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Old Feb 28, 2014, 02:34 AM
No slope for richardo
Endless Sloper's Avatar
Seattle, WA
Joined Jun 2005
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Epoxy / fabric ratio

Hey all, me again. This time a question about the wetness of a layup. The last set of wings that I bagged came out heavier than expected. My scale died on me the other day, so didn't have a chance to weight materials vs. finished product (new one on the way). I have the Barnes vids and have watched end to end, but didn't see anything other than "It shouldn't pool up". I'm more of a numbers guy at this point than a 'feel' guy.

So the question:
Is there a golden ratio for fabric type and weight to epoxy? I have a feeling my epoxy got a bit cold and might have thickened up more than usual. I squeegeed then fabric after whetting out on the Mylar, but didn't dry out with paper towels.

Thanks,
-Rick
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Old Feb 28, 2014, 08:05 AM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
Joined Apr 2002
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Here is a question and reply I got from Adam at Wyoming Wind Works a long time ago.

My question:
I had always thought that a single 1.7oz Kevlar layup would be lighter than say 2 or 3 layers of 1.4oz fiberglass. But I had someone tell me that isn’t so. They said that the Kevlar soaks up much more epoxy and it’s more difficult to get a lighter layup with it than with the fiberglass.
Thoughts?

His answer:
Kevlar doesn't soak up any more resin than any other fiber does. People get confused because they think that 1.7 oz Kevlar and 1.7 ounce glass contains the same amount of fiber. The 1.7 ounce Kevlar when saturated will always be heavier than 1.7 ounce glass. Why? The specific density of Kevlar is 1.44 grams/cc while the density of glass is 2.55 g/cc. A 1.7 ounce Kevlar contains a higher volume of fiber than a 1.7 ounce glass. Those extra fibers require more resin to have the same saturation level as 1.7 ounce glass. The 1.7 ounce Kevlar will be thicker than the 1.7 ounce glass. More fiber=more thickness. Most people always think of fiber:resin in terms of weight, but it really helps to think of it in terms of volume of fiber and volume of resin. When laminates are tested for building specs. the fiber:resin ratio is typically communicated in terms of volume ratio. This tells you how well the fibers are saturated regardless of the fiber or resin used. The fiber:resin ratio by weight doesn't give you a good comparison between fibers. If you wanted a 50:50 ratio by volume (equal saturation) you would do glass at 70:30 by weight, carbon at 62:38 by weight, and kevlar at 57:43 by weight.

Make sense?

Adam Pequette
Wyoming Wind Works
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Old Feb 28, 2014, 11:44 PM
No slope for richardo
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Seattle, WA
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Very well put, thanks Curtis (and Adam!).
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Old Mar 02, 2014, 01:43 PM
No slope for richardo
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Seattle, WA
Joined Jun 2005
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Here are the latest results. I used a paint roller to apply the epoxy this time and used a seam roller and paper towel to dry out the lay up a bit. My calc for each wing is a bit off as the fabric was weighted before any trimming and I cut one set a little larger than the other. The finished weight was the same for each wing, so I'm going to say that the fabric (1.7 Kevlar and 3.7 uni-web carbon) weight was 12g each. That makes the all up ratio 12:11 ratio by volume. Still a little heavy on the ratio, so need to work that out a bit. Also, not super accurate as there are two different fabrics in the layup, but the finished product is more than expectable IMO.

Previous wings (did 6 sets) came out averaging 44g per wing, this set came out at 35g per wing, quite the weight savings.
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Old Mar 02, 2014, 05:13 PM
MrE
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United States, WA, Gig Harbor
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Looks good!
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Old Mar 02, 2014, 05:45 PM
Red Merle ALES
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United States, Mt, Helena
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Why yes, that's a huge decrease in weight! Great job.

Curtis
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Old Mar 02, 2014, 09:04 PM
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Australia, VIC, Mitcham
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What are the approximate dimensions of your wing, Rick? (Chord, Semi-span, Thickness) I'm just trying to get a handle on what a good composite wing should weigh.
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Old Mar 02, 2014, 11:31 PM
No slope for richardo
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Seattle, WA
Joined Jun 2005
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It's based off the CRRC Apogee plan-form: http://www.charlesriverrc.org/articl...hlg.htm#Molded Wing. I'm calling mine the "Apologee" . Mainly because this is my first attempt at a bagged wing / one-piece fuse.

It has a 40" span, 190 sq. in. wing area. I changed up the airfoils a bit, but shouldn't change the numbers that much. All up weight is supposed to be around 119g, but I'm going to be over budget as the wings alone are 70g as is. The plans call for 52g wings for reference.
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Old Mar 03, 2014, 02:28 PM
Make Flying Fun
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United States, MD, Lutherville-Timonium
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Sloper, I make my F3K wings with a Carbon Dbox and Kevlar skins. After wetting out I now bag them with paper towels rather than dry rolling to debulk and it works great. I wet them out with 3" rollers and flatten with wall paper roller for tightening the fabrics weaves and put them in the bag for around 4 1/2 minutes on full vacuum. (This requires some trial and error but 4 was not enough and 5 was to dry, your mileage might vary) I place the mylars in with paper towels between and in the release plastic/breathers I use to bag the wings! I now do very consistent layups for the 59" wings this way.
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Old Mar 04, 2014, 02:13 AM
No slope for richardo
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Cool tip Kevin, I'll give that a shot on the next set!
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