Here is a picture of a bottom P-51 style cooling intake on a Chev
V8 powered extended BD4 that has been around for many years. The 
rad is directly above this in the fuselage mounted near vertical 
so the air must make an immediate 90 degree turn, make another 90 degree 
turn to go through the rad and two more 90 degree turns to get out 
the flapped exit which is on the other end of this thing. 

Must work as the guy flew it up from Texas to OSH. 

Not exactly low cooling drag I suspect.

Kind of ugly too. 

The rectangular channel in front of the intake and on the bottom 
of the fuselage contains the cooling pipes to and from the engine.

Paul Lamar
 
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