My engine choice is currently a TSIO-550. I want to make sure that I have plenty of cooling air available to the engine and oil cooler. Recall that I am not electing to install an oil cooler in the nose of the aircraft.
I am planning to modify the factory-provided NACA ducts to enlarge the entrance height to 3.5″ and also allow for a 1/4″ thick airfoil lip at the duct entrance plane. I don’t really want to extend the length of the duct because of the headspace encroachment in the cabin. The concern is that the ramp angle for this configuration is about 9.6 degrees. This is higher than the recommended range from the old NACA literature.
To give myself some confidence, I ran some flow simulations in Solidworks and validated the performance of my modified NACA duct for takeoff conditions (0 MSL, 15C, 100 kts TAS) and medium cruise (10,000 MSL, -4C, 160 kts TAS). The unmodified (factory stock) duct provides 78% of free-stream mass flux and the enlarged entrance duct provides 77% of free-stream mass flux – only a tiny decrease in efficiency, but with a 1.75 times larger mass flow due to the enlarged area.