The most detailed, and what I believe to be authentic, analysis of the P-51 scoop legend is that there is no net thrust – ever.  There are certain conditions where it is basically 1 : 1 offsetting drag (top speeds, and in dives); and yes, the outlet velocity may get slightly higher than inlet, but the air is less dense so has less momentum per unit volume.  For most operations it was in the 1 to .85-.95 range, which is excellent.

 

Anything we can do to accelerate the air back toward the free stream and velocity, and have it exit in the direction of the free stream, helps.  An efficient converging duct with an outlet area in the range of 1.3 to about 1.6 is good, and the ratio of choice depends on the temperature increase of the air – which depends on the mass flow rate through the core. (Higher DT, higher ratio). Analysis that I have seen (and verified) suggests minimum drag and radiator weight is achieved with DT in the range of 50 – 75F, core thickness 2 1/4 – 3”; and yeah, there are set of assumptions involved, so YMMV.

 

That’s my story; and I’m sticking to itJ.

 

BTW, ED, thanks for the tip on the D; that is very cool.

 

Al G