Corrie wrote:

Back in the early 80's I was a co-op student (read,
engineer-in-training) at Fairchild-Swearingen. The group to which I
was assigned was studying the flow through the oil cooler for a PT-6
version of the Metro / Merlin line. They were using NACA-shaped ducts
on the sides of the nacelles to get the air into the plenum to cool
the oil.

It wasn't working, until I dug down into the original NACA references
to discover that the duct lip had a bump - not just a plain radius.

You should've kept reading. The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Russell Kent