----- Original Message -----

From: Ernest Christley

To: Rotary motors in aircraft

Sent: Friday, March 10, 2006 10:00 AM

Subject: [FlyRotary] Re: NACA's, Cooling and Sport Aviation Mag..

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  |

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