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Hi Joe
I was reading the posting regarding Kevlar for the belly tank and yes on both counts. Kevlar would add abrasion resistance to the tank and yes, it is different than doing a normal layup (but not by much).
I have to make an assumption here that you want abrasion resistance in case you don't get the toes down for a landing? Kevlar 49 is great stuff in the right application but I suspect the tank will shear off before abrasion becomes an issue. However, if you want to continue with that plan, here are some things to consider. If you have to fair in the Kevlar on the bottom of the tank i.e. sanding, life won't be worth living! Kevlar doesn't sand worth a damn (remember you are applying this to add abasion resistance) so some suggestions I have would be to first, make sure the underside is as fair as possible so for don't have to fair into the Kevlar. Second, consider covering the layup with a layer of sacrificial glass and third, taper the under laminate of Kevlar so that you don't have to sand out big lumps in the new laminate. By that, I mean don't lay one layer on top of another. Instead, figure where you need bthe thickest layup and put all the layers there but shorten each layer evenly so that you have only one layer at the edges. This is structurally more efficient and makes fairing easier.
It is inevitable that you will end up sanding through in some areas so here are a few hints:
1. Use new sandpaper
2. You will have to go through several grind/prime cycles to get rid of that cursed yellow fuzz in order to make it smooth. If you go too deep into new laminate during an intermediate sanding cycle, you get to start again so control your sanding and make SURE you have the shape you want before you start to prime!
3. Make sure you aren't resin starved. Kevlar doesn't give the tell-tale signs of insufficient resin like glass. You might want to vacuum bag the laminate under low bag pressure to compact it and make sure it is void free. Use normal peel ply/bleeder ply/breather ply layup.
4. Consider making a plug over the tank in some cheap foam then fiberglass over and prime. Next splash a mold over that, then layup your Kevlar in the mold. Once done, bond the Kevlar on to the tank and feather in the edges and prime. I'd still go with an overlayer of at least 6 oz plain weave glass against the mold surface but this will minimize sanding into the expensive yellow stuff.
5. Think about sacrificing a lamb or cow to the Gods of Lamination and Sanding.
Kevlar doesn't have a surface finish like silane or volan for glass and dry-uncrosslinked epoxy for carbon. It therefore tends not to have very good peel strength. We don't know why DuPont hasn't worked on that but they feel it's adequate. Another characteristic of Kevlar is that it is hydrophyllic and absorbs water much like wood does. As the water enters, the fibers swell circumferencially (just like wood) and can cause micro-cracking along the interface of the resin. This is exacerbated by the freeze - thaw cycles a plane sees on almost every flight. This was found out about awhile back after much grief in the aircraft industry. This is why there aren't many Kevlar aircraft structures being built anymore.
You might also want to consider using Kevlar pulp to make a putty-like mixture that is remarkably tough and pretty cheap instead of a full blown layup. I got some from DuPont because they were researching new areas of use for Kevlar and they asked if I'd give it a try. I got some and found it had some interesting properties but somehow lost the few pounds I had left, otherwise I'd give it to you. It is a remarkable thixotrope, however. A VERY small amount of fiber thickens a lot of resin remarkably well but it is not very homogeneous like fairng putty is. It is tough but very different to work with from usual fillers. Basically it's like the superman version of milled glass fibers or flox.
If you want more info on Kevlar or getting some pulp (they may comp you a bag), call their technical line at 1-800- 453-8527 (that's 1-800-4 KEVLAR...pretty cool, huh?)
But you can further cloud the issue with the addition of other super-fibers like Spectra (Allied Signal) and Zylon (ICI I think) for abrasion resistance. I don't have abrasion resistance of the various fibers but I am working with the Stanford Research Institute on adding ballistic protection into aircraft. Some data suggests that Spectra and Kevlar are about equal on ballistic impact but Spectra is only 2/3 the density. Note that the specific gravity of Spectra (long chain, highly tensilized polyethylene) is .96 and Kevlar is 1.41 so Spectra floats. But that the equivilent weights of fabric will hold different amounts of resin, so the extra bulk of say a 6 oz. Spectra vs. a 6 oz. Kevlar will have a higher percent of resin. Anyway, the ballstic protection of the Zylon is almost twice that of Kevlar, (very dependant on weave, fiber diameter and resin). So while these are not quite abrasion numbers, they are related.
One last thing, these fibers cannot be easily cut by normal scissors. You need to have cerated, ceramic edge scissors that have an added sawing action (they go back and forth as the scissor is opened and closed). And that is still a workout! It's not so bad once some resin hardens where you are cutting but normal dry fabric cutting is really REALLY hard!
Good luck!
Dan Newland
ES #61
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