X-Mozilla-Keys:                                                                                 
Fred;

Thanks for further info.

Keep in mind that the compression of air ("diffusion" - it comes from the
19th century) occurs in two phases: a slowing (and compressing) in front of
the inlet due to the bluntness of the front of the airplane, and further
slowing after passing through the inlet into the diffuser itself.  The first
phase is frictionless and is thus preferred, if you want to ignore all other
things.  Just make the airplane real blunt in front and make a large inlet.
You will get good overall pressure recovery.  The second phase has friction
(which causes boundary layers and flow separation and turbulence, etc.) which
reduces the pressure presented to the front of the radiator.  If you build a
pointy airplane (and I hope you do -- they are faster) then you need to take
air in through a smaller inlet (to keep it pointy) and then do more of the
slowing in the diffuser.  So faster airplanes (pointy airplanes) need better
diffusers since more pressure is recovered in the diffuser instead of in 
front of the airplane.  So.. you are likely to get good overall pressure
recovery in a blunt airplane without working hard, but you have to work hard
to get good pressure recovery in a pointy airplane with small inlets.

Yes, I have a very pointed airplane; a characteristic made possible by
the
fact that the engine is in the rear (Velocity, canard pusher).  It's not
feasable to have a coolant inlet in front or on the leading edge in a
zone
of compression.  So we're left with having ram scoops sticking out into
the
free stream, or NACA type submerged inlets, and I think the diffuser
design
becomes a little more critical.  This is what lead to my questions in
trying
to understand the applicability of your data and guidelines to this
configuration.  I think what you provided is still quite directly
applicable; just a different problem in front of the radiator.

I would think that not being able to take advantage of the compression
in
front of the inlet, one would like to be able to vary both the inlet and
exit openings to be able to design for the hot day climb, and not have
to
pay the drag penalty for all the cool day cruising.

Taking all this into account, and placing my finger wetted and into the air,
I arrived at the figure of 75%.  (In case you did not know, without testing,
diffusers are a large part guesswork.)

3) The inlet area is done by guess, (slow the air stream to 50% of the free
stream velocity at the inlet, based largely on the work of Miley and
Lopresti) but does not enter the analysis directly.  Instead I use the
overall diffuser efficiency to compute pressure available at the front of the
radiator.  The inlet area by itself is not enough to compute inlet
performance.  It depends on shape, location, etc.  If you are looking for a
real rough rule of thumb to size inlets based on the radiator area, I
discourage you from going so.  It can be misleading.  If you insist, I would
say make the inlet 20-25% of the radiator frontal area.  Better to compute
the volume of air needed (which I did for each of these analyses) and then
select the inlet size that produces velocity that is 50% of the free stream
at the inlet.  Unfortunately I did not provide enough information for to do
so.  It varies depending on the effectiveness of the radiator (how well the
radiator heats the air and thus dumps heat).  I can go back and come up with
some recommendations.  I can also compute some radiator face velocities if
that would be helpful.

I've sized the inlet based on flow needed for cooling, and assuming 85%
of
airspeed as inlet velocity (a guess, depending on scoop design and
boundary
layer thickness).  A radiator face of 4 - 5 times larger seems gives
about
the right area, and would seem to provide good pressure recovery. 
Here's
where I think we try to integrate the analysis with some data points
from
similar existing airplanes.

Good explanation of air density effects.  Thanks.  I'm planning normally
aspirated, so I think my assumption of not needing added air cooling
capacity for higher altitudes is OK.  My limiting desgn points become
high
power climb and waiting on the taxiway.

Al Gietzen

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