STEEL SPAR CAP TO WOOD SPAR LOW COST

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by dougwanderson, Sep 12, 2018.

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  1. Sep 12, 2018 #1

    dougwanderson

    dougwanderson

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    Good or bad idea? instead of carbon fiber spar caps on a wood spar what about a normalised carbon steel flat bar 100,000 psi yield. clean sand and epoxy onto wood spar. maybe a layer of glass over top of steel to help with seperation?
    more heavy than carbon fiber but lighter than wood alone spar. Also with steel easy to laminate 2 or more bars at hi load areas.
     
  2. Sep 12, 2018 #2

    Dana

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    Problems with the different rates of thermal expansion might be a problem.
     
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  3. Sep 12, 2018 #3

    TFF

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    Pick a material and build with it. Mixing is generally bad without even more engineering. What is so hard about a slab of wood? Stenson Reliants had steel truss spars.
     
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  4. Sep 12, 2018 #4

    BBerson

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    Safer to make the spar all steel. A thin steel web welded to the caps makes it into an I beam.
    I am planning to do some tests on home made steel I beams.
     
  5. Sep 12, 2018 #5

    poormansairforce

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    Awesome, I've been thinking about trying Steel in a spar also. The idea of welding fittings, etc where needed is an attractive option as opposed to making sure you get a good glue joint or good rivet. I've also looked at stacking tubes inside the spar caps to get strength exactly as needed.
     
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  6. Sep 12, 2018 #6

    dougwanderson

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    200 inches at 120f steel 200.14 for wood 200.053 .08 difference is well within epoxy elongation capabilitys.
     
  7. Sep 12, 2018 #7

    dougwanderson

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    well weight space and cost. as for pick a material and stick with it. why? so foam and fiberglass or wood carbon fiber.
     
  8. Sep 12, 2018 #8

    dougwanderson

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    i agree but weight cost complexity and source of thin hi carbon steel for shear web. a truss would and has worked but back to complexity, cost and weight.
     
  9. Sep 12, 2018 #9

    TFF

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    The answer to sticking with a material is mainly to get away from the "ain't gon'na happen " to the I got the design ready to build. I know I'm not smart enough to pull odd mixes off. Asking if it will work instead of building test coupons means it ain't gon'na happen.
     
  10. Sep 12, 2018 #10

    dougwanderson

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    i do plan on doing a full load tests section and load to destruction. just reaching out to see if it has been done or any observations and challenges to the idea before testing.
     
  11. Sep 12, 2018 #11

    blane.c

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    Up on the North Slope Oil Fields in Alaska during the early days a man cut a steel beam for a roof support it was around 90 feet long, then brought it inside out of the cold and placed it. It grew in length over 3/4" of an inch when it warmed up. This was a problem in the roof structure.

    The difference of coefficient of expansion between wood and steel is going to be problematic for gluing. Steel movement at a 70 degree F temperature change, 70 degrees F x 0.0000072 (coefficient) = 0.000504, 0.000504 x 240" (20') = 0.12096". Wood pine 70 degrees F x 0.0000028 = 0.000196 x 240" = 0.04704" so 0.12096" - 0.04704"= 0.07392" over a 1/16 of an inch difference. Steel is generally bolted to wood to allow for expansion and contraction. There are some flexible glue products used in marine applications to glue wood to steel that allow the different rates of expansion to occur without separating the glue line, they are more like rubber when cured not epoxy.
     
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  12. Sep 12, 2018 #12

    Dan Thomas

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    And with a flexible adhesive like that, there goes the shear strength needed to keep the steel straps in correct relation to each other. If one uses a rigid glue, the thermal changes could, at a minimum, cause small shearings in the wood at the glue joint. Enough of that and the spar fails.

    Any tests need to incorporate loading in temperature extremes as well as numerous thermal cycles.

    It's so much easier just to make the whole thing of wood. Wood is lighter than steel for a given tensile strength anyway.
     
  13. Sep 12, 2018 #13

    BBerson

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    Strojnic bonded aluminum angles (not bars) to plywood shear webs. Plywood is a better shear web than plank.
    His book explains it, expansion wasn't a problem. Getting a good bond is critical. How would you know when it might disbond?
    I don't bond metals.
     
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  14. Sep 12, 2018 #14

    Geraldc

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    Steel carton strapping could work for this.High tensile strength and a choice of widths and thicknesses.You could even put a lengthways band around the spar and join it with the standard strapping tools.
     
  15. Sep 12, 2018 #15

    BoKu

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    I doubt it. I'd guess that the Young's Modulus for plywood (especially if used on the +/-45 as you'd want for a shear web) is low enough that the steel would probably just stretch and shrink the web a little bit, and that the resulting stresses would be relatively trivial. If sixty thou of length change is an issue, then there's not much chance that they'd play nice enough to go to limit or ultimate load factor.

    That said, mixing wood and steel in this manner still gives me the willies. I'm not really seeing an upside to it in ether the short or long term.

    --Bob K.
     
  16. Sep 12, 2018 #16

    pictsidhe

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    It's the bonding that bothers me. Your adhesive has to maintain limit load under all conditions, including extreme temps. Glueing steel to wood has a poor history. You need to look into why that is.
     
  17. Sep 12, 2018 #17

    dougwanderson

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    Epoxy has a very good history with maintaining load is is the bonding to steel that is the question. And I have not found any reference to bonding steel to wood so what history do you write about?
     
  18. Sep 12, 2018 #18

    Jay Kempf

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    Regardless of what you do the weak part is the wood just below where you bonded the strong stuff to it. Best to have better attachment between the spar cap and web so the cap doesn't pull out like that little string on a cigarette pack. Carbon Unidirectional tape or fabric to make tape out of is not that expensive for what you need. But if it was me I would be just using a lite ply box or equiv as a sort of in place mold to make a C channel or double I beam spar by wrapping and interleaving the plies between shear web and cap just to get the whole thing to be a bit more of one piece after curing. How will you inspect the steel parts for corrosion if they are under plies that attach them to the wood parts?
     
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  19. Sep 12, 2018 #19

    dougwanderson

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    Steel banding may may work with layers. I always thought they would be great for antidrag wires in a wing and fusalge in place of wire and turnbuckle.
     
  20. Sep 12, 2018 #20

    dougwanderson

    dougwanderson

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    Was thinking of a built up spar with plywood shear web and wood with groove for the steel on top and bottom for good surface area. Corrosion should not be a problem with thin layer epoxy on top.
     

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