X-Mozilla-Keys:                                                                                 
Fred wrote:

Remember that the pressure available is proportional to the square of the
speed.  So if you slow the flow to 50% of the free stream velocity in front
of the inlet (which is what Lopresti seems to do) you recover 75% of the
available pressure energy before you enter the inlet.  Not a bad deal since
this is what is called isentropic compression which is frictionless and thus
lossless.  Same thing that happens in front of your pitot tube.

The job of the inlet diffuser is then to recover as much of the remaining 25%
as possible, and distribute it where it is needed.  The drag penalty which
arises is due to the difference in the inlet momentum (air flow times free
stream velocity) minus the exit momentum (air flow times the exit velocity)
as the air exits the cowl hopefully through a nice nozzle pointed backwards
so the velocity is maximized and the direction is aft.

Fred Moreno

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