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RE: fuel vent line

> From: Simon Aegerter[SMTP:aegerter@dataway.ch]
>
> whatever the KPC says (I don't have it in front of me): forget about NACA
> scoops in this case. NACA scoops are dynamic. They work with air
> throughput. There is no appreciable air flow through the vent lines; not
> enough in any case to result in pressure inside the scoop.

My understanding of NACA ducts is that they use pressure recovery, not 
flow. They're supposed to turn dynamic pressure into static, which is what 
we need for the tank vents. Kirk Lindberg has been flying with NACA ducts 
on the bottom of his strakes for four years now, with no problems, even in 
icing.

The original 1940's papers talked about submerged inlets not working well 
when large volumes of air were required (e.g., turbine inlets), and a ram 
scoop or other opening in a high-pressure area (wing leading edge) being 
more appropriate for that application.

I've seen very few inlets on Velocitys that come even remotely close to the 
dimensions defined by NACA fifty years ago (7 degree ramp slope, 4 to 1 
width to depth ratio, airfoil shaped lip, sharp intersections between the 
ramp & walls and walls & skin, very specific wall divergence.) Most end up 
being just oddly shaped openings in the fuselage, with probably some very 
bizarre airflow. The factory's attempt at using pseudo NACA ducts for rear 
cabin ventilation in the Elite prototype were so bad that air actually 
flowed OUT of them. But, they weren't anything like the NACA specs. They 
got it right on the XL roof duct.

One thing to watch out for if you're sticking a pipe out into the airflow 
is the possibility of icing. Make sure there's an alternate air inlet 
somewhere (hole in the back of the tube, for example). Cessna mounts theirs 
behind the wing strut for just that reason. In theory, NACA ducts shouldn't 
ice up, because the moisture particles are supposed to be too massive to 
make the turn along the ramp. This seems to be borne out by Kirk's couple 
of inadvertent icing sojourns, but you never know ...

 - Chuck

P.S. For more information about NACA ducts, take a look at:

	http://naca.larc.nasa.gov/reports/1945/naca-acr-5i20/
	http://naca.larc.nasa.gov/reports/1948/naca-rm-a7i30/
	http://naca.larc.nasa.gov/reports/1951/naca-tn-2323/

Ugh, they're in PDF ...