Re: Thermal Conductivity , Ice & Me (or You, too)

[Montblack]

(Greenwavepilot, the OP, just posted this over on rec.aviation.student)

You are missing something. I typed the wrong temperature. The overnight low
was 30*F, not 40. So, it WAS below freezing for a period of time.


So one mystery is solved.

Now about those black wings at night...


Montblack

[greenwavepilot]

Thanks for bringing this over Montblack. I really wish I would have
caught my typo because I think it pulled away from my main
point/question, which is, "What is the difference in thermal
conductivity (as it relates to ice/icing conditions) between a
composite wing and an aluminum wing."

Obviously, at certain extreme temperatures and conditions (NH in
January), both an aluminum and composite wing may act similarly, or in
other words, the therrmal conductivity/icing differences may not be
distinguishable.

But in marginal temperatures and conditions, (like SC in February),
those differences MAY be quite distinguishable, as the experience I
posted, and may be a point of concern for composite drivers.

Or I may be completely bonkers. But the only logical explanation I
could reason/infer from my experience was that the thermal conductivity
differences between aluminum and fiberglass were to blame.

This probably breaches usenet protocol, but I shall reprint my original
post from r.a.student that Montblack refers to below, with the
temperature typo corrected.

"I am training in a Diamond DA-20 C1, incidentally, the only
composite airplane on my flight schools ramp. I am flying in upstate
SC. This morning, at 8:15 the top surfaces of the wings on the C1 were
iced significantly, as was the nose and fuselage (tail boom). Outside
air temp was 41*F/Overnight low was 30*F. Plane is tied-down, morning
sun was directly on wing surfaces, no intervening shadows. My lesson
was delayed, of course.

Curious, I checked the other planes on the ramp-all of which are
aluminum. NONE had icing on any surface. Through a very unscientific
"hand touch" test I determined the composite surfaces "felt" much
colder than the aluminum surfaces.

I would be very interested in learning more about the heat/cold
transfer dynamics of aluminum versus composites. Pure speculatin'
though, I would bet from my limited experience that the composite will
ice faster or retain ice longer than similarly exposed aluminum. But,
there's always someone who knows more about it than me-so maybe they
will chime in.

So, I did some research and found the following thermal
conductivity values (Note these figures are for a standard temperature
of 25*C):

Aluminum,
Pure=237 watts/meter*Kelvin

Fiberglass,
Paper Faced=.046 watts/meter*Kelvin

So, what I should have inferred from my non-aviation experience with
these materials is confirmed by the above thermal conductivity values.

That is, aluminum is a good heat conductor-it can either gain or lose
heat very quickly. Fiberglass on the other hand is a good insulator.
It does not lose or gain heat very rapidly. Thus once "set" at a
temperature, it will tend to remain there longer than aluminum.

Therefore versus fiberglass the aluminum surfaces
will cool to icing temperatures faster, and conversely will heat to
non-icing temperatures faster. The composite will cool more slowly,
but once cooled, will retain that temperature much longer, meaning like
I discovered this morning, my composite plane will/may be iced when the
Cessnas, Pipers and Mooneys won't.

During the preflight "Hand Checking Of All Surfaces" has added meaning
for a composite driver (especially one like me who has scheduled 8am
lessons so that I can at least pretend there is a usable workday
left;).

WPR
Student Pilot"

[Larry Dighera]

On 11 Feb 2005 13:50:34 -0800, "greenwavepilot"
> wrote in
om>::

>Therefore versus fiberglass the aluminum surfaces
>will cool to icing temperatures faster, and conversely will heat to
>non-icing temperatures faster.

Yes, exactly.

>The composite will cool more slowly,
>but once cooled, will retain that temperature much longer, meaning like
>I discovered this morning, my composite plane will/may be iced when the
>Cessnas, Pipers and Mooneys won't.

Consider also, the mass of composite v aluminum necessary to achieve
the same strength. I'd guess the composite skin thickness exceeds
that of the aluminum skin.

[Steve.T]

The composite materials are used as insulation in certain cases. Think
of fiberglass.

Next, look at the paint used for the a/c in question. The composites
are painted below the surface while the metal a/c have painted
surfaces.

Now, given the insulative qualities of the composites, and the surface
area with which to "absorb" energy (heat), the aluminum a/c will shed
ice much faster when placed in a heated hanger than will a composite.
Now if you place a fan in that heated hanger, the composite will de-ice
at a faster rate.

OTOH - at altitude and with air speed, I do not know if the composite
will ice faster than the aluminum, because the aluminum is not (well, I
wouldn't think it) as smooth as the composite. So water has something
to adhere to which will start the process (talking about rime). Because
of the smoothness of the wing, I don't know, and have no experience
with clear ice and its ability to stick to the wing. But because my
step-son is close to test flying his Vari-eze (I think that is the one
he is building), we may be able to find out by this time next year.

So you other composite drivers, how about practical experience?

Later,
Steve.T
PP ASEL/Instrument

[George Patterson]

"Steve.T" wrote:
>
> The composite materials are used as insulation in certain cases. Think
> of fiberglass.

Fiberglass, in and of itself, is not a particularly good insulator. When it's
spun into a sort of glass wool and traps a lot of small air bubbles, that
changes. The air *is* a good insulator.

George Patterson
He who would distinguish what is true from what is false must have an
adequate understanding of truth and falsehood.

[Casey Wilson]

"George Patterson" > wrote in message
...
>
>
> "Steve.T" wrote:
>>
>> The composite materials are used as insulation in certain cases. Think
>> of fiberglass.
>
> Fiberglass, in and of itself, is not a particularly good insulator. When
> it's
> spun into a sort of glass wool and traps a lot of small air bubbles, that
> changes. The air *is* a good insulator.
>
Depends on how you look at it. In the long-wavelength infrared
spectra the atmosphere is virtually transparent. In that regard, air is an
excellent conductor since it poses no resistance to thermal radiation. And
it is the radiation that accounts for the type of heat loss involved in
these discussions, not conduction. So it is the thermal emissivity of the
wing surface that really matters -- not the molecular conductivity.

[Larry Dighera]

On Sat, 12 Feb 2005 05:18:37 GMT, "Casey Wilson" <N2310D @ gmail.com>
wrote in <NkgPd.2805$uc.202@trnddc01>::

>So it is the thermal emissivity of the
>wing surface that really matters -- not the molecular conductivity.

If the molecular conductivity is unimportant, once the surface changes
temperature, how does the rest of the material change temperature
without conduction?

[Casey Wilson]

"Larry Dighera" > wrote in message
...
> On Sat, 12 Feb 2005 05:18:37 GMT, "Casey Wilson" <N2310D @ gmail.com>
> wrote in <NkgPd.2805$uc.202@trnddc01>::
>
>>So it is the thermal emissivity of the
>>wing surface that really matters -- not the molecular conductivity.
>
> If the molecular conductivity is unimportant, once the surface changes
> temperature, how does the rest of the material change temperature
> without conduction?

Go back and read it in context. I didn't say there was no conduction.
What I said was (in different words) that conduction is not the process of
the wing surface losing thermal energy. Here, I'll put it in another term so
you can do some more nit picking: Because the atomosphere is transparent in
certain important wavelengths, the heat of the wing is being sucked into
outer space.

[Larry Dighera]

On Sun, 13 Feb 2005 04:34:40 GMT, "Casey Wilson" <N2310D @ gmail.com>
wrote in <ANAPd.26325$uc.19139@trnddc04>::

>
>"Larry Dighera" > wrote in message
...
>> On Sat, 12 Feb 2005 05:18:37 GMT, "Casey Wilson" <N2310D @ gmail.com>
>> wrote in <NkgPd.2805$uc.202@trnddc01>::
>>
>>>So it is the thermal emissivity of the
>>>wing surface that really matters -- not the molecular conductivity.
>>
>> If the molecular conductivity is unimportant, once the surface changes
>> temperature, how does the rest of the material change temperature
>> without conduction?
>
> Go back and read it in context. I didn't say there was no conduction.
>What I said was (in different words) that conduction is not the process of
>the wing surface losing thermal energy. Here, I'll put it in another term so
>you can do some more nit picking: Because the atomosphere is transparent in
>certain important wavelengths, the heat of the wing is being sucked into
>outer space.
>

Yes. I understood what you said. And I thank you for the insight
into the arcana of IR radiation.

However, I believe the question that I posed provides further insight
into the RATE of temperature change that we were discussing.
Obviously conduction is required to move the heat to the surface.

[Casey Wilson]

"Larry Dighera" > wrote in message
...
> On Sun, 13 Feb 2005 04:34:40 GMT, "Casey Wilson" <N2310D @ gmail.com>
> wrote in <ANAPd.26325$uc.19139@trnddc04>::
>
>>
>>"Larry Dighera" > wrote in message
...
>>> On Sat, 12 Feb 2005 05:18:37 GMT, "Casey Wilson" <N2310D @ gmail.com>
>>> wrote in <NkgPd.2805$uc.202@trnddc01>::
>>>
>>>>So it is the thermal emissivity of the
>>>>wing surface that really matters -- not the molecular conductivity.
>>>
>>> If the molecular conductivity is unimportant, once the surface changes
>>> temperature, how does the rest of the material change temperature
>>> without conduction?
>>
>> Go back and read it in context. I didn't say there was no conduction.
>>What I said was (in different words) that conduction is not the process
>>of
>>the wing surface losing thermal energy. Here, I'll put it in another term
>>so
>>you can do some more nit picking: Because the atomosphere is transparent
>>in
>>certain important wavelengths, the heat of the wing is being sucked into
>>outer space.
>>
>
> Yes. I understood what you said. And I thank you for the insight
> into the arcana of IR radiation.
>
> However, I believe the question that I posed provides further insight
> into the RATE of temperature change that we were discussing.
> Obviously conduction is required to move the heat to the surface.

Ahh, now I see your point. You are obviously correct that it is
a systemic process.

[AES]

In article <ANAPd.26325$uc.19139@trnddc04>,
"Casey Wilson" <N2310D @ gmail.com> wrote:

> Go back and read it in context. I didn't say there was no conduction.
> What I said was (in different words) that conduction is not the process of
> the wing surface losing thermal energy. Here, I'll put it in another term so
> you can do some more nit picking: Because the atomosphere is transparent in
> certain important wavelengths, the heat of the wing is being sucked into
> outer space.

Well, I'll pick a tiny, purely semantic nit:

The heat of the wing isn't being "sucked" into space; it's being
actively _radiated_ into space -- but space, being cold (3 degrees K in
the limiting case), isn't radiating back.

But this is only to quarrel with your phrasing, not at all with your
basic argument.

[Jose]

> The heat of the wing isn't being "sucked" into space; it's being
> actively _radiated_ into space -- but space, being cold (3 degrees K in
> the limiting case), isn't radiating back.
>
> But this is only to quarrel with your phrasing, not at all with your
> basic argument.

Actually, I like the imagery of the heat being sucked into space. :)

Jose