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Re: Reinforcing Balsa Wood

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  • lynn_b_nutz
    Agreed. If you *really* need lower viscosity, heat it up a bit and deal with the shorter pot life. The best plan is to leave the chemistry alone. I ll take
    Message 1 of 32 , Apr 1, 2008
      Agreed. If you *really* need lower viscosity, heat it up a bit and
      deal with the shorter pot life. The best plan is to leave the
      chemistry alone.

      I'll take two minutes and tell all y'all everything I think I know
      about composites. It may not even that long. :)

      Just coating something with glue, any glue, won't do much for
      strength. Glue adds strength when it's sticking something TO
      something. And just sticking something to the surface of something
      else won't do much, either. Surface bonds with no saturation depth
      don't have anything to work with, so to speak.

      The deal with laminating for strength, and especially for obtaining a
      high strength-to-weight ratio, is to use the strongest combination of
      glue and laminate so you can use the least amount of it, and to use
      it over the lightest substrate that offers the highest *compression*
      resistance suitable for the intended purpose and loads.

      Tensile strength is good for substrates to have--and occasionally
      very necessary in some designs--but substrates with high tensile
      strength typically are heavy. Aircraft plywood is much heavier than
      hot-wired styrofoam, for example. Compression strength is easier to
      achieve with very light materials, so the tendency is to design for
      laminated surface to carry the dynamic load and depend on the
      substrate to carry only the processing load--generally compressive,
      such as seen in vacuum bagging, for instance.

      A couple of good candidates for substrates are styrofoam and end-
      grain balsa. Thus, it's no accident that Scaled Composites uses a
      lot of styrofoam and end-grain balsa, right? And 'Vettes have a
      section of their chassis floor pan that's end-grain balsa sandwiched
      with carbon fiber. Marvelously light, strong, and rigid.

      For relatively simple part shapes, the substrate (foam or whatever)
      contributes the general shape of the piece and a bit of compression
      resistance. That's it. The laminations do all the work. They
      supply the strength, the durable surface, and the place to put the
      shiny paint.

      Copy paper over thin balsa fins works because just about anything you
      can do to thin balsa fins will help to a degree, probably a large
      one. The copy paper itself isn't all that strong, right? But when
      saturated with wood glue, it becomes quite strong after it's all
      dry. A composite, if you will. But would copy paper and wood glue
      be as good as autoclaved carbon and epoxy on the 'Vette floor panel?
      Nope. A matter of degree, certainly, but the principle is exactly
      the same.

      In the basic statics/solids/strength-of-materials courses, one
      discovers that in any structure, the load travels through the
      stiffest path. It sounds odd at first, but it makes sense. The path
      that's not the stiffest flexes until the stiffest part of the
      structure is carrying the load. In composite work, that'll almost
      always be the outer surface. Which is cool, because those same
      engineering courses also teach that the load in a structural member
      is greatest at its outermost fiber. Again, that's perfect for
      composite construction.

      A bit of trivia: It was that tidbit of info that drove the invention
      and development of monocoque construction in the early days of
      avaiation. The earlisest birds were built like buildings were at the
      time: a strong frame with a relatively flimsy covering. (Early
      aircraft designers apparently missed part of the genius of the Wright
      Brothers. Much of the strength of their Flyer came from the cloth
      surfaces. The ones who got it tended to use the cloth to reinforce
      in one direction only. The Wrights used it in two directions.
      Marvelous insight and ingenuity, that.) Digging around in the
      emerging science of load path dynamics led to the understanding that
      airplanes could be built the other way around with a stronger skin
      that would allow a much lighter supporting framework (not to mention
      the whole new range of shapes that could be experimented with).

      Looking at drawings for WWII aircraft will show that the strongest
      parts of the airframe were the engine mount(s) and the wing between
      the landing gear. The big honking engines only had a very few
      attachment points, so all the dynamic load had to go through these
      few, very strong places. Attaching the engine load to the airframe
      required that the load "fan out" from the discreet attachment points
      quickly to hook up and distribute it to the relatively thin, but
      complex, sheet metal airframe. Brilliant design work there. And the
      wings between the landing gear absorbed all the landing loads which,
      under combat conditions and especially for carrier-based aircraft,
      could be brutal. And it still is. Some things never change, right?

      The copy-paper-and-wood-glue lamination works because the relatively
      flimsy paper becomes pretty much saturated with the liquid wood
      glue. When it dries, the fibers in the paper are held rigidly in
      close proximity with each other. That's precisely the mechanism that
      makes all composite construction work. Is fiberglass cloth all by
      itself particularly strong? Not really. And it's certainly not
      rigid, right? But wrap a layer or two around a thin slice of really
      flimsy styrofoam, saturate it with a decent grade of laminating
      epoxy, ignore it until it's properly cured, and you suddenly have a
      control surface for a home-built aircraft that's good for more g-
      forces than its human pilot can withstand. Seeing what could be done
      with glue, cloth, and foam was just as amazing to me as discovering
      that the human body *requires* salt that is made from sodium and
      clorine, both of which are deadly to humans singly. Amazing.

      But the key to composite construction is saturation. Every fiber of
      the laminate must be completely stuck to the other fibers around it.
      For our purposes, we can usually see bubbles and dry spots or other
      anomalies and work them out of our layups. And if we miss a few, the
      things we build generally are over-built enough that it makes no
      difference at all. (Except for that dreaded--and usually real
      loud, "Dude, your new carbon bird is killer, but did you know you got
      a big boogery spot here in your layup? See? It's right here."
      Don't you hate when that happens?)

      However, for the thick, complex, dense layups the pros use in the new
      monster passenger aircraft and other aerospace apps, a bubble or a
      dry spot or a de-lam can spell disaster. Hence the expensive X-ray
      and other in-depth sorts of inspection that high-risk designs require.

      Again, it's all about the saturation. Every fiber must be stuck
      firmly to every other fiber.

      Which brings up fiber orientation. Last year, we were given some uni-
      directional carbon fiber. A contractor had some left over after a
      big highway project was completed, and it was either throw it away or
      give it away. Somebody told them about us, and we got a very nice
      surprise! In our experiments with some scraps and end pieces, our
      kids laminated two small pieces together with the "grains" aligned
      and two more with the grains parallel. Saturated them with some
      Aeropoxy, put 'em between a couple of pieces of plastic wrap, stacked
      some books on 'em, and ignored the mess over a weekend.

      Come Monday morning, the perpendicular grain sample was stiff but
      still slightly flexible. If you tried real hard, you could bend it a
      little with your bare hands. The kids liked that a lot.

      The parallel grain specimen, however, was incredibly stiff across the
      grain. Couldn't budge it with bare hands. Rock solid. But it
      snapped like a potato chip along the grain. No kidding. Very little
      force required. Snap, and that was it. We were amazed at how little
      effort it took.

      The kids picked up the perpendicular grain sample and said, "Well, I
      guess we found our layup pattern." :)

      Uni-directional carbon fiber is good, but be paying attention when
      thinking through your layout plan. Not much forgiveness built into
      uni. Food for thought.

      But fiber orientation aside, saturation is the key. And thinning the
      epoxy to get more soak-in might not do anything for you but add
      weight. Saturating the layers of lamination is essential, but the
      lamination just needs to be stuck to the substrate. That's it. If
      you thin the epoxy enough to soak a lot of it into the substrate,
      you've just added weight. And the stuff that soaks in is pushing air
      out the other side. Capillary action pulls the liquid in, but the
      liquid displaces the air in the voids. It's the voids that make
      balsa light, right? Fill the voids with epoxy, and it's not light
      any more. And that air has to go somewhere, probably out the other
      side in the form of bubbles under the laminate. Vacuum bagging might
      be the answer, or part of it, but what you want is a good surface
      bond between the laminate and the substrate. The substrate supplies
      the initial shape and a bit of compression resistance. The laminate
      itself needs to do almost all of the work.

      Bryant's Law: If some is good, and more is better, then too much is
      just right!

      That works for lots of stuff, but not necessarily for laminating.
      Too much epoxy is just that, too much. Generally, for hobby work we
      try to shoot for a fifty-fifty ratio by weight between epoxy and
      cloth. Does more epoxy make it stronger? Nope. The cloth fibers
      need to be stuck to each other hard, and the whole mess needs to be
      stuck to the substrate hard. However much glue it takes to make that
      happen is what you need, no more, no less. If you need more
      strength, add more fibers. And just enough more glue to stick it all
      together.

      If memory serves, Mick mentioned a while back that the aerospace pros
      routinely get glue-to-fiber ratios down to percentages in the mid-
      thirties or so. Also if memory serves, heat had a place in the
      process that gave those results. Elevated cure temps were required
      to get the desired physical properties from the particular epoxy used
      in the process, and the reduced viscosity from the heat aided in
      squeezing out the last drop of excess glue. Doing composite parts in
      heated pressure molds gets the strongest and lightest parts, but
      that's possible only with unlimited funding. Your tax dollars at
      work. :) Not cheap, but certainly effective.

      Hope this helps.

      Lynn B.





      --- In CompositeRockets@yahoogroups.com, "Warren Musselman"
      <wbmussel@...> wrote:
      >
      > One rule - at least with West Systems and Aeropoxy - never thin
      with more than 10%
      > Acetone by volume or you risk damaging the ability of the polymer
      to crosslink.
      > Best way, according to West Systems, is to heat the epoxy in a
      double boiler to
      > 115 degrees F just before application. This will shorten cure time,
      but will allow
      > the epoxy to be a lot runnier and soak in better. Also be aware
      that you will
      > probably get outgassing from the balsa wood that will produce
      bubbles in the
      > surface of the epoxy.
      >
      > If you don't apply fabric, you're not gaining much strength at all.
      >
      > Warren
      >
    • lynn_b_nutz
      Sorry, pulled the trigger too soon. Maybe next year for the lathe, if plans gell. Wish me luch! Lynn B. ... up ... out ... 4 ... a ... know ... that ...
      Message 32 of 32 , Apr 3, 2008
        Sorry, pulled the trigger too soon.

        Maybe next year for the lathe, if plans gell. Wish me luch!

        Lynn B.

        --- In CompositeRockets@yahoogroups.com, "lynn_b_nutz"
        <lynn_b_nutz@...> wrote:
        >
        > Hi, John,
        >
        > I'd like nothing betther than to have the wherewithal to just fire
        up
        > a lathe and carve out a billet nose cone from aluminum.
        >
        > --- In CompositeRockets@yahoogroups.com, <jbolene@> wrote:
        > >
        > > probably easier if you have to machine it, is to make that cone
        out
        > of AL
        > >
        > > I made a hollowed out one for my 2" diameter rocket, and made a
        4"
        > one out of 2x4's glued together and hollowed with a 2" long AL top
        > cone that was threaded and acted as the top nut to which the
        > retaining eyebolt was screwed into
        > >
        > > ---- lynn_b_nutz <lynn_b_nutz@> wrote:
        > > > Thomas,
        > > >
        > > > I've never even thought of using Apoxy for a nose cone, but now
        > that
        > > > you mention it, I don't see why it wouldn't work. If you need
        a
        > > > heavy nose for stability purposes, give it a shot and let us
        know
        > how
        > > > it works. But if you need light and thin, I'd use something
        > else.
        > > > It'd be great for the male plug for a mold, though. I like
        that
        > idea
        > > > a lot.
        > > >
        > > > If you need to machine something anyway, try machining
        styrofoam,
        > > > then 'glassing that. Fast, inexpensive, easy, and the results
        > can be
        > > > absolutely first class.
        > > >
        > > >
        > > > Lynn B.
        > > >
        > > >
        > > >
        > > > --- In CompositeRockets@yahoogroups.com, "kernes" <tkernes@>
        > wrote:
        > > > >
        > > > > Lynn-
        > > > >
        > > > > this sounds like pretty good stuff. I have a CF airframe
        that
        > I am
        > > > building that has a non-
        > > > > standard airframe size...so I need a nose cone. Do you think
        > this
        > > > stuff could be cast and
        > > > > then machined for a NC?
        > > > >
        > > > > let me know your thoughts. I have pictures of the bird in
        > progress
        > > > if you want to see it as
        > > > > well.
        > > > >
        > > > > Thank you.
        > > > >
        > > > > Thomas Kernes
        > > > >
        > > > >
        > > > >
        > > > > --- In CompositeRockets@yahoogroups.com, "lynn_b_nutz"
        > > > <lynn_b_nutz@> wrote:
        > > > > >
        > > > > > By the way, I recommend the "natural" shade. My teaching
        > buddy
        > > > > > ordered the black version for his L3 bird. It's carbon and
        > black
        > > > > > paint, so why not? Well, the color is okay for that
        purpose,
        > but
        > > > the
        > > > > > stuff stains surfaces that the natural finish won't.
        > > > > >
        > > > > > By all means, get whatever color you need, but be aware
        that
        > the
        > > > > > natural isn't as messy to use. All of it's good, but use
        the
        > > > natural
        > > > > > if you don't really need a color. That'll save some clean-
        up.
        > > > > >
        > > > > > Also, I've used both Sculpt and Fixit. I can't tell any
        > > > difference
        > > > > > between the two, at least for rocketry purposes. Also,
        it's
        > > > great
        > > > > > for building up airfoil-shaped standoffs for rail buttons
        to
        > keep
        > > > > > rail rash down to a minimum. You'll be amazed at what you
        > can
        > > > use
        > > > > > this stuff for. We'll use a pinch or two of it Friday to
        > give
        > > > the
        > > > > > last tiny tune up to the pointy end of our home-made nose
        > cone,
        > > > for
        > > > > > instance. Good stuff.
        > > > > >
        > > > > > Lynn B.
        > > > > >
        > > > > >
        > > > > > --- In CompositeRockets@yahoogroups.com, "lynn_b_nutz"
        > > > > > <lynn_b_nutz@> wrote:
        > > > > > >
        > > > > > >
        > > > > > >
        > > >
        > http://www.avesstudio.com/Products/Apoxie_Sculpt/apoxie_sculpt.html
        > > > > > >
        > > > > > > I used it for the edges and fillets on my L3 bird. Giant
        > Leap's
        > > > >
        > > >
        > > >
        > > >
        > > > ------------------------------------
        > > >
        > > > Yahoo! Groups Links
        > > >
        > > >
        > > >
        > >
        > > --
        > > John Bolene
        > > http://mostlymissiles.com
        > >
        >
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