Rationale behind vacuum instruments

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. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

--
Transpose mxsmanic and gmail to reach me by e-mail.

On Apr 9, 8:17 am, Mxsmanic wrote:
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. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

--
Transpose mxsmanic and gmail to reach me by e-mail.

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

-robert, CFII

"Mxsmanic" wrote in message
...
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. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond
what
was required by the engines themselves. However, a vacuum pump seems no
more
reliable to me than an alternator. What are the reasons behind it all? I
see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that
vacuum
is subject to partial failures, whereas an electrical failure is usually
much
more obvious.


In your case it makes no difference. The vacuum pump in your desk is
electrically driven.

On Apr 9, 9:17 am, Mxsmanic wrote:
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. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Because your head is an infinite source of vacuum...

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

Driving gyros with air pressure is much simpler, cheaper, and
more reliable from the gyro point of view than using small electric
motors. Venturis and engine-driven wet pumps are real reliable
and not too expensive too. The major problem is engine-driven
dry pumps which have a nasty habit of failing unexpectedly.

I
cannot see any special reliability of a vacuum pump as opposed to an electric
motor.

Your vision is impaired. Spinning a free-in-space gyro (like the AI or
DH) with a motor is complicated. On the other hand, a restrained gyro
like the Turn and Bank is much easier (and these are typically electric).

The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves.

The above is only partially true. While it certainly starts there, the
answer is really that electrical gyros are much more expensive and not
that much more reliable.

There are some real nice units coming on the market these days with
a electric Attitude Gyro with a battery backup. But they are still
MUCH more effective. You can actually have two engine driven vacuum
pumps for redundancy in many cases and two independent gyros for what
they cost.


However, a vacuum pump seems no more reliable to me than an alternator.

A wet pump driving a air gyro is MUCH more reliable than an alternator
driving the entire electrical system driving an electric gyro.

Anecdotally, I've had perhaps a half a dozen electrical failures in
several different aircraft over the years. I've had one dry pump
go on me.

Evaluating the options. I have the following:

A dry pump feeding the AI.
The vacuum guage prominently located next to the above.
A electric turn coordinator feeding the autopilot.
A real nice IFR GPS with fast update rates.

One major issue with the vacuum failure and transition to parital panel
is realizing that the gyros have failed. While IFR pilots are taught
to contantly verify the indications between the various instruments
a prominate failure flag or annunciator is a big help.

Frankly, the IFR GPS really makes partial panel a lot easier. Zoom
up the thing so you can instantly see any track changes and watch the
altimeter and you'll find it's not too rough maintaining flight.
Punching the autopilot on is almost cheating.

(See what happens when you ask a question nice rather than throwing
out bull****).

"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.

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.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Ron Natalie writes:

Driving gyros with air pressure is much simpler, cheaper, and
more reliable from the gyro point of view than using small electric
motors. Venturis and engine-driven wet pumps are real reliable
and not too expensive too. The major problem is engine-driven
dry pumps which have a nasty habit of failing unexpectedly.

Which types of pumps are used in most modern small aircraft?

There are some real nice units coming on the market these days with
a electric Attitude Gyro with a battery backup. But they are still
MUCH more effective. You can actually have two engine driven vacuum
pumps for redundancy in many cases and two independent gyros for what
they cost.

How many small aircraft have redundant pumps and gyros?

Are any small aircraft using RLGs?

Anecdotally, I've had perhaps a half a dozen electrical failures in
several different aircraft over the years. I've had one dry pump
go on me.

How hard is it to spot a pump failure? I've gotten the impression from what
I've read here and elsewhere that vacuum pumps may fail gradually and
insidiously, whereas (presumably) an electric motor fails in a much more
obvious way.

One major issue with the vacuum failure and transition to parital panel
is realizing that the gyros have failed. While IFR pilots are taught
to contantly verify the indications between the various instruments
a prominate failure flag or annunciator is a big help.

Does a failure involve the gyro coming to a stop, or can it just slow down and
thereby cause problems?

Frankly, the IFR GPS really makes partial panel a lot easier. Zoom
up the thing so you can instantly see any track changes and watch the
altimeter and you'll find it's not too rough maintaining flight.
Punching the autopilot on is almost cheating.

But how does that help you if the AI has failed? The GPS wouldn't tell you
the attitude of your aircraft.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Snowbird writes:

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

Since you use the past tense, are you saying that an electrical drive is more
reliable today?

--
Transpose mxsmanic and gmail to reach me by e-mail.

"Mxsmanic" wrote ..

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

Since you use the past tense, are you saying that an electrical drive is
more
reliable today?

No, I'm not saying that, although I've observed that recent marketing
activities in aviation publications tend to promote electrical solutions. I
leave that judgment to qualified aviation professionals.