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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-773629@logan.com>
Subject: Re: [FlyRotary] Re: NACA scoops
Date: Wed, 19 Oct 2005 14:20:25 -0400
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Hey, I'm not against NACA - if they work and you like them and they fit you 
need, then go for it.
The report did not say they would not work, just that they would not work as 
well as other types (streamline duct for instance) if you need pressure 
recovery (decrease airflow velocity and increase pressure recovery from 
dynamic energy).

However, I don't think using a streamline duct after a Naca duct is going to 
avoid the "bullet", the problem is any thing that causes a pressure recovery 
is going to cause "back pressure" to the air flow.  From what I think I 
understood the Naca duct is great where you keep the air velocity up (like 
it being sucked into a jet engine) but don't have a lot of pressure 
recovery. But whatever you have (radiator, streamline duct) that causes back 
pressure tends to adversely effect the performance of a Naca duct because it 
slows down the airflow.  At least that is what I think I understood.

Since I have my ducts right up front and am not faced with some of the 
challenges of the pusher type installation - I can say I don't have a dog in 
this fight {:>)

Ed A


----- Original Message ----- 
From: "Ernest Christley" <echristley@nc.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, October 19, 2005 2:02 PM
Subject: [FlyRotary] Re: NACA scoops


> 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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