.......... If the APU(Aux. power unit)caught on fire during a start, the
NACA duct

immediately closes. This door was the only acceptable means to do

this.

It was

not a function of getting more air to the APU. ...........>
Doug

I guess this could be done just as well with a regular scoop (like a

belly

cowl flap just facing forward, .....sorry but my Rhino-prowess still
sucks!), and the ram effect would probably be a lot better. Or a

straight in

scoop on the vertical stab leading edge with a internal shutter or ball
valve. I guess there are plenty more solutions.
Any Boing engineer on the list? Like to comment?

So what's the verdict?
Is there any valid use for the NACA duct, where there is NO better (less
drag....) alternative?

Thomas J.

If there ever was a good place for a NACA scoop on the front of the
cowling
is it. As Tom Parkes found out it did not work well here either. Once
you raise the lip it is just another bad scoop taking in the boundary
layer.
Ed Anderson had a similar experiences five years ago.

Paul Lamar

The only place I can think of is where the boundary layer is thin (as on the
front of the aircraft) and the thing the inlet is connected to sucks air
into the inlet. This could be a Jet engine or engine air intake. I don't
think you would get as high a pressure recovery with the NACA inlet as a
standard inlet. One guy used NACA ducts on an E-racer effectively by placing
vortex generators in front of the scoops. The Vortex  from the VGs energized
the boundary layer and caused the air to flow into the scoop. He was using
an air cooled aircraft engine and there is a huge bluff body low pressure
area behind an E-Racer so don't go getting any ideas.

Monty

Speaking of bluff bodies there was a guy named Kamm that came up with the
idea of cutting off streamlined cars to reduce their length. Normally the aft
sloping surfaces should be less than 7 degrees to maintained an attached
flow. This made a perfectly streamlined car very long with a lot of skin
friction surface area. Kamm cut the body off just behind the rear wheels 
more or less to reduce the skin friction surface area. The current Corvette 
is an an excellent example. 

The resulting negative pressure behind the car sucked down the boundary layer 
on the body just forward of the cut off point. It also caused some vortices 
around the edge which caused the air to roll around and to impact the 
flat aft surface. This further reduced the drag as it resulted in a 
small forward force. Nature abhors a vacuum so the inflow from the 
surrounding fuselage boundary layer air reduces somewhat the low pressure 
area behind a such a body. Bottom line; you may not get the degree of 
suction you are hoping for.

Paul Lamar
 
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