Hi Mark, I too support your conclusions on using different collars on KR's, although my own tests were less scientific. I plan on using "Nevada silver" as a base and considering a checkerboard pattern on the nose. I have fuselage mounted spring gear with no load on the wings also. As A side note, do you see any problem using feather fill over smoothprime? Tim KR2 N7038V ----- Original Message ----- From: "Mark Langford" <[email protected]> To: "KR builders and pilots" <[email protected]> Sent: Friday, October 17, 2003 8:07 PM Subject: Re: KR>paint
> Y'all can just go ahead and call me stupid, but my airplane's gonna be red. > Again, I'll say that when the plane is sitting on gear, there are no > stresses on any composites, other than the Scotchply 3M gear legs. My wings > are covered with two layers of carbon fiber (three on the leading edge), > which you might remember, are supported by 3/8 to 1/2" of urethane foam, > which is supported by an inner layer of fiberglass. > > So let's just say my composite skin gets up to Tg, the temperature at which > the strength of the epoxy is significantly reduced. For Aeropoxy, that's > 194 degrees F for layups cured at room temperature. It's normally good for > something like 45,000 psi. I'd call "significant" something like 75%, but > lets just be conservative and say that the strength is cut in half. Now > it's down to ONLY 22,500 psi. But wait, I have a half inch layer of foam > being supporting by another layer of fiberglass that's at least 30 degrees > cooler (my inner skin). And don't forget, there is absolutely no external > influence on this surface, other than gravity pulling down on something that > weighs just about nothing. > > I actually called myself "post curing" my stub wings right after they were > constructed, by parking the plane out in the sun one hot day that summer, > shortly after construction. I taped (using clear tape) a Fluke 52 digital > recording thermometer's thermocouple to the black, unpainted surface, and > the best I can remember, the highest temperature I ever saw was 128 degrees > F. Don't forget, it's "air-cooled", just sitting there! > > Given the rule of thumb that your Tg will be raised to 50 degrees over the > post cure temp, I was probably wasting my time at that "low" temperature. > So when is it going to see 194 degrees? Never. And what effect would it > have if it DID see 194 degrees? None. And how much strength is required > of the skin, just sitting there on the tarmac? Absolutely none. And how > long does it take something as thin and massless as a .020" thick layer of > carbon fiber layup to cool off with a 30 mph breeze blowing on it during > takeoff? About 5 seconds. > > The few horror stories you hear about planes "melting" in the sun are > referring to airplanes with composite parts such as spars and stressed skins > supporting the load while parked. Neither of these apply to my plane. > > Sorry, but I have a bad habit of getting the facts, considering all aspects > of the situation, and making my own decisions. It's gonna be red, and if > y'all don't like it, you can just get over it... > > http://home.hiwaay.net/~langford/03101703m.jpg > > Mark Langford, Huntsville, AL > N56ML "at" hiwaay.net > see KR2S project at http://home.hiwaay.net/~langford > > > > _______________________________________________ > see KRnet list details at http://www.krnet.org/instructions.html
