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?

Submerged NACA ducts do not allow much air diffusion; they're for feeding
large quantities of air to jet engines. Radiators work best with highly
diffused air (large dynamic pressure recovery). See pgs 18-19 of:

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

If you're determined to have NACA submerged ducts, you might want to study
these NACA reports to get the best duct shape:

http://naca.larc.nasa.gov/reports/1948/naca-rm-a8a20/
http://naca.larc.nasa.gov/reports/1948/naca-rm-a7i30/
http://naca.larc.nasa.gov/reports/1942/naca-report-743/

Russell Kent