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Old Dec 12, 2006, 01:55 PM
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Spead Eagle, WI (about 2 hours north of Green Bay)
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Guidelines for first fiberglass fuse layup (and some other molding questions)...

Hi everyone I have been working on a plug for a plane that I plan on using for AP as a first composite rc project for awhile and I am about ready to put together an order for some epoxy and fiberglass. I am wondering how I can determine the layup I will need for this fuse...it is 36" long and I want to make it as light as possible but still relatively strong. I will try and get a pic up of the plug soon since my description probably isn't very helpful. Are there any guidelines I should follow as to how many layers and what weight fiberglass would work best for this fuse or is it mostly a matter of trial and error to find the right balance between strength and weight? I don't know anything about fiberglass weights/weaves so maybe someone could post a good starting point as far as what fiberglass I should purchase?
For the mold I just plan on brushing a light coat of epoxy over the plug followed by some heavyweight layers of fiberglass and epoxy until it is stiff enough.
Thanks
Dan
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Old Dec 12, 2006, 11:14 PM
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Quote:
Originally Posted by Picoflyer
Hi everyone I have been working on a plug for a plane that I plan on using for AP as a first composite rc project for awhile and I am about ready to put together an order for some epoxy and fiberglass. I am wondering how I can determine the layup I will need for this fuse...it is 36" long and I want to make it as light as possible but still relatively strong.
That is the hardest part. You have to learn how to make a decent part and then you have to figure how to maximize the strength and minimize the weight. You dont need a degree in operational analysis, but there might be some trial and error. The shape of your fuselage is important in determining the best layup. The arch is still the strongest structure known to man. My point is that you will have to determine where the weak points are and use that to schedule a layup. Very light weight cloth will give a smooth finish and conform to the most difficult shapes, heavier cloths are for strength, and resin doesnt add strength, so blot and bag to save weight. Remember that the weight of a cloth is per sq yard and that is alot of material, so maybe make your mold and determine with paper templates what your sq yardage will be and that will tell you how heavy the cloth you will use will be. If your resin is blotted up properly then you will probably end up with about 30% resin to 70% cloth ratio.

Quote:
Originally Posted by Picoflyer
For the mold I just plan on brushing a light coat of epoxy over the plug followed by some heavyweight layers of fiberglass and epoxy until it is stiff enough.
Thanks
Dan
That could be a problem, you want enough goop over the plug to get good coverage. Generally, people mix graphite powder and silica with the epoxy to make a thick paste. Doing this ensures that as the mix sags some it still covers and gives a good surface for molding. They call it a tooling coat/surface. If it is too runny then you will have thin spots that will break through when you wax, or you might even get spots that are just air.
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Old Dec 13, 2006, 12:00 AM
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Spead Eagle, WI (about 2 hours north of Green Bay)
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Thanks for the response...I kinda figured it was something that you have to play around with until you get it right...do you think you could post something like an example layup for something lightweight around the length of my fuse just so I can get an idea of what weights/how many layers I am looking at since right now I have no clue what to start with.
Quote:
That could be a problem, you want enough goop over the plug to get good coverage. Generally, people mix graphite powder and silica with the epoxy to make a thick paste. Doing this ensures that as the mix sags some it still covers and gives a good surface for molding. They call it a tooling coat/surface. If it is too runny then you will have thin spots that will break through when you wax, or you might even get spots that are just air.
Yeah I did plan on thickening it in someway just didnt mention that...I was thinking that brushing on a very light layer of unthickened epoxy to duplicate the surface as perfectly as possible followed by a few brushed on layers of thickened epoxy and heavy glass/epoxy after that would work.
I have a few more questions about molding that maybe you guys could answer in this thread as well...I want my mold surface to come out as glossy as possible after removing the plug and I plan on using pva (sprayed on w/ an airbrush) as a mold release after wet sanding and buffing the plug surface...I read somewhere that using pva can make the surface come out not as perfect...will using pva be a problem? If so is there a wax that will allow the mold to release just as well but not interfere with the surface quality? Also...when I mold my fuse I want to use the method where you spray paint inside the mold and then do your layup after...I just dont see how this would work out...wouldnt the paint bead up/not lay down consistently on a buffed and waxed surface like the inside of a mold? Is there some trick to avoiding this that I am missing here?
Thanks
Dan
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Old Dec 13, 2006, 08:26 AM
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The is a hard question because it is really many questions. Read about it before you understand many questions. What is the fuselage area? What kind of strength? Tension strength? Compression strength? With thin structures, and compression load, it fails with buckling far before maximum compression strength.
http://www.djaerotech.com/dj_askjd/d...ons/nylon.html
http://www.djaerotech.com/dj_askjd/d...ions/fuse.html
http://www.djaerotech.com/dj_askjd/d...ns/kevlar.html
http://www.djaerotech.com/dj_askjd/d...arbontalk.html
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Old Dec 13, 2006, 12:20 PM
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There's more:

See three pages of fiberglass from .6 oz. to 18 oz. and thickness from 0.001" to about 0.03".
http://www.cstsales.com/fiberglass.html
and
http://www.cstsales.com/fiberglass_data.html
A good efficient layup uses an about equal weight fiberglass and about equal weight of laminating epoxy.

So, you calculate the area times the thickness times fiberglass times epoxy weight equals weight of the fuselage shell.

The compression strength is very roughly half the tension strength. It is hard find the real compression strength data because it depends on the craftmanship and layup process. If that was very hard to calculate, then the buckling strength depends on shell thickness.
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Old Dec 13, 2006, 12:45 PM
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Some of the experts claim that you can get a good smooth coat of PVA(and yes, definitely use it)by smearing it on with a cloth instead of spraying. I've read that spraying can cause fisheyes and irregularities if droplet size is too big. I've only applied it with a cloth and that works great.

30-70 resin cloth ratio is ambitious and as Ollie says 50-50 is good and probably more realistically achieved at home. The important thing about that ratio is dont fall into the trap of believing that lots of resin is good.
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Old Dec 13, 2006, 03:17 PM
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The problems are looks vs. strength vs. weight. It depends on YOUR values of looks vs. strength vs. weight.

I give up looks (no gel coat and no paint) and high strength and low weight like MY values. Then I make the shell small in minimum cross section and high thickness for an fuselage budget weight compared to the whole aircraft budget weight. I give up volume of the fuselage shell so that has tight equipment mounting.

Other people like easy equipment mounting so that they like more fuselage shell volume and give up some strength and higher weight.

What are YOUR design values? You can't eat the cake now and keep it for the future too.
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Old Dec 13, 2006, 05:22 PM
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Good info Ollie. MY design values are utilitarian in the sense that weight is bad, strength is good, looks get better with altitude, and low drag is usually esthetically pleasing.

Also, my budget is low so I tend to not throw money at a project and try to only make purchases for materials when I know that is exactly what is needed. I think that is what Pico is trying to do here, avoid buying cloth that might be wrong. Youre links should be a big help to him and others, myself included.
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Old Dec 13, 2006, 05:44 PM
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Picoflyer,
I would 2 Oz. or 3 Oz. fiberglass. I would two layers first. I would judge the cured skins. Weigh the skins. If they are too thin and too flexible then, I would add one or two new layers (and cured) till I thought enough stiffness and not too much weight. Sneekup on the thickness question and the weight question. Measure and record everything as you learn.
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Old Dec 13, 2006, 08:55 PM
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Spead Eagle, WI (about 2 hours north of Green Bay)
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Wow thanks for all the information! Looks like I better start reading all those links...
Thanks again
Dan
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Old Dec 13, 2006, 09:27 PM
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Ollie,

You wrote:

"So, you calculate the area times the thickness times fiberglass times epoxy weight equals weight of the fuselage shell."

In order to estimate the finished part weight, the composites technician would need to know the exact area of cloth needed for the layup, and the fabric weight per area, to calculate the weight of glass cloth needed for the finished part. But how do you know what the epoxy weight will be? If you assume a 50:50 ratio of resin to cloth, the resin weight would equal the weight of the glass cloth. The sum of the two would be the total weight not the product. And thickness becomes the number of layers. Right?

Again, how do you know what the epoxy weight will be? If you use a heavier weave, the layup may have larger glass fiber voids (between the bundles) that end up filled with epoxy. This isn't 50:50. If you use numerous fine weaves, and minimize the void spaces, this isn't 50:50 either. So, how do you estimate the epoxy weight?

Sean

BTW. A 30:70 epoxy to glass ratio is impossible.
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Old Dec 14, 2006, 12:20 AM
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30-70? Thanks for the correction. If you dont want weight, then dont use excessive epoxy, that was all I really wanted to say. I'm still shooting for my first complete composite success, but I've learned alot from my failures and each experiment puts me closer. Estimates will get you far.

Here is a good article on bagging a float:

http://www.kitplanes.com/magazine/mi...us/164-1.phtml

They claim a resin to cloth ratio of 40-60, but how they minimize weight and estimate the amount of epoxy needed is explained.
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Old Dec 14, 2006, 08:38 AM
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Sean,

I'll demostrate an example of a fuselage skin (shell) with an computed weight.
1. The fuselage length of 36" and streamline with maximum 2" diameter. The area of the fuselage skin area is about 160 square inches.
2. In this design, I assumed 4 layers of 1.5 ounce of square yard fiberglass and a one layer thickness of 0.002". Four layers of fiberglass have a total thickness of 0.008"
3. The fiberglass area is 160 x 4 = 640 square inch area.
4. One square yard area has 12 x 12 x 12 = 1728 square inches. (correction: One square yard area has 36 x 36 = 1296 square inches.)
5. Therefore the weight of the 1.5 oz. fiberglass has 1.5 x 640/1728 = 0.556 oz. (correction: 1.5 x 640/1296 = 0.74 oz.)
6. Just enough epoxy to wet the fiberglass has about 0.556 oz. (correction: 0.74 oz.)
7. Therefore the total weight of the fuselage skin (shell) is about 1.11oz. (correction: 1.48 oz.)

Thanks, Sean, for your correction to my brain fart.

I assumed the equipment of 1.5oz motor, 4oz. battery, two 0.5 oz servos and 0.5 oz receiver. Misc. bulkheads, wing jointers, etc. and equipment and for the fuselage skins (shell) and for total weight of about 9 to 10 oz.
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Old Dec 14, 2006, 03:22 PM
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Sean said:

"If you use numerous fine weaves, and minimize the void spaces, this isn't 50:50 either."

So, with Sean's comment, I could improve the layup using one layer of G1671x, 6.00 oz, 8HS Satin thickness of 0.0060" rather than four layers of 1.5 oz. fiberglass and save ~25% epoxy weight, fiberglass cost by half and labor time.

Thanks to Sean!
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Old Dec 14, 2006, 09:03 PM
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I think it's worth mentioning that 50/50 by weight is not even close to the same thing as 50/50 by volume. The 60% fiber, 40% resin you see quoted for industry stuff is for volume. It's the fiber volume fraction.

Using typical density values for glass and epoxy, 50/50 by weight is only only a 30% fiber volume (70% resin by volume). This becomes obvious when you apply a vacuum bag to what you *thought* was a dry layup. Suddenly, the fabric is swimming in resin as the laminate consolidates.

I think it would be an interesting exercise to develop some kind of guidelines for small scale, wet layup parts made without vacuum or pressure systems. Not tonight, though.
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