Why are airplanes white?

Another aspect of the issue is painting things in the engine
compartment:

http://www.sacskyranch.com/paint.htm

This source concurs with an earlier poster in stating that it matters
whether the object is warmer or colder than its surroundings. The
author also points out that what the paint is made of matters: since
most of the energy transfer at temperatures we're interested in occurs
via conduction a paint's insulating properties may be more important
than its color, especially for outward energy transfer, where there's
almost no reflection of radiation at the component/paint interface.

wrote:
By the way I don't know for a fact that
the surface finish makes any difference
to the outgoing radiation.

I agree. I have taken it that the infrared emissivity (the capability
to radiate energy) is fairly high for most objects. The wavelength of
such infrared, corresponding to 300K (30 C or about 85F, the
temoperature that a body's surface becomes in the sunlight) is about
five microns (visible light wavelength is 1/2 micron). I mean that this
1/2 vs 5 microns wavelength difference is so great that the emissivity
for infrared is completely different than the reflectivity of light by
paint. The reality is that the pigments (any color except white) used in
paints will absorb light, while white (which is not a color at all but
the capability to scatter light to go back where it came from) does not
absorb light.

Sunlight is a broad band of wavelengths, but perhaps 90% of its energy
is in the visible range (1/3 to 2/3 microns), were pigments are
purposefully chosen to absorb. Of curse, black paint absorbs best, but
only yellow and the pastels will absorb noticeably less than black.

I knew a Bonanza owner years ago that had moved from Alaska to Ohio,
bringing the Bonanza with him. He had already painted it black so that
the snow would melt most quickly in winter.

I have heard that it does but I don't know
from first hand measurements or through any
reliable schooling (which would presumably have
involved first hand measurements).

Years ago, I rented an old closed bed truck for a special job on
company property, parked all the time. Its box body was red, it got
mighty hot inside the box while we we using it. I had the techs paint
the box body white. The temperature in the box was consistently 15F
cooler as a result.

Angelo Campanella

Tony writes:

Your assumption is that solar heating is a factor. If the aerodynamic
heating is high, one could argue radiative heat transfer would be
enhanced with black body radiation. The SR 71 is, you might notice,
black, but that may be the result of long wavelength (longer than
optical at least) absorbance considrations.

I seem to recall that one reason for painting the SR-71 black was indeed to
improve heat radiation, but I'm not sure. However the flight envelope of the
SR-71 is quite different from that of an ordinary jet (and even more so from a
small piston aircraft). Aerodynamic heating might raise the temperature of an
ordinary jet by tens of degrees, whereas in the SR-71 it raises it by hundreds
of degrees, AFAICR.

Jay Honeck writes:

I thought black increased surface temperature?

When it absorbs radiation, yes, it does. When it radiates radiation, it
lowers temperature.

A black-painted object is an approximation of a blackbody, a theoretical
object in physics that absorbs all radiation hitting it. A blackbody can emit
any radiation it can absorb, and since it absorbs all wavelengths, it emits
all wavelengths.

So a black airplane can be heating by solar radiation hitting it, but it can
also radiate heat to outer space with greater efficiency. If the primary
source of heating is solar radiation, as it may well be with ordinary
aircraft, a light color will keep the aircraft cooler. If the primary source
of heating is aerodynamic friction, as it might be with a very high-speed
aircraft, a dark color will help radiate that heat away from the aircraft.

Astronauts always wear white because it keeps temperature more even. The
white color reflects sunlight, reducing direct heating by sunlight; and the
white color also radiates heat poorly, keeping the part of the astronaut
that's in the shade a bit warmer.

My Mooney was orange and white. The white on top of fuselage helped
keep cabin temp down on the ramp in the summer and the combination was
easy to see in the air and also in summer vegetation (a la Faccett)
and winter snow if bird went down.

Combination looked good like and airplane should, so best of all
worlds.

Big John
************************************************** *******



On Sat, 3 Nov 2007 17:35:15 +0000 (UTC), Bertie the Bunyip
wrote:

Jay Honeck wrote in news:1194095788.943020.167300
:

My daughter innocently asked that question the other day, and I gave
her the usual "because white reflects heat better" answer -- but that
really doesn't add up, does it?

Automobiles, motorcycles, boats -- you name it -- all come in a
rainbow of colors. Why are 99.5% of all planes (except for some cool
old classics and some neat homebuilts) white?
--


My last one wasn't and my next one won't be..

Bertie

Mx, who earlier wrote

Jay Honeck writes:
My daughter innocently asked that question the other day, and I gave
her the usual "because white reflects heat better" answer -- but that
really doesn't add up, does it?


Actually it does, at least for high-speed aircraft. They are already
heated
externally by aerodynamic friction, and letting the sun beat down on
them at
high altitude with a black paint job would only heat them up more.


finally got a physics question right.


For what it's worth, the radiation transfer depends on the difference
of the absolute temperature raised to the 4th power, something like
K*(T1^4 -T2^4), where T1 and T2 are the absolute temperatures. Think
of T1 as the temperature of the incremental part of the body in
question, and T2 some kind of weighed average of the environment it's
radiating into. K is a measure of the emmissivity of the bodies -- if
the body is black it's higher, so if the airplane is hotter than the
radiative ambient you would want it black. By the way, it's worth
remembering the radiative ambient is NOT the temperature of the nearby
air.

Can't believe I remember that stuff!

Dudley Henriques wrote in
:

I do believe this thread is getting to be quite colorful.

I'd have said off-color...

On Nov 3, 6:41 pm, Mxsmanic wrote:
Jay Honeck writes:
My daughter innocently asked that question the other day, and I gave
her the usual "because white reflects heat better" answer -- but that
really doesn't add up, does it?

Actually it does, at least for high-speed aircraft. They are already heated
externally by aerodynamic friction, and letting the sun beat down on them at
high altitude with a black paint job would only heat them up more.

God you're a moron.



However, some black liveries are in service, so apparently it's not enough to
cause a problem. The old Playboy corporate jet was entirely black, and I
think there have been a few others.

For small GA aircraft, I can't think of any obvious reason why they should be
painted white.


Not knowing anything never stopped you mouthing off about it before.


Bertie

On Nov 4, 1:49 pm, Mxsmanic wrote:
Jay Honeck writes:
I thought black increased surface temperature?

When it absorbs radiation, yes, it does. When it radiates radiation, it
lowers temperature.

A black-painted object is an approximation of a blackbody, a theoretical
object in physics that absorbs all radiation hitting it. A blackbody can emit
any radiation it can absorb, and since it absorbs all wavelengths, it emits
all wavelengths.

So a black airplane can be heating by solar radiation hitting it, but it can
also radiate heat to outer space with greater efficiency. If the primary
source of heating is solar radiation, as it may well be with ordinary
aircraft, a light color will keep the aircraft cooler. If the primary source
of heating is aerodynamic friction, as it might be with a very high-speed
aircraft, a dark color will help radiate that heat away from the aircraft.

Astronauts always wear white because it keeps temperature more even. The
white color reflects sunlight, reducing direct heating by sunlight; and the
white color also radiates heat poorly, keeping the part of the astronaut
that's in the shade a bit warmer.



God you're an idiot.


Bertie

On Nov 4, 1:45 pm, Mxsmanic wrote:
Tony writes:
Your assumption is that solar heating is a factor. If the aerodynamic
heating is high, one could argue radiative heat transfer would be
enhanced with black body radiation. The SR 71 is, you might notice,
black, but that may be the result of long wavelength (longer than
optical at least) absorbance considrations.

I seem to recall that one reason for painting the SR-71 black was indeed to
improve heat radiation,

Nope;


Bertie