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
WP Marsh wrote:
Gentlemen,
I'm wondering if the PW J-58 (powering the SR-71) would have been
able to achieve 32k+ lbs of thrust each with NACA inlets. I think
Kelly Johnson and his boys at the Skunk Works had the right idea:
Mach 3+ RAM AIR. As I recall, Ben Rich spent a good deal of time
optimising the movable inlet cone to gain maximum inlet ram air
advantage.
IMHO, a NACA inlet is a passive, scavanging inlet, suitable for
cockpit cooling. If you want HP, go RAM.
For a simple illustration, fly straight and level at full smash,
then pull your carb heat. Notice an RPM drop? You've just gone
from ram air induction to passive (NACA-style) induction.
As for the images of the NACA induction for the Boeing APU, wasn't
there some kind of diverter in that setup that attempted to redirect
ain into the NACA scoop. That looked like a pretty blatant
admission that Boeing was willing to sacrifice a bit of drag in
order to emulate the performance of the more effective ( but less
sexy) conventional scoop.
wp marsh
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