Rationale behind vacuum instruments

On Apr 9, 10:40 am, "Snowbird" wrote:
"Mxsmanic" wrote ...

Can someone tell me why vacuum power is popular for certain instruments?

Historical. Aircraft electrics tended to be a) more unreliable than today
and b) add unnecessary weight.

Of course, one shouldn't forget that a usable vacuum "supply" was
available without a pump when an external venturi was installed.

Of course, one shouldn't forget that a usable vacuum "supply" was
available without a pump when an external venturi was installed.

I was wondering how long it was going to take for someone to divulge the
REAL reason for vacuum instruments. External venturi tubes were the only
form of energy available to drive gyros on airplanes in the 30'3 and 40's
which had no electrical systems. After WW2 here were so many vacuum
instruments out there nobody had the courage to change.

Rod

Aircraft of old use venturies mounted on the fuselage to
power vacuum gyros. They were fairly reliable and did not
require any source of electrical power. The disadvantage
was that the AC had to be moving before the gyros would
spin up.

Later on vacuum pumps replaced the venturies for more
consistant vacuum, but one could argue the reliability
over a venturi... other than icing up perhaps.

The advantage of Vacuum instruments is that you could
still have a venturi as a backup vacuum source in case the
pump failed. Alternatively you could use some manifold
pressure (vacuum) as a backup source.

Until recently electrical instruments and electrical
systems were not considered as reliable. Nowadays you
can easily have battery backed up electrical gyros.

Either way, if a gyro-power source fails there are
systems that can immediately identify that fact to the pilot.
The gyros of course would spin down slowly and without
some immediate way to alert the pilot hwe my get erronious
information until the fail in an apparent way.

Usually, a gyro will show signs of needing a rebuild before
is actually "fails" such as slow to spin up/recet, excessive
precession, etc., but not always.

ktbr writes:

The advantage of Vacuum instruments is that you could
still have a venturi as a backup vacuum source in case the
pump failed. Alternatively you could use some manifold
pressure (vacuum) as a backup source.

Why not use manifold vacuum as the primary source, instead of a separate
vacuum pump that provides an additional point of failure? The manifold vacuum
would be there as long as the engine were running, without the need to rely on
a pump.

Usually, a gyro will show signs of needing a rebuild before
is actually "fails" such as slow to spin up/recet, excessive
precession, etc., but not always.

What type of bearings do the gyros use, and how long do the bearings last?

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Mxsmanic wrote in
:

Can someone tell me why vacuum power is popular for certain
instruments? I cannot see any special reliability of a vacuum pump as
opposed to an electric motor.

Of course you can't


Bertie

Mxsmanic wrote in
:

Robert M. Gary writes:

If I tell you are you going to tell me I'm wrong??

If it conflicts with other information I have, I may question it, but
I don't have much other information. If it sounds odd I may ask for
further explanation.

Send me fifty bucks and I'll explain it to oyu.


bertie

"rod" wrote in
:

Of course, one shouldn't forget that a usable vacuum "supply" was
available without a pump when an external venturi was installed.

I was wondering how long it was going to take for someone to divulge
the REAL reason for vacuum instruments. External venturi tubes were
the only form of energy available to drive gyros on airplanes in the
30'3 and 40's which had no electrical systems. After WW2 here were so
many vacuum instruments out there nobody had the courage to change.

No, actualy, that's not it.


Bertie

Mxsmanic wrote in
:

ktbr writes:

The advantage of Vacuum instruments is that you could
still have a venturi as a backup vacuum source in case the
pump failed. Alternatively you could use some manifold
pressure (vacuum) as a backup source.

Why not use manifold vacuum as the primary source, instead of a
separate vacuum pump that provides an additional point of failure?

God you;'re an idiot.
No wonder you can't fly.



Bertie

Mxsmanic wrote:

Why not use manifold vacuum as the primary source, instead of a separate
vacuum pump that provides an additional point of failure? The manifold vacuum
would be there as long as the engine were running, without the need to rely on
a pump.


It sounds like a free lunch doesn't it? :^O Well, tt works in a pinch
of course, but having to maintain a power setting that produces a
manifold pressure difference (between MP and outside static pressure)
just so that the gyros can spin sort of limits your flexibility.

kontiki writes:

It sounds like a free lunch doesn't it? :^O Well, tt works in a pinch
of course, but having to maintain a power setting that produces a
manifold pressure difference (between MP and outside static pressure)
just so that the gyros can spin sort of limits your flexibility.

Yes. I guess I got confused, thinking there would always be vacuum available
from the engine.

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