Dave Jeltema wrote: >>Has anyone coverd the wings with either carbon or kevlar? if so how was the >>process different from glass?<<
Yes, see http://home.hiwaay.net/~langford/owings.html . The only difference in using it is it's a little more difficult to tell when the fabric is completely wetted out, and you can't see through it afterward, but otherwise it's the same. Saving weight is not a good reason for using carbon fiber on wings (I know you didn't ask that question, but a previous poster did). I used it, but I had a good reason (adding torsional rigidity because I have huge flaps). The weight of 5.85 ounce carbon fiber is the same as the KR cloth that's called for in the plans, so there is no opportunity for weight reduction, except in places where there are two layers called for, and there aren't many of those on the KR. On the other hand, what carbon fiber DOES do for you is make the skin a lot tougher and stiffer, so it's less likely to deform and separate from the foam over time, and if you drop a screwdriver on your wing, it just bounces off. The canopy and/or door frame and control surfaces are one place that I would definitely use carbon fiber on, as well as the cowling. My carbon fiber cowling weighs 4 pounds complete, compared to the 14 pound fiberglass Revmaster cowling. Kevlar is another story, and I wouldn't touch that stuff with a YOUR ten foot pole. For more on that, go to the KRnet search engine at http://www.maddyhome.com/krsrch/index.jsp and enter "kevlar" as the keyword, and langf...@hiwaay.net in the FROM box, but I've enclosed one of them below. List-Post: krnet@list.krnet.org Date: Oct 19, 2000 5:37 PM From: Mark Langford <langf...@hiwaay.net> Subject: Re: KR> carbon or kevlar > First of all which is better? I have 20 ydrs. of > kevlar 1.2 oz. per yd.I do not have carbon yet.The > kevlar is the same strength as 6 oz. glass. Vincent, The opportunities to save weight using carbon fiber aren't all that great, since most of the plane is covered with one layer of whatever cloth you use, and both KR glass and the CF that I used weigh the same. What you CAN get is much stronger parts. The weight savings will be in places where you use several layers, like the canopy frame or wing walks. I'd also use it on ailerons and elevators for flutter resistance. Below is something I posted a while back, again. I think there's a reason why Kevlar is so cheap. ---------- While I can't lay my hands on any tables of comparative strengths at the moment (I could, but I'm really not in the mood to do any more homework at the moment), from memory I'll say that carbon fiber does fail quickly, but much further up the stress/strain curve than the point at which fiberglass fails, so it is in fact stronger than glass. But you're still talking about force levels like you'd experience in a crash here. The point of using carbon fiber is reduced weight for the same strength as glass, or improved strength with the same weight. Last time I looked, 282 carbon fiber has a strength to weight ratio about 3-5 times higher than regular 7533 "KR" glass, but then it also costs 6 times as much. And let the record show that I'm not saying the KR2S even needs ANY carbon fiber, but it's a great way to make things stronger and stiffer while saving weight in the process. Also, from "Composite Aircraft Design" by Hollman: 1) "Although fiberglass is the least expensive material, graphite fibers are the most promising for aircraft structures because of their low weight, high strength, and high stiffness as shown in Figure 3. The Starship and Voyager are completely built out of graphite and honeycomb and we can expect to see more and more complete aircraft built of this material." 2) "...this is especially true for Kevlar, which has a tensile strength of 60,000 psi and a compressive strength of 23,000 psi. Because of this low compressive strength, Kevlar is almost solely used for fairings, wheel pants, engine cowls, and other fairings in aircraft structures." 3) "However, because of Kevlar's low crompression strength, Kevlar has found limited structural application in aircraft primary structures. Kevlar is difficult to work with and special tools are needed. The above quotes are not where I formed my opinion about Carbon Fiber vs Kevlar, just the first ones I came across to support my argument. Engineering data from many different sources is where I formed my "opinion". You guys are welcome to carry on this debate, but I really need to get back to the basement... ----------------------- Personally, there's no amount of benefits of Kevlar that could possibly offset the frustration of trying to work with it. That one little "tracer" thread on carbon fiber rolls has driven me almost to insanity (well, maybe ALL the way, many would argue). That thread invariably ends up on the overlapping joints where there's a material overlap (like wings). Once you sand that little thread, all the fibers stick up, and refuse to go away. With CF or glass they just sand right off. With Kevlar, you're gonna have to sand that thread entirely away, or have a really ugly joint that looks like a line of fuzz. I eventually buried it under Aeropoxy Lite, but in the future, I'll always ensure that the overlap is ON TOP of that thread, rather than below it... ------------------------ I know an engineer who built a Defiant. He built the first cowling out of Kevlar, and the second out of carbon fiber. He swore he'd never touch Kevlar again. ------------------------- Mark Langford, Huntsville, AL N56ML at hiwaay.net see KR2S project N56ML at http://home.hiwaay.net/~langford
