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Old Oct 14, 2012, 09:26 AM
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Adding CF to wing spars...... again

OK, I know this topic has been discussed ad nauseam, and there are sever differing opinions on where (top or bottom) and how much (thickness) CF should be used to reinforce spars on built-up wings.

My question basically asks the same thing, but the wings are foam core.... The cores have slots cut in the top and bottom to accept the spar caps, and the foam inbetween ostensibly acts as the spar web. In addition, some of the wing's strength comes from the skin bonded to the cores.

So, in a foam-core wing as I have described, should CF be added in a similar manner as in a built-up woodie wing?
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Old Oct 14, 2012, 02:09 PM
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You probably should give us some dimensions, and the intended use, and we need to know what kind of foam. Not that I could calculate easily what the shear strength needs to be, but if the shear web is foam only. Hiload would probably take a lot more than, say, bead foam. Of course the sheeting might help with spreading that around.

I'm guessing in a lot of cases it would be worth adding at least some carbon. If the sheeting is balsa or other wood, perhaps it would be a good idea to have something like a 1 inch wide strip of light glass, preferably on the bias, to spread the shear load to adjacent foam. I think this is probably the direction I'd go in, if, say, I was doing a two meter with wood sheeting.

DLG wings often have a carbon rod under a glass skin, and they're pretty strong for the size. By memory, I think on the order of 1/16" or a bit less diameter. A use for your busted carbon pushrods, perhaps.

Another thought is to make holes and put in dowels or something through the core, under the caps, every half inch or something, at least near the root. Lots of work, though. A commercial product I've heard of is, I think, some kind of fiber (fiberglass, carbon??) sewn diagonally through foam, takes up epoxy when laid up with other composites. Can't remember where I saw it, was being pitched for use in wind turbine blades. Maybe could be done in a foam core by sewing it in with a long needle.

A technique I've heard of is to make little slots in the core and put in thin strips of carbon on edge. Or maybe some of the rods I mentioned, spread around slightly. Mostly near the root, of course.
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Old Oct 14, 2012, 11:36 PM
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Originally Posted by LVsoaring View Post
OK, I know this topic has been discussed ad nauseam, and there are sever differing opinions on where (top or bottom) and how much (thickness) CF should be used to reinforce spars on built-up wings.

My question basically asks the same thing, but the wings are foam core.... The cores have slots cut in the top and bottom to accept the spar caps, and the foam inbetween ostensibly acts as the spar web. In addition, some of the wing's strength comes from the skin bonded to the cores.

So, in a foam-core wing as I have described, should CF be added in a similar manner as in a built-up woodie wing?
Cut the entire spar-width span of foam away from the cores. Bond with weight or bagging the carbon to some wood OR the foam itself.

Final spar should consist of carbon or carbon/wood + foam to the same thickness of the material removed from the core.

Wrap with kevlar tow, tight in the middle 1/3rd and further apart as one moves outward spanwise.

Re-insert complete / wrapped spar into core. Tack in place with 3M-77 spray. Layup core as one would ordinarily do for bagging.

The above is shorthand for this:

http://charlesriverrc.org/articles/s.../sparbuild.pdf
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Old Oct 15, 2012, 05:21 AM
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As windlord says, this is the method used on the bagged wing Supra.

Yes, the skin can add a LOT of strength. A well know way to build foam fore bagged wings is via the stressed skin method used by Phil Barnes on the Thermal Dancer and many other gliders. He used a carbon skin on foam with a 1/2 " ID carbon tube for the wing joiner tube. Amazingly strong for a wing with no true spar.
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Old Oct 15, 2012, 11:46 AM
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Well let's back up a moment and clarify a couple of things... the wing in question is for my Quasoar, old-school white foam. Don't know how much strength difference there is between this and newer foams, but I intend to use the original materials. I don't want to cut into the foam any more than necessary as I'm not really skilled at working with foam. I don't need a winch-proof wing, just something that can take a moderate launch with some tapping.

So, with those parameters stated, is it, or is it not worth adding CF to the spar caps? Or would it be better to add biased FG strip?

One last thing, while not really "bagging" in the traditional sense of mylars, wet layup, etc, I will be putting the skinned cores in the bag for curing. Weights just don't seem like they'd be good enough.
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Old Oct 15, 2012, 12:01 PM
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Carbon will be much stronger than glass. You would not want it on the bias. You want the fibers to run along the spar.

The recommendation to wrap with kevlar is just an extra safety step for competition glider to insure the carbon does not delam from the foam or wood spars.

So, is the core all foam, or is there a wood spar that will be embedded also?

I had a foam core wing that was sheeted in Obechi. I crashed it one day so I took a look. There as carbon strip, about 1" wide under the obechi that was bonded to the foam to help handle the bending forces of launch.

Carbon is easy to add and the cost is fairly low so there is really no reason not to add it.

Typically if there is a wood spar of some kind you sand down the wood spar equal to the thickness of the carbon stip, such as what is found on this page:
http://www.cstsales.com/carbon_laminates_48.html

Something like this would typically be epoxied to a wood spar cap or into a channel in the foam. comes in various thicknesses, widths and lengths. You might put .021 on the top of the spar/core in the center pannel of a three piece wing .014 on the bottom as the greatest stress is on the compression on the top skin/spar. Then maybe .007 top and bottom on the tips, or nothing at all. The center takes the most stress. For a contest grade spar you would likely use thicker stuff, but this would probably serve your goals You are not trying to make it winch proof but more goof proof in case you over pedal it a bit.

Top - Just and example

Rectangular Carbon Fiber Laminate
0.021" x 0.5" wide x 48" long C1224 $6.40
or
Rectangular Carbon Fiber Laminate
0.021" x 1" wide x 48" long C1214 $10.65

Bottom - to match a wood spar or just embed in the foam.

Rectangular Carbon Fiber Laminate
0.014" x 0.5" wide x 48" long

The real builders on the forum will give you better advice. You can look at the construction of the Supra wing to see how it is done for contest grade wings ( The charles river link above) but as I said you don't have to cut out the foam to create a supra grade spar, just trim out a little foam, embed the carbon strip with epoxy, then put on your skin. It will add a huge amount of strength for relatively little work.
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Old Oct 15, 2012, 12:48 PM
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I've built many a foam core wing for power planes but never for a "glider". I know the glider wings are much thinner. If no carbon is in the wing you can't beat Gorilla Glue or it's equivalent. A light coat is needed like glassing a wing. If it looks wet you have to much on type of thing. It really penetrates the foam for a EXCELLENT bond. GG would be perfect in a bag. I don't know how it would work with carbon. You might have to use epoxy to get bonding to the carbon???????

Ken
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Old Oct 15, 2012, 01:55 PM
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@ aeajr, to answer your question, there are notches cut ito the coers along the span to receive 1/8 x 3/8 (maybe it's 1/2, not sure) spruce spars.

Your comment on the ease of adding and the low cost is duly noted, and pretty much what I was thinking, but wanted to get some more input before going the extra mile of ordering the CF.
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Old Oct 15, 2012, 02:07 PM
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Perfect. Sand the wood spars, to take the carbon and end up the same thickness. The carbon is so much stronger than that wood for this purpose.

Then lay the spar on the bench, apply epoxy and lay the carbon on it, square it up, then put wax paper over it and apply weigth to be sure you get a tight bond.

Post 81 has some documents on adding CF to a wood spar.
http://www.rcgroups.com/forums/showp...3&postcount=81

If you want, wrap the result with carbon thread, kevlar thread or, I use unwaxed dental floss. . this insures the wood and carbon can not separate. About 3-4 wraps per inch is good. Make it tight, then apply a little CA to bond the thread. But again, for your purposes, this is optional.

Should help keep you from overstressing the wing.

What size joiner rod will you be using? A lot of the old birds used 3/16 rod which is very thin. Now that you have a stronger wing, you can go up to 3/8" rod.

This is how the spar of a bagged wing Supra is built. Probably more work than you want to do but it may give you some ideas. Notice how he caps the carbon with some balsa for sanding purposes to conform to the shape of the foam. If you need to shape, this should work well.
http://www.charlesriverrc.org/articl.../sparbuild.pdf
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Old Oct 15, 2012, 03:56 PM
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When I mentioned the glass strip on the bias, the purpose was to spread the extra shear load from the carbon to more foam. White foam is very weak compared to extruded foam, especially something like Highload 60. White foam is not supposed to be bagged at more than 7" Hg or something, which is on the order of 4psi! Regular extruded foam is rated, by the manufacturer, to around 15 to 25 psi, depending on the density. Highload 60, surprise surprise, is rated to 60 psi. There are also 40 and 100 psi versions, which are lighter and heavier, respectively. So be careful if you are trying something normally used on bagged foam wings. On the other hand, that wood skin is probably much stiffer locally, so it may help make up for the weakness of the foam.

BTW, as I recall, the Quasoar is ok for moderate launches with tapping, just the way it is. It's meant for that. Carbon just gives you a bit more strength.

An easy way to add strength would be to make the sheeting for the inner foot or two of the semi span, at the thickest place on the wing, out of really hard balsa or even basswood, spruce, etc. Maybe make it tapered so that there is no hard spot at the outer end. Or, after bagging the sheeting on, you could rout out little slots over the spar caps and put in some 1/16" carbon rod (square would be best) with epoxy. Then fill over and sand. This takes advantage of almost all of the thickness of the airfoil.


Yet another idea: sand a thin spot in the sheeting and put flat carbon in there ahead of time, on the inside. Lots of ways to do it.

I suspect if your glue is good, the only wrapping really needed is over the joiner. I also suspect that sevaral times more unwaxed dental floss might be needed, compared to kevlar or the like.

I haven't seen a lot of test data on spar construction. I suspect many of the methods are not as good as we think. I know Mark Drela did some testing. Possibly posted in the Allegro Lite Yahoo group?
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Old Oct 15, 2012, 06:31 PM
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Originally Posted by lincoln View Post
When I mentioned the glass strip on the bias, the purpose was to spread the extra shear load from the carbon to more foam.
That's what I was thinking. A strip of bias cloth an inch or so wide would do wonders for both spreading the shear load out to the foam, and for gathering the skin loads into the carbon on the spar. The carbon must, of course, be spanwise.

I was actualy thinking of laying the glass in to the slot first, then the carbon, then the original wood, then the skin? Having the carbon on the outside is better, I know, but I was thinking about the cloth wrapping up the sides of the original wood so that the carbon and wood spar couldn't separate. (similar to the kevlar thread, but easier?)

As to testing- Anker did some terriffic test work a couple of years ago, it's around here someplace, maybe the Thermal forum?
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Old Oct 15, 2012, 07:41 PM
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Thanks for all the input guys. I hope to get back to it in a couple of weeks, and will document the progress.
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Old Oct 15, 2012, 08:21 PM
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If you are talking about a wing in excess of about 2.5 meters and you are contemplating winch launching you may be disappointed with the spar design you are contemplating.

Carbon cap spars derive most of their structural properties from the compressive strength of the spar web and the tensile properties of the spar wrap. Some relatively dense foams can be suited as web material, but end grain balsa the width of the spar caps is hard to beat. Kevlar tow, Kevlar cloth and dental floss are acceptable spar wraps IF THE CORNERS OF THE CAPS ARE PREPARED WITH ADEQUATE RADIUSES TO ENSURE THAT THE WRAP WILL NOT BE CUT BY SHARP EDGED CARBON.

Mark Drela has published detailed information on spar design. And without regurgitating all of his wisdom it is worth understanding that carbon caps without webs and wraps are really not much stronger than similar unsupported spruce spars.

Your observation that DLG's with carbon rods are quite rigid is correct. But that strength goes away very, very rapidly as the span and launch load increase.

Happy Landings,

Don
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Old Oct 15, 2012, 09:12 PM
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Rigidity and strength are two different animals... A wing can be rigid but not very strong. Fibrous materials like wood and cf are (as a general rule) twice as strong in tensile as they are in compression. It will not matter how well they are braced with webbing and wrapping if the top cap is overloaded in compression. That's why the Hawk and others of my design have twice the thickness on the top cap as on the bottom. Some will argue that for modern winch launches that they need to be equal. Perhaps for their launch techniques, they are correct. For the way I do it, that's not so. During my launches, my wing never has a negative load, because I use a very shallow bucket.

Just my .02 worth... added to what mi amigo from Oklahoma said above... Hi Don!

Jack
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Old Oct 15, 2012, 09:38 PM
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It all depends. If your carbon is good enough, you end up designing for stiffness more than strength. In that case, the caps should both be the same size.

As far as the compressive strength of the web and the tensile strength of the wrap, they may not matter much beyond a certain point. Much of their utility is in preventing buckling, and for that you need stiffness. Come to think of it, if we are trying to make things ridiculously strong, we can put .007" on the inside surface of the spar cap, and thicker carbon on the outside surface. This will make the cap and the carbon much less prone to buckling.

Let's say you have a 120 inch wing, with the carbon part of the caps 0.75 inches apart. Load on the airframe is 100 lbs. We'll assume that gives us something like 1200 inch lbs. Force in cap is 1600 lbs. If the cap is .035" X 0.5", that's only about 91kpsi. Carbon can take a lot more than that if it's really straight. So much of that carbon is just there for stiffness.
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