The answer depends on the resin system and core materials that are used.

All polymer materials have a glass transition temperature (Tg). If the
temperature is raised above this temperature then the mechanical properties
reduce rapidly and irreversible changes occur in the material. See
MIL-HDBK-17 for much more detail info.

The FAA and MIL-HDBK-17 recommend a buffer of 50 deg F (27.7 deg C I think
from memory) between the maximum inservice temperature and the Tg of the
material (i.e. Max Service Temp (deg F) = Tg - 50). You need a bloody good
reason if you want to break this rule.

That said you need to find out what the Tg of the materials in your airplane
are. Get this from the material manufacturers - if they are materials meant
for an aircraft they will have the data. When you have this calculate the
maximum in service temperature. WARNING ... make sure you understand the
data. What humidity, test methods, cure cycles, post cure cycle etc ...
these can a BIG influence on the Tg. The data you have needs to correspond
to the manufacturing processes used in your aircraft. If you want to test
for Tg you can do that but you need a fairly expensive bit of test equipment
but there are labs that can do the testing for you.

If this were an FAA Certification program you would then be required to
demonstrate (probably by test with thermocouples that the specified
temperatures were not exceeded) however this is not practical for a
homebuilt. Fortunately you can get a rough idea by charts that are available
from a number of sources. JAR VLA has a chart in the ACJ's at the rear. This
was taken from a NASA report (NASA CP-2036 and CR-3290 ). One of these NASA
reports in available for download on the NASA Tech Report server. Not sure
which. If I recall correctly this data was from Lockheed and I have a vague
recollection that it may be for vertical surfaces (not sure). Horizontal
surfaces would probably get hotter and it would be advisable to allow a
little extra buffer for this. The temperatures are for 38 deg Ambient, sun
at Zenith, cooling after 3 minutes taxi, zero wind. The chart gives 54 deg C
for white surfaces. Light Green / Yellow is approx 68 deg C and dark colours
are approx 80 deg C. So if you have a typical wet layup epoxy resin system
with Tg = 85 deg C. Then 57.3 deg C would be the max surface temperature.
White is just OK and all other colours are out. Of course you can get room
temperature cure resins with higher Tg's (up to 300-400 deg F in specialised
cases). Prepreg materials generally have Tg's 200 deg F depending on cure
temperature. Some go to the mid to high 300 F range.

A few cautions:

Tg is dependant on the moisture content of the resin. Tg decreases with
moisture content. Since composite aircraft are generally designed for an
ambient humidity of 85% RH (FAA and MIL-HDBK-17 requirements) then you
should use Tg's that correspond to this condition.

The other major concern is that the mechanical properties of the material
are a function of temperature. It is normal to design composite aircraft
with the allowable stresses and strains that correspond to various
environmental conditions including max operating temperature and humidity. I
doubt (know) that some homebuilts do not consider the effect of environment
on mechanical properties. They just design with the room temperature
allowables. If this were the case then I would want to keep the thing as
cool as possible ... even if the Tg was such that you could paint the plane
black I would paint it white to keep the temperatures low and hence
mechanical properties as high as possible.

This is a complex issue and to provide any more specific advice one needs a
lot more information such as info on the resin system and details of the
stress analysis and material allowables used.

Ask if you have more questions.



"firstflight" wrote in message
...
I have a composite kit aircraft, and it is time to paint! But I just hate
the idea of painting it WHITE (like most all the others). I understand
that
HEAT is a big factor in this decision, and that white attracts the least
amount of heat which could disrupt the Epoxy over time. Since my plane is
held together with Epoxy, this seems like a valid concern.

What do you think people? Does it have to be white?? Could one choose a
light yellow, light gray, silver,etc....... and not be pushing the
envelop
too much?

I happen to live in a very cool part of North America, so I am not too
worried about regular heat (like someone in Arizona might be).

Thoughts?