Message-Id: <5.2.1.1.0.20040524123426.04cc82c8@mailsrvr.al.noaa.gov>
Date: Mon, 24 May 2004 13:26:44 -0600
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Two problems solved, 2 more pop up...
In-Reply-To: <list-86615@logan.com>
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         How "hot" something feels to the touch is only loosely related to 
its temperature. The "thermal effusivity" plays a large role in what you 
perceive. The thermal effusivity is the thermal conductivity times the 
specific heat times the density. It is the ability of a material to 
transfer heat to (or from) its surface.

         Aluminum and copper have a very very high thermal effusivity. 
Plastic and wood have a very low thermal effusivity.

         Put a copper block and a piece of wood in your oven at 350 
degrees. After an hour, reach in and grab the wood block with one hand and 
the copper block with the other. They are both at the identical temperature 
of 350 degrees, but the copper block will burn you severely, while the wood 
block will just be mildly uncomfortable to hold.

         Aluminum works great as a heat reflector, even as it ages. Not so 
with white paint.

         The best heat reflector for use in ambient air is gold. Gold leaf 
might not work too badly for a reflective coating inside the cowl. It would 
certainly dress it up a bit. :^)

At 01:27 AM 5/24/2004 -0500, you wrote:
>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?
> >