Vacuum Gyro Question

This is one of those questions where there must be a simple answer.
This is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?
The reason I'm wondering is because about 2" of suction is
sufficient to power a turn needle, and while attitude gyros require
more, about 5", either of those amounts is still much less than
standard atmospheric pressure, meaning that the absolute pressure
inside the instrument is still close to ambient pressure.

Note other than seeing one on an instrument panel, I've never
actually taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!

This is a fantastic question. I sure can't think of any good reason.

"Jim Carriere" wrote in message
...
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead? The
reason I'm wondering is because about 2" of suction is sufficient to power
a turn needle, and while attitude gyros require more, about 5", either of
those amounts is still much less than standard atmospheric pressure,
meaning that the absolute pressure inside the instrument is still close to
ambient pressure.

Note other than seeing one on an instrument panel, I've never actually
taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!

Jim Carriere wrote:
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?

Well, they are. They are driven by a pressure differential between the
input and output.

As for why we lower the output pressure rather than raising the input
pressure...I'm guessing that a venturi in the airflow is
easier/better/more efficient than trying to use some sort of ram air
device to raise it.

nafod40 wrote:

Jim Carriere wrote:
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?



Some systems ARE designed to work off of pressure. Depending on the system
designer.
John

UltraJohn wrote:
nafod40 wrote:

Jim Carriere wrote:
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?



Some systems ARE designed to work off of pressure. Depending on the system
designer.
John

If the carbon vanes in a vacuum pump fail, any debris will be blown
overboard. If the pump pressurizes the instruments, the debris will be
blown into the instruments, ruining them.

UltraJohn wrote:

nafod40 wrote:


Jim Carriere wrote:

This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?



Some systems ARE designed to work off of pressure. Depending on the system
designer.

Hmm, I learn something new every day.

I figured maybe there was some technical reason they had to use
vacuum, since most of the instruments one can buy seem to be either
electrical or vacuum. From your answer, I suppose there is no real
reason, maybe most of the non electrical ones are vacuum because that
is just the way things are.

Colibri wrote:
If the carbon vanes in a vacuum pump fail, any debris will be blown
overboard. If the pump pressurizes the instruments, the debris will be
blown into the instruments, ruining them.

This is a good point, although hopefully a filter would protect
against this expensive possibility.

Hmm, I learn something new every day.

I figured maybe there was some technical reason they had to use
vacuum, since most of the instruments one can buy seem to be either
electrical or vacuum. From your answer, I suppose there is no real
reason, maybe most of the non electrical ones are vacuum because that
is just the way things are.
Colibri's response is why most are driven off the vacuum side but it isn't a
requirement. Like was mentioned the filter provides some protection.
According to an article in the newest Private Pilot magazine on twins. The
Beech Duchess Model 76 that they review is run off the pressure side of the
pumps (page 52, February 2005). They also mention most Beechcrafts are done
that way.
John

They _are_ being driven by air pressure. The air is sucked in from the cabin
and through an air filter by the vacuum generated by the pump (or venturi).
The filter is inside the cabin. This setup comes from the fact that when
faced with the choice of air from the outside (or from the engine
compartment), or air sucked in from the cabin using the vacuum generated by
a venturi or pump, the latter is considered more suitable to contribute to a
longer instrument working life.

"Jim Carriere" wrote in message
...
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead? The
reason I'm wondering is because about 2" of suction is sufficient to power
a turn needle, and while attitude gyros require more, about 5", either of
those amounts is still much less than standard atmospheric pressure,
meaning that the absolute pressure inside the instrument is still close to
ambient pressure.

Note other than seeing one on an instrument panel, I've never actually
taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!

Most succinctly expressed!

Juan Jimenez wrote:

They _are_ being driven by air pressure. The air is sucked in from the cabin
and through an air filter by the vacuum generated by the pump (or venturi).
The filter is inside the cabin. This setup comes from the fact that when
faced with the choice of air from the outside (or from the engine
compartment), or air sucked in from the cabin using the vacuum generated by
a venturi or pump, the latter is considered more suitable to contribute to a
longer instrument working life.

"Jim Carriere" wrote in message
...
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead? The
reason I'm wondering is because about 2" of suction is sufficient to power
a turn needle, and while attitude gyros require more, about 5", either of
those amounts is still much less than standard atmospheric pressure,
meaning that the absolute pressure inside the instrument is still close to
ambient pressure.

Note other than seeing one on an instrument panel, I've never actually
taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!