X-Mozilla-Keys:                                                                                 

Al Gietzen wrote:

The air flow required can be similarly computed; and as it turns out, for a
50 F temp increase in the air (a seemingly reasonable number that Paul
chose), the air flow in cubic feet/min (cfm) is very close to being
equal to the heat rejected in Btu/min.  So for a 20B I'd need 
about 7000 cfm for a 50 F delta T.  
That's a lot of flow (and drag).  Maybe the direct air-cooled
engines can get by with less flow because their average fin temp is
probably considerably higher than we can accept with a water coolant loop.
I had always heard the liquid cooled was better from the standpoint 
of cooling air required.  Maybe we need to design for a 
higher air temp increase.

Jeff Spitzer wrote:

I always do the first estimates based on the expected outlet temp of the
air being not more than 20 deg less than the coolant temp.  At altitude
that gives higher delta T's for the thinner, colder air.  That balances
things more realistically.  Assigning a delta T to the air will not work
because of large differences in inlet temp and density.

Jeff Spitzer

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