This is another of Gary Hunter's postings. Worth reading IMHO. He has more
experience than most of us and is kind enough to pass it on.
Graham
> WHAT'S THE REASON FOR POST CURING?
>
Post Curing or the heating of the composite article after the initial cure
at ambient temperature conditions - is to further the curing process beyond
what it is capable of achieving without it. Epoxies do not cure
completely
under ambient temperature conditions alone. For some applications, this
is OK. And for the most part, depending on the conditions when we build
our airplanes, it is OK not to post cure your airplane too.
Post Curing will increase certain performance properties of the epoxy
resin.
Most notably will be the resins chemical resistance (fuel resistance in our
case) and the glass transition temperature (heat or desert sun resistance).
To a lesser degree, there will be improvements and/or changes in some
physical strength characteristics. Generally, absolute values for tensile,
flex, compression, and impact properties will improve. In some resins, the
tensile strength may go up slightly, while the strain to failure will go
down - slightly. Similarly with flex and compression. This is not
terribly significant and is more than compensated for in the design safety
factors used in composite aircraft (a fair amount of overkill in my
opinion).
Two things the design engineers cannot design in a safety factor for is
chemical & heat resistance. The fuel tanks will gain the most benefit from
a post cure. Physical strength characteristics of composite laminates are
not absolute and are very dynamic with temperature. They diminish as the
temperature approaches the resin's glass transition temperature. On a hot
summer day, if your wings are anything but the purest of whites in color,
the skin and subsequently, the spars will warm up considerably. Without a
post cure, some of the resins we use barely have enough heat resistance to
withstand the extreme summer day conditions in the desert. Theoretically,
but not probable, a quick departure into turbulent conditions may result in
buckling of the upper wing skins or worse if your wings are not white.
Yes, the composite will post cure itself by being baked in the summer sun
much the same as it would in an oven. One small difference - only the top
side is getting baked and you stand a extremely small chance of warpage
due to differential shrinkage, and blah blah blah.
The point is, controlled Post Curing is a plus in every way. But not
mandatory.
> SHOULD YOU TAKE OUT AS MUCH METAL AS POSSIBLE (CONTROL SYSTEM, FORWARD
> BRAKES ETC.)
>
NO
> THE CONTROL SURFACES WILL TRY TO LEVEL.
>
WHAT ?
> CAN YOU LEAVE IT ALL TOGETHER?
>
Everything but the wings.
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