[caldrail]

~S~
I have read that the only plane who's speed increased when radiator
was fully opne was the P-51. In the D model the speed actually increased
by 5-8 mph faster. I guess the radiator was design as not to influence the
drag, when the P-51 radiator is fully open the hot gas push out the rear of the radiator which caused the increase in speed. At what altitude did
this happen I am not sure, however it must be above 18-20,000ft.

The increase in speed is actually caused from a type of 'RAM AIR' effect as air enters through the intakes and is forced out the exhaust at the back of the aircraft.
There is a name for the effect, but it escapes me at the moment.
To my knowledge, it wasn't effected by alt, but by speed.

Again, no. That can't happen because the the radiator impedes airflow through the channel. Therefore there is always less air coming out the back than striking the front of the radiator shroud. In other words, an aerodynamic bow wave is set up (though this doesn't become a huge problem until transonic speeds)

What does happen is two-fold. Firstly the coanda effect provides a smoother flow of air under the fuselage because it can follow the curve of the scoop (as opposed to radiators like those on Hurricanes or Yaks, that just sit in the airstream like bricks). Secondly, the air exiting the radiator scoop forms a crude boundary layer control, simply because of the arrangement, and that provides airflow around the bottom of the fuselage and rudder that is smoother and less draggy that in other aeroplane designs.

At first sight it seems a little odd - you would think the air exiting the radiator is somewhat turbulent - but the idea works. The P51 was after all a very efficient design aerodynamically for a number of reasons including this one. The Napier Heston racer, which used the same style radiator scoop as the P51 sadly never proved the point because the oil line broke on its first flight and the engine siezed, causing a crash.