and right were built. In this case I gave in to the design needs and fabricated a molded box beam. The cross section was split mid way up the web and a series of trim lines and joggles were incorporated for a nearly self jigging assembly. The wing was assembled in a large one piece upper wing skin tool, which automatically aligned and positioned the skins and the spar pieces.

    If I were to do that again, I'd go back to the one piece wing.
     
  14. Sep 13, 2012 #14

    Aerowerx

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    No, my question was: Do you save anything over using 'built up' ribs and spar?

    We just ate at McDonalds. Since this question was in my mind, I noticed that their chairs were made of molded plywood, maybe 1/2 inch thick. But I don't know if it was molded in one piece, or if two or more sheets were laminated in a mold.

    I had some difficlty deciding what forum to post my question in, since it covers several topics (composites, wood, design).

    I remember seeing an old thread, where Orion first posted that picture, and someone else made the statement that maybe it would also work in aluminum. That and Orion's statement about 'metal think' got me thinking about 'wood think'. Why do wood wings always have to be built the same way?
     
  15. Sep 13, 2012 #15

    Aerowerx

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    Have you looked at this Sticky in the wood forum?

    There is also this report on plywood webs.

    And this web site has a pdf titled "Aircraft woods : their properties, selection, and characteristics"

    Isn't Google wonderful?
     
  16. Sep 13, 2012 #16

    WonderousMountain

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    Wonderful would be someone actually having a working knowledge of the subject matter you're interested in, rather than citing large volumes of classic texts as if that meant something. Why not just categorize all the pertinent references, and then we can reduce this site to what it's really here for, unproductive political tangents, harsh pointless criticism of others projects, and big videos of planes we wish to play with.

    For instance,

    Birch
    Mahogony
    spruce
    douglass fir

    Anyone KNOW these for common specific strengths for any moisture content?

    Blessings,

    Wonderous Mountain
     
  17. Sep 13, 2012 #17

    autoreply

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    Buckling of the shear web is (I think) a major issue. I originally planned to go with Orion's wing-construction technique, but quickly figured out that concentrated loads (detachable wings...) were a nightmare to design without advanced FEA. Still planning them for my tail though, if only to avoid molding and aligning two separate spars plus a close-out for the elevator and rudder nose.

    Apart from that, you have to prevent the shear webs from buckling. Orion simply solved that by using it in a big, bad, very strong airframe that's beyond my budget (drool...), but that's simply too heavy for most GA applications. Sandwich shear webs is trivial, but it does require your shear web to be straight between both skins. I don't think you can bend wood sharp enough to do that. A balsa/wood laminate sounds like an interesting option if you're destined for wood.
     
  18. Sep 13, 2012 #18

    SVSUSteve

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    WM, no one trusts their memory for this sort of thing. You refer to the manuals simply because of the fallibility of human memory and the wide range of numbers between different testing methods/modes and moisture content. Not to mention that you don't learn as much if we simply tell you the numbers.
     
    Topaz and bmcj like this.
  19. Sep 14, 2012 #19

    Aerowerx

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    As far as that goes, no one actually "knows" the properties of any material---aluminum, steel, wood, or composite!

    It is all based on statistics. You can never "know" the exact force required to break your aluminum spar, for example. It is just that the properties of aluminum vary over such a narrow range that that variation can be ignored.

    And there is that safety factor of 2 that everyone uses. If you "knew" for certain what the forces were going to be, and "knew" for certain what the properties of the material are, then you could use a safety factor of 1.000001 and fly safely.
     
  20. Sep 14, 2012 #20

    SVSUSteve

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    At a minimum for critical structures. For non-critical structures, I have seen factors of safety/stupidity of 1.25 to 1.5.
     

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