How fast does the skin of the airplane cool to surrounding temperatures?

Peter R. writes:

A question for those of you more adept at chemistry/physics than I: How
fast does the aluminum skin of the standard single engine GA aircraft take
to cool to surrounding air temperatures? For example, how long would it
take for the skin to cool from a heated hangar at 65 degrees F to outside
air at 20 degrees F?

Nor more than a couple of minutes. Aluminum is an excellent conductor
of heat.

This is my first winter where my airplane sits at my destination airport
(Buffalo, NY) all week in a heated hangar. The problem I just inherited
is that if I desire to depart during a lake effect snowfall event, falling
snow could melt on the wings and fuselage and then turn to ice.

If the wings are cold enough to freeze water, falling snow won't melt
on them.

I suppose that if snow fell on the wing while it was still above
freezing, it might freeze into ice as the wing cooled, but since the
wing will cool so quickly and since ice is hard to melt, I don't know
that this would be much of a problem.

--
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Mxsmanic wrote:
Peter R. writes:

A question for those of you more adept at chemistry/physics than I: How
fast does the aluminum skin of the standard single engine GA aircraft take
to cool to surrounding air temperatures? For example, how long would it
take for the skin to cool from a heated hangar at 65 degrees F to outside
air at 20 degrees F?

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space. We regularly see
that here, and it's what causes frost to form so quickly.

Dan

writes:

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.

The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

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Mxsmanic opined

writes:

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.

The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing. Taxiing will sometimes melt frost.

A clear night sky is /cold./


-ash
Cthulhu in 2005!
Why wait for nature?

Ash Wyllie writes:

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing.

What was the temperature profile of the air during the preceding
hours, and at the exact time of the observation?

If the air is warming up and moist, metal surfaces might well be below
that temperature and below the dew point.

A clear night sky is /cold./

Yes, because large masses of water vapor have a moderating effect on
temperature, making warm days cooler and cold days warmer.

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Mxsmanic opined

Ash Wyllie writes:

Actually, it can. I have seen frost on metal surfaces (like wings) when the
air temp is above freezing.

What was the temperature profile of the air during the preceding
hours, and at the exact time of the observation?

If the air is warming up and moist, metal surfaces might well be below
that temperature and below the dew point.

It was before dawn, and the temp had dropped over night (and was still dropping).

A clear night sky is /cold./

Yes, because large masses of water vapor have a moderating effect on
temperature, making warm days cooler and cold days warmer.

Look up _radiational cooling_ . Space is in the single digits absolute, and
the atmosphere is pretty much transparent to radiation. Objects will radiate
energy trying to heat up interstellar space, and cool in the process.

You might drop in on some of the amatuer telescope groups. They have the same problem.


-ash
Cthulhu in 2005!
Why wait for nature?

Ash Wyllie writes:

Look up _radiational cooling_.

I'm familiar with it, and with blackbody radiation.

Space is in the single digits absolute, and the atmosphere is pretty
much transparent to radiation.

The atmosphere not only absorbs and reflects the blackbody radiation
emitted by most objects on the ground at ordinary temperatures, it
also emits radiation of its own (as does anything with a non-zero
absolute temperature).

Objects will radiate energy trying to heat up interstellar space,
and cool in the process.

The surface of the planet is not part of interstellar space.

Everything on the surface of the Earth cools through emission of
radiation, but aircraft do not do this to any greater extent than
anything else, and they don't cool to subfreezing temperatures in air
that is above freezing.

--
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"Ash Wyllie" wrote in message
...

Look up _radiational cooling_ . Space is in the single digits absolute,
and
the atmosphere is pretty much transparent to radiation. Objects will
radiate
energy trying to heat up interstellar space, and cool in the process.

Ash, I'd like to expand a bit on your statements.
In the 3-5 and 8-12 micrometer region of the electromagnetic spectrum,
atmospheric transmission is, like you said, close to 100% transparent. Outer
space, in those bands, is to photons like a Hoover is to dust in the
carpet - it just sucks heat right out.
The rate at which the transfer of photons occurs is dependent on, among
other things, the emmissivity of the surface and the thermal conductivity
from the mass to that surface. The perfect case is a gaussian emmissivity of
one, and since perfection is difficult, the closest you can come is a bunch
of nines behind the decimal point. A pure white specular surface is at the
opposite end of the scale.
One good example to back up your statement is in the winter time. We
have all felt the phenomena when the sky is clear that the nights are much
colder than when the sky is overcast. That is because interstellar space is
sucking the heat off the planet's surface. The adjacent atmosphere loses a
lot of its energy to the surface and its temperature decreases also.
Please, don't confuse the cloud cover effect with the so-called
"greenhouse" effect.
I always used the term "radiative cooling" (not radiational) to keep in
tune with NASA's glossary:
(http://eobglossary.gsfc.nasa.gov/Lib...seg=q&segend=s)
radiative cooling Cooling process of the Earth's surface and adjacent
air, which occurs when infrared (heat) energy radiates from the surface of
the Earth upward through the atmosphere into space. Air near the surface
transfers its thermal energy to the nearby ground through conduction, so
that radiative cooling lowers the temperature of both the surface and the
lowest part of the atmosphere.

Oh, did I mention that I spent the better part of two decades doing infrared
measurements using a Michelson Interferometric Spectrometer? [Don't try
saying that with more than two drinks under your belt.] A lot of that dealt
with radiative transfer.

"Ash Wyllie" wrote

Look up _radiational cooling_ . Space is in the single digits absolute, and
the atmosphere is pretty much transparent to radiation. Objects will radiate
energy trying to heat up interstellar space, and cool in the process.

You might drop in on some of the amatuer telescope groups. They have the same
problem.

Why on earth are you arguing with this BoZo? He doesn't know **** from shinola,
and has proven it on many occasions and many subjects, yet still he argues with
people that do know, and then tells them that they are not qualified to make
statements on xyz subject.

You only contribute to him overstaying his welcome even longer!
--
Jim in NC

On 8 Dec 2006 18:39:30 -0500, "Ash Wyllie" wrote:

Mxsmanic opined

writes:

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.

The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing. Taxiing will sometimes melt frost.

As usual, mxsmanic is both right and wrong (as in 'a little knowledge can be
a dfangerous thing"). When the skin radiates, it cools down, and will cool
the air in contact with its surface. This creates a thin inversion layer,
which (in the absence of any appreciable wind) is stable. The air
temperateure a few millimeters above the skin will be significantly warmer.


A clear night sky is /cold./


-ash
Cthulhu in 2005!
Why wait for nature?