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David, your observation was correct on the temperature of aircraft.
However white paint would make be a bad shielding for the inside of a
cowl.
Here is what I got from my heat transfer book:
A=solar absorptivity
E=infrared emissivity & absorptivity
Surface A E
AL polished .09 .03
AL foil .15 .05
Stainless Steel Polished .37 .6
SS Dull .5 .21
Black Paint .97 .97
Wight Paint .14 .93
These numbers are very interesting. For shielding we want the lowest E.
For trying to block radiant heat AL is clearly the best. Stainless steel
is between 4 and 12 times worse at blocking radiation that AL foil.
Trying to keep objects cool under sunlight is a special case. We want it
to have a low A so that it does not absorb much solar energy but a high
E so that it easily radiates any acquired energy away. Note white paint
is great for this.
The bottom line is that white paint is great for solar shielding on the
outside of but bad for infrared shielding under cowlings and such. Al is
the best for IR shielding.
I am missing all the Greek letters so a am kind of mangling the
radiation formula but here it is.
Qrad= a*G*A + a*E*O(Tinf^4-Tsurf^4)
Qrad=heat transfer (W/M^2)
a=area of surface (M^2)
G=solar radiation (W/M^2)
A= solar absorptivity
E= infrared emissivity
O=5.67E-8 (W/(M^2*K^4)
Tinf= temperature of the surroundings (K)
Tsur= Temperature of the surface (K)
NOTE temperatures must be in Kelvin
Alex Madsen
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Alex Madsen
Sent: Monday, May 24, 2004 1:28 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Two problems solved, 2 more pop up...
I am skeptical of this. Where the light colored aircraft composite? This
would make a difference. The higher thermal conductivity of the AL would
make it seem much hotter than composites. Both aircraft must be
constructed of the same material. Were they? Anyway off to look at my
heat transfer book to see what it has to say on this topic.
Alex Madsen
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of David Carter
Sent: Sunday, May 23, 2004 11:49 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Two problems solved, 2 more pop up...
I've noticed that shiney polished aluminum airplanes are quite hot in
the
sun, whereas white and light cream colored painted exteriors are quite
cool.
I used to think and "aluminum" finish would be best for reflecting heat,
but
I no longer think that. I think the same situation would prevail inside
the
cowl.
David
----- Original Message -----
From: "Chad Robinson" <crj@lucubration.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, May 23, 2004 9:25 PM
Subject: [FlyRotary] Re: Two problems solved, 2 more pop up...
<snip> Given that the primary goal is reflecting radiated heat (rather
than
"real"
> thermal insulation) what's wrong with a coat or two of shiny silver
paint?
> It's not as good as polished aluminum but it's decent, and high on the
> reflective scale. Sherwin Williams (and others) sells a product called
> "Silver-Brite" that is marketed as "heat reflective" and "dry heat
resistant
> to 700degF". I haven't tested its adherence to epoxy so it might need
a
primer
> coat to help it stick better (it's designed for metal) but that's not
a
> painful step. In any event, it would sure be a lot easier to apply
than
either
> foil OR fiberfrax, especially to an oddly-shaped cowl.
>
> Unless real thermal insulation properties are necesary?
>
> Regards,
> Chad
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