Vacuum Gyro Question

[Jim Carriere]

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!

[x]

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!
>

[nafod40]

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.

[UltraJohn]

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

[Colibri]

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.

[Jim Carriere]

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.

[Jim Carriere]

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.

[UltraJohn]

>
> 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

[Juan Jimenez]

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!
>

[Jerry J. Wass]

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!
> >

[JDupre5762]

>> 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.

The Piper PA 31 Navajos for one use a pressure system with a filter between the
pumps and the instruments.

John Dupre'

[Don Hammer]

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.


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[Jim Carriere]

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.

[Jerry J. Wass]

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.

[UltraJohn]

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

[Cy Galley]

Most airplanes that have de-icing boots use a pressure system. Even some
Bonanza even without the boots.


"JDupre5762" > wrote in message
...
> >> 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.
>
> The Piper PA 31 Navajos for one use a pressure system with a filter
between the
> pumps and the instruments.
>
> John Dupre'