I have to agree with you argument below. It has been
argued that since so little thrust can be recovered
downstream of the radiator, that it should be assumed
that nothing can be recovered. So, in that case, the
lowest mass flow is the lowest drag. Well, that would
be true in that case, but the inital assumption is a
bit extreme. The only way to achieve it is to put a
black hole just downstream of the radiator!

All the mass flow that enters the inlet will exit the
outlet. The effect of the exit flow is significant and
I don't think it can be ignored.

[The next part of this email is optional reading
material!]

Conservation of mass says that the mass flow rate
entering the inlet will equal the mass flow rate
exiting the outlet.

Mass flow rate = density x area x velocity

Consider an example with the inlet and exit areas the
same.

Then, density times velocity is the same at the inlet
and the outlet. The density at the outlet will be
lower than at the inlet because the air has been
heated. So, the velocity must be higher at the outlet
than at the inlet. This is true for this case
regardless of the drag created by the radiator, or the
efficiency of the system, as long as inlet amd outlet
areas match.

(Actually, if drag is increased too much, the inlet
will spill air and the mass flow rate will be lowered
to below that of a full flowing inlet. A smaller
streamtube of air would be captured, there would be
external diffusion prior to the lip of the inlet, and
the effective inlet area (or capture area) would need
to be used instead, which is less than the physical
area of the inlet.)

More sophisticated examples can be done for non-equal
areas, but then it becomes more involved. The point is
that the air exits with significant velocity as long
as there is significant mass flow and the effects
can't be ignored.

Ron

--- David Leonard <wdleonard@gmail.com> wrote:

> I do not believe that useful pressure recovery is a
> pipe-dream.  Some report
> that the P-51 even gained thrust from the cooling
> exit (I don't think they
> mean net thrust from the system thought).  Also,
> both Bill Eslick and I have
> the same experience that our cowl flaps want to pull
> themselves wide open by
> the pressure available at the cooling exit (well
> into the slip stream).
>
> Oops, sorry, couldn't help myself.
>
> --
> David Leonard
>
> Turbo Rotary RV-6 N4VY
> http://N4VY.RotaryRoster.net
> http://RotaryRoster.net
>