Of course all this is neither here nor there as we are dealing with
water cooled engines and the vast majority of canards flying today
are air cooled. As I said over and over and over and over on here
many times before what works for an air cooled engine might not work
for a liquid cooled engine. It totally escapes me why this subject
keeps coming up. Perhaps the desire for a magic cooling solution is so
strong it shuts down people's ability to reason.
SNIP<
Paul Lamar
Tracy Crook wrote:
Have to disagree on the air-cooled vs water-cooled thing.
On aircraft, both aircooled & watercooled systems need *exactly* the same
thing. I suspect you do agree on what that thing is.
Don't have time to cover the whole subject here but if there is one key to
cooling system success, it is pressure recovery in front of the heat
exchangers. Doesn't mater if the heat exchangers are cylinder & head fins
or radiator fins. Several RV builders have backup airspeed indicators fed
from a pressure tap located in the pressure plenum on top of the cylinders
at the back of the engine (Lyc). They read within a couple of MPH or less
of the airspeed read from the pitot tube. This means good pressure
recovery. The Lycs that don't have this pressure also don't cool well.
Again, pressure recovery is the 400 lb gorilla of efficient aircraft
cooling. Doesn't matter how you get it as long as you do. This being the
case, I think it makes sense to utilize the one place on every aircraft
where high pressure is naturally found. It's as plain as the nose on your
face : < ) This won't help you pusher guys so you have to seek the next
best points.
Tracy Crook
You too are missing the point. I don't know how to explain it any other
way. SOME SCOOPS WORK WITH HIGH RESTRICTIONS IN THE DUCTS AND SOME
DON'T. MASS AIR FLOW THROUGH THE HEAT EXCHANGER AND THE RATE OF HEAT
TRANSFER TO THAT AIR IS THE NAME OF THE GAME. NOT THE PRESSURE GENERATED
IN THE DUCT PER SE. THE MASS FLOW RATE VARIES WITH THE HEAT EXCHANGER
RESTRICTION AND NOT JUST THE PRESSURE IN THE DUCT.
Statements in a recent Aug. Sport Aviation article "Cool It"
by Neal Willford not withstanding which you appear to have just read.
He is comparing apples to apples as most certified AC engines
have the same LOW restriction to air flow. Yes the engine manufacturers
specify a pressure drop across the cylinders. This is to insure
sufficient mass airflow.
Have you every heard of a fluidic(s?) amplifier? If not look it up
on the internet and study it. Small pressure differences in
a trigger pipe can force the flow to switch to one of two other pipes.
http://americanhistory.si.edu/scienceservice/057001.htm
http://fluid.power.net/techbriefs/hanghzau/4_33.pdf
http://www.interdip.polito.it/meccanica/English/staff/pers_hmpg_en/viktorov_en.htm
The same thing happens in a NACA duct. If the pressure gets too
high in the NACA duct the boundary layer in front of the NACA duct (due to external
diffusion) is thickened and air is triggered to flow around the duct
opening rather than in to the duct opening. This is fundamentally different
than a perpendicular opening in a P51 scoop. The pressure at the P51 opening
will always be more or less a fixed ratio with dynamic pressure.
This is what the NACA engineers that invented the NACA
duct were referring too when they said "DON'T USE NACA DUCTS
WITH HEAT EXCHANGERS".
Sheessss this is getting as bad as the 2.6L rotary equals a 3.9L six
cylinder.
I am getting ready to cut this subject off as a waste of time.
Paul Lamar
They also said that NACA inlets *would* work for low restriction
applications like jet intakes. Seen any?
Tracy
They don't work but not for the reasons you think they don't work.
A NACA duct by nature is mounted on a surface more or less parallel with
the air flow. If you think about it for awhile what happens when the aircraft
yaws or pitches? The flow could then be separated which reinforces my contention that
the NACA duct works like a fluidic switch. The NACA engineers that invented
the duct may not have been pilots so they may not have been aware
that aircraft yaw and pitch under steady state conditions. You don't
want your compressor to stall every time you kick the rudder.
BTW Further reason not to use it on the roof of a Velocity cabin structure.
When the aircraft is flying at a high angles of attack at low speeds during climb
the NACA duct is then likely to suddenly stop working just when you need
it the most.
Paul Lamar
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