Dave Hyde wrote:

Russell Kent wrote:

The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

It's my understanding that the NACA submerged duct was designed to feed a
jet engine, and as such if the velocity of the air in the duct is not a
significant fraction (like 70%) of the free airstream velocity, then the
duct "looks" like a wart on the fuselage, and the free airstream flows
around it. See NACA-ACR 5i20 at

http://naca.larc.nasa.gov/reports/1945/naca-acr-5i20/

Specifically:
The data obtained indicate that submerged entrances are most suitable for
use with internale-flow systems which diffuse the air only a small amount:
for example, those used with jet motors which have axial-flow compressors.
Where complete diffusion of the air is required, fuselage-nose or
wing-leading-edge inlets may prove to be superior.

And later (pgs. 18-19):
Submerged inlets do not appear to have desirable pressure-recovery
characteristics for use in supplying air to oil coolers, radiators, or
carburetors of conventional reciprocating engines. The required diffusion
of the air and the range of inlet-velocity ratios is too great to give
desirable characteristics at all flight conditions.

If you're determined to use submerged NACA ducts, you might study these
papers to get the best performance:

Russell Kent