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Old Mar 19, 2011, 04:43 PM
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Disser questions

Hi guys

I've recently started experimenting with wing bagging and disser layup. For now I bagged a couple of wings using carbon tow for +/-45deg local strips and 1.6oz glass on top. After I get the wings from vacuum I always get the "ridges" of the tow, I mean the surface is not smooth but rather with bumps at the carbon locations. I always get this no matter how hard I flatten the tows with a roller.

Is there a way of getting a smoother glass surface?

Another question is, did someone tried to layup the wing with +45deg tows on top and -45deg tows on the bottom? rather then running it +/-45deg on both surfaces. Thought that this will save weight and time without significant stiffness penalty.


Thanks
Dani
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Old Mar 19, 2011, 07:13 PM
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Thicker mylars that are thinned down in just the leading edge areas
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Old Mar 19, 2011, 07:44 PM
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Quote:
Originally Posted by Dani123 View Post
Another question is, did someone tried to layup the wing with +45deg tows on top and -45deg tows on the bottom? rather then running it +/-45deg on both surfaces. Thought that this will save weight and time without significant stiffness penalty.
Twist works on both sides of the wing so if one side is not strengthened in the direction of the twist, that side will shear even if the other side is strengthened against the twist.

It's the same for a bias skin or a disser, or a strange bias skin that's only on one side, or only in one direction. Think of a cylinder between both your hands and twisting it one way, then the other. Which way do all the fibers have to be under tension when twisted in that particular direction? What happens if only one side has fibers? You should come up with what I have in my PDF (zoom in to 200% to see which lines are solid and dashed).
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Old Mar 20, 2011, 10:57 AM
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talonxracer, thanks that sounds like a good idea. what typical thickness mylars do you use?

DKNguyen, thanks for the info. if the tows are glued good to the core, woulds they hold the twist in compression? or there is a chance of local buckling?

Dani
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Old Mar 20, 2011, 01:12 PM
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I think you don't rely on compression at all in this case. Fibers strong in the direction of teh twist all go into tension. If you twist it the other way those fibers go into compression and they would buckle, but you don't really worry about that because you are supposed to have the other fibers that go into tension for the direction of the second twist.

I don't think the foam can stop such a narrow strip from buckling (or peeling). Even the large area of foam in a full skin sometimes isn't enough. There's a discussion going on in another thread in the composites forum where wyowindworks suggests that even though the thin glass backing for a carbon disser doesn't contribute to the stiffness because it is so much more flexible than the carbon disser so the carbon deform and load to failure far sooner than the glass does, that it might help the narrow disser from peeling because the glue area is much increased. That's peeling though, not buckling. It seems buckling is a non-issue though if you have a full disser since you are really only taking the loads in tension (with a tiny bit of help from compression).
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Old Mar 21, 2011, 12:42 PM
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A half disser would only save a few grams. You need to use 1.2oz glass to save ~10-15g over 1.6oz glass depending on where you switch fabrics, personally you need 1.6oz glass or kevlar on the leading edge, the 1.2oz glass gets eaten up just landing in grass. Use 25 psi foam over 60 psi foam for a 20g savings (theoretical), but with glass I stick with 60 psi foam for dent resistance. 40 psi would be a good compromise and save 10g.

What tow are you using for disser? Anything greater than 1k will likely have bumps. I'm using 0.014" thick mylar and shave down the leading edge with a knife. I vacuum down my mylars prior to bagging to remove excess resin and pull the fabrics tight.
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Old Mar 21, 2011, 03:42 PM
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Thanks for the input guys.

samc99us: you are right, there is no need of getting this heavy, I will use a lighter cloth on my next wing, for now this what I got around.
I'm using 6k tow, and this probably is my problem, for disser you probably don't need anything greater than 1k.
I used maybe half thickness mylars. What do you by vacuuming them prior to bagging?

Attached are my SG2 wings, first attempt, 65 and 66 gr.
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Old Mar 21, 2011, 08:19 PM
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wow,nice job for a first pair. You seem to have the weights figured out.

6K tow will never flatten out completely. 3 k will though.
@SAM.. Do you ever consider how much more resin 25 psi foam soaks compared to 60 psi for example?

I havent seen a great weight savings in the softer foams
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Old Mar 21, 2011, 11:43 PM
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thank you, great inputs guys, just what I needed.

dani
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Old Mar 22, 2011, 01:13 PM
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Very nice first pair, my last set (wing #7 or such, I've lost count) were 83 grams per panel, so you're beating me on weight depending on the layup.

I take my wet panels, put 2 layers of paper towels on the wet side, put them in the bag and vacuum down for 30 seconds or so, pulls everything flat and takes out the excess resin. I didn't do such a good job on this last set, which is why she is heavy, plus I have 1" of fiberglass to trim off the TE, a bit more than usual.

I haven't really figured resin soaking into the equation, its a 20g savings just on foam density, I don't see 40 psi soaking more resin than 60 psi for example. 25 psi might but I haven't bagged a part using it before.

Finally, what layup was this? 1 oz kevlar or 1.7 oz kevlar?
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Old Mar 22, 2011, 01:36 PM
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Very workable set of wings!
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Old Mar 22, 2011, 02:18 PM
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What I noticed was my lay-ups looking dry and I couldnt figure it out.I even went to the point of weighing the wet mylars before inserting the taco.What I finally figured out was that when I switched from 60 psi foam to 30 psi DOW SM the foam was soaking in more resin. I had to leave my lay-ups a bit wetter to get the same surface finish when using the 30 psi foam.

I get consistent 65 gram SG panels using 1.6 glass/kevlar LE and 3k disser using the 30psi foam.
I know the cores are about ten grams lighter but from what I can tell the savings is about 5 grams per panel.

Highload 40/60 are twice the money in my location or I would use it ahead of anything else.
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Old Mar 22, 2011, 03:51 PM
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samc99us: it is actually 1.6oz glass. if you are referring to the color, it is because the foam is yellow . Here it is called rondofan, and it is the easiest to get around here. As far as I know it is locally made (Israel). I don't even where to get the pink and blue stuff here..

Dani
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Old Mar 23, 2011, 09:14 AM
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Your foam is probably the same as what we have, different regions will produce identicle products with differing coloration for a myriad of reasons.
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Old Mar 29, 2011, 11:43 AM
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Hi Guys

Another question on the same subject.
I've been looking around here on some disser layups and pretty much everyone that I looked uses the disser carbon on the outside, like here for instance: http://www.rcgroups.com/forums/attac...mentid=2605556

I'm not mistaken? This is (from the inside) core, glass and carbon tows???
I always assumed the tows go on the core and then covered with glass/kevlar? This way there is no way for the tows to get separated in any way..

Dani
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