Conference vmszoo::rc

Glass has a big advantage over carbon fiber - I've got some!

>                Putting a strip of glass between two strips of balsa does
>        very little to  stiffen the spar.  To stiffen it you add material
>        of little stretch on  the top and bottom of the lamination.  This
>        could be anything that has  a  lot  of  shear strength and little
>        elasticity:  hardwood, glass, carbon fiber,  etc..  Watch out for
>        materials with no elasticity, they may crack on a severe landing.

Anker your argument doesn't seem to be based on fact - I've seen some articles
about glider wings that laminates carbon fiber between the wood.  Why should
it be better to put it on top and bottom?  Also several expertly build
wings have a single layer of carbon fiber on one end (top or bottom) of
a spar.  Some builders only carbon fiber the top spar or the bottom spar.

It would appear that the experts only know that carbon fiber adds strength
and that they don't know the optimal way to apply it.  Or maybe it really
just does add strength and the method of application has little effect
on the results.

Soooooooo let me put my question in another way.  Can thin Fiberglas
laminated to balsa spars add significant strength and/or rigidity?

Bye          --+--
Kay R. Fisher  |
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    Re: .58
    
    Sandwiching it in the spar will add strength without adding rigidty.
    You don't want rigid glider wings, you want them flexible. Just look at
    a glider going up a hi-start with it's wingtips pointing away from the
    tow... To make the spar more rigid you want it top AND bottom. I always
    liked using glass for stiffness but you need to get it on evenly so you
    don't have weak spots. I would thing you could do the same thing by
    putting hard 1/16th" balsa upright between/across the spars with the 
    grain going top to bottom (spar to spar). I've built several models that 
    have called for this and it always helped on my CL combat planes.

    I use CF tape on the bottom of the spar in the wing of my big
    electric glider.  I did this because I had a wing fold while
    pulling out of a dive on an earlier version.  The net effect of
    adding the Carbon Fiber is the same as when you add something like
    nylon tape to the bottom of a foam wing on a plane like a Scooter.
    You are forcing the upper part of the spar to take a compression
    load when the spar bends, but the lower part of the spar is
    releived of the expansion load by the CF, which is bonded to the
    spar and incapable of stretching.  The advantage is, of
    course, that the wooden spar can withstand a much higher
    compression load than it can an expansion, and so the spar
    survives higher stress levels without breaking.

    The spar becomes even stronger when you laminate some CF to the
    top too, since now, the CF resists bending moments in both the
    upward and downward directions.

    Laminating some CF between two spars is an inbetween alternative
    to the top and bottom method.  The spar on top of the laminated CF
    is strenghtened when the ends of the spar are pushed upward, much
    like a spar with a single laminate along its bottom, and the spar
    on the bottom is strengthened in the opposite direction.

    CF is easy to apply, I just use CA to glue it down, it adds little
    weight (20 ft of the stuff probably doesn't even weigh an ounce),
    takes up next to no room (so you don't have to rework all of your
    wing ribs a lot to accomodate the lamination on the spar), and
    makes a noticable difference in strength.  I recommend it.  It is
    much more suitable than fiberglass for reenforcing spars.

    Tower sells someones Carbon Fiber tape. I can't remeber the Man.,
    but if youcan't find it, lemme know and I'll go home and dig the
    original package out of my building stuff.  Basiclly (how do you
    spell that word, anyway?)  It comes as a one inch wide "tape",
    about 6 feet long, wrapped around a 5 or 6 inch long piece of
    cardboard.  The "Tape" is actually just a *P_SSLOAD* of wicked
    thin strands (like hair size, or smaller, even) kind of jumbled
    together so that they form a coherent mass thats, like I said,
    about an inch wide, and maybe a 16th thick, by 6 feet long.

    You open the package, unwrap the tape to its full length, and
    separate however much you want to use from the edge of the bundle.
    You wre-wrap whats left, and then glue down the chuck you removed
    from the whole.  You don't need all that much, as you can confirm
    to yourself by taking something like a 1/16th bundle of strands
    and trying to break them by pulling (you will find it quite
    difficult, if not impossible).

    With the CF tape, you don't really need to be as careful as you do
    with the CF composite pieces (the ones that are a solid piece like
    a piece of balsa stock, rather than what I'm describing, which is
    loosely bundled hair-like strands).  The composites often splinter
    when you cut them.  The splinters, which have been known to fly
    about when they break free, are quite small, very sharp, and
    incredibly strong, and so really necessitate the use of gloves and
    saftey goggles to ensure no unfortunate mishaps (if you get a
    splinter in your finger, you'll never be able to get it out with
    tweezers, and it will cause a lot of irritation, if you get one in
    your eye, the results could be considerably less pleasnat.

    I bought some carbon fiber tape at Tom's Hobby Korner in Chelmsford.

                                - Dan Miner

    Well, it's been a while now, and we haven't heard the final verdict. 
    Is this tape found at the loading dock the real thing or not?  Maybe we
    haven't been hearing anything because indeed it really IS carbon fibre
    and those involved have gone underground to protect their "free" source
    of this stuff ;-}.  Just kidding.  
    
    But please let us know one way or the other.
    
    Incidentally, an earlier note suggested that even using a plastic or
    nylon tape on top/bottom of spars can be better than the raw spar. 
    I've been tempted to try using some nylon tape myself, but it would not
    adhere with CA glue, and I've been too busy to try a sample with epoxy. 
    Has anyone else done this, and if so, what are the results?  
    
    As far as CF is concerned, I just bought a 1"x24" piece from Tower in
    the $4-5 range.  I had a sample from Aerospace Composits (not sure of
    company name) and it works great for repairs of spars and fuse sides
    and other high-stress areas.
    
    Regards,
    Joe

        Re:                      <<< Note 415.15 by CIVIC::MARRONE >>>

        Joe,
        
                Nylon is  totally  useless  for  this  purpose because it
        stretches.  The whole point is that the reinforcing tape prevents
        the spar from bending  to the point where the wood pulls apart.
        
        Anker

    I can categorically state that the packing/strapping tape is not
    carbon fiber. It is some sort of plastic, polyethelene, PVC, or
    related material.
    I would hate to think of the cost of using CF as strapping tape,
    as well as the lacerated hands from handling it. 
    
    Using CF as spar reinforcement is only as good as the glue joint.
    I've seen crashes where the CF pulled cleanly away from the spar
    without even taking any wood with it. It obviously contributed little
    or nothing to the overall spar strength.
    
    Another way to reinforce spars is to wrap them with Kevlar cord.
    This may be impractical on certain types of foam wing structures,
    but if the entire spar assembly is completed before installing in
    the wing, then it's feasible.
    
    Edmonds Scientific sells a large roll, ~700 ft., of small diameter
    Kevlar cord at a reasonable price, ~$10-15, that is good for wrapping
    purposes. 
    
    Wrap it around the spar spacing each wrap ~1/8" apart, then hit
    it with thin CA. That portion of the spar will be intact long after
    the rest of the plane has disintegrated.
    
    Terry

    Kay got a sample of the stuff from the guy in the base note and we
    checked it out at one of our lunchtime sessions. It seems to be some
    type of plastic coated braid but we couldn't tell what it was. Probably
    nylon or another plastic. It was very strong and stretch resistant but
    I didn't feel comfortable with it as a CF replacement.
    
    I just bought a 2"x48" piece of carbon fiber strip and almost died when
    it rang up as $15+ at Tom's. I hadn't paid attention to the prices in
    the magazines. My previous CF was the Dave Brown "horsehair". I thought
    the ribbon would be better (read "more uniform") under fiberglass when
    vacuum bagging so I picked it up.

    Carbon fiber tip to tip on the bottom of the spar....  That broke too..
    
    Speaking of which, Ken B (the CD from the contest) told Mike Stains
    that C.F. is only good in compression...  He says that you should run
    the C.F. on the top of the top spare, not the bottom of the bottom
    spare....   I find that very hard to believe...  Anyone care to 
    dispute it??
    
    jeff
    

    I've heard that also. My general plan is to use it on both sides. Might
    want to fly inverted to lose some altitude.
    
    My S3014 Gnome foam wing (say that three times fast) just had CF on the
    bottom and failed by compressing the foam (and folding the wing)

        re: .233
        
        One piece of CF is pretty weak. Two, with something between them
        form a VERY rigid I beam structure. The "something" can be foam,
        balsa (end grain shear webbing would be best), spruce, or
        whatever. Failure of this structure usually comes from destroying
        the something in between, not from the breaking of the carbon
        fiber.

    I'm about to build another built-up wing. I'm going to use the CF
    laminate on the spars. The wing will be a "D" tube design. I'm playing
    with ideas for the CF on the spars. The June 91 MAN had a section on
    using vertical CF laminate as spars in foam wings. This got me thinking
    about the following design:
    
    
    
    
                     bbbbbbbCssWbbbbbbb               b = 1/16" balsa
                       ^    CssW   ^                  C = Carbon Fiber lam.
                       |       W   |                  W = Ver. grain shear web
    leading edge sheet +       W   + Cap strips       s = 1/4" sq balsa spar
                       v    CssW   v
                     bbbbbbbCssWbbbbbbb
    
    Trying to bend CF laminate widthwise should be stiff enough for
    launches. How would this compare to using the same laminate on the
    inner (or top AND bottom) spar surfaces?

    Jim,
    
    Based on the way most successful spar structures are being built
    now, I'd say your layout would work if you're looking for the absolute
    lightest structure but the one thing that will kill you is the balsa
    spars.
    You're asking the CF and the shear webs to take nearly all the load.
    Even 1/4" balsa is so much lower in tension or compression strength
    that it's only along for the ride. 
    I suspect what would happen in failure mode is that the CF would
    buckle forward or aft, away from the spar, and blooie!
    If you substituted 1/8" X 3/8" spruce for the balsa, you'd have
    more strength and more ability for flexing before failure.
    
    Here's a cross section of the Legend spar in the center 8" of the
    wing:
                                c = .007 CF
                      ccc
                     xsssx      s = 1/8" X 3/8" spruce
                     xpppx 
                     xpppx      p = 3/8" X 1/2" ply
                     xpppx
                     xsssx      x = 1/16" x 3/4" ply
                      ccc
    
    Everything runs full span on the center spar, ~40", except the 3/8"
    ply is replaced by 3/8" balsa vert. grain, between the spars, outboard
    of the center 8". 
    The whole assembly weighs 142 grams, 5 oz., and would make a nice
    baseball bat.
    But it can live under a full bore winch launch with no visible flexing.
    
    Terry

>    Trying to bend CF laminate widthwise should be stiff enough for
>    launches. How would this compare to using the same laminate on the
>    inner (or top AND bottom) spar surfaces?

I'll add some controversy since I already told Jim the following:

You don't want to stop it from bending (how else do you measure stress
on tow) what you want to do is stop it from breaking.

Stick with the carbon fiber on the top spar.

OK - ready for rebuttals.

Bye          --+--
Kay R. Fisher  |
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    Ok, back into the shop and check out the supplies... I've got some
    1/8"x3/16" spruce, some 3/16" square, and some 1/8" square left from my
    last bulk buy. I'll put this into perspective. This is a 2 meter
    replacement wing for my GL. Different airfoil (S3021) since I saw how
    well a S3021 did this weekend in iffy conditions. How about this:
    
          ss            cc
    	  cc		ss		c = CF laminate
    	  bb		bb              s = 1/8"x3/16" spruce spars
    	  bb	 or	bb		b = 3/16" vertical grain shear web
    	  bb		bb
    	  cc		ss
    	  ss		cc
    
    I like the CF on top and bottom since I have been known to fly inverted
    to get down (with flaps extended it's a fast alternative but looks
    funky). Is it better to put the CF inside the spar sandwich or on the
    outside? I'll go with 1/16" ply dihedral braces on both sides and only
    sheet the leading edge of the wing
    
    BTW: Jim Tyrie broke his Falcon 880 wings on the winch yesterday
    BECAUSE he went to the "can't bend" wing rods. He was bending the
    old ones on every flight. He didn't bend the rods but the force that
    they were taking bent the wings outward from the wing rod tubes.

    re .814
    
    Jim,
    
    The version on the right should work fine for a 2 meter wing.
    It's easier to put the CF on the outside, and avoids the possibility
    of building a wavy spar. Strength wise, outside is just as good
    if your careful to get an even glue coat over ALL the bonding surface
    of the CF. Use slow set CA and press the spar down against the bench
    until the CA cures.
    
    Terry

    Look at the limb of a modern laminated bow. What you find are two
    layers of fiberglass or graphite, laminated to the top and bottom of a
    piece of wood. When the bow string is pulled back, one side of the bow
    limbs are stretched, and the other sides are compressed. The center is
    there just to hold the two sides in the proper relationship to each
    other.
    
    When a bow limb breaks, it is usually because the limb becomes
    delaminated, either the glue bond breaks down, or the wood between the
    two layers of fiberglass/graphite sheers apart.
    
    The same kind of forces are exerted on a wing spar. If you could build
    a spar with carbon on the top and bottom, and fill the space in the
    middle with a material that could be bonded well to the carbon and that
    has good sheer strength, then you could build an almost unbreakable
    wing.
    
    I don't have enough experience building models to tell you how to do
    this.
    
    	Marty Sasaki

    I wouldn't attempt the scroll saw bit unless you have some way to
    maintain a perfectly straight feed, down the entire length of the
    strip, AND wear a nose mask.
    
    The best , but tedious way, is to tape a long metal straight edge
    on top of the CF strip, then repeatedly and lightly score the surface
    with a single edge razor blade or X-acto, until it's cut through.
    .007 shouldn't be too bad. I've done it with .14 ". Wear a mask
    and look out for splinters.
    
    Terry

        I sometimes use a balsa stripper to cut CF strips. Another
        favorite is my old darkroom paper cutter - it works real well on
        CF.

    LeRoy Satterlee, the soaring columnist for RC Report and the
    re-designer of the Chuperosa, has an excellent article on building
    spars for foam wings in the October '91 issue, page 98.  This article
    alone earns the price of the issue.  I now assume that the spars in the
    Chuperosa kit are his design. (The pre-RCM-article Chup had a built-up
    wing.)

    The Chup kit uses the foam as the shear web.  Flat spruce spars are
    glued to the top and bottom of the web, and then the foam is slit fore
    and aft for the dihedral brace.  The design is very, very strong. 
    Several of us consider this wing to be almost bullet proof.

    The article in RC Report deals with the fabrication of conventional,
    thickness-of-the-wing spar structures for insertion into a foam wing. 
    The problems he addresses are: 1) building the spar with the correct
    thickness and taper and 2) jigging the insertion process.  He also
    discusses the structure for wing rods, but that is incidental and, in
    my opinion, not as well thought out.  As for carbon, he recommends
    putting it on the bottom of the top spruce and the bottom of the bottom
    spruce.

    The fabrication process would apply even to kits that do not anticipate
    spars.  He cuts out a top-to-bottom slot **and saves the scrap**.  The
    scrap is then laid on its side on a building board and sandwiched
    between two straight edges of the same thickness.  Then the scrap is
    removed, leaving the straight edges as a form for building the spar. 
    Note that this technique could be slightly modified to apply to wings
    that were not to be straight (e.g. the Hobie Hawk's) or uniform taper.

    The jigging is utterly trivial; he uses the foam core beds.