X-Mozilla-Keys:                                                                                 
Bill Freeman wrote:

   BIG SNIP

BTW I don't really think external diffusion is totally free.
If you have more air going around the outside of the cowl you have
more velocity and drag from that source.

Paul Lamar

It can be free if you do not increase the frontal area beyond
that needed for the people & motor.  Certainly, adding a
protruding scoop is never free.  But, picking off low speed
air at the front of a nicely streamlined body *is* essentially
drag free, i.e. no *additional* drag beyond what the body
would have to incur.   Also, I contend that external diffusion
is much more efficient, since the air goes where it needs to,
and nobody had to figure out the correct cooredinates of
a diffuser to make the air "do the right thing".  My guess
is that the Lopresti cowls are mostly external diffusion,
with less than half the pressure rise occurring inside the
cowl.   There is just not enough distance and delta area
available for good effiency (internally), and they *are*
getting good efficiency, e.g. there is a lot of external
diffusion going on, like in front of every normal
cowling.  The biggest problem that Cessna and Piper
made (IMHO) with they're 'bad old' cowls was not
having a wide enough lip on the top, so they captured
way too much high speed air, diffusing it poorly and
causing a lot of drag.  My 64 C182 is a good (bad) example.
A Bonanza, OTOH, has its inlets well below the top edge
of the cowl, where the velocity is low, and they probably
take much better advantage of the external diffusion which
*is* occurring ahead of the fuselage.

Bill

Mississippi State U not Lopresti :-).

Paul Lamar
 
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