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The original NACA divergent inlet called for a ratio of width to height
of 3 and 5 to 1
In other words , if you had a 15" wide inlet the opening should be 3-5"
high. That 1-3/4 x 15-3/4 is not optimum . I doubt it allows full
expansion so there is not full pressure developed in the duct and the
air is not entering the cowl . (See CSA NewsletterJuly 1990, p2)
I suggests you cover the area around the inlet with dirty oil and go fly
45 minutes. I'll bet the oil flows will show the air enters the duct but
comes back out and continues back along the cowl's surface after the
inlet lip. If you take pictures of the flow, please send them to me.
The original NACA duct required an internal expansion area that VERY few
people install. That omission reduces pressure recovery in the cowl and
reduces cooling an appreciable amount. On my old Lycoming 235
installation I'd see a 50 F CHT change with the expansion area attached.
I know you are running a rotary but cooling is cooling.
Terry Schubert
Central States Association
Newsletter Editor
On Sun, 6 Jun 2004 19:57:37 -0400 "John Slade" <sladerj@bellsouth.net>
writes:
> the inside of the scoop measures 1-3/4" high by 15-3/4" wide.
That's strange. My NACA scoop is built per plans. How come yours is 1 3/4
wide? Are we measuring the same thing? From the floor of the scoop to the
inside edge of the airfoil mine measures 3.25 * 15.4 for a total square area
of about 50 sq inches. I don't have any additional scoops and so far my
cooling seems adequate.
> Today when flying straight and level, when I increased
> throttle, and increased speed, I also increased in oil and coolant
> temperature, which tells me that I don't have enough air flow, or
> velocity, or pressure, or whatever the correct term is.
I think this would be expected. Increase power and temperatures will go up,
at least initially. Does it stabilize at an acceptable temperature, or does
it continue to climb?
> As it turns out, I have some time to work on the scoop, especially since
> I'll be doing a little fiberglass work anyway.
You could be going in the wrong direction here, Steve. I didn't do much in
the way of contouring the intake areas, but I did make a bit of a duct.
Maybe that's helping. The augmented exhaust may also be contributing.
I'd suggest more testing at different speeds and outside air temps before
hacking at the scoops.
Regards,
John
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