However millions of trucks and thousands of WW I airplanes successfully
used unducted radiators and they cooled fine. The myth of slowing the air
flow through the  core to ostensible improve the heat transfer is actually
counter productive.

Paul Lamar

Paul, I am not opposed to the simple system, but you must admit the 
unducted radiators you are talking about here are either
HUGE, (trucks), or flying at much slower speeds and HP numbers, 
(WW1 aircraft) Bill Jepson

Actually the WW II P40 had un ducted rads as does a very modern  Extra 400.
Neither air plane was/is slow. I think the P39 is in that category
as well. So too the P38. All the rads in these airplanes were quite 
thick. In the  case of the P51 it was eleven inches. Good luck getting 
that to cool at 100 MPH. In the case of the P39 it too was maybe as much
as 12 inches thick. Most of these airplane had the bunch-of-soda-straws
type of rads that I suspect were lower drag than the modern fin and tube.

I define un ducted rads as those rads that have scoops and inlet ducts
with the same cross sectional frontal area as the rad.

The saving grace as far as drag in these airplanes was concerned is the 
phenomenon of external diffusion wherein you close the cowl flaps
and these large scoop openings become self streamlining due to a large
pressure build up in front of the scoop. The classic example for
external diffusion  is the Corsair where the addition of a large 
spinner added nothing to the top speed.

Some of these airplanes were only slightly slower than a P51 at the
lower altitudes. the P51 scored at high altitudes because of the
superior two stage super charger compressor on the Merlin. 

BTW The early P51 had a defective duct and it was not fixed until
after the war on the last prototype XP51J built.
Probably after they had a talk with Kuchemann & Webber :)
 
Paul Lamar
 
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