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. Just behind
this NACA scoop and above it there is green P51 type scoop with an
opening 
that is perpendicular to those straight lines those tiny marbles 
are traveling in.

1: Which duct is scoop to gather more marbles?

2: What force or forces causes the marbles to go into the NACA scoop
if any?

3: What force or forces causes the marbles not to go into the NACA
scoop?

4: What force or forces causes the marbles to go into the P51 scoop?

NACA scoops  require suck like a jet engine or a carb air intake to
work.
It is the force that deflects the marbles into the NACA scoop.
Ironically they don't even work for jet engines very well. You never
see them 
on jets. The fastest RV's in the world don't use them for carb air 
intake either. Boeing had to use this kludged up fix to make them 
work for 737 APU air intakes as much as someone at Boeing wanted them
to work.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2003 All world wide rights reserved.