Q-talk 29 - COMPOSITE TIPS
- Category: Q-Talk Articles
- Published: Saturday, 31 August 1991 07:11
- Written by Mike Bergen
- Hits: 1687
Have you ever wondered why "the Book", on page 3-14, tells us to paint our plastic air ships white opposed to some other color? Then we see the Air Force's plastic, fantastic, invisible airplanes painted black, or the Lancair painted red! If some of you boys are as curious as I tend to be, I would like to offer you a little crash course in "Composites 101, Temperature versus Structural Properties". You may want to refresh your memory by referring to the chart extracted from SOARING that is given on page 3-14 in our plans. Pay close attention to the temperatures.
Most organically derived materials have that which is known as the glass transition temperature (Tg). This number is used to indicate the temperature at which the material starts to soften and lose the "solid" properties. The number that the designer pays attention to is the heat distortion temperature (HDT). The HDT is typically ten to fifteen degrees less than the Tg. Now, how does this concern us?
The ol' SAFE-T-POXY, now HEXCEL EPOLITE 2410 & 218X (because it isn't ...)is what is known as your basic room temperature amine cured bisphenol-A epoxy. It enjoys a "mighty" HDT of about 165 degrees F. In addition, empirical results (lab tests) have shown the compression strength to be cut in half at a dry 145 degree F temperature. In the presence of moisture (humidity), this could be worse! Don't sweat yet boys, a good safety factor can make up for this. The question is, what did our boy Gene use as a safety factor to allow for this and builder sloppiness? Builder slop would be things like low fiber volume fraction (i.e. wet layups), high void content (air in between the plies or poor wet out of the fibers), and poor alignment of the fibers, as mentioned in the closing of my last article. How else can we achieve better properties? Simply by "post curing".
There has been some discussion in the past about solar post curing of the wings. There is some merit to this! Post curing while the resin is still a little "green" causes full polymerization of the resin crystalline matrix and raises the Tg or the HDT a few degrees. The chemistry that we are working with has its limits though. You can't get the Tg over about 190 degrees F, I don't care what Chuck Ritchie says (Scaled Composites, YES, Burt's boy)!
Now how does the Air Force do it? They throw a lot of money at it, of course! Despite the fact that it is low bid! The epoxy materials that they use go for something like $45 to $100 a pound. The HDT is somewhere in the upper 300 degree F range.
Now how does Neibauer do it? Certainly not with the same stuff. The Lancair is made of an oven-cured preimpregnated epoxy. The HDT is somewhere around the low to mid 200's. I'm just not sure which resin he uses. Me thinks it is something like HEXCELL F-155 which is a good, touch epoxy, good for 250 F.
Well, what can we do? Bite the bullet, paint the bird Caucasian and be conscientious in our construction. HEXCELL does have out a real sweet resin system that is a bisphenol-F known as EPOLITE 2315 that has some very good properties (over 200) and is without the nasty MDA (dunt ask me to rite it out, not a nuff room on the paper) that OSHA is trying to ban (SAFE-T-POXY features this wonderful stuff). The only problem is the blue foam starts to go gooey at 200 F. Sooo, we lose anyway.
In closing, if you are thinking about post curing, DO NOT use the stupid black tempera paint trick (ooops, did I offend someone?)! The unequal heating of one side to the other causes stresses within the composite that is very undesirable, maybe even warps the wings. In addition, one can't be sure to cure both sides the same. Build yourself a solar oven using the black plastic and wooden frame trick. Just figure a good, reliable way of monitoring the internal temperature and circulate the inside air just a little bit. Any of you geniuses have any design ideas? Malechek, where are ya good buddy?
ED. NOTE: By now I should be infamous for giving certain engineers some grief. Since Brian Martinez gets it and takes it in stride, I suppose it's only fair play for me to give some to Bergen (at least a glancing blow). Some timid builders are always looking for an excuse for NOT working on their airplanes. The need to build a curing oven is a perfect offering for these characters. The problem with engineers is that most of them work for megabucks commercial aerospace or governmental institutions that encourage them to design to 0.0001 tolerances and beyond (I call it designing warts for a gnat's ass). Now I cured my Quickie airfoils in the sun. It flies fine. If I incurred a wing warp of 0.0001 or 2, what do I care? I'll just apply 0.0001 pounds more of rudder pressure to whichever one needs it. The additional stress won't kill me. The point is, learn from these experienced and savvy engineers, but don't let the exquisite detail of their knowledge override the practical conduct of your life. FINISH THE DAMN PLANE. Use care, but remember, you're not building a space shuttle (or a gnat's ass).
You can order a PDF or printed copy of Q-talk #29 by using the Q-talk Back Issue Order Page.