Foam vs. honeycomb for carbon fiber sandwich

Discussion in 'Composites' started by birdus, May 17, 2018.

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  1. May 17, 2018 #1

    birdus

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    What are pros and cons of using foam vs. honeycomb (nomex) when building carbon fiber sandwich panels for wings and fuselage? One concern I have is getting foam to bend to fit curves since it's stiff.

    Thanks,
    Jay
     
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  2. May 17, 2018 #2

    Voidhawk9

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    You don't have to bend foam, you can hotwire it, Rutan style. Or a sheet of PVC will bend readily if sufficiently heated. But - yes it depends on the details of what you are trying to accomplish.
     
  3. May 17, 2018 #3

    BoKu

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    It's not that stiff. And when you really need to bend the foam so it conforms (which helps things stay in position until you get vacuum), you just hit it with a heat gun. But we made our first few wing sets without curving the foam.
     
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  4. May 17, 2018 #4

    birdus

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    For a plane with roughly 24' wingspan, 40" diameter fuselage, 2,500 gross weight, what thickness foam would you use for the fuselage? Can that be bent to a 20" radius? What about tighter curves? Just use thinner foam, I assume. You didn't mention honeycomb. Is that not even something you'd consider? What thickness foam for the wing panels? Again, would you not even think about honeycomb? If not, why not? It would certainly be easier for curved parts.

    I'm in early design stages. Just trying to learn.

    Thanks,
    Jay
     
  5. May 17, 2018 #5

    Tantrum1

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    Interesting you ask this, I just ordered 2 pieces of 2' X 4', 1/8" thick nomex honeycomb to experiment with! I was thinking about using is in a canopy enclosure.
     
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  6. May 17, 2018 #6

    TFF

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    My opinion is nomex core requires professional facilities and skills. Have both or want to develop both, great material. Foam get 85% of the fancy core with 50% of the precision needed. That's a pretty high gross weight for wingspan. 700 lbs higher than a RV7. What is the configuration?
     
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  7. May 17, 2018 #7

    wsimpso1

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    Foam works just fine for room temp cures and nominal ramp temps (as long as your ship is white or pastel colors). My ship is 29 foot span, 2150 pound gross weight, wing skins and fuselage skins are vacuum bagged fiberglass on 3/8 PVC foam. Wing skins and most fuselage skins we left the PVC flat, curving it with the vacuum. Wing skins, we did inner and outer fiberglass facings without bending in one layup session. If we were working in graphite fiber, we would have worked in 1/4" foam, and it would have been even easier. Only where the fuselage has small radii curves did we heat bend. You can heat bend with an oven or a heat gun. Be patient about heating, foam is a good insulator, which means it is slow to warm up and to cool down. For the tighter places we installed glass against the mold and foam in the first step, did the other layer of glass in a second vacuum bag layup. Some folks find it necessary to do all three layers in separate sessions.

    If your fuselage is elliptical cross section, you will probably want to prebend foam for much of it. It does not have to be perfect, just sort of follows the mold. The vacuum bag will take care of the rest. Many of us build with sides and floor closer to rectangular through the wing and people space, and those areas won't even require pre-bends. I have even vacuum bagged large radii compound surfaces with unbent foam... Three bag operations, and apply vacuum slowly so you can work everything down.

    Foam is not to be scared of for making curved shapes.

    Now, onto honeycomb. Wet lamination? Nope. Epoxy runs off of the skin lamination and into the honeycomb, making for heavy cores and/or resin lean areas in the lamination. It works fine if you are laying up your skins using unidirectional pre-preg tapes and film adhesives on each side of the honeycomb. That means oven cure with all molds and all materials rated for the oven cycle.

    If you are working in graphite fiber, one other characteristic comes into play. Graphite fiber has a negative thermal expansion rate along the fibers, but a positive rate across the fibers. And the value for this varies with the source. For the graphite fiber composites to come out shaped like the cold mold, the molds and the layups must be built of the same materials and fiber orientation schemes as the parts. When the mold and the part become solid and match each other intimately at the end of the cure cycle, they will be a different shape than the cold mold, but if you match fibers and orientation ratios, the part and mold then change shape together back to room temp, giving the correct shape.

    Just making room temp cure tools and making room temp cured foam cored parts in them is a big learning curve. You gotta need that last couple of pounds of airframe weigh pretty badly to take on graphite tools and oven cure cycles just to use Nomex cores. I say spend a little more time designing in graphite fiber and optimizing the heavier pieces to get that weight instead.

    Billski
     
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  8. May 17, 2018 #8

    Tantrum1

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    WSIMPSO,
    So for honeycomb core, I have a fuselage shell that's carbon and is finished shape. I want to add rigidity to the canopy surround I just cut out. Initially I was going to use divincel 1/8 foam to do this as I've done before. But was thinking of doing something different just for an experiment. Could I lay down a film mixture of thickened west systems with microlight fill, then lay in the 1/8" honeycomb and allow it to room temp cure, then "cap" it with a wet carbon layup then vacuum bag it and heat cure? What I'm talking about is the underside of this piece.

    IMG_1447.jpg
     
  9. May 17, 2018 #9

    wsimpso1

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    Core thickness needed is a function of how much bending stiffness and strength you need in the skin panel and which fiber you are using. You will have to bone up on composite plate theory... Or use the monkey-see, monkey-do approach. Lots of glass skins with 1/2" and 3/8" cores, lots of graphite skins with 3/8" and 1/4" cores, and some of the sailplane guys go even thinner on some cores. There is a big difference in stiffness and in strength between a panel made with 1/4" core and 3/8" core. A lot of how stiff and strong the panels need to be has to do with what your Vne is. The faster the airplane, the higher the forces pulling the skins away from the rest of the airplane, and the forces go with V^2. Bending moments in the panels then goes with q and panel size. I do not like the idea of guessing here. If you must copy, at least make sure your q at Vne is no higher and your panel dimensions are no bigger than what you are copying, then remember that you may be jumping off the same bridge as the one the other designer jumped off... Better to KNOW by calculation, then do the laugh check of seeing what other guys did in the same speed and size range...

    Billski
     
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  10. May 17, 2018 #10

    birdus

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    It's been a while since I've looked at my numbers. It actually might be closer to 2,000, and I'm not exactly sure about wingspan yet. So, rough guestimates right now. Some of my initial estimates are based on the Radial Rocket. What I'm working on is a 70% Corsair, all carbon fiber.

    Jay
     
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  11. May 17, 2018 #11

    BJC

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    It has been done.

    The Glasair III with standard wing has a span of just over 23 feet, and a gross weight of 2400 pounds.

    A Glasair II, (with extended wing tips) configured for long range flight, had a gross weight of 2500 pounds, including a non-stop flight from Hawaii to Florida.


    BJC
     
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  12. May 17, 2018 #12

    birdus

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    Okay. So, the epoxy runs into the honeycomb cells, so you need pre-preg, so you need big ovens, so that's out. That's the kind of info I was after.

    It's going to be dark blue, so I'm probably going to need to do a cure at elevated temp (Aeropoxy at ≈250, I think). Will need to figure out how to do that.

    I'm hoping Vne will be pretty zippy. Wouldn't mind 300 mph. Too early to know. Based on what you were saying, Bill, maybe I'll need to look at some thicker foam in the fuselage. 1/2"? Parts of the fuselage—e.g., turtledeck—will have much tighter curves, so I hope it's okay to switch to thinner foam in those areas. I'll get to that point sometime. Sounds like 1/4" in wing panels might be adequate. As you said, the carbon fiber fabric I use will influence things. Jim Marske talks some about the different recommended fabrics in his book.

    Speaking of Jim Marske, I'm going to use his book in helping design the spar. I've gone through a few different ideas about how to make the spar(s). At this point, I'm thinking of making it one piece (if possible). The question is if the Graphlite rods are flexible enough to do the bends in the gull wing. Maybe I'll be able to achieve that by using a greater number of smaller rods vs. fewer larger ones.

    Thanks,
    Jay
     
    Last edited: May 17, 2018
  13. May 17, 2018 #13

    BBerson

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    Foam can be scored on the inside for tight radius curves.
     
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  14. May 17, 2018 #14

    TFF

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    Plenty of Glasairs around here. Fast and slow are always the topic in airplane design. Weight too. When you say you have short wings and high gross, you are talking a pretty hot landing plane. They are usually the most entertaining to watch landing.
     
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  15. May 17, 2018 #15

    BoKu

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    That could probably be made to work okay. It will take rather a lot of time per unit area, but it will probably work.

    One of the hard parts will be getting a uniform thickness of the first coat of resin. A trick we often use is to lay down a piece of window screen, squeegee on a coat of the thickened resin, then lift away the window screen. That leaves a bunch of tiny square patches of uniform height that you can moosh together with a roller. For the closeout ply, I'd saturate the carbon medium wet on the workbench with peel ply saturated onto one side, then flop it onto the honeycomb (peel ply side up, of course).

    Here's some advice I've heard from the folks who repair ASK21 gliders with honeycomb: Arrange your work that the air pressure inside the honeycomb cells is stable or if possible slightly decreasing during the cure. You can either heat the workpiece before applying the closeout ply, or you can cool it after applying that ply. Or maybe start work in the warmest part of the day and let the cure run after that. That way ambient air pressure pushes the closeout down onto the honeycomb for a good bond with the cell tops. What you don't want to do is to put on the closeout ply and then heat the workpiece. That causes the air in the cells to expand and push the closeout ply off the top. I've seen the result, you get areas of delamination (or rather, non-lamination) where a bunch of cells have pushed the closeout ply up away from the honeycomb.

    --Bob K.
     
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  16. May 17, 2018 #16

    Tantrum1

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    How would it work with heat curing it but while under vacuum? Would that alleviate the delamination/non-lamination issue?
     
  17. May 17, 2018 #17

    BoKu

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    I don't know, I really don't have that much experience with honeycomb. You'd probably have to be careful with how much vacuum you apply so you don't just pull the closeout ply down into the cells.

    If you can do a vacuum cure, you might as well just use PVC foam. You can easily shape it and bevel the edges, and cut it into little segments if you need to.

    --Bob K.
     
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  18. May 17, 2018 #18

    wsimpso1

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    You can use either type core. Doing this in a wet open layup over honeycomb may not even be a weight save, as resin will drain from the cloth into the honeycomb. Then there is how you deal with the cloth at the edge of the honeycomb. Much of your part will be edges of the core, so if the method you use is not really light, your part would be lighter in foam. I would use foam.

    Billski
     
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  19. May 17, 2018 #19

    wanttobuild

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    If you feel you you can get a good bond by rolling out a film, what would stop you from flipping it over for the other skin. This would work for curved panels as well.vacuum clamp away! Cure it at a very high temp and you don't have to paint it stinking white. Pm me for SUPER high temp epoxy sources.

    Ben
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    When the Green Light pops the BS stops
     
  20. May 17, 2018 #20

    BJC

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    Why not share the specs and sources with everyone here?


    BJC
     
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