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I agree that the 'automatic' assumption that a NACA will not work for a rad/oil cooler is not always valid. Equally incorrect is the 'automatic' assumption that the NACA scoop is lower drag. Drag is a measure of the directed energy of the air in vs the directed energy out. It’s about the design of the scoops, the core and the ducting – both in and out.

All we know for sure is that a ram scoop has a better chance of successfully providing the cooling you need because you have a larger pressure head to work with.

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ernest Christley
Sent: Friday, March 10, 2006 7:01 AM
To: Rotary motors in aircraft
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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