You have to look at how heat is transferred *both ways*.  Aluminum *is* a
good reflector, especially in the infrared where the exhaust heat is
radiating.  When an object is out in the sun, it is being bathed in visible
light in addition to some infrared. The temperature that a body or paint
goes to in the sun is a function of how much it absorbs, reflects, and
emits.  For normal paints and aluminum, the relationship

Absorptance = Emittance = (1 - reflectivity) holds, for any given
wavelength.

Aluminum has a reflectance of about 0.8 to 0.9, meaning that 10% is
absorbed, but it also means that its emmisivity is only ~.1 also, so that
energy that gets absorbed is not reemitted until it gets hot.

White paint, on the other hand has a reflectance of 0.9, however the paint
is bound together with an organic binder that has an emittance close to 0.9
to 0.95 in the infrared.  So the white paint can get rid of the heat
deposited from the visible spectrum faster than aluminum, but will "appear"
black (i.e. absorb and emit well) in the infrared.

Bill Schertz
KIS Cruiser # 4045
----- Original Message -----
From: "David Carter" <dcarter@datarecall.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, May 23, 2004 11:48 PM
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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