Tracy Crook wrote:

I think we are talking about two different kinds of efficiency here.  I
agree that for a given fin area, the thin radiator is more efficient in
terms of dropping the water temperature a given number of degrees.  It is an
efficient radiator.  I was addressing the aerodynamic efficiency of the
cooling system as installed in the airplane, i.e.,  having less overall
cooling drag.    It WILL take more fin area in the thick radiator to do the
same job (air at the back side is hotter and therefor less temp differential
between air & water).

Jeff has a valid point in saying that the air could be slowed down in the
thin radiator to accomplish the same thing but there are a number of factors
which work against this approach, especially in aircraft applications.  Not
only does the size get out of hand but the same factor (low air velocity
leading to laminar flow, etc.) starts to work against the thin approach.
Plus, if you slow the air down to the point where the air is heated as much
at its rear as in the thick radiator (less temp differential between air &
water), what is the thin radiator's advantage?

To avoid a long winded discussion of all the steps in between, let's look at
the inputs and outputs of the cooling system.   First we have to agree that
less CFM (of air) through the cooling system will equal less drag, all else
being equal.  I will assume we agree here.  If not, stop reading here and we
will discuss this area first.   Remember I am talking about total aircraft
cooling drag, not air resistance of the heat exchanger itself.  One does not
equal the other.  It may not be intuitively obvious, but If you have
understood the discussion on air diffusion & ducts so far, you will already
know that the airplane would have less drag if you plugged up the airflow
through the radiator.

   For a given number of BTU  heat rejection, the system which uses the
least number of CFM to do the job will be the one which heats each one of
those cubic feet of air the most.  i.e., You can reject a certain amount of
heat by heating 100 CFM of air to 50 degrees above ambient, or, 200 CFM of
air if you only heat it to 25 degrees above ambient.

 It is my contention that in the context of an airplane, the job of heating
the air the most is best accomplished by a thick and very dense (lots of air
turbulators as Jeff described) radiator.  It is absolutely true that you
will need more core (fin) area with this approach  but minimizing core area
was not the goal, cooling the engine with minimum cooling drag is.    Again,
such a radiator requires a relatively high differential air pressure and
this is why the past discussion on pressure recovery in the cooling inlet
ducts is important.

I now await the storm of contrary opinion  :-)

Tracy Crook
 rws@altavista.net

I hear you Tracy but what we are trying to do here is put some numbers
on these things. We need to do some testing. Either in the airplane or
on the ground. Both duct testing and radiator testing. I know you
are buzy but we could get some data by instrumenting your radiator
set up with a manometer. I suggest putting it in the aft cockpit
and have a person take pictures and keep a log of the pictures.
Or trigger the camera remotely. You could use your static system
as a reference pressure.
 
Paul Lamar
 
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