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Date: Wed, 19 Oct 2005 14:02:52 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: NACA scoops
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Ed Anderson wrote:

> Hi Thomas, afraid not exactly an old wives tale.   Here is an extract 
> from an  old NACA report that appears to take the position that 
> submerged (NACA?) ducts are not the best suited for radiators or 
> anything requiring much diffusion or pressure recovery. 


Ed, I took my lunch break to read the 5I20 report.  What FUN!!

I think we're tripping over semantics here.  There is a subtle 
difference between "supplying air to a radiator" and "using a NACA inlet 
for pressure recovery."  The subtlety is the root of the argument.  Just 
because it isn't good a pressure recovery doesn't mean that it will make 
a bad cooling inlet.  It only means that you will have to do some 
pressure recovery in some other way, like a streamlined duct.  "We don't 
THINK it will work for radiators, because there isn't enough pressure 
recovery" IS a long way from "it can't supply air to a radiator and you 
shouldn't waste your time trying ***because the inventors said so***".  
I'm only objecting to the very last clause.  The authors might fully 
agree that a submerged inlet is a perfect compliment to a streamlined 
duct. The submerged inlet collects air in a low drag manner which it 
does well, and the streamline duct is responsible for pressure recovery 
which IT does well.  But we don't know that, 'cause they don't make any 
attempt to speak to it.  I think it would be appropriate to assert that 
the authors would agree with the statement that sticking a radiator 
against the back end of a NACA inlet is only marginally better for 
cooling than hanging it out the bottom of the fuselage.

Having said all that,  Thomas' original statement is spot-on.  The 
report authors did indeed recommend against using the NACA inlet as a 
pressure-recovery device.

Jim, the report Ed and I are discussing has a section on drag.  As I 
read it, they didn't bother with a drag study because it is so low with 
a submerged inlet.  All the losses due to drag end up as pressure where 
it's needed.  If those NACA guys are to be believed, even though the 
downdraft scoop was half the size, it will still have more drag.  Half 
of somethin' will always be greater than nothin' 8*)

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