Vacuum Gyro Question

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!

Originally, at least, the vacuum came from a venturi in the
slipstream. It was a natural to use a vacuum pump with the same
instruments.


Posted Via Usenet.com Premium Usenet Newsgroup Services
----------------------------------------------------------
** SPEED ** RETENTION ** COMPLETION ** ANONYMITY **
----------------------------------------------------------
http://www.usenet.com

Don Hammer wrote:
Originally, at least, the vacuum came from a venturi in the
slipstream. It was a natural to use a vacuum pump with the same
instruments.

Right. I've seen a venturi on an old aircraft. You can still buy
them. They cost about as much as beer money, and are pretty reliable
unless a bird hits them or they ice up. I understand from the
designer's point of view the drag they create hurts performance much
more than an engine driven pump...

Thanks to all who provided examples of current aircraft that use
pressure pumps. Looking more closeley in my Aircraft Spruce catalog,
it has a couple pumps with separate part numbers- for "vacuum system"
and "pressure system." Most everything else is simply described as
vacuum.

Also, thanks to the two or three guys who gave short physics lessons
without being condescending, although I already understood that "push
here" = "pull over there"

Lots of good help on this group. Keep the fender washer jokes coming.

Jim Carriere wrote:

Don Hammer wrote:
Originally, at least, the vacuum came from a venturi in the
slipstream. It was a natural to use a vacuum pump with the same
instruments.

Right. I've seen a venturi on an old aircraft. You can still buy
them. They cost about as much as beer money, and are pretty reliable
unless a bird hits them or they ice up. I understand from the
designer's point of view the drag they create hurts performance much
more than an engine driven pump...Thas Right! A venturi doesn't add
much, percentage wise, to a slow,draggy, wire-braced biplane---but on a
modern fast streamlined job would cause a significant speed loss.



Thanks to all who provided examples of current aircraft that use
pressure pumps. Looking more closeley in my Aircraft Spruce catalog,
it has a couple pumps with separate part numbers- for "vacuum system"
and "pressure system." Most everything else is simply described as
vacuum.

Also, thanks to the two or three guys who gave short physics lessons
without being condescending, although I already understood that "push
here" = "pull over there"

Lots of good help on this group. Keep the fender washer jokes coming.

Jim Carriere wrote:
Right. I've seen a venturi on an old aircraft. You can still buy
them. They cost about as much as beer money, and are pretty reliable
unless a bird hits them or they ice up. I understand from the
designer's point of view the drag they create hurts performance much
more than an engine driven pump...



Yes they hurt performance but if you don't have an engine with a vacuum pump
and/or don't have an electrical system (and we all know how much more
electrical gauges cost!) them really there is not much choice. Venturis
work and hey they look cool!
John