I had the same thoughts as Charlie but may have part of the answer.  Bernie
has done a lot of pressure & velocity measuring on his plane and came to the
conclusion that in the case of conventional inlets (holes in cowl on either
side of the prop spinner), practically all of the diffusion is *external*
and almost zero was taking place in the duct.  That is what led him to his
"open chamber" plenum for the oil cooler & rad.

Paul Lamar proposed the possibility of external diffusion and I think he was
right.  This would explain why Finn Lassen's cooling system works OK.  He
has absolutely no ducting at all, just a hole into a sharp cornered box in
front of the rads.  If it were not for external diffusion, he would have
burned up years ago.

This factor is very significant in choosing cooling inlet location.  Note
that the external diffusion is only available in *very* few locations on the
airplane. This is why I'm sticking to the stock cooling inlets on my RV-8.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

External diffusion happens anywhere flow into the duct is restricted.

If the P51 type duct is on the aft end of the airplane where the boundary layer
has had considerably time to build up and there is insufficient
distance between duct opening and skin surface the airflow will
merely be diverted around the opening. This in effect is what happens
to the NACA duct when used with a flow restriction like a radiator
even if located where the boundary layer is thin. If the duct is
out of the boundary layer and the flow into the duct is restricted,
say by too thick of a rad, external diffusion will also take place.

Once the flow enters the duct and the rad is not perpendicular to the
axis of the duct the wedge shaped effect comes into play. The air pressure
will build at the back of the duct due to the inertia of the air.
If the duct is NOT a wedge shape more or less most of the air will try to
flow through the rad closest to the back of the duct. The part of the rad
nearest the front will have less flow.

If the rad is perpendicular to the flow and a bell shape is not used there
will be separation of the flow on the duct walls and most of the air will
impact the center of the rad with a consequently loss of rad flow
around the edges. If the duct is not about 2-1/2 times longer
than the max dimension on the rad a small opening (25% to 30%) can not be used.
In that case a larger and larger opening as the duct gets shorter and shorter
will be required. The limit to this is no duct at all with just the rad stuck
out into the air stream perpendicular to the flow like a WW I fighter plane.
Any air that cannot flow through the rad for what ever reason such as
an object placed too close behind the rad will cause external diffusion
and the air will be diverted around the rad with high cooling drag.

Not however as high as intuition and flat plate drag theory would have
you believe. That was what I was referring to when I said external
diffusion reduces the drag. The WW II Corsair cowl is a good example.
The Corsair was no faster when a spinner was tried because of external
diffusion. Later radial engines were made with longer crank cases and
a longer internal  bell shaped radial engine cowl could then be employed with a
reduction in cooling drag over the Corsair configuration. These had a smaller
diameter opening of less area. The Lockheed P2V and late versions of the
Constellation among others are good examples. Here is a picture that
dramatically illustrates the difference between the old and new
radial engine cowlings.

Bottom line; good design is better than external diffusion.

Paul Lamar

BTW I forgot to mention. IMHO the reason Finn's cooling system is 
working as well as it is... is because he is not running the cowl cheek
fairings on both sides of the fuselage.  Consequently the cowl is 
well ventilated. The air can exit the rad relatively unobstructed
which is known to enhance the flow through the rad.

Left side of this  picture.

Paul Lamar
 
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