Al wrote;

I recall a NACA scoop on a Velocity that was somewhere back on the
fuselage under the wing for fresh air into the back seat.  Opening the
vent produced no air coming in.  Apparently improperly designed scoop,
or bad location; or both.

Another problem w/ Naca ducts [ or any ducts into the cockpit] is that if
there is no place for the air to get out then the air has no reason to go in
the duct. We've run into this problem w/ Lancairs where they cockpit is so
sealed up that they didn't get the airflow they expected through the cabin.
By enlarging rudder cable openings etc the problem was soon rectified to the
point that now they where getting too much air. As long as there is an exit
in a lower pressure region [or a exit duct creating a low pressure region]
the air WILL come in [usually around the canopy seals and every where else
that a person hadn't thought of :-) ]

Food for thought
Jarrett Johnson