Tracy Crook wrote:

Paul L. wrote:

IMHO Internal diffusion implies a long entrance duct like the
P51. The rads being so close to the front of the airplane there
is no room for a long ducts so you use a big opening and hope
that it is just the right size so it does not spill over
and increase the overall drag of the airplane.

IMHO The exit duct has little to do with the term external
diffusion other than the nozzle must apparently be larger
to encourage the flow through the radiator. I was hoping
that would not be quite so necessary because I was exiting
the air in a low pressure area over the wing and not to
a neutral pressure area.

Add a hole in the cowling and an exit area to cool the engine and
exhaust pipes. :-)

I'm not in my area of expertise here but I can state with certainty that
what is going on internally to the short ducts is VERY important.   When the
duct was basically a chamber, my engine overheated immediately.  When shaped
as shown in your drawing it cools fine.   That led me to think that internal
diffusion is important even on this short duct.

Tracy Crook   rws@altavista.net

By defination internal diffusion is a slowing of the air with an 
increase in pressure entirely within the duct. The entrance 
velocity should be near airspeed. 

(P1 X V1) / T1 = (P2 X V2) / T2

If you build a couple of small pito tubes out of 1/16 copper tubing
you can check the velocity and static pressure in several places
and determine a map of air velocity. I suspect you are merely
distributing a little more air to the corners of the evap cores
by reducing separation.

If you find the air velocity is much less than airspeed at
the entrance of the hole than you have mostly external diffusion.

Evidense of this is the large size of the hole relative to the 
area of the core. Most internal diffusion systems are down around
10 to 20% as a guess.
 
Paul Lamar
 
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