Originally Posted by
ezbrit
Hi Bob and Ed,
Bob, your are correct in your assumption that I'm using 'old school boat building methods' and that is because the subject fuselage we are discussing has been produced using 'old school boat building methods' and until a company develops a temperature sensitive, highly unstable, flexible epoxy resin to match the characteristics of polyester resins, there will always be a performance mismatch between the two resin systems. I whole heartedly agree that almost any epoxy adhesives will out perform the polyester/cotton flock/fumed silica method I suggested, but the reason that I made that suggestion is because I know that the polyester adhesive will exhibit exactly the same characteristics of the polyester laminate. I'm sure you are well aware of the low temperature sensitivity(T of G) of polyester, normally around 110 degrees. As the temperature increases past this point, the polyester softens and weakens considerably and becomes unstable. Now lets say that the formers have been bonded into the polyester fuselage using a normal off the shelf 30 minute epoxy and a reinforcing laminate using finishing resin, both with a 160 degree T of G which is pretty normal. Because this is a warbird, we are of course, going to paint it in dark, flat camouflage colors and when finished we will take it to a warbird fly in where it will sit in the sun with surface temperatures reaching 140 degrees. At that temperature, the polyester is going to become highly unstable and flexible, yet the epoxy adhesive will remain unaffected which will result in surface distortions around where we have used epoxy to bond and reinforce areas. This is the reason I gave the advice I did, because I didn't want to see Keith spend countless hours building a show stopping warbird, only to discover that it has a 'starved horse' appearance after it's first outing.
Ed, As I stated before, I'm no fan of polyester and hate using it and there are far better alternatives with epoxies. When the model manufacturers quit using polyester to build models, then we can quit using polyester to repair them and I'm fine with that. I never questioned your ability and obviously you are skilled evidenced by the work in your photos. My background is that I'm a composite fabricator/fitter with 16 years of experience in racing, aircraft, marine, musical, military, medical and space exploration fields as well as this composite field. I'm just trying to pass on my hard learned experiences and I'm happy to help out anyone who wants it.
Hope you both have a great weekend,
Ian
Ian,
You mean the (CTE) coefficient of thermal expansion, yes I am well aware of it's effects, I use the CTE almost on a daily basis in my calculations of machining metal layup blocks; carbon steel or 6061 T6 aluminum for use of manufacturing high temperature composite aerospace components.
Your statement of the polyester resin weakening and turning to rubber at roughly 12 degrees higher then our body temperature (98.6 F./110 F. is in fact only fiction, I have seen much higher heat yields from polyester resin systems, ISO or Ortho either one. Like you I have also been in the game for a minute and know that the heat distortion of polyester is considerably higher then you stated, so at 140 F. their fuselage is not going to turn to rubber. By the way a dark surfaces like carbon black sitting in the sun can and does reach much higher temperatures than 140 F. More like of around 240-280 F. Hot enough to fry an egg in the Mojave desert, but that is another story. If you are seeing distortion around formers in any fuselage it's generally caused from wood being used as the former's inside of a relatively thin skin layup with no core as seen in most all old school stuff, using dissimilar materials edge bonded inside a thin skinned no core fuselage made from an epoxy layup will show rib transfer after a couple of outing in the sun, especially if painted dark. There are many reasons the unknowing have these kind of issues, but having a fuselage that begins to turn to rubber around 110 F. from the use of polyester is not one of them, don't get me wrong, I too stopped using poly nearly 30 years ago, but am well aware of what you can, or cannot do with the stuff.
Have a nice day!!!
Bob