Iced up Cirrus crashes

("Ron Garret" wrote)
Not quite. They have to reach thermal equilibrium if there is no heat
flowing in or out of the system. But, as you correctly note, heat can
(and doe) flow in and out via radiation. Surfaces can "soak up" the
cold of the night sky (actually, they radiate their heat into the night
sky) and become colder than the surrounding air, just as they can "soak
up" the heat of the sun and become warmer than the surrounding air.
Eventually some of the cold/heat does get transferred to the air. This
is why clear nights tend to be colder than cloudy ones (and why clear
days tend to be warmer, all else being equal).


So on the 41F night in question, and having imaginary temp probes build into
the composite wing surface, we might see overnight wing temperature readings
of say 29F or 30F?

Is there a way to (WAG), in advance, what different surface temps will be on
the night in question? (41F overnight and 40F at 8:15 am)

Knowing air temp, humidity, cloud cover, wind, etc - could someone predict
that the composite wing will be in the 25F - 31F range overnight, whereas
the aluminum wing might only get briefly down to say 35F? Aluminum being
willing to give up its heat to the air more readily than the composites?


Montblack

In article ,
"Martin X. Moleski, SJ" wrote:

On Fri, 11 Feb 2005 15:12:27 GMT, George Patterson
wrote:

W P Dixon wrote:

That's pretty cool (Pardon the pun ) Where can I read up on that George?

I read about it in the 70s and don't remember where; probably a Science Fact
article in Analog or Popular Mechanics. As I recall, the technique is to dig
a
hole large enough to keep your water container completely below ground.
Cover it
during the day and insulate it (the Romans used straw). Leave it open to the
night sky. It will freeze in a few days. The article said it only works in
areas
where the night sky is usually perfectly clear (ie. the desert).

This site gives a reference:

http://www.madsci.org/posts/archives/nov99/941723540.Sh.r.html

Marty

Except that particular reference isn't very clear about the relative
roles of heat transfer to the air (and into the surrounding ground) and
radiative transfer to the sky -- e.g., it says

". . . at night, the pit would be uncovered so that it could lose
heat to the desert air."

You might take that to imply that the air temp in those African and
Palestinian deserts goes below 32 F at night? (Anyone know?)

You guys are into some interesting physics here. Do the plane wings
frost even when the surrounding air is above 32 F because their net heat
transfer to the surrounding above-freezing air is not as strong as their
net radiative heat transfer to the cold sky? (which should also imply
equally strong net heat connectivity to the ground below, is that not
so?)

Or do they acquire and retain frost because some colder and therefore
heavier below-freezing air just fell down on them at some earlier time?

I'll wait for an authoritative answer, but bet on the latter explanation.

("Ron Garret" wrote)

It is worth noting also that dark surfaces absorb and radiate more
readily than light ones, and so they get hotter during the day and
colder at night. Cirri are all painted white in order to take advantage
of this phenomenon and keep the skin from getting too hot in the sun.
(You'll never see a non-white Cirrus. It's part of the certification
conditions to paint the white.) Accordingly, Cirri are less prone to
radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.


I get the sun heating darker surfaces up (many degrees!) more than an
identical white surface.

What I don't get is: Two wings of identical design and an identical starting
temp, both sitting out on a cold February night (no sun). How does the dark
wing get colder than the white wing?

If it's a microscopic temperature difference because of star twinkle and
ambient light pollution from the surrounding city, I can see that. However,
a number of degrees between the white wing and the dark wing at night? Nope,
I still don't get it.


Montblack

What I don't get is: Two wings of identical design and an identical
starting
temp, both sitting out on a cold February night (no sun). How does the
dark
wing get colder than the white wing?

Visible light and infrared radiation (heat) are both forms of
electromagnetic radiation, they just have different values of
frequency/wavelength. Objects that are absorptive or reflective of
radiation in the visible spectrum can (but don't necessarily) also
exhibit the same or similar properties of absorption or reflection of
radiation in the infrared spectrum. I think.

-R

AES wrote:

You guys are into some interesting physics here. Do the plane wings
frost even when the surrounding air is above 32 F because their net heat
transfer to the surrounding above-freezing air is not as strong as their
net radiative heat transfer to the cold sky?

No, it's not necessary for the heat transfer to the night sky to be
"stronger", just for it to be a significant effect. According to:
http://www.efunda.com/formulae/heat_..._enclosure.cfm
the radiative temperature of the clear night sky is about -150 F, so
if the wing surface got no heating from its surroundings and reached
thermal equilibrium with the night sky then it would cool to -150 F.
On the other hand, if it didn't lose any radiational heat to the sky
but was in complete equilibrium with the ambient air temperature that
dropped to a low of 40F, then that would have been its lowest
temperature. In reality, both effects occur. It gets some heating
from the surrounding air and also conduction from other parts of the
wing and plane, but it also loses some heat through radiation to the
cold night sky. The resulting temperature is therefore somewhere
between +40 F and -150 F and in practice probably around 25F - 30F on
a perfectly clear night and very close to 40F on a cloudy night.

(which should also imply
equally strong net heat connectivity to the ground below, is that not
so?)

Yes, for the bottom surface of the wing. I wouldn't expect ice to form
on that surface on nights where neither the air nor ground temperatures
drop below freezing.

Or do they acquire and retain frost because some colder and therefore
heavier below-freezing air just fell down on them at some earlier time?

In that case a thermometer registering the ambient air temperature
should also record this below-freezing temperature sometime during
the night. But the lowest temperature recorded on the night in
question was reported to be +40F.

I'll wait for an authoritative answer, but bet on the latter explanation.

How much?

I don't follow the workload issue. Yes, the Mooney may require a bit
more skill to land but in cruise I've not noticed it flying much
different than an Arrow (just faster). I've had both, I love my Mooney
best. The Mooney is the ideal plane for tall guys like me. My partner
is of average size and finds it difficult to find the rabbit on the ILS
and see over the glareshield as well as trying to reach the rudders.
Short guys beware.

-Robert

"T o d d P a t t i s t" wrote in message
...
"Montblack" wrote:

What I don't get is: Two wings of identical design and an identical
starting
temp, both sitting out on a cold February night (no sun). How does the
dark
wing get colder than the white wing?

The dark wing radiates heat better than the white wing, so
it cools more rapidly.

I thought that bodies radiated EM energy based on the relative temperature
irrespective of color? Darker bodies *absorbed* more visible spectrum
radiation but there was no difference in radiating energy. Is this wrong?

Mike
MU-2

Mike Rapoport wrote:

I thought that bodies radiated EM energy based on the relative
temperature irrespective of color? Darker bodies *absorbed* more
visible spectrum radiation but there was no difference in
radiating energy. Is this wrong?

Nope, you're right. See www.azsolarcenter.com/design/pas-1.html for an
explanation. One relevant quote from that site:

"The extent to which a material emits thermal energy depends both on
the temperature of the material and nature of its surface. Polished
metal surfaces are poor emitters and poor absorbers of thermal energy."

Maybe not clear from the brief snippet I quoted, but when they talk
about "nature of the material surface" they don't mean color.

Jim Rosinski

I agree. Having lived 40nm south of Reno for five years, I would say that
it is no place for IMC flying in any piston single, by any pilot. The only
IMC there consists of thunderstorms or clouds containing ice (or both) along
with lots of turbulence (tow planes and gliders have ended up facing each
other head on!). The current theory around Reno (I was there yesterday) is
that the pilot thought he could climb on top since the tops weren't forecast
to be too high. This strategy probably would have worked during daytime
when the clouds are visible. When I had a Turbo Lance, I used to use this
strategy to cross the mountains on top VFR at about 15,000 and then descend
near Sacramento where tops were often below 5000'. I never depended on
being able to see clouds at night over unlit terrain. In the case of the
accident airplane the tops forecast was wrong and the pilot didn't realize
it until he had bet his life.

There is nothing wrong with Cirrus' deicing system. A TKS type sytem is
pretty much immune from being overwhelmed by icing because the fluid runs
back and protects the entire wing. In that respect it is superior to boots
or heated leading edges where ice can form behind the protected surface.
Granted, TKS doesn't have unlimited duration but that should not be a
problem in practice. THe only way it doesn't work is if it is broken, out
of fluid, turned off or not installed.

Mike
MU-2




"Colin W Kingsbury" wrote in message
link.net...

"houstondan" wrote in message
oups.com...

i like that analogy. i'm sorta suprised noone has gotten on the cirrus
guys' comment that they have a really excellent icing system...the
saying it's only good for an hour or so to find somewhere to land.
huh?? do you suspect they might phrase that a little different in the
sales talk?? i would be interested in knowing how many times that pilot
had made that trip in those "approximate" conditions, relied on that
"excellent" icing system and did just fine.


Night, mountains, ice, inexperienced pilot? Could be just simple task
saturation followed by panic followed by disorientation with 'chute
deployment too late to make a difference. It's awful soon to speculate the
deicing system is bum.

-cwk.

This is a classic heat transfer problem from college engineering. The
heat transfered by radiation is proportional to the emissivity of the surface.
Most paint emissivities range from .98 for flat black to as low as .8 for some
very shiny paints. Oxidized aluminum, like my plane, would run .25, while a
highly polished aluminum surface could be as low as .04.

The other part of the heat transfer equation is that the transfer is
proportional to the ratio of the temperatures to the fourth power. That is
why something as far away as the sun is such a great heat source. It's
temperature is very high.

Finally, the radiant heat transfer is effected by the "view" of one surface to
the other. This part is very complicated to calculate, depending on the
geometry. I never was worth a crap at this part of the calculations.

Have fun,
tom pettit

radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.


I get the sun heating darker surfaces up (many degrees!) more than an
identical white surface.

What I don't get is: Two wings of identical design and an identical starting
temp, both sitting out on a cold February night (no sun). How does the dark
wing get colder than the white wing?