It still sounds like he is dealing with an air cooled engine.
Apples and oranges folks. What has all this got to do with
radiators? NACA themselves, the inventors of the NACA duct,
say: "don't use them with radiators!"

Paul Lamar

A properly designed and executed NACA scoop should have pressure recovery
approaching 80%, so with a effective diffuser you'd think could be expected
to work for a radiator; depending on the flow and pressure drop.  Probably
not for a P-51 rad, which I think is about 6" thick.

Velocity fliers using NACA inlets at the top of the fuselage just forward of
the engine cowl find they work fine for downflow aircooled engines; and at
least one person found significant improvement in cooling air flow by
strategically placing a couple of vg's forward of the scoop.  Apparently if
properly oriented they set up a vortex that diverts free stream flow into
the scoop.

Al  Gietzen

The 85% number is when feeding a jet engine.
The NACA duct WILL NOT WORK when the velocity of the air going though
the duct is less then 70% of the free stream velocity.
Save yourself a lot of grief.

DO NOT USE IT FOR RADIATORS!!!!!

Paul Lamar
 
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