I have had a lot of trouble in the past with this subject.
I have posted the NACA scientist's and engineer's cautions that
invented the device back in 1945 on using it with heat exchangers
many times in the past. Some people ignore that.

Engineers by nature tend to qualify statements just in case they
are wrong. In other words they do not generally make dogmatic
or black and white statements even if they are sure they are
absolutely right about something. Unfortunately this leaves
an opening for the disbelievers.

Consequently that statement has NOT been enough to dissuaded the
proponents. They WANT to believe in NACA scoops because they LOOK
like they have little or no drag. Besides that... they look cool.
This then becomes  emotional belief in NACA scoops.

Therefore I thought of a way to illustrate in an intuitive manner
the problems of a NACA scoop starting with the first principle of
Physics. Namely Newton's First Law.

******************************************************************
Every body continues in its state of rest, or if in uniform motion
in a straight line, unless it is compelled to change that state
by force impressed upon it.
********************************************************************

It is not just a good idea...... IT IS A FUNDAMENTAL LAW OF NATURE!!!!

Now think of air molecules as a zillion tiny marbles moving at 200
MPH parallel to a surface that contains a red NACA scoop.

Paul;

I got this far and stopped; because right here is a factor that leads

to

a lot of confusion in dealing with aerodynamics.  The "tiny marbles"

in

your anology are NOT moving.  They are standing still and have no
momentum. The plane is moving past, and pushing them around, but they
begin at rest.

Al

That is true Al but what counts is the relative motion. I used that
simplified reverse analogy to make the situation more understandable.

The fuselage is at first separating the air causing a disturbance
which is forcing the once stationary air molecule away from the center
line.
If the NACA scoop is located on a slopping front surface it should
work fine
as long as the pressure in the NACA scoop is not higher than the
dynamic pressure.
A blockage in the duct behind the scoop such as a radiator could
easily
build up enough pressure in the mouth of the scoop to persuade the air
molecules to move out of the way. They don't have very far to go. A
little molecule motion is enough to avoid the NACA scoop unlike the
swift motion required for an air molecule to avoid being ingesting
into a conventional scoop opening.

If the conventional scoop duct is restricted enough, say with a cowl
flap,
so that pressure builds in front of the mouth then the air molecules
move aside
and do indeed bypass the conventional scoop opening. In that case it
is
called external diffusion.

By the time the pusher NACA scoop gets to a given stationary air
molecule
it is moving parallel to that molecule.

"Every body continues in its state of rest"....... "unless it is
compelled
to change that state by force impressed upon it."

Where is the force coming from to make the air molecule translate
vertically upward into the NACA scoop? Obviously at that point there
is no force on the molecule.

Paul Lamar

Using the relative motion is fine; the key is to know the precise
'direction' of the relative flow; which is sometimes more easily
visualized by changing the frame of reference to the air.  The pressure
distribution along the surface where the NACA is placed is a factor in
the effectiveness of the scoop.  If the pressure is increasing, there is
a component of velocity toward the entrance.

More importantly, the shape of the NACA is critical and causes a low
pressure area in the throat which provides the force to move the air
molecule into the scoop whence it is 'captured' by the opening.  It
doesn't take much pressure differential to make air molecule move.  The
shape of the outer 'lip' at the entrance is also important to maximize
the effectiveness of the scoop.  And then depending on the thickness of
the boundary layer buildup you can add diverter ledges. . . blah, blah,
blah...

So; a properly designed NACA will produce positive pressure; and if
everything is done right, can achieve (as I recall) about 75% of the ram
pressure of a ram scoop.  Good for lots of purposes; but, as you say,
for max pressure recovery, you've got to go ram.

I recently had to reconsider the use of a NACA scoop for inlet air to
cool the tangential muffler because I have the large ram scoop feeding
my in-cowl radiator.  The exit area from the cowl is about 60% greater
than the ram scoop inlet.  Even after the pressure drop thru the rad
core, will the pressure in the cowling render the NACA ineffective?
Probably.

Al

 
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