CriCri Composite

Discussion in 'Composites' started by ULF, Jun 16, 2019.

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  1. Jun 16, 2019 #1

    ULF

    ULF

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    Hi,
    I am getting serious about a CriCri built, not in 2024 alclad, but made out of composits.
    Right now I am working on cutting the wing and flaps cores out of pu-foam, and I plan to use the negatives as a mould for the wing skin.
    The wing will be a hollow composite with a glass- carbon skin, having a balsa core and again, glass on the inside.
    The spar will be made of carbon rovings and glass shear web on balsa and pu-foam core.
    There will be 4 ribs, to hold the 4 flaperon brackets, plus a rib at the root.
    I calculated, that the weigh of the 2024 T3 original wing sheeting would be nearly cut in half, going composite.
    I think, it should be reasonable, to reduce the overall weight of the entire airplane, without sacrificing strength.
    What do you people say to this approach?
    How about using some inner part of the hollow wing as a fueltank? This way I`d get rid of some more weight?

    Are there any CriCri piloits? How does the fuelsystem work?
    Is there any electric fuel pump, or does the engines run a pump, or is it only suction?
    How would you make a fueltank inside a hollow wing? Do I need special epoxy, or does regular laminating epoxy do the job? And is there a standpipe in the tank?
     
    Last edited: Jun 16, 2019
  2. Jun 16, 2019 #2

    Scheny

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    You will find a lot of info in the fuel section. In short: epoxy is not enough for gasoline. There is some treatment (sealing layer) needed at least.

    As for balsa: not the best choice. Much better materials available like PVC foam (Balsa more easily gets core failure), but: with fuel next to it, foam is not the best solution so you would rather go with honeycomb.

    What is the reason for the glass/carbon mixture? Most people use glass as either coupling layer for honeycomb, or electrical isolation. Otherwise mixing makes less sense, unless you want some failsafe when carbon breaks (but then you would need Aramid fibres instead of glass).

    Best regards, Andreas
     
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  3. Jun 17, 2019 #3

    MadRocketScientist

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    The fuel system just uses the pulse carbys to suck the fuel up from the tank between the pilots legs. Some people have added a low pressure pump (5psi or so) to help fuel delivery. The tanks has a flop tube in it to allow aerobatics.

    The original wings are around 8kgs each and the spar is rated to 9G ultimate.
     
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  4. Jun 17, 2019 #4

    Marc Zeitlin

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    Thousands of EZ type aircraft with 40 years of experience with only epoxy on the inside of the fuel tanks would beg to differ. You have to choose the right epoxy, and apply it correctly, but it's more than adequate when done right.
     
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  5. Jun 17, 2019 #5

    Hephaestus

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    Um... Yes sort of.

    Epoxys fine in gas, as long as there's never ethanol that ever gets into the tank. So in some ways if you like to roll the dice that fuel tankers are properly rinsed between loads and nobody makes mistakes because all other fuel gets ethanol now... Because if you do get ethanol into that tank you'll probably be rebuilding fuel systems...

    Vinylester resin is the proper one for fuel tanks.
     
  6. Jun 17, 2019 #6

    mcrae0104

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    Building a composite Cri Cri is like a pyramid made of 2x4s. Nothing wrong with pyramids or 2x4s, but you'll be starting from scratch, except for the outside mold line.
     
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  7. Jun 17, 2019 #7

    Tiger Tim

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    Wasn’t there a video of a carbon Cri Cri posted here maybe a year ago? It had the engines relocated to the wings and may have been electric. Anyways, that may be a builder worth tracking down.
     
  8. Jun 17, 2019 #8

    TFF

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    You might as well design a different shape with all that work. It’s kind of a restrictive shape for composite.
     
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  9. Jun 17, 2019 #9

    Victor Bravo

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    Why carbon rovings instead of carbon pulltrusion strips? Why give up that strength AND reliability?
     
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  10. Jun 17, 2019 #10

    Marc Zeitlin

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    Certainly there are some epoxies that are susceptible to ethanol, which is why I explicitly said "...the right epoxy". There are many folks using the EZ10/87 epoxy system in fuel tanks that use car gas that may or may not have ethanol, and there are some folks that have been soaking and testing numerous epoxy samples in ethanol containing gasolines for over 20 years now, with no indication of degradation of the layups.

    I do not disagree that Vinylester may be a better choice all around for fuel containers, but that wasn't the claim to which I was responding. Had you said "vinylester is best for fuel tanks, or an epoxy with a polysulfide sealant on the inside, with certain neat epoxies being acceptable as well", I would have said "yup".
     
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  11. Jun 17, 2019 #11

    pictsidhe

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    End grain balsa cores are tricky to keep light when thin. They sponge up the epoxy if you aren't really careful. Another idea that I ditched as the reality was difficult to do...
    I would highly recommend perfecting your construction techniques on test pieces before beginning actual construction. It will result in much less scrap...
     
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  12. Jun 17, 2019 #12

    Hephaestus

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    I just remember the 80s when we sure didn't seem to have near the fuel issues in aircraft we see now. Maybe I'm more exposed - maybe it's just that extra 30yrs of age...

    Somehow I'm not buying it...

    So easier to point people at what's rated than what might survive until they add something else new.
     
  13. Jun 18, 2019 #13

    ULF

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    Yes that one is on youtube, you can find it searching for carbon CriCri. That one actually got me to the idea.
    I doubt it will ever fly though, looks like the paintjob is hiding a lot, I mean from what I saw on the video didn`t look very airwothy to me.
    Hi, Thanks for the thought on the fuelcell, I fond a paper from Swiss components, about making a fueltank, that one I`ll kook into.
    About the hollow carbon fibre wing built, I thought, of using the tsame technique as they use to build F3B glider wings,with a carbon spar, there is enough literature and experince values of how to calculate the weight and strength.
    I thought to calculate the spar for the load, and test it with sandbags before I even put it into the shell.
    I am not inexperienced building F3B wings.
     
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  14. Jun 19, 2019 #14

    ULF

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    Thanks, thats good to know! (plenty of space to experiment, with an MTOW of 80Kg as a upper limit) - do we really need 9g`s? And the costs could be that of a model airoplane.
     
  15. Jun 19, 2019 #15

    STDJantar2

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    Thousands of glass gliders wings are built that way... carbon or glass roving for most manufacturers, LaK uses pultrusions but their wings are not lighter and they flex too much. Yes, I have assembled a friend´s Lak 17, also had a Std Jantar (heavy wings!!! made out of glass, surely overbuilt) and a Ventus a (wings almost the same weight as Lak´s). Spar made out of rovings tend to be cheaper and usually stiffer since the safety factor is majored because of builder variation. I´d make a first set of wings this way.
     
  16. Jun 19, 2019 #16

    ULF

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    Thats what I thought too.
    I had the same thought.
    A industrial fabricated carbon rod at least guarantees a optimal fibers/resin = optimal strength.
    In my case I need the highest trust strength, not the tension strength.
    Its mainly a question of application, to use industrial fabricated strips, or to use rovings, that are pressed with embedded tissue, and nylon peel ply fabric, to take off excessive resin.
    So far, with my equipment and knowledge, I probably feel to be on the saver side, to go for the roving process. (maybe embed some carbon stripes, but have to calculate-if I manage).
    Here a set of formulars, I would use toi calculate the spar: see Elefnat -Spar. pdf
     

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  17. Jun 19, 2019 #17

    ULF

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    Thanks, greate info!
    John
     
  18. Jun 20, 2019 #18

    Tiger Tim

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    I had thought the same but I’ll be the first to admit I have no experience with composites and no way to properly judge.
     
  19. Jun 20, 2019 #19

    MadRocketScientist

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    From what I saw the layups were very dry in places, the spar didn't look like it used uni fibers, but maybe it was wrapped. The locations of some of the part join lines seemed odd. It would make a great land based go kart but you wouldn't catch me trying to fly it. I am not sure why everything wasn't vacuum bagged and resin infusion used. It gives a much better finish and weight to the parts.
     
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  20. Jun 20, 2019 #20

    bmcj

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    Marc, are you referring to any/all epoxies, some (but not all) epoxies, or just the EZ10/87 in this statement?
     

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