On 17 Jul 2003 09:34:54 -0700, (Jay) wrote:

On the Meredith Effect. Intuitively I don't see how its supposed to
work. You take air in the front of the scoop, you heat it up, and it
expands (ideal gas law). How does the air know thats only supposed to
go out the rear?

Jay, let's review the path the air takes through a radiator type
aviation cooling system and what happens to it.

Before I begin, you should understand that the air from the cooling
system MUST BE EXHAUSTED INTO A LOW PRESSURE AREA. If you do not
understand this first point, you will have huge problems with any
system you attempt to develop.

So let's presume we have situated the exhaust opening such that it
empties into the afore mentioned low pressure zone somewhere on the
fuselage, and we're moving through the air at cruising speed.

The air is directed into the intake opening because we designed it
such that it scoops in air and is pointed at the airflow. It flows
into the intake duct and the duct expands at anywhere from 7 to 15
degrees or so until it gets to the radiator, which is necessarily
larger than the intake opening. While the duct is expanding to the
size of the radiator, the air has lost some velocity but it's gained
pressure. We have selected a fin density compatible with the speed we
have slowed the air down to, so as to allow the air time to absorb
heat from the fins.

It flows through the radiator and removes heat from the fins. The air
is now heated much more than it was when it entered the duct. We've
sealed the space surrounding the radiator because air is lazy, it
doesn't want to go where you want it to go, it will seek easier
passages if you let it. If it can bypass the radiator, it will, thus
robbing you of it's ability to remove heat from the radiator.

Now the air is behind the radiator and it's greatly heated and
expanded. Why doesn't it want to blow forward? Because it's lazy,
remember? It wants to take the easiest path and that path is toward
the low pressure area we first stipulated was necessary (high pressure
at the inlet, low pressure at the outlet). But the duct is now
narrowing. This compresses the air and accelerates it. It cannot do
anything but head to the rear at ever increasing velocity, blowing out
the exit outlet parallel to the slipstream.

It's the exit parallel to the slipstream in it's accelerated state
that recovers the drag imposed while it entered the inlet and pushed
through the radiator face.

In effect, it's a low grade pulsejet engine with the radiator and the
rearward velocity of the air through the system acting as the vanes
that normally slam shut against the combustion of a real pulsejet,
forcing the products of the combustion out the tailpipe.

Does this clear things up?

Corky Scott

PS, the system described above is optimal. The reality is that the
confines of the engine compartment, protrusions and the usual
narrowness of the fuselage contribute to making almost all cooling
systems less than optimal. That's why I emphasized that in the real
world, the most important thing is that the system cool properly
first, and worry about the drag it produces later.