CF wingtips and cowling

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Will Aldridge

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I have a kitfox model 1 (850 lbs max gross and 100 mph Vne). I'm going to build new wingtips to replace the fairly heavy(3.5 LBS each) FG droop tips and I'll be building a new cowling as well. I'm not set up for vacuum bagging so it'll be wet layup.

I am looking for suggestions on what kind of cf fabric and how many plies to use for the wing tips and cowling. I have Rutan BID and UNI and if doing a wet layup is comparable in weight to FG then I'll just save some money and use what I've got. I think even a careful FG wet layup can save some weight over what I've got.
 

wsimpso1

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I do not know the weight of your cowling. I can advise in detail on glass, then interpret to graphite.

Min glass for wing tips is 3 BID, 4 BID for the cowling with no sandwich core. Wet layup and excellent technique, that is around .37 and .49 pounds/square foot of skin area. Go vacuum bag on it and that goes to about .31 and 0.42 psf. Reinforcements, fairing, finishing, and paint will be the same for all, so I am sticking to structure weights.

Go graphite fiber and no vacuum bag, and I am guessing 3 plies of 6 oz for the pants, 4 for the cowl. Wet layup weight in excellent technique is about 0.24 and 0.33 psf. Go vacuum bag, and you will get to more like 0.21 and 0.28 psf.

These all might might be a tad fragile and feel kind of flimsy, but those are standard thicknesses for this sort of thing on faster airplanes. If you want the cowling to feel more substantial, you can include a foam core, but then you must vacuum bag, or you can add another ply and scale up the weights. Given your low Vne, you might be able to drop a ply on each but that starts to feel really insubstantial when demolding, handling and in maintenance.

At the lines where you will cut and be adding hardware to attach the cowling to stuff, you will be adding plies. Part of the game.

Personally, I would not even consider the cowling without vacuum bagging. The old school method of wrapping the engine, foaming things, shaping the foam, then laminating four plies ends up heavier than my above weights. Trying to do that with a core and open layups? My guess is it will end heavier than you want. One trick is to layup 2 plies, demold, set it back on the mold and brace up the outside with wood and bondo so it will hold shape off the mold, then you can low vacuum bag cores inside and follow with a couple more plies inside all that. Do not get me wrong, this will still be heavier than vacuum bagging inside a good mold.

Oh, and when you add a core to these parts, 2 BID is still a min on each side - less than and it is just a damage magnet. In graphite fiber, the sailplane guys get by on one ply of 6 oz per side of cored skins.

I would take female molds off the existing parts. Once you have the mold, you might as well vacuum bag... just add a pump and some practice. On the wing tips first, then on the cowling. You can go as light as you want, if you damage it too easily, build the next one with another ply in the fragile areas.

Oh, and as usual, this is all dependent upon your technique on everything... Have fun.

Billski
 
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Will Aldridge

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Thanks Billsky. I've been shying away from vacuum bagging for awhile mostly because I have absolutely zero experience and it intimidates me. It's time to change that.

I'm going to be creating brand new shapes for all of these parts, so unfortunately can't use the old ones as molds.
 

wsimpso1

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Thanks Billsky. I've been shying away from vacuum bagging for awhile mostly because I have absolutely zero experience and it intimidates me. It's time to change that.

I'm going to be creating brand new shapes for all of these parts, so unfortunately can't use the old ones as molds.
Gougeon Brothers has a small book on their website as a .pdf - https://www.westsystem.com/instruction-manuals/ that is the best single reference I know on the topic. After that, I like my writings on the here about how I have done mine.

Billski
 

TFF

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You are going to want female molds to cut down on finish work and weight. It takes more time up front because you have to do it twice in essence, but less at the end, and if you are looking at weight reduction, you do not want to be trying to smooth out a rough exterior because the only way is adding weight of filler to fix it.
 
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