Hi Finn,
I was hoping
someone with better memory/understanding would jump in, but
I'll take a swing at 'general principles'.
If memory
serves, the guys who have applied the paper to hardware
recently have said something like this:
The sharper
edged lips are the most efficient shape where the flow is
'perfect' into the diffuser, and there's no spillage over the
outside of the lip. But in our typical situation we need lots
of flow at low airspeed (climb profile; high power) and less
flow (relative to freestream) at high airspeed (cruise). If
airspeed is high enough that some of the air in front of the
inlet can't get in and must divert around the lip, the sharp
edge of the lip will cause turbulence and drag on the outside
of the airframe (at high airspeed, where it hurts the most).
So, we can't truly optimize the lip. If a cowl flap will be
used to increase cooling flow at low airspeed and reduce it at
high airspeed, that means there will be significant spillage
around the lip at high airspeed. So while the fatter lip is
less efficient in the ideal flow situation, it ends up being
better in the real world, because we must be able to cool at
low speed and we want minimum drag at high speed.
There's also
the 'internal vs external diffusion' issue. I think Bernie
Kerr was the 1st person that talked to me about that. Internal
(K&W duct, for instance) is theoretically more efficient,
and could use a sharp edged lip, but it's really difficult for
us 'measure with a micrometer; cut with an ax' builders to get
perfect. So the safer thing is to go big on the inlet to
ensure plenty of flow, and then throttle the outlet to match
actual flow to conditions, with a cowl flap. Since we'll have
a lot of spillage around the inlet with the flap closed, that
means a fat lip on the inlet to minimize drag when there's
lots of spillage.
Or I could
have an advanced case of oldtimer's, and I'm remembering it
wrong.
I hope
someone will correct me if that's the case.
Charlie