Paul,

   Let me add my two cents to the cooling exit discussion.

   While there may be a theoretically optimal inlet/exit ratio for drag,
   wouldn't it be more important not to put more cooling air through the
   cowling than is needed? I assume that liquid-cooled engine have
   thermostats, and so the engine getting too cold is not a problem. But
   the inlet has to be sized for the most adverse hot-day climb condition,
   and so it will be able to admit far more air than is needed for, say,
   economy cruise at altitude on a cool day. If you determine the outlet
   area by some rule of thumb, say 80% of the inlet area, then you will be
   on the thermostat and will be pumping air through the cowling for
   nothing.

   There is a sort of mania these days for small inlets, but I think you
   would agree that there is no harm in a large inlet, provided that it has
   a well-formed lip. Pressure recovery occurs before the inlet, and
   un-needed air flows smoothly around it. That's why radial-engined planes
   still go fast. Opening and closing the outlet moves the stagnation line
   on the inlet lip, and in the case of maximum spillage it actually yields
   some duct thrust, just like on your inflatable shrouds.

   My plane has about 60 sq. in. of inlet area (200-hp engine), and the
   outlet area can be adjusted from 98 to 18 square inches. I normally
   cruise with the cowl flaps fully closed.

   Peter Garrison

   http://www.melmoth2.com/texts/NACA%20inlet%20sizing.htm

   I agree Peter. In fact I suggest a thermostat NOT be installed and the
   engine temperature controlled with cowl flap alone. I too cruise the
   C182 with the cowl flaps fully closed. Of course there is a lot of
   leakage
   around the crude C182 cowl :)


   Paul Lamar

Hi Peter

On the Glasair I’ve decided to control engine cooling via the cowl flaps
for two reasons.
First as Paul suggests to control cooling via air flow and thus
minimizing cooling drag.
And secondly in the Glasair the thrust line is very high and it was
easier to remove the thermostat from the Renesis engine.

Steve Izett
Perth Western Australia