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

"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

For that matter, spinning iron should have gone away a long time ago,
too. Laser gyros draw less power, are more consistent, don't precess,
and have no moving pars save a stream each of electrons and photons.

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

Grieg wrote:

For that matter, spinning iron should have gone away a long time ago,
too. Laser gyros draw less power, are more consistent, don't precess,
and have no moving pars save a stream each of electrons and photons.

Solid State sensors are an option too but I want a way to backup vacuum gyros.

--
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

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.

Chris

Some of the Century Series Fighters had a RAT (Ram Air Turbine). It
was mounted on a hinged door and was deployed by a handle and cable
from the cockpit. It gave enough hydraulic pressure for minimal
control of the A/C with loss of engine hydraulic pressure.

If you connected a ventura on a hinged spring loaded door that
could be extended into the slip stream when needed your problem might
be solved simply and with not a lot of extra weight or drag in normal
flight?

You also could put a vacuum pump, that is propeller driven, on door
that could be extended if and when needed.

You of course need to mount door to open in a high pressure location
in the slip stream.

Big John

On Sun, 26 Oct 2003 08:20:49 -0600, Chris W wrote:

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?

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.

The Precise Flight (I'm not sure they are still in business) device tapped
the left rear intake riser on the Lyc O-360 with a tube mounted into the
rubber hose . It was manually engaged with a pull knob operated valve that
switched the vacuum source from the vacuum pump to the manifold tap. Of
course, both the vacuum pump and the manifold vacuum sources used the vacuum
regulator to deliver the right vacuum to the instruments.

I was surprised that it provided full instrument vacuum at full throttle -
but it did. Vacuum gyros only need about 3 - 5 inches of water column and
even a clean new engine air filter produced at least this amount of pressure
drop.

It worked extremely well.

Bill Daniels

Bill Daniels wrote:

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.

The Precise Flight (I'm not sure they are still in business) device tapped
the left rear intake riser on the Lyc O-360 with a tube mounted into the
rubber hose . It was manually engaged with a pull knob operated valve that
switched the vacuum source from the vacuum pump to the manifold tap. Of
course, both the vacuum pump and the manifold vacuum sources used the vacuum
regulator to deliver the right vacuum to the instruments.

I was surprised that it provided full instrument vacuum at full throttle -
but it did. Vacuum gyros only need about 3 - 5 inches of water column and
even a clean new engine air filter produced at least this amount of pressure
drop.

It worked extremely well.

Bill Daniels

Aha! I was thinking 3 to 5 inches of mercury!