How does an attitude indicator know which direction is up?

OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to
the correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...

"Tman" x@x wrote in message
. ..
OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to the
correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the plane
is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...

The attitude indicator has an internal device which gradually resets the
instrument back to plumb to compensate for precession errors.

Tman x@x wrote in :

OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to
the correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...


Yeah, it references to gravity and averages it al out. If you maintain a
gentle turn for a long long time it will eventually settle on the bank you
maintain as being level flight.. That's acomplished by a couple of little
doors hung around the enge of the wheel in the gyro that are arranged so
that when the gyro isn;t aligned the doors open and a little puff of air
comes out and applies a precessional force to right the gyro. Since nobody
flies in circles all the time it pretty much averages itself out.
Electric ones have a simlar electronic pendulum.

Bertie

"Tman,"

Congratulations for asking an excellent question!

I'm not qualified to give you an authoritative answer myself, but I found a
good explanation in the FAA's AC 65-15A, The A&P Airframe Handbook (page
5040. I think it's available on-line. Search this document or Google for
"pendulous vanes."

Jon

"Tman" x@x wrote in message
. ..
OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to the
correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the plane
is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...

Correction: page 504.

"Jon Woellhaf" wrote in message
...
"Tman,"

Congratulations for asking an excellent question!

I'm not qualified to give you an authoritative answer myself, but I found
a good explanation in the FAA's AC 65-15A, The A&P Airframe Handbook (page
5040. I think it's available on-line. Search this document or Google for
"pendulous vanes."

Jon

"Tman" x@x wrote in message
. ..
OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to the
correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...

On Sun, 5 Oct 2008 21:05:32 +0000 (UTC), Bertie the Bunyip
wrote:

Tman x@x wrote in :

OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to
the correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...


Yeah, it references to gravity and averages it al out. If you maintain a
gentle turn for a long long time it will eventually settle on the bank you
maintain as being level flight.. That's acomplished by a couple of little
doors hung around the enge of the wheel in the gyro that are arranged so
that when the gyro isn;t aligned the doors open and a little puff of air
comes out and applies a precessional force to right the gyro. Since nobody
flies in circles all the time it pretty much averages itself out.
Electric ones have a simlar electronic pendulum.

Bertie
**************************************************

I've flown vacuum gyros that if they had not erected by time for take
off you could pull a knob on instrument and manually erect the gyro.

Bertie may have time and place of those gyros?

Haven't see one for years.

Big John

Tech Support wrote in
:

On Sun, 5 Oct 2008 21:05:32 +0000 (UTC), Bertie the Bunyip
wrote:

Tman x@x wrote in :

OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset"
to the correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum
pump powered 172. One vac pump is shot, and it did not build up
sufficient vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...


Yeah, it references to gravity and averages it al out. If you maintain
a gentle turn for a long long time it will eventually settle on the
bank you maintain as being level flight.. That's acomplished by a
couple of little doors hung around the enge of the wheel in the gyro
that are arranged so that when the gyro isn;t aligned the doors open
and a little puff of air comes out and applies a precessional force to
right the gyro. Since nobody flies in circles all the time it pretty
much averages itself out. Electric ones have a simlar electronic
pendulum.

Bertie
**************************************************

I've flown vacuum gyros that if they had not erected by time for take
off you could pull a knob on instrument and manually erect the gyro.


Bertie may have time and place of those gyros?


What, the cageable ones?

They're still around. They standby horizons in commercial aircraft are
like that as well as some lightplane ones.


Bertie