Posted for "David Leonard" <Daveleonard@cox.net>:

Ok, I'll submit an opposing viewpoint.  Though I point out that from the
beginning I am proved wrong by the real experience of NASCAR.  I will
also agree that if there needs to be a choice between efficient rad and
efficient airplane - choose the efficient airplane!

I think the key to the argument for a thin rad lies in the fact that
parasitic drag is a function of the velocity SQUARED.  Implying to me
that passing air quickly through a rad pays a disproportionately higher
drag penalty than passing a slightly larger amount of air slowly.
Comparing 2 rads of the same volume; one thick, one thin:  if we are
going to, say, pass 10 cubic feet of air through in 5 seconds, it will
need to be going faster in the thicker rad (with proportionately less
surface area)- as already mentioned.  As you point out above the thicker rad will allow each packet to get
hotter because it will be spending more time crossing the thicker rad
(so less air is needed).  But this is offset somewhat by the fact that
the slower speed in the thin rad allows each packet to gain more heat
per inch and spend almost as much time in the rad.

Paul once illustrated it best for me when he said the ideal would be to
take a 'small' packet of fast air and use an efficient diffuser to
expand the cross section and slow it down (now higher pressure), pass it
slowly and efficiently through the rad.  Yes, most energy is now lost,
but obviously not ALL of it (otherwise it would never leave the cowl),
and use an efficient diffuser to go back to small cross section and high
speed.

The other key point then becomes "efficient diffuser."  IMHO, the happy
medium is to use the broadest rad that will fit without disturbing the
profile of the aircraft - and still leaves room for efficient diffusers
(whatever that is) (7 degree rule and all I suppose). David Leonard
The Rotary Roster:
http://members.aol.com/_ht_a/rotaryroster/index.html