[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: fuel vent line
> From: Simon Aegerter[SMTP:firstname.lastname@example.org]
> 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
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 ...
P.S. For more information about NACA ducts, take a look at:
Ugh, they're in PDF ...