Received-SPF: softfail
 receiver=logan.com; client-ip=171.68.10.86; envelope-from=echristley@nc.rr.com
Message-ID: <44119493.9070001@nc.rr.com>
Date: Fri, 10 Mar 2006 10:00:35 -0500
From: Ernest Christley <echristley@nc.rr.com>
Reply-To: echristley@nc.rr.com
User-Agent: Mozilla Thunderbird 1.0.7-1.4.1 (X11/20050929)
MIME-Version: 1.0
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: NACA's, Cooling and Sport Aviation Mag..
References: <list-1029114@logan.com>
In-Reply-To: <list-1029114@logan.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Bulent Aliev wrote:

> Bob, if the cabin does not have exhaust path for the incoming air,  
> the cabin pressure will build up and the NACA scoops will be  
> ineffective.
> Buly

That is correct.  But it is also correct for any other type of inlet 
you'd care to mention.   I'm not trying to be a smarta$$, just trying to 
point out that there is so much sound and fury around NACA inlets, but 
without a system approach it all signifies nothing.

The radiator doesn't care what sort of scoop is out front.  And it has 
no idea what sort of exhaust is behind it.  All that matters is the 
pressure DIFFERENTIAL across it.  Differential implies that there are 
TWO values to consider.  You could have a working system with negative 
pressure compared to ambient in front of the radiator, if and only if 
you had a much more negative pressure behind it.  Flatly stating that a 
NACA will or won't work is like talking about voltage without a 
reference ground. 

The Honorable Mr. Crook has done us all the favor of showing how to 
create a water manometer for less than the cost of a Coke at the 
movies.  The only number for pressure differential that I've seen for a 
working system is Tracy's. I recall that to be 5" H20, so let's go with 
that and make up a few more numbers.  You need 5" of pressure across the 
radiator to get adequate cooling.  A P-51 style scoop stuck out in the 
wind could probably give you 4" of ram pressure.  A properly designed 
exit could possibly give you -2".  There you go. Your done.   You'll get 
more than enough airflow to cool the engine.

But you want to cut the drag down, so you consider an submerged inlet.  
Use John Slade's approach, the partially submerged inlet.  Don't just go 
straight for the fully flush inlet, but start slowly sinking the scoop 
into the skin.  As it moves in, the positive pressure in front will 
drop.  You still have the -2" on the back, but if you drop below 3" on 
the front you won't have adequate cooling.  You start to slowly pull the 
scoop in, but before it is even halfway in you hit the 3" mark. 

Hmm?  Maybe work on the exit.  Change the shape a little, clean it up 
and maybe it will push the exhaust pressure down to -3".  Now you only 
need 2" on the front, and you can get the scoop down to only half the 
original obstruction.  What else?  Maybe you can fit a K&W streamlined 
duct in before the radiator.  Now that your duct is using the air it 
does have more efficiently, the frontal pressure is higher with the same 
scoop.  Mabybe you have 2.5" instead of the 2", and you can sink the 
scoop just a little more.

Hmm?  But what happens if you scoop out a little bit of the air frame 
and put the scoop in the rut that is formed?  Would that let you sink 
the scoop even further?  You have the same sized opening, but it isn't 
sticking out in the wind as far for less profile drag.  What if you gave 
the rut a carefully designed shape so that air will get a little extra 
pull into the rut instead of just flowing right over the top?  Could you 
sink it still further?  Maybe you can even play with negative pressure 
gradients and vortex sheets.  Damn, now we're having to head over to 
naca.larc.gov to pull up old studies where 50 years ago they derived 
actual equations to predict what will happen.

I guess my point is to not think of the NACA scoop as anything more than 
one end of the spectrum that starts with a pot-belly stove flue sticking 
out the belly.  I will be using a scoop that will be eerily similar to a 
NACA, except that it isn't.  Due to it's location just below the leading 
edge on the thick airfoil of the delta wing, it will work much more like 
a traditional scoop at high AOA.  During cruise, it will flatten out and 
begin to work more closely but not exactly like the submerged inlet.  
The exit will be on the top of the wing, just behind the max thickness.  
I have high hopes, but the water manometer will tell the true story.  8*)

-- 
         ,|"|"|,               Ernest Christley     |
----===<{{(oQo)}}>===----     Dyke Delta Builder    |
        o|  d  |o          www.ernest.isa-geek.org  |