2) The scoop MUST be

out of the boundary layer for the internal diffuser to work

properly. I

suggest some reasonable

lip radii and a splitter to separate the boundary layer.

Jeff Spitzer

Mike Wills wrote:

  How significant is the boundary layer effect in a tractor config with

the

rad under the engine and a chin scoop directly under the spinner?

Seems to

me that the further back on the airframe the inlet is, the more

significant

would be the effect. Is it really necessary to use a splitter with a chin
scoop? There seems to be some confusion on the part of industry as

well as

Ive seen a number of turbine powered aircraft with chin inlets and no
splitter, as well as some that do incorporate a splitter.

Mike Wills

Bill Freeman wrote:

The chin scoop RIGHT under the spinner will be naturally in an area of
substantial external diffusion due to the fuselage.  Also the boundary layer
will be very thin on the sides of the fuselage near the front.  I would

imagine

that a well rounded inlet just below the prop will work well.  The issue of
boundary layer splitters is really for scoops protruding into the airstream,
where you are likely to want high energy air going into the inlet, so you
want to keep the low energy boundary layer air from "clogging up" the
inlet.  It doesn't literally clog it, but the lower speed air will limit the
massflow into the inlet and prevent any pressure gain in the diffuser, since
the dynamic pressure of slow air is low (by definition).  The boundary layer
on the front 25% of the airframe is probably on the order of 0.1" or less,
my guess.  Anyone with accurate numbers on this, please jump right
in.

As you move the chin scoop down you can rapidly reach an area of
flow which is more like freestream and would require internal diffusion
to efficiently capture the dynamic pressure while slowing the air.
Of course the whole point is to blow the air over the heat exchanger
at lower velocity since drag is proportional to the square of the
velocity.   Halve the velocity at the heat exchanger face and expect
to cut the drag of the flow to be reduced by a factor of four.
But you already knew that, I'm sure.

Bill

Jeff Spitzer wrote:

More significant then the "clogging" is the inability to do internal
diffusion if you ingest the boundary layer.  The reason is the boundary
layer air will easily separate from the wall of the internal diffuser.
That's one area of inlets/diffusers often overlooked.  The "allowable"
diffusion rate for any diffuser will be a function of the boundary layer
thickness/profile/energy that exists at the beginning.  Pitot inlets
generate a fresh new (thin, high energy) boundary layer that is capable of
high diffusion rates.  For those of you who don't already know, the whole
purpose of the special "hourglass" shape of a NACA inlet is to generate a
pair of strong vorticies (just like the vortex lift of a delta wing or F-18
chine) that simultaneously scrub the boundary layer from the ramp surface
and entrain free stream air due the vortex rotation.

Jeff Spitzer

 
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