X-Mozilla-Keys:                                                                                 
Bill Freeman wrote:

Fred wrote:

By the way, on the diffuser chart you sent out, it is interesting to note
that the "good" diffuser was able to get about 45% of the total available ram
pressure, while the theoretical maximum for the area ratio is 84% meaning
that the good diffuser is only about 45/84= 54% effective.  This shows the
need to do some diffusion in front of the inlet.  If one slows the flow to
50% of the free stream velocity in front of the inlet, you get about 75% of
the ram pressure because deceleration in front of the airplane is
frictionless (but requires some bluntness in the airframe and inlet to
distort the streamlines to get the deceleration).  The diffuser then collects
54% of the balance (about 13%) giving a total overall diffusion effectiveness
of 75+13 = 88% which is a very good value.  I typically use 75% overall for
first estimates.

Fred

I agree whole heartedly, and really believe that very few  tractor
installations are primarily internal diffusion.  From what I have read,
at speeds well below the mach, the difference in overall drag from a
fairly blunt object and a very pointed object with a similar overall
shape (i.e. leading edge radius is the primary difference) is very small.
I have suggested in the past and reiterate that external diffusion is
in most cases "free", i.e. you already have paid the drag penalty and
cannot avoid paying it, so take your air from the low velocity flow
available at the front of the fuselage.

I ran into some captured WWII German studies (by NACA)  on external
expansion inlet designs years ago, and was quite enamored, once I really
grasped the advantages of external expansion.  In some ways it is analogous to
the linear aerospike rocket motor, i.e. arrange your system to
let the fluid do what it really wants to do (expand as it needs to, satisfying
continuity) and tap into that natural flow pattern to get what you want to
accomplish.

Bill

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