NACA duct ventrui

Here is why I was asking about the reverse NACA duct. I am thinking
that a simple venturi would be a great backup to an engine driven pump.
Naturally no one wants to add the drag that bolting a venturi tube on
the side of the plane would give. So I was trying to think of a way to
make it retractable. Then I started thinking about using a NACA duct
and keeping the venturi inside the plane, probably in the rear
fuselage. You would open a vent when you needed it and keep it closed
when you didn't. So the question I have now is how to make sure there
is no drag penalty when the venturi is not in use. Would it be
necessary to cover both entry and exit holes or would simply shutting a
valve to prevent air flow do the job?

--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania