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Just some more napalm for the bonfire:
Ever stop to think about the real cause of
cooling drag? It seems that there is this bedrock truth out there with
all these superfluous arguments swirling about all around it in a cloud of
incomprehensible gibberish.
Whenever encountering such conundrums it is best to
start over from scratch and think things through.
The bottom line is making pressure costs you
drag. I think there was a guy named Bernoulli who had something to say
about this. He has a fairly famous equation that is good to know. I says
basically :No matter what type of catchers mitt you hang out into the stream of
baseballs in order to catch a few, you are going to feel it.
Don't let the con artists tell you that external
diffusion is the holy grail. Don't believe me? Try this experiment: Drive down
the road at 100 mph and place your palm flat to the onslaught. Do you feel
anything? Yes!- external diffusion.
Now place your hand at a knife edge into the
wind. Is there a difference?
Coolers, be they cooling fins around a
cylinder or cooling fins in a radiator require some amount of pressure to get
the baseballs through the sieve. The amount of pressure for a given flow of
baseballs is the key. If you inhale a lot of baseballs at a low pressure (Mack
truck radiator) or a few baseballs at a high pressure (thick rad) you
have....drumroll please......DRAG!!! TA--DA!!
Now some would argue that you close the outlet and
let the baseballs stack up until you are pushing this pile of baseballs, so that
baseballs are bouncing off the pile and going around the outside, totally
screwing up the baseball flow. Remember the hand!
Does this sound like the route to low drag
Nirvana?
This all sounds very simple until some smarty pants
thermo guy comes along and starts yakking about the P-51 and all the thrust they
made with their special catchers mitt and gets everybody all hot and bothered.
The keyboards begin to click away as arm chair aerodynamicists the world over
imagine infinite speed from a lawnmower engine in their very own laminar light
plane. flying at 250 mph on a teaspoon-full of gasoline per hour. Talk of exotic
devices like augmentation tubes and flush inlets are flung about with
reckless abandon. (I have my own theories about why male pilots are
ever fascinated by these devices ;-).
So how does this work?? The theory is: If I can
throw the baseballs away faster than I catch them, I can make thrust! Even
better, If I can keep all the energy I get when I catch it and put it back in I
won't have to throw so hard. Imagine I catch it and let my arm fly around
in a circle and let it go in the opposite direction. EUREKA! I can just
snatch baseballs out of the air and throw them away and propel my self about the
room!
So lets say I have this really slick design and I
want to use this principle to go really fast. Lets say I have an airplane which
will do 200 mph on 100hp. There are a few that can so this is within the realm
of possibility.
Now the P51 went closer to 400 mph so lets say this
is how fast we want to go to keep the math simple.
The energy in the baseballs we are catching is
proportional to the square of the speed V^2. This means the baseballs the P-51
is catching have 4 times the energy of the baseballs I am catching at 200.
Which is a lot. But it gets worse. To go 400 mph I
would need more power. How much? Power increases with the CUBE! So I will need
2^3 or 2*2*2=8 or 800 hp! This means I have 4 times the energy and 8 times the
heat rejection to work with by simply doubling the speed. So something that is
significant at 400 mph is decimal dust below 200mph. We just don't have
energetic enough baseballs or a big enough arm to help ourselves out
here.
So how does this help us. We have now eliminated
50% of the cloud of incomprehensible gibberish. You are not going to make a jet
engine from your piston single no matter how hard you try.
There is still some confusion about the catchers
mitt however that results in much argument.
Some want the catchers mitt hung right out in the
flow of baseballs, some want it cleverly hidden so the baseballs just roll into
it. One group says that only real men fly with their catchers mitt in the air
the others are just out to lunch.
Remember the hand!
Making pressure cost you no matter how you do it.
The ideal inlet, whether flush or conventional, will be sized to catch just the
right amount of air with a diffuser to build the pressure AFTER the inlet. There
is NO external diffusion. The problem is the ideal inlet is only ideal
for 1 flight condition. Change the temperature, pressure, altitude, speed, or
engine operating point and it is now less than ideal.
Most airplanes have a flight envelope, not a flight
point.
It turns out that you need a bigger inlet for
climb, because you are going slow and making a lot of power. Then you get to
altitude and speed up and make less power. The ideal inlet changes. The typical
way to deal with this is to size for climb with an open cowl flap. Then close
the flap and tolerate some mild external diffusion in cruise. This works
regardless of the inlet type.
Either inlet will make more pressure at lower inlet
velocity ratios. Vi/Vo is how much slower the air going in the inlet is vs. how
fast the airplane is flying. So if I am flying at 100 mph and my velocity ratio
is .8 then the air through my inlet is doing 80mph. Now not all airplanes
have a convenient place to put a pitot inlet and this necessitates a scoop type
inlet. the scoop has a lot of drag because it messes up the airflow all around
and behind it plus it adds a lot of wetted area in order to properly fair it. If
you decrease the inlet velocity ratio so that you start getting a lot of
external diffusion, and you do this on a laminar wing or cowl, you really make a
mess.
The following graphs may be found in NACA RM
A7I30
The problem with the flush inlet is that it makes
up 85% of the free stream pressure at around .2-.6 inlet velocity ratios.
FIG 18
Now this is pressure before the diffuser. When they
placed a long diffuser on the inlet they got more losses at the higher inlet
velocity ratios. This is easily attributable to mixing losses. There is some low
hanging fruit here if you use one of the shorter diffuser types.
Now look at the drag
FIG 25
At an inlet velocity ratio of .8 the drag is ZERO!!
And I can make at least 75% of the free stream pressure with the proper inlet
size. Now at reasonable altitudes and cruise speeds that means 75% of around 8
in of water. So I can make around 5 in water at 200 mph with no inlet drag. Now
I am still catching baseballs and they are still scrubbing along the ductwork
and it is still making drag, I am just not disturbing the external flow or
making a mess around the inlet. You can do this with a pitot or scoop inlet as
well, and you will get a higher pressure recovery with a well designed diffuser,
but you have to hang your mitt out in the air and disturb all those other
baseballs.
Now to size this for cruise, it will be woefully
small for full power climb. You will have to figure out a way to deal with that
to make this work. If you size it for climb you will be able to make more
pressure at cruise, but your inlet velocity ratio will go down and your drag
will go up, just like it would with a scoop or pitot inlet. If you use an
augmenter to increase your inlet velocity ratio above 1 you may even get some
THRUST! or negative drag from the inlet! Now you still caught the baseball and
it is still scrubbing along the duct work so you probably will not make thrust
overall, but you will definitely make less drag! Now this is not only true of
the flush inlet, but is also true of the pitot or scoop. You will have less
pressure recovery with the flush inlet the higher the inlet velocity ratio IF
YOU USE A LONG DIFFUSER! Note, that is where the loss occurs, in the diffuser.
This is due to mixing. If you use a shorter diffuser and use the turbulence to
your advantage you may get better results.
Now if you can tell me where on a typical single
engine airplane you can place a pitot type inlet with a proper diffuser leading
to a vertical radiator and then have a perfect exit duct without making a mess
on the wing or the front of the airplane, I would agree that it would be the
ideal setup. The only place I can think of to do this is on a twin engine
pusher. Then you might be able to put the Ideal setup to work. Otherwise it is
all a compromise dependent on what you want the airplane to do.
So there is no free lunch. But you can make a NACA
flush inlet work and you can even do it without VGs, protruding lips or other
Band-Aids.
Your mileage may vary.
Monty
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