REFLECTOR:NACA inlet shape

[Al Gietzen]

Al,

That was a great explanation as to what inlet, be it a ram scoop or NACA

, fits what task.

Al Gietzen wrote:
> The original Velocity arm pit scoops do not qualify as well designed
ram 
> scoops.

While I agree the lip of the arm pit scoop is not ideal, it does appear 
to work well. Isn't the proof in the pudding?

Scott

I don't know the details, but I think generally they did work OK.  I
know there were cases where they did not.  It may depend on what the
builders did going in from the scoop.  I think the main issue beyond the
sharp edged entrance was that the cross--section actually decreased a
little going into the scoop, and then (again, depending on individual
builders) either had a rapid expansion, or further contraction, and/or a
90 degree turn.  Easy to set up conditions for spilling at the entrance.
Generally you'd like a slightly rounded or airfoil edge on the entrance,
and once inside the air should be seeing an expanding x-section, slowing
down and converting dynamic pressure to static pressure. 

I'm no expert but I've just observed and learned from whatever info I
could get a hold of.  My understanding is that the main reason for going
to the NACA scoops on top is that they work better.  To some it also
seems counter--intuitive that the submerged scoops on top would work,
but studying pressure distributions over similar shaped fuselages show
that the lowest pressure on top is just back of the windshield and
increases going back, with good recovery by the time you get to the cowl
line.  I suspect that the increasing pressure is also causing some
boundary layer flow to go toward and down the sides toward the lower
pressure above the strakes.  Could be helping in lessening of the
boundary layer thickness at the scoops.

Sometimes it's helpful to think from the frame of reference of a
molecule of air with an airplane speeding by, rather than the usual
reference of the speeding airplane.

Al