The only purpose of the example was to show that when
you switch from a thin to thick radiator, or large
frontal area to a small frontal area, or both at the
same time, that the mass flow can change significantly
even though the inlet opening did not change. There
was some debate over that.

--- Tracy Crook <tracy@rotaryaviation.com> wrote:

> The example is just fine but it has little to do
> with an aircraft
> engine cooling system.  I was waiting to pounce on
> you for suggesting
> that the lowest drag would result from a rad with
> the lowest
> restriction but you didn't go there.
>
> Consider this example:  The drag brakes installed on
> some aircraft are
> essentially small flat plates extended into the
> airstream.  Those
> plates have large HOLES in them.  They make more
> drag that way.  Is
> that intuitive?
>
> What I'm getting at is that ANY practical radiator
> is going to chew up
> virtually all of the energy in the airstream that
> passes through it.
> It is only a pipe dream that a free(er) flowing
> radiator is going to
> give less drag than one with a higher pressure drop.
>  For other
> reasons, the opposite is usually true.
>
> Tracy (brain dead at 02:21 )
>
>
> On Nov 13, 2007 9:17 PM, Ron Springer
> <ron2369@sbcglobal.net> wrote:
> > How's this example?
> >
> > Let's say you have an inlet opening of a given
> area
> > and downstream of that is a constant area duct at
> the
> > same area. At the exit of the duct is a door flap
> that
> > can be set to anything from fully open to fully
> > closed.
> >
> > When the door is fully open you will get the max
> flow
> > through the duct. It might be 99%+ of the
> freestream
> > flow at that same cross-sectional area.
> >
> > When the door is fully closed you get zero flow
> > through the duct. All flow streamlines steer right
> > around the inlet opening. This is a pitot tube
> > basically.
> >
> > When the door is set to an intermediate position,
> you
> > can get any flow you want through the duct
> (between
> > zero and max) and the inlet area is the same.
> >
> > Changing door positions is like swapping in and
> out
> > different radiators and ductwork that have
> different
> > flow resistances. When going to a higher
> resistance
> > radiator (for instance, one with smaller frontal
> area,
> > a thicker core, and the same fin spacing) it is
> like
> > closing the door a bit. The air flow will
> decrease.
> >
> > So, my point is that you can get any flow from
> zero to
> > max flow through an inlet by changing the
> downstream
> > flow losses.
> >
> > Ron
> >
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