"Looking at your photos it appears that the air can flow from the inlets directly to the rear cylinders by traveling around the valve covers as well as over the top."

 

Yes, and this is common in many Continental installations.  The space between valve cover attachment plane and outside edge of cowl is commonly used as part of the volume to slow and distribute air as evenly as possible.   This baffle design also allows the rubber seals to rub against the side of the cowl instead of the top which may or may not be a benefit.

 

"It does look like a portion of the air has to flow upward before entering the plenum. Would there be any advantage to placing the inlets higher so the air could go straight into the area over the cylinders?"

 

Yes, and this has been done on the  Columbia inlets which are raised so that the center of the inlets are 1.5 inches above the centerline of the crankshaft.  I did the same after studying a Columbia closely.  It also has the boundary layer splitters I have previously mentioned to prohibit the low velocity flow off the splitter and behind the blunt prop blade shanks from screwing up the inlet velocity profile across the inlet.  I did not invent these features.  I stole them.   Conventional GA inlet designs that come right up to the spinner side suffer from major velocity distribution distortions, and as noted, on Bonanzas, the flow even reverses and goes out of the cowl in this region.  But the look pretty, so stylists love them.

 

"Perhaps the inlets could then be downsized to meet the flow needed for cooling with a separate inlet for the engine air intake?"

 

I suppose, but why bother?  When the airplane operated in the deep subsonic regime (most of us operate at Mach 0.35 to 0.5) there is no penalty to being blunt on the front end, contrary to what intuition may suggest, as long as that blunt shape is reasonably contoured and smooth to prevent separation at the lip of the inlets.  Pointy and sharp are for high transonic and supersonic flow conditions and sex appeal.  Make your choice.

 

  "Along those lines might NACA scoops on the top of the cowl be an option? My cowl isn’t mounted but it seems there’s a positive slope to the upper surface so that they could potentially work."

 

There is no point and no benefit.  The front facing inlets work better than NACA scoops.  NACA scoops are great for areas that need flow some of the time, but not others, such as cooling air for the cabin. When you do not need flow, NACA scoops create very little drag because nothing sticks out into the flow.  But NACA ducts do not recover ram pressure as effectively as well designed front facing inlets.  The only place you will find NACA ducts flowing air into engines is on the early turbine Lancair IVs, and they were clearly inferior to conventional front facing inlets that appear on every other medium or high performance turbine engine.   So if you need air all the time as with cooling air for a piston engine or intake air for a turbine, use a front facing round inlet to get best ram pressure recovery.  When air flow varies from zero to max, or from max down to something less (like induction air for your piston engine), a NACA duct is a satisfactory solution that also fits airplane styling nicely. 

 

Fred Moreno