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Posted for "Tracy Crook" <lors01@msn.com>:
I'll embed my comments in the appropriate places below. This is an
abreviated version because I don't have the patience to rewrite the first
version which took 2 hours to write and was lost to the "This program has
performed an illegal operation and will be shut down" message.
Tracy
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.
Keep in mind we are not flying a radiator through the air, it's the
AIRPLANE. Unless you can show why more air through the system causes less
drag, you have to conceed the point that less air means less drag.
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.
As soon as you use the "slower air" through the thin rad argument, you loose
the vaunted higher efficiency of the thin rad. Paul often makes the point
that the rear part of the thick rad is not working as well due to the
pre-heated air. If you slow the air through the thin rad, the same thing
will happen in the thin 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.
Major and widespread error here. Please explain why a *high pressure* in
front of a thin rad will result in *slower flow* through the thin rad core.
The lower velocity in the aft end of the diffuser is being confused with the
velocity through the core of the radiator. They are NOT the same thing.
The rad (even a thin one) is a restriction and the air is rushing FASTER
through it due to the pressure recovered by the diffuser, not slower.
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).
Compare a 7 deg. difuser for an 18" x 18" thin rad and compare it to one for
a 9.5" x 9.5" thick rad. Do you have a prayer of fitting the big one under
the cowl?
Another reason for using thick rads.
Thanks for the opportunity to debate this issue in a non-censored
environment David. No offense is taken or meant
Tracy Crook
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