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If you don't know the characteristics
of the NACA curved-divergent inlet for its proper design, I would advise you to
instead use a stagnation/pitot inlet, that is, one which projects out from the
cowling pointing forward and with a streamlined aft body. See Hoerner's
"Fluid Dynamic Drag" for some inlet shapes and their drag coefficients. You
might also want to look at the NACA parallel-wall submerged duct which has a
square outlet that is much easier to interface to a round duct. It also has
better pressure-recovery characteristics than the curved-divergent inlet at
duct velocities greater than half of the free-stream velocity. Look at your
oil-cooler's spec sheet from the manufacturer, pick the cooling flow from the
middle of the graph of heat-rejection vs flow, size the inlet for that flow
at cruise, then add 25% to the area. Say the center of your spec sheet
graph shows 300 cfm/ 5cfs, and your cruise speed is 230 mph. Then your inlet
area is 5 * 144 / (230 * 22/15) = 2.13 square inches. Increase that by
25% to get 2.67 sq. in. 1 5/8 " square, 2 5/16" X 1 3/16" rectangle or 1
7/8" diameter round, all with nice, rounded lips. Then provide a
diverging duct from that inlet to the oil cooler to slow the flow and increase
the pressure, and then a converging duct from the cooler to the cowl outlet,
pointed toward the rear, with an outlet area of about 3-3.5 sq in to get the
flow back up to freestream velocity.
A three blade prop can be considerably
reduced in diameter relative to a two blade of the same mass-flow
characteristic, so you might want to go that route. Each blade sweeps an area
based on the diameter, so a three blade would sweep the same area at
a reduced diameter. Carried to the extreme, a 59" three-blade would
intercept the same area and mass as a 72" two-blade. A three-blade
would also give you better static and climb thrust, as well as a bigger hole in
your bank account. But what the Hell, it's only money! My three-blade is
only 63" diameter, and has very good climb and
cruise efficiency!
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