Airframe cooling rate

RyanB

RyanB

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A recent discussion with @Ravioli, brought up an interesting topic about cooling rate and airframe ice accumulation.

I was schooled on the fact that the airframe itself, has to be cooled below freezing in order for icing to occur. So this brings up a good discussion question for the rest of the group.

Let’s say for example you’re preflighting the airplane and the ambient temperature is currently 20deg (F) and no wind. You anticipate to fly through visible moisture shortly after takeoff in the form of clouds and icing conditions have been forecasted. The airplane you’re flying has been sitting in a heated hangar overnight at 70deg (F). You finish the preflight, pull the airplane out, perform the run-up and are airborne in less than 10 minutes. At what point will the temperature of the airframe cool below freezing to the point where ice accumulation would be probable? I assume there must be a formula for this somewhere.

What theories do we have floating around out there?
 
I don't think it takes long at all to cool. On a warm day, put cold fuel in then feel the temperature of the skin over the tanks, and the skin over empty wing. The skin is thin and I think cools quite quickly - certainly in a 10-min taxi with propwash over the wing.
 
Specific heat of aluminum is not much. Cooling is for all practical purposes instantaneous. OK, maybe not instant, but at the speeds we are going, I would measure it in seconds rather than minutes.
 
My other, UNINFORMED, thought was if there is some coefficient to account for the friction over the surface that extends or shortens the cooling time.

and to @EdFred - I did pose the question of composite vs aluminum. Not that there is much difference to be found there.
 
PDQ, pretty damn quick with a 100 mph cold wind flowing over it. 20F and knowing you need to fly through visible moisture is an absolute no go for a nonfiki airplane and a serious what am I getting into for FIKI plane.
 
Ravioli said:
My other, UNINFORMED, thought was if there is some coefficient to account for the friction over the surface that extends or shortens the cooling time.
Click to expand...

One of the things I learned in my IFR training is that increased velocities increase skin temps, so an airliner going through a cloud at 300 knots reporting no ice is not a reliable icing potential indicator for a Cessna going through the cloud at 100 knots.
 
If you pull it out of a heated hangar into 20°F air with NO wind, it will be below freezing long before you take off, and maybe even before you start it. If you don't believe me, buy one of those cheap infrared thermometers and take some readings. Wing and fuselage skins are thin. Really thin. And aluminum is not only light but it transmits heat rapidly.
 
Ryanb said:
A recent discussion with @Ravioli, brought up an interesting topic about cooling rate and airframe ice accumulation.

I was schooled on the fact that the airframe itself, has to be cooled below freezing in order for icing to occur. So this brings up a good discussion question for the rest of the group.

Let’s say for example you’re preflighting the airplane and the ambient temperature is currently 20deg (F) and no wind. You anticipate to fly through visible moisture shortly after takeoff in the form of clouds and icing conditions have been forecasted. The airplane you’re flying has been sitting in a heated hangar overnight at 70deg (F). You finish the preflight, pull the airplane out, perform the run-up and are airborne in less than 10 minutes. At what point will the temperature of the airframe cool below freezing to the point where ice accumulation would be probable? I assume there must be a formula for this somewhere.

What theories do we have floating around out there?
Click to expand...

Only the airframe surface need be that cold, not the entire airframe.
 
I know this: If you take a warm can of beer and spin it for 90 seconds in a bowl of ice and water it will be plenty cold enough for drinking. FWIW.
 
PaulS said:
One of the things I learned in my IFR training is that increased velocities increase skin temps, so an airliner going through a cloud at 300 knots reporting no ice is not a reliable icing potential indicator for a Cessna going through the cloud at 100 knots.
Click to expand...

We had a discussion about ram rise in a prior thread. I'll attach this link to my comments in order to not repeat the spiel. BL, ram rise is a function of true airspeed, and generally a delta of 20deg C is needed to escape the entire icing temp band if you're starting at the very coldest (-22C or so). That comes out to 400-450KTAS. 300 is not gonna cut it in the high teens, where the biggest statistical probability of SLD occurs. Also note, ram rise is a theoretical measurement at the singularity point of static pressure, aka the apex of the wing leading edge. Everywhere else the temperature rise of the skin will be a lower component than the calculated ram rise. So different geometries of the flying body will derive different icing outcomes even at the same flying speed.
 
Need more speed -

temperature.gif


http://www.aerospaceweb.org/question/planes/q0199a.shtml
Data source not stated.

"These temperatures rose even higher at the Concorde's maximum rated speed of Mach 2.2, with a peak nose temperature of 153°C (307°F). If those temperatures aren't impressive enough on their own, just consider that the outside air temperature at the Concorde's cruise altitude of 17,000 m (56,000 ft) is only -57°C (-70°F)!"
 
hindsight2020 said:
We had a discussion about ram rise in a prior thread. I'll attach this link to my comments in order to not repeat the spiel. BL, ram rise is a function of true airspeed, and generally a delta of 20deg C is needed to escape the entire icing temp band if you're starting at the very coldest (-22C or so). That comes out to 400-450KTAS. 300 is not gonna cut it in the high teens, where the biggest statistical probability of SLD occurs. Also note, ram rise is a theoretical measurement at the singularity point of static pressure, aka the apex of the wing leading edge. Everywhere else the temperature rise of the skin will be a lower component than the calculated ram rise. So different geometries of the flying body will derive different icing outcomes even at the same flying speed.
Click to expand...

Hindsight, the world doesn't always operate at the extremes and icing does not only occur at the extremes. I've seen icing at -2C going 145ktas, a jet liner, at 300 would probably not see icing there, that is the point.
 
It will end up with frost on it, look up cold soaking.

Pulling a warm plane out into your common 20f outside can cause some issues, ideal would be to spray some warm fluid on it before you pull it out.
 
James331 said:
It will end up with frost on it, look up cold soaking.

Pulling a warm plane out into your common 20f outside can cause some issues, ideal would be to spray some warm fluid on it before you pull it out.
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This just seems SO wrong (unless maybe you mean deicing fluid).
 
James331 said:
It will end up with frost on it, look up cold soaking.

Pulling a warm plane out into your common 20f outside can cause some issues, ideal would be to spray some warm fluid on it before you pull it out.
Click to expand...

James are you based in a cold area? I've never seen that happen and it gets cold around here.
 
Have never seen a formula for airframe cooling. My question would be ,if you do pick up ice ,will it sublimate quicker from aluminum or composite?
 
Ryanb said:
I was schooled on the fact that the airframe itself, has to be cooled below freezing in order for icing to occur.
Click to expand...

True.

PaulS said:
One of the things I learned in my IFR training is that increased velocities increase skin temps
Click to expand...

Also true.

But the most important truth is, that unless you're flying something that burns stinky fuel and has no props, neither of these things are going to make a significant difference. The plane is going to be cooled below freezing before the wheels leave the ground, and the ram air heating is only going to give you a couple degrees' rise (KTAS^2 / 7569 is the formula).
 
frfly172 said:
Have never seen a formula for airframe cooling. My question would be ,if you do pick up ice ,will it sublimate quicker from aluminum or composite?
Click to expand...
That was also one of the things we discussed. I would assume aluminum but it probably wouldn’t be by much.
 
PaulS said:
James are you based in a cold area? I've never seen that happen and it gets cold around here.
Click to expand...

I've seen it happen a lot. It all depends on a close temperature/dewpoint, and a clear sky. You can get frost a few degrees above freezing at night with a clear sky if the dewpoint is close enough. The metal radiates its heat into the sky rapidly and can drop below both the airand deqpoint temperatures, causing frosting. It's basic physics but there are people here that don't believe it. They've never seen it and don't believe the linked articles I've posted here before.
 
flyingcheesehead said:
True.



Also true.

But the most important truth is, that unless you're flying something that burns stinky fuel and has no props, neither of these things are going to make a significant difference. The plane is going to be cooled below freezing before the wheels leave the ground, and the ram air heating is only going to give you a couple degrees' rise (KTAS^2 / 7569 is the formula).
Click to expand...

Agreed, the point is an airliner flying 300 knots through a cloud at 0C probably won't get ice and could pirep no ice, a 172 going through the same cloud a few minutes later at 100 knots could definitely get ice.
 
EdFred said:
Specific heat of aluminum is not much. Cooling is for all practical purposes instantaneous. OK, maybe not instant, but at the speeds we are going, I would measure it in seconds rather than minutes.
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Aluminum is a great heatsink, Ed is right all most immediately

Sent from my SM-N920V using Tapatalk
 
It’s gonna depend on a few variables....skin color, airfoil shape, OAT, and Reynolds number.
 
James331 said:
when you put one thing that’s XYZ temp into a area that’s a very different temp it’s something to be aware of, heck even fuel to skin temp difference after landing can cause it.
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This is one of the coolest things I've seen.. was flying on a Continental United jet back from a work trip from Boston to Houston.. upon landing in Houston and taxing to the gate the whole Wing developed at least a 16th of an inch of frost over the top

Now, it wasn't absurdly hot day in Houston and very humid.. both up near 100, I figured I knew what was happening but asked the crew upon deplaning and they confirmed that the cold fuel in the tanks caused it
 
PaulS said:
James are you based in a cold area? I've never seen that happen and it gets cold around here.
Click to expand...

Dan Thomas said:
I've seen it happen a lot. It all depends on a close temperature/dewpoint, and a clear sky. You can get frost a few degrees above freezing at night with a clear sky if the dewpoint is close enough. The metal radiates its heat into the sky rapidly and can drop below both the airand deqpoint temperatures, causing frosting. It's basic physics but there are people here that don't believe it. They've never seen it and don't believe the linked articles I've posted here before.
Click to expand...

Yup. I've had it happen many times. Generally late fall/early spring, when the temp is cool and the humidity is high, and it's a calm, clear night. In fact, that's practically a guarantee of it happening. Paul, where are you located? This would be rare in dry/windy locations. However, what the OP describes is pretty much a perfect recipe for frost.

Now, leaving the airplane outside all day if it's below freezing might help - Not as much energy to radiate into space would mean less of a skin temperature drop.
 
flyingcheesehead said:
Yup. I've had it happen many times. Generally late fall/early spring, when the temp is cool and the humidity is high, and it's a calm, clear night. In fact, that's practically a guarantee of it happening. Paul, where are you located? This would be rare in dry/windy locations. However, what the OP describes is pretty much a perfect recipe for frost.

Now, leaving the airplane outside all day if it's below freezing might help - Not as much energy to radiate into space would mean less of a skin temperature drop.
Click to expand...

I live in Massachusetts near the NH border. I think there are about 4 different sub threads going on here, but the skin dropping below the ambient temp deal and frosting up was hashed and disagreed on in another thread. This one was about an airplane pulled out of a heated hanger into a 20 degree day, you preflight and are airborne in 10 minutes, if you fly into a cloud will the residual heat be enough to stop the airplane icing in that cloud. The answer is a resounding no.
 
PaulS said:
I live in Massachusetts near the NH border. I think there are about 4 different sub threads going on here, but the skin dropping below the ambient temp deal and frosting up was hashed and disagreed on in another thread.
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On the third page of this:https://aircrafticing.grc.nasa.gov/2_2_2_1.html

...we read:

Anticipating Contamination Cues

Frost

Expect frost to form on your aircraft if temperatures are cold enough and there is sufficient moisture.

Specifically:

  • the aircraft skin temperature is below freezing (Note – the aircraft skin temperature can be colder than ambient due to radiation cooling or cold soak fuel.)
  • the air temperature is close to the dew point temperature (within 3°C or 5°F)
  • the dew point is below freezing
And either one of these conditions is/was present:

  • a cloudless sky with calm winds1
  • a warm front bringing warm, moist air2
1 due to the radiation cooling mechanism
2 due to the advection mechanism
 
Dan Thomas said:
On the third page of this:https://aircrafticing.grc.nasa.gov/2_2_2_1.html

...we read:

Anticipating Contamination Cues

Frost

Expect frost to form on your aircraft if temperatures are cold enough and there is sufficient moisture.

Specifically:

  • the aircraft skin temperature is below freezing (Note – the aircraft skin temperature can be colder than ambient due to radiation cooling or cold soak fuel.)
  • the air temperature is close to the dew point temperature (within 3°C or 5°F)
  • the dew point is below freezing
And either one of these conditions is/was present:

  • a cloudless sky with calm winds1
  • a warm front bringing warm, moist air2
1 due to the radiation cooling mechanism
2 due to the advection mechanism
Click to expand...

Yeah, that was the argument on the last thread, why do you want to continue it here, go back there if you have something new? FWIW, I don't believe it (your bolded text), for a few reasons I posted back in that thread, go look them up there if you want to hash it up again, I would be interested in seeing some supporting data/ research for the bolded text. And fwiw the radiation theory is not applicable to the original post, he talks about pulling an airplane from a 70f hangar into 20f air and taking off in 10 minutes.
 
PaulS said:
Yeah, that was the argument on the last thread, why do you want to continue it here, go back there if you have something new? FWIW, I don't believe it (your bolded text), for a few reasons I posted back in that thread, go look them up there if you want to hash it up again, I would be interested in seeing some supporting data/ research for the bolded text. And fwiw the radiation theory is not applicable to the original post, he talks about pulling an airplane from a 70f hangar into 20f air and taking off in 10 minutes.
Click to expand...
This is a new link, a new article about the phenomenon, and it IS applicable here. We would pull airplanes out of the heated hangar on a clear, cold winter morning and watch the frost accumulate in no time at all, long before startup, and would have to get the deice fluid on it. It would otherwise burn off as the sun got higher, but we had flying to do right away.

NASA isn't good enough for you as far as supporting documentation? You know, rocket scientists and all? The guys that have to deal with frost before launches?
 
Dan Thomas said:
This is a new link, a new article about the phenomenon, and it IS applicable here. We would pull airplanes out of the heated hangar on a clear, cold winter morning and watch the frost accumulate in no time at all, long before startup, and would have to get the deice fluid on it. It would otherwise burn off as the sun got higher, but we had flying to do right away.

NASA isn't good enough for you as far as supporting documentation? You know, rocket scientists and all? The guys that have to deal with frost before launches?
Click to expand...

NASA? They pull rockets out of 70 degree hangers 10 minutes before flight? You know Dan, at the end of the day, it doesn't matter. I've never seen it, you apparently have seen it lots of times, I'm not worrying about it. If my plane frosts up, I deal with it. If it ever happens to me, I'll come back and update you, until then, have a good one.
 
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