Idea for covering fins

Discussion in 'Techniques' started by KILTED COWBOY, Aug 14, 2019.

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  1. Aug 14, 2019 #1

    KILTED COWBOY

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    I was watching a guy build a balsa model airplane.
    When I was a kid,we used to cover with tissue and put on dope to make it strong and rigid
    He was using a plastic film that has an adhesive on it which is activated by heating it with an iron it shrinks it a bit and adheres to the balsa frame.
    Instead of papering fins I was going to try on my next build to cover the fins with this stuff and see how it works. It is very light and strong as it is used on flying models.
     
  2. Aug 14, 2019 #2

    dhbarr

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    monokote?
     
  3. Aug 14, 2019 #3

    kuririn

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    That I believe is heat shrink monokote. Can be ironed on with a special iron and sock or a heat gun can be used. I asked about the use of this a while back. The experts here said something about you might get warping of the airframe when the film shrinks. Model airplane frames are much more rigid than cardboard tubing.
    Sounds like it's worth investigating.
    Cheers.
     
  4. Aug 14, 2019 #4

    mbeels

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    If the standard Monokote results in warping, there are low temperature and low-shrink options that may work better. Someone here (perhaps it was BABAR?) uses the self adhesive trim monokote, it is basically a large sticker. It wouldn't expect to add much strength, though.
     
  5. Aug 14, 2019 #5

    K'Tesh

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  6. Aug 14, 2019 #6

    mbeels

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    Looks like a good method.

    My favorite is to use a very thin layer of white glue for papering, and then iron it flat. The heat re-activates the PVA glue and you can get all the bubbles out, as well as a dead flat fin. Iron out each side (flipping back and forth) until all the warps are gone. It seems to add a lot of strength and results in a smooth surface.
     
  7. Aug 14, 2019 #7

    BABAR

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    I have covered fins with trim Monokote and with Mylar tape. Cosmetic results are great, I don’t think it adds significant strength. I have no experience with heat activated shrink Monokote.

    Those that have used it on airplane models feel free to chime in, I suspect it acts differently stretched across a wooden spar frame than a solid piece of balsa.

    White glue and paper adds a LOT of strength, it is more challenging and messier.
     
  8. Aug 14, 2019 #8

    jlabrasca

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    [​IMG]

    40# sketch vellum and finish-cure epoxy over plywood frames. I'll fly it in September and post a flight report somewhere on TRF.

    As for the Monokote (or some other polyester film) over balsa -- papering the fins improves the bending strength because paper isn't very elastic and has high tensile strength. It doesn't stretch, so papered fins are rigid (they resist bending -- since bending the fin means stretching the paper skin on one side of the fin). There are different kinds of polyester films used for model airplanes. Try to find one that is NOT elastic.
     
  9. Aug 14, 2019 #9

    mbeels

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    I've covered a fair number of airplane models with monokote (and its cousins). It is capable of warping built up frames as well as solid sheets of balsa, depending on the strength of the structure and the technique. One good method would be to tack down the heat activated covering material with even tension, and shrink the top and bottom of a surface at the same time to avoid warping. Small warps can be corrected by applying more heat, and holding the warp out until it cools. However, after sitting in a hot sun, the warp is liable to return. Furthermore, solid sheets of balsa are very easy to warp, so getting good results with rocket fins may be tricky.

    I think for rockets, the glue + paper method will be stronger and stiffer. But I've never tried monokoting rocket fins, perhaps it is worth a try!
     
  10. Aug 14, 2019 #10

    KILTED COWBOY

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    Sounds like it is more trouble than it is worth.
    Just an idea,
     
  11. Aug 14, 2019 #11

    neil_w

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    I religiously paper my fins with adhesive label paper, and have documented my process here many times. It works well and is easy and quick. No ironing, no pressing, no warping, no bubbles, no fumes. It adds *some* strength but not as much as glued paper. That's usually fine, but if I were in a situation where I really need maximum strength (and needed to keep the fins thin) then glued paper is the way to go. Glued cardstock even better (albeit heavier).
     
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  12. Aug 14, 2019 #12

    dr wogz

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    I've covered many a plane with Ultracote (Monokote's main competitor). I prefer Ultracote to Monokote as I find it's easier to work with. There are other brands, and Ultracote (Hangar 9 / HorizonHobby) has a cheaper cousin; Econocote..

    It is strong, it is robust, and will add tremendous strength. But it does take practice to apply & use. And it eats X-Acto blades & single sided razor blades! (dulls them quickly)..
     
  13. Aug 14, 2019 #13

    MALBAR 70

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    +1 On the label paper, only I burnish it down with the handle of my X-Acto knife,and seal the edges with thin CA. Works great.
     
  14. Aug 14, 2019 #14

    Mugs914

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    I used Sig dope and medium silkspan tissue on my Star Orbiter with good results, but it was a lot of work. (Just like it is on a model airplane!)

    Stiffended up the tube only a little, but added a lot of strength to the fins for very little weight. (Just like it is on a model airplane, oddly enough :rolleyes:)
     
  15. Aug 14, 2019 #15

    Bruiser

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    I've read about people using MonoKote, Ultracote and the low temps iron on covering on rockets. Some people even have used them on the body tubes. I guess if you want a shiny finish and don't like to paint or can't get good results they are an option...

    When I was looking into electric R/C models I discovered those coverings are considered too heavy. A product called Doculam seemed very popular. It is still applied with an iron but it is lighter, clear and much cheaper than anything listed above.

    With any iron on the big thing is that just like on R/C airplanes you need to remove it from wherever you are gluing so you can get a good bond. So that means having wood showing at the fin root so the glue will bond to the wood. You could cut the film away but that is hard to do without scoring the wood too. Best way is to not cover that area in the first place.

    After the fillet is set up you have a choice of painting the fillet or you can cut a small piece of covering and iron it on over the fillet.

    Me, I'll just stick to Avery laber paper with thin CA around the edges :)

    -Bob
     
  16. Aug 14, 2019 #16

    BABAR

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    Love the repurposed Pringles can (hard to get the motor mount to stick to that foil inner wrapper, though!)
    What'd you use for a nose cone?
     
  17. Aug 15, 2019 #17

    K'Tesh

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    Wouldn't matter if you go TTW. But you could also try to peel the lining.

    Nosecone looks like it's 3D printed.
     
  18. Aug 15, 2019 #18

    jlabrasca

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    Didn't want hijack the thread -- the OP mentioned stick-and-tissue airplanes, and I built a rocket with stick-and-paper (and epoxy) fins.

    The build thread is scattered on the forum -- mostly on the "rocket wise today" thread

    #284
    #21760
    #21781
    #21998
    #22027
    #22042
    #22052
    #22123
    #22221

    If anybody wants the STL for the nosecone, or any of the F360 files, send me a PM. We now return to regularly scheduled programming, already in progress...
     
  19. Aug 15, 2019 #19

    K'Tesh

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    I don't follow that thread, it's too hard to keep track of what's going on. My guesses were based on my eyeballs (the nosecone has that 3D printed graininess that I recognized), and my best guesses on how I'd try to do it.
     
  20. Aug 15, 2019 #20

    mcderek

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    Applying high quality tissue, shrinking and then sealing with dope will add considerable strength. It works great when you want a high strength to weight ratio. Prime consideration in Free Flight aeromodeling. Not quite as important in rocketry (at least the rockets I build). Regular paper glued to fins will be heavier and even stronger than tissue.

    The problem with tissue is that it will wrinkle with humidity which robs a bit of the strength it provides. In aeromodeling the shrunk tissue provides a taught stressed skin which is the source of its strength. Early morning damp will cause the covering to wrinkle and can lead to wing flutter which isn't a problem when the tissue covering is tight.

    One of my tissue covered rubber powered airplanes. This is tissue over mylar which solves the humidity problem mentioned above.

    [​IMG]

    Launching the plane for a max-

    [​IMG]
     
    Last edited: Aug 15, 2019
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  21. Aug 15, 2019 #21

    mcderek

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    Sorry, I got a little carried away there:rolleyes: This is the stuff I use for papering fins.

    [​IMG]

    Apply to both sides of the fin wet. Pin down so it won't warp till dry. The tape shrinks as it dries. For a bit more strength and waterproofness I seal with super thin superglue. Sand with fine sandpaper and paint as desired. Works great! If you leave an extra bit on the fin root it can be use to strengthen the joint.
     
  22. Aug 22, 2019 at 5:50 PM #22

    jqavins

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    I saw some of this thread, and it's very nice work. The frames are plywood, as you said. What about using paper over balsa stick frames, just like the airplanes do? That would even allow for shaping the airfoil of the fins exactly as the wings are shaped on the planes. That's sort of what I did with my Ellipse rocket to make the body tube, but I used basswood sticks and cardstock, not tissue. Doing that for the fins and body tube could make for a really light rocket.
     
  23. Aug 22, 2019 at 7:08 PM #23

    jlabrasca

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    If someone has the patience and dexterity to cut and assemble 40 pieces from 3/32 balsa, I've attached a drawing >smile<.

    Actually, if I had easier access to a laser cutter I might have tried balsa. Plywood is a more forgiving material on the CNC. Also, this will have to get by the RSO -- and "aircraft plywood" seems likely to be an easier sell when trying to rack a (frankly) b*tsh!t design.

    As for whether balsa sick & paper fins would be/could be strong enough, sure. Since I only launch with a club, which means an audience and potential collateral damage, I'd hedge by using c-grain and/or soaking it in thinned epoxy (Minwax Wood Hardener or similar), but balsa is actually a very tough material (under bending stress parallel to the grain/shear stress perpendicular to the grain) and well-adhered paper of sufficient weight would stiffen the frame.

    It'd be worth the time to run the simulations. Since the frame is a small volume of the fin, the weight reduction by using balsa might not do much to improve performance.

    For the Pringles rocket, I am less concerned with it shredding than I am worried about getting it to the field in one piece (although, I am pretty sure it'll be confetti if I load an F67)

    EDIT: There is nothing to stop you from making air-foiled fins from plywood.
     

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    Last edited: Aug 22, 2019 at 7:22 PM
  24. Aug 22, 2019 at 8:22 PM #24

    Rex R

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    I've done 'stick & tissue' fins...works best when the fins don;t extend past the body tube. stones and pavement are not overly forgiving :). i coated mine w/ white glue then shot them w/ clear coat.
    Rex
     
  25. Aug 22, 2019 at 9:01 PM #25

    jqavins

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    Oh, of course there isn't. I was just thinking that, if you're cutting ribs, preferably by laser, then gluing in the sticks (what's the right word? Is it spars?), it'd be no additional work to shape the ribs for an airfoil and root-to-tip taper. Sanding such a shape into a solid fin is most definitely a bunch more work than leaving it flat, or rounded, or even rounded leading and beveled trailing.

    Since I now have access to a laser cutter, I've talked myself into trying this. Some time. Probably with basswood or 1/8" plywood rather than balsa to reduce breakage during assembly; I had a lot of that on the Ellipse.
     
  26. Aug 23, 2019 at 6:32 PM #26

    Ez2cDave

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    Rex,

    I sure hope you meant fins that don't extend BELOW the Body Tube ( swept fins ) - Otherwise, they would have ZERO span . . . LOL !

    Dave F.
     

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