Its true that the larger proportion of the air from the propeller
never goes onto the cowl, but it's still a lot.

A surface radiator isn't necessarily a flat plane, what shape is the
cowl on the airplane you fly? And the air stream is directed onto the
cowl at an angle as is necessary to flair around the engine/cowling.
The cowl would look similar to the current cowls except it would be
more stream lined and have no large holes to duct in air. And of
course in big red letters "HOT SURFACE- DO NOT TOUCH"

"Ron Natalie" wrote in message om...
"Jay" wrote in message om...

I'm not sure its fair to compare a 10" 1/4 HP electric fan to a 150HP
propeller.

That 150HP propeller is developing some useful thrust as well. For efficiency
the last thing you want is a large amount of it impinging on some flat plate.
That 1/4HP fan is shrouded and delivers almost all of it's flow directly to the cooling fins
of the radiator.