Bill Daniels wrote:

"Chris W" wrote in message
...
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


I think a better way is to tap engine intake manifold vacuum. I had a
Precise Flight system on my Piper Archer that provided full instrument
vacuum to 12500 feet at full throttle. I couldn't think of a situation that
would be an issue with that sort of backup. (If the engine quits while in
hard IMC, you don't need a vacuum backup - you need a parachute.)

Better still, get electric instruments. Powering gyro instruments with
vacuum should have gone away long ago. Add an avionics bus standby battery
that will power the panel longer than the gas will last.

Bill Daniels

I am not familiar with the Precise Flight system, but would expect poor
suction based on manifold vacuum at full power.

Venturi vacuum, which is also available from some carburetors, works well
at most power settings including full power; but is probably not adequate
at idle. Therefore, if carburetor venturi vacuum is used, descents and
approaches would require partial power to keep the gyros spun up.

I don't know whether this is a common feature of aircraft carburetors, and
possibly the suction source that Precise Flight uses, as the carburetors
I have seen that provided the second suction source have been automotive.