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Message-ID: <1b4b137c0711121418i36509410pc82e9c0d2c803562@mail.gmail.com>
Date: Mon, 12 Nov 2007 17:18:02 -0500
From: "Tracy Crook" <tracy@rotaryaviation.com>
Sender: rwstracy@gmail.com
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Diffuser Configuration Comparison
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References: <list-2467994@logan.com>

I won't accuse you of being a contrarian Al, besides, we need a few of
those anyway :>)

True, Over simplification is a barier to success. But so is over complicati=
on.

 In general, we really don't have a lot of choices in radiator core
styles (unless you are willing to pay big bucks).   The P51 did have a
different style rad but OTOH,  it was only one layer in a stack of
oil,water & intercooler heat exchangers that totalled about 11" thick.
  I don't have the individual dimensions on each part at hand.

"I can say that generally increased pressure drop equates to increased drag=
."

Now there I can generally disagree!  And this is a point where a lot
of radiator discussions go off into the weeds.   Assuming that the
radiator is enclosed in an aerodynamically streamlined enclosure  (not
a  Pietenpol) The lowest (aircraft) drag will occure with the highest
pressure drop across the radiator.  Best case is at the point where
airflow is completely blocked through the rad.

Unless you are prepared to fully understand the physics and math of
any subject, rules of thumb (simplifications) are absolutely
necessary.  I will repeat my favorite R.O.T. cooling mantra:  Every
CFM passing through the cooling system represents drag.  Unless I have
missed an important point somewhere, more CFM will always result in
more drag.

Tracy


On Nov 12, 2007 1:37 PM, Al Gietzen <ALVentures@cox.net> wrote:
>
>
>
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: Diffuser Configuration Comparison
>
>
>
>
> I especially liked the comment in the report where it says that the
>
> higher the pressure drop across the core, the higher the diffuser
>
> efficiency.   I interpret this as 'thicker core is better than
>
> thinner!'   Of course there is a point of diminishing returns where
>
> flow is simply too low as Ed has pointed out but in an application
>
> where diffuser efficiency is such an important factor, this pushes
>
> that point in the direction of 'thick'.
>
>
>
> Tracy (back from the dragon and sorry to bring up the thick vs thin
>
> thing again :>)
>
>
>
> Again I feel compelled to caution against over-simplification and
> generalities (I'm not just trying to be a contrarian; really!).  In a myr=
iad
> of factors, drawing that conclusion based on one configuration and one
> parameter can certainly be misleading. What is your criteria?  I can say
> that generally increased pressure drop equates to increased drag. Also
> "thickness" and "pressure drop" are not equatable (is that a word?).  The
> very thick P-51 rad with large hexagonal passages is in no way comparable=
 to
> the same thickness of modern radiators with =BD" between tubes and 16 fin=
s/in.
>
>
>
> The most detailed, sophisticated analysis of radiators for our applicatio=
n
> that I have seem was done by Fred Moreno back in '99; but unfortunately i=
t
> also was done for a specific case of 220 KTAS and 10 fins/in. What it
> suggests is there is an optimum thickness (minimum drag), and that is
> roughly 3".  That's for 10 fins/in.; I concluded that my 16 fins/in rad
> should be thinner, and went with 2.5" thckness.  It works exceptionally
> well.  The 2.5" to 3" thickness seems common for rads made by Ron Davis a=
nd
> Griffin for racing applications.  That seems consistent.
>
>
>
> But likely, obtaining minimum drag is beyond the sophistication that most=
 of
> us can achieve in our applications. If we can get it to cool adequately
> we've done a good job; and that is probably mostly a function of scoops,
> diffuser and exit configurations.
>
>
>
> FWIW,
>
>
>
> Al G