Bill Freeman wrote:

Make sure that the scoop is thin with knife edges since
you want to avoid any external diffusion with this internal
diffuser.  External diffusion would be caused by wide
rounded lips on the scoop and would increase the inlet
pressure, but at the expense of mass flow.  With a
narrow inlet like that you need full freestream velocity
into the inlet to get the massflow you need, and then
slow it down and increase the pressure in your nicely
shaped internal diffuser.

Bill

Mike Wills wrote:

Bill,

  I think I understand your point above about the impact of external
diffusion on mass flow. I think your comments assume that we are certain
that a proper internal diffuser design has been achieved. Im not sure how
you can guarantee that without lots of testing.

  When discussing inlets with Fred Moreno last year he recommended large
radius lips on the inlet. I think his thinking was that it was highly
unlikely that an eyeball engineered internal diffuser would be successful.
Particularly so in the case of a tractor type airframe with a large rad
located under the engine. He felt that an external diffusion scheme was
much more likely to be successful on a first attempt.

  He recommended the ramped inlet that has been discussed here recently to
insure even flow across the rad core. He further recommended a fairly large
inlet (for my RV4 he recommended 60 - 70 sq in) with large radius lips.
Recommended outlet size was about 80% of the inlet but with a large cowl
flap to allow lots of airflow during climb. Your opinion on this scheme
would be appreciated.

Mike Wills

 
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