A different way to build a wing

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Apr 10, 2019.

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  1. Apr 14, 2019 #41

    wsimpso1

    wsimpso1

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    I can talk about foam in a slightly different sense. I can take a chunk of blue 2 pcf foam with cross section 3-1/4x7 inches and it will stand between 2600 and 4000 inch pounds of bending moment. This average looking 63 year old can not quite break it in two with his hands. Other higher density foams are stronger, roughly in proportion with their density, but that is still close to nothing. In comparison, I have a 3-1/4 by 7 inch fiberglass channel that is designed to carry over 850,000 inch pounds of bending, 11,700 pounds of shear, and take whatever torsional deformation the wing generates. That is 300 times more strength than if we used solid foam in the same dimension, and is on the order of what we need in our airplanes.

    Plastic foams do so little for strength that we can ignore it in our computations. Plastic foam allows us to make reasonably light, precisely dimensioned composite parts of incredible stiffness and strength so we include it...

    Billski
     
    Last edited: Apr 14, 2019
  2. Apr 14, 2019 #42

    Sockmonkey

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    Yeah, I assumed the fiberglass was most of the strength, but fiberglass wings usually have a spar of something stronger yes?
     
  3. Apr 14, 2019 #43

    Vigilant1

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    Solid core fiberglass wings would have fiberglass/epoxy wing skins and a spar. The spar would have caps of fiberglass/epoxy or carbon fiber/epoxy and these caps are joined by a spar web of (usually) the same material as the caps. With solid foam cores it is practical to lay up the spars caps and web directly on the foam, then apply the skin.
     
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  4. Apr 14, 2019 #44

    wsimpso1

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    I suspect some really large concept errors if you are asking that question... In factory made fiberglass airplanes or kits, yeah, some use graphite spar caps. Others use fiberglass. What makes you think that you need really strong material to make the spars? Why are spars special?

    If you need to carry 30,000 pounds in a rod under tension, you can carry it with 3 in^2 of 10 kpsi material of 1 in^2 of 30 kpsi material or 0.40 in^2 of 75 kpsi materiel or 0.10 in^2 of 300 kpsi material. All will just barely do the job. Which one is lightest might be your next question...

    There have been a bunch of airplanes made with 6061 aluminum tubes as main spars with a yield strength of about 35 kpsi, and many many airplanes with spars made of Sitka Spruce. Are they inadequate? Nope. If you build your wings in aluminum, 7075 -T6 is about as good as it gets, and yield strength is about 63-69 kpsi.

    My fiberglass wing has fiberglass spars and spar caps. Lots of VariEze, LongEz, and derivatives use E-glass caps... Lots of Glasair I, II, and III used glass too.

    Breaking strength of unidirectional E-glass-epoxy in the direction of the fibers is around 70 kpsi, while unidirectional graphite rods are about 300 kpsi. If strength were the only thing going on, you would need a little over four times the cross sectional area of uni glass as you do uni graphite rods. E of the uni E glass composite is about 4.4 Mpsi while graphite rods are around 24 Mpsi, so if stiffness were all that mattered, you would need a little over 5.5 times the area in glass than in graphite.

    Billski
     
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  5. Apr 16, 2019 #45

    rv7charlie

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    Given the discussion of spar webs in this thread, I found it an interesting coincidence that I got this 'recommendation' from youtube yesterday:


    Not saying they're right, or that it is directly related to this discussion. Just a funny coincidence.

    Charlie
     
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