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
Hey Paul, just a short comment.You are absolutely correct. LIST, Please
listen
to Paul on this. Commercial aviation needed fixes like this to fix the
expensive goofs. The NACA vent on the 737 was used for one exclusive
purpose
only. 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. The reason for the paddle
in
the front of the NACA inlet was only used to help at high altitude. The
vane
increased the air pressure in a low pressure environment. It served no
purpose
on the ground. On the 737-100 and 200 models they did not have the vane.
It's
starting envelope max altitude was 31K. On the 737-300/400/500/600/700/800
models the max altitude was 37K because the vane was added. The NACA vent
is a
poor source of positive pressure air. Another example was the DC-8. The
NACA
vents were added to the front of the nose to use ram air force. In the
initial
design of the Douglas DC8 they had serious problems cooling engine bleed
air
for the PACK(Passenger Air Conditioning Kit) They could not cool the heat
exchanger in the belly of the aircraft to cool the cabin efficiently.
Anybody
remember the Diesel 8 days?
727,737,757,767 Captain
Doug Mueller
RX-6 13BT
N900DM
Henderson,NV
Joe Hull wrote:
Maybe a dumb question but...here goes!
Isn't the air moving capability in a cowling just as dependent on the exits
as it is on the entrance? On pushers - where the NACA is used - aren't there
usually some flat-plate openings facing aft that in a sense suck air out of
the cowling? As 200MPH air races over the cowling and past the opening at
the aft end it creates significant low pressure that sucks air out. Add to
that the prop in the back pulling air toward it - and there is low pressure
all over the openings at the rear of the cowling.
Paul's marble analogy doesn't account for ALL of the forces working on those
marbles. It is acknowledged by everyone that some "marbles" will spill into
the NACA by design and create some positive pressure. Now if we factor in
the low pressure forces BEHIND the NACA opening perhaps that low pressure
will suck more of those marbles in! A NACA doesn't provide much positive
pressure but it MIGHT be adequate where there is either low resistance (read
"air cooled engine") or low pressure caused by a good venting strategy.
We don't need or want air streaming through the cowling at 200MPH. So, I
guess the real question is would the NACA with its partial positive pressure
and the aft vents with their low pressure provide adequate flow?
Joe Hull
Cozy MkIV #991 (working on Engine & Cowling)
http://www.maddyhome.com/canardpages/pages/JoeHull/index.html
Yes you are absolutely right. If you can somehow lower the pressure
substantially less on the back side of a rad mounted in a duct
connected
to a NACA scoop it will flow air. Assume for the moment you get the
pressure down to atmospheric at the exit opening. A difficult thing to
do
as there is always an eddy that curls around and builds a bit of
dynamic
pressure on the backside of any object moving through the air. After
all
that is how the so called pressure recovery fuselages and wheel pants
work
sort of. Also assume there is a slight amount of dynamic pressure in
the NACA scoop even though the lip is in the boundary layer down below
the high speed marbles.
Unfortunately I don't think this is going to work near as well as a
P51 scoop
capable of recovering 80% of dynamic pressure on the front side of the
rad
with the same outlet configuration. The next thing you can do is
arrange
the out let to obtain suction energy from the prop as Perry Mick is
doing with
his ducted fan configuration. Perry Mick's 5,000 RPM ducted fan sucks
a lot of air.
It does seem to work for air cooled engines with low pressure drops
across the
cylinder fins. Occasionally even those people have problems judging by
the
number of article you see on the Internet and in the home built
magazines
attempting to improve air cooled aircraft engine cooling. It takes a
lot
more pressure difference to get air to flow through a heat exchanger
(radiator). A large frontal area thin radiator will help.
So far nobody has got a radiator to work using the conventional (non
ducted
fan) prop suction and a NACA scoop. The prop does not move much air in
a
longitudinal direction near its hub. If we could see the moving air in
the
vicinity of the hub and in the wake of the fuselage it would probably
be whirling
around in a circle.
My suggestion if you are absolutely sold on the use of a NACA scoop is
to place
two large rectangular openings as far outboard as you can get and as
close to the
prop blades as you can get. The area of these openings should be near
as large
as the rad area. You could use cowl flaps to reduce the area for lower
drag and better prop efficiency during cruise. it can be similar to
this
system used on the Me109. Make sure these flaps are very robust and
are
not going to come off and go through the prop as needless to say as
they
will be subject to a lot of turbulence from the prop. BTW the Me109
cooling system would have worked better if they had used a P51 type
scoop
with boundary layer air bleed :)
Paul Lamar
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