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

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


Tony wrote:
Dan, it's not an important point, but from the physics/theromdynamics
side of the issue, the top surface of the wing is really at risk of
radiational cooling. The wing's leading edge's shape would allow
convective warming, as the warmer air in contact with the surface would
cool and flow downward.

And that's where we see frost forming: on top of the wings,
fuselage and stabilizer.


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

You really should read the textbooks. Try this website:
http://www.islandnet.com/~see/weathe...nts/frost1.htm

I quote from this article from the above site:

"Frost forms first on rock, glass, or metal surfaces that lose heat
more rapidly through radiative cooling than the surrounding air. This
is why car windshields frost over before frost forms on surrounding
vegetation. If the surface on which it forms has a temperature below
the frost point, frost may even appear when the officially reported air
temperature is above freezing."

The aviation weather textbooks teach the same thing. Another website,
this time by NASA:

http://aircrafticing.grc.nasa.gov/co...4_2_1a_RI.html

Now try to tell people the dry surface temp won't drop below the
air temp.

Dan

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?

Are you wimping out simply because it's a transient thermodynamics
problem that involves coductive, convective, and radiative heat
transfer and a little fluid dynamics?

To that I say, you're a wise man! When the experiment is easier to do
than the calculation, do the experiment! (So long as it can be done
safely, of course.)


On Dec 8, 2:10 pm, Nomen Nescio wrote:
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From: "Peter R."

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?In theory................NEVER.
It will approach 20 deg but never quite get there.

Now that I've made my heat transfer professor proud...........................

The practical answer is very complex.
First question is how close is close enough (now we're in the realm
of "engineering" as opposed to "science")
21 deg? 25 deg? Below freezing?

On an infinite plate, heat transfer is analagous to Ohm's law (V=IR).
[Temp(side1) -[Temp(side2)]= [Heat flow] [plate's resistance to heat flow]

Ok, that's easy. BUT.....................
Now you bring in convective heat transfer (that' a bit more tricky), in a
dynamic system (even more tricky), in a non-uniform system (now we're
approaching engineering hell).

So what would an engineer do to get a working answer?

I'd say tape a thermometer over the surface with the most thermal
inertia (probably over the fuel tanks) and insulate the bulb from the
environment (a washcloth folded and taped over the bulb would
probably be good enough), record the time it takes for the surface to
reach an acceptable temp. Do this with full tanks to get a maximum
time.

Caveat:
Agitation of the fuel tanks will change the whole heat transfer equation.
You could get the surface to an acceptable temp., and then have it rise
above an acceptable temp when the plane is being moved.

So there's the basic scientific answer, and the basic engineering answer.

Basic piloting answer........
Observe.....apply a little intuition.......
de-ice if it's questionable.

Basic capitalist answer.......
I'll do a full heat transfer profile for you for about 100 grand.

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


Tony wrote:
Dan, it's not an important point, but from the physics/theromdynamics
side of the issue, the top surface of the wing is really at risk of
radiational cooling. The wing's leading edge's shape would allow
convective warming, as the warmer air in contact with the surface would
cool and flow downward.

And that's where we see frost forming: on top of the wings,
fuselage and stabilizer.

Yea, same here. Often we'll get lots of frost on the top of the plane,
almost never on the bottom. I wasn't sure if the difference was temp or
moisture.

-Robert

writes:

You really should read the textbooks.

I have.

Try this website:
http://www.islandnet.com/~see/weathe...nts/frost1.htm

I quote from this article from the above site:

The above site is not a textbook.

"Frost forms first on rock, glass, or metal surfaces that lose heat
more rapidly through radiative cooling than the surrounding air. This
is why car windshields frost over before frost forms on surrounding
vegetation. If the surface on which it forms has a temperature below
the frost point, frost may even appear when the officially reported air
temperature is above freezing."

Surfaces such as metal and glass cool more during the night because of
radiative cooling. However, they emit in the infrared, and water
vapor, CO2, and other gases in the atmosphere absorb it, so it doesn't
get very far.

Early in the morning, as temperatures rise, the air may be filled with
moisture but slightly warmer than surfaces that have cooled greatly
during the night and conduct heat well (such as metal). The moisture
in the air may condense (and even freeze) on such surfaces. But there
isn't much radiation loss at ground level unless the air temperature
is lower than surface temperatures. Remember that not only does the
atmosphere absorb infrared, but it also emits it.

The aviation weather textbooks teach the same thing. Another website,
this time by NASA:

http://aircrafticing.grc.nasa.gov/co...4_2_1a_RI.html

This site requires Flash, so I can't read it.

Now try to tell people the dry surface temp won't drop below the
air temp.

I already have.

--
Transpose mxsmanic and gmail to reach me by e-mail.

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.

--
Transpose mxsmanic and gmail to reach me by e-mail.

In article ,
says...
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?

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.

Put some fuel in them wings for the night. Take a while to bring all
that lot down to freezing level when you head out.

Indeed the reverse happenned to me a couple years ago, I was down in
Omarama in winter and stayed the night. Had to scrape the ice of the
wings in the morn, but they kept freezing up even though it was a lovely
sunny morning. Had to leave it for a couple of cups of coffee and let
the wing warm a bit (and all the fuel in the wing) before I was happy to
fly.

--
Duncan

You'll notice some have claimed to observe frost forming as I had
suggested it might, When theory and observations differ, it's the
theory that should change.



On Dec 8, 7:47 pm, Mxsmanic wrote:
writes:
You really should read the textbooks.I have.

Try this website:
http://www.islandnet.com/~see/weathe...nts/frost1.htm

I quote from this article from the above site:The above site is not a textbook.

"Frost forms first on rock, glass, or metal surfaces that lose heat
more rapidly through radiative cooling than the surrounding air. This
is why car windshields frost over before frost forms on surrounding
vegetation. If the surface on which it forms has a temperature below
the frost point, frost may even appear when the officially reported air
temperature is above freezing."Surfaces such as metal and glass cool more during the night because of
radiative cooling. However, they emit in the infrared, and water
vapor, CO2, and other gases in the atmosphere absorb it, so it doesn't
get very far.

Early in the morning, as temperatures rise, the air may be filled with
moisture but slightly warmer than surfaces that have cooled greatly
during the night and conduct heat well (such as metal). The moisture
in the air may condense (and even freeze) on such surfaces. But there
isn't much radiation loss at ground level unless the air temperature
is lower than surface temperatures. Remember that not only does the
atmosphere absorb infrared, but it also emits it.

The aviation weather textbooks teach the same thing. Another website,
this time by NASA:

http://aircrafticing.grc.nasa.gov/co.../4_2_1a...This site requires Flash, so I can't read it.

Now try to tell people the dry surface temp won't drop below the
air temp.I already have.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Yea, same here. Often we'll get lots of frost on the top of the plane,
almost never on the bottom. I wasn't sure if the difference was temp or
moisture.

The bottom is subject to radiation heating from the pavement.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

"Jose" wrote in message
. net...
Yea, same here. Often we'll get lots of frost on the top of the plane,
almost never on the bottom. I wasn't sure if the difference was temp or
moisture.

The bottom is subject to radiation heating from the pavement.

Jose

Zactly - radiation depends on what the surface is exposed to. I park my
mini-van next to a big pine tree in the driveway - the tree side is clear in
the morning- it "see's" the tree. The other windows are frosted over (even
if the overnight low is still a bit above freezing) - they are exposed to
the sky. It's a real problem with telescopes too. Since they are pointed
directly at the sky (on clear nights) the objective lens cools off rapidly
and will fog up. Look up "telescope heater".

The earth surfaces cool first through radiation, which, in turn, cools the
air (which is reasonably "transparent" to a lot of wavelengths). That's what
makes clear nights colder than cloudy nights - more heat loss to space via
radiation. And having the air cooled by the surface of the earth is what
causes surface winds to often die down at sundown - you get a layer of cold,
heavy, air that just sits there under the warmer air that is moving around
the high and low pressure areas.

During the daytime, of course, radiation from the sun warms the surface,
which then warms the air. The nice thing about that is that it is the
mechinism that triggers the thermals that keep glider pilots aloft (dragging
it back to aviation content, eh?).

But to get back to the orignial post - just ask the guy in the next hanger,
he/she ought to know if they have been in the area a few years, right?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.