Does it have to WHITE!!!

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?

"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?


If your kit manufacturer recommends white, I suggest you follow that
recommendation. At some time, you'll probably want to fly it to Texas,
California, or Florida, and wouldn't it be a shame if you either chose not
to because of the color you used, or (even worse) the aircraft was damaged
due to heat build up.

Don't let vanity or the desire to be different get in the way of common
sense and/or good engineering.

KB

firstflight wrote:

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?
There are charts out there that give the relative effect of different colors
on heat build up. Check them out I think you'll find at least a few choices
that have minimal heat build up compared to white. Usually the light pastel
colors will not be a problem. The other possibility is to paint it white
and use color trim to liven it up! Also consider getting a reflective cover
for the plane so when you travel (or at home if you don't hanger it) you
can protect it better.
John

It could be white, or aluminum or other light color. Try different color
paints on similar material of the same size. Lay them in the sun and take
their temperature. Lowest temperature wins. Usually you can feel the
difference.

You can dress up a white airplane with stripes, circles, squares or
diagonals.

If it is going to be in a hangar or under cover, the paint is less a problem
than if it sits out all the time.

You can also give it a two-tone job. Light on the top and darker on the
bottom. People on the ground will see the darker colors.



Colin

On Wed, 22 Jun 2005 20:34:32 -0400, "firstflight"
wrote:

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?


there was an architectural book out of england in the early 80's which
gave a rundown on the infrared characteristics of various paint
colours. It turns out that colour does not directly equate to infrared
transmissability. additives to a paint may achieve what you seek.
I dont have access to the reference, I only remember that it was a red
book. I just point this out to give a pointer to where the answer may
lie. you want the infrared transmissability data not the colour data.

Stealth Pilot

Stealth Pilot wrote:

.. you want the infrared transmissability data not the colour data.

Stealth Pilot

Actually its not so much the IR.. as much as it is heat. Dark colors
cause higher heat absorption and can cause shrinkage of the underlying
foam core used in moldless construction on the fast glass birds. That is
why the light colors - heat reflection and rejection.

Dave

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?

The rule of thumb for painting composite AC is that you can
paint them any color you like as long as its white.

--

FF

The heat is in the IR region that you do not see with your eye. In the
IR spectrum things look very different. If you've ever looked at
things using an IR sensitive camera you'll see what I mean. I think
bees see in IR, and I noticed that flowers reflect a lot of IR even
though they may seem like very dark rich colors to the human eye.
Typically darker colors do absorb more IR however, but its more of a
coincidence than a requirement. Those handheld non-contact temperature
sensors are really cheap these days. You could make a sample sheet
with swatches of color, put it in the sun for 30 minutes, then shade
and measure immediately. See what kind of differences you get.

If anyone is wondering why I was looking at flowers with IR sensitive
cameras, it was part of testing a series of IR cut filters for a camera
we were designing. A group of flowers is a stereo typical test subject
for a color camera.

You could make an IR pass filter (daylight filter) from a piece of
exposed film negative. Put it in front of a black and white TV camera.
Then point it at your test subjects. The ones that are the brightest
are relecting the most IR and should be the coolest.

BTW, the touch test may not be a very good test because your finger is
really measuring the amount of heat rather than the absolute
temperature. Put another way, touching a 150 degree F piece of
plywood, or a 150 degre F piece of aluminum, you'll think the Al is way
hotter because the Al will transfer more heat to your finger.

firstflight wrote:
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?

Fred

You forget about Henry Ford who said you can have a Model T in any
color as long as it's black.

Older than mud

Big John
`````````````````````````````````````````````````` ``````````````````````

On 25 Jun 2005 09:33:00 -0700, wrote:


The rule of thumb for painting composite AC is that you can
paint them any color you like as long as its white.