Unimportant Question

[Matt Young]

OK, I know this doesn't make a bit of difference, but I was always that
kid that kept asking why and occasionally a strange question pops into
my head and I thought I'd throw it out for the flying gurus to take a
stab at if I can explain my question.

The AI works on the principle that a spinning gyro will remain in the
plane in which it spins, right? If taking a long flight, ie
transatlantic, why does flying around the curvature of the Earth not
affect this. The plane that parallels the ground in NYC won't be the
same plane that parallels the ground in London. Does gravity make it
precess gradually or what?

[Paul Tomblin]

In a previous article, Matt Young > said:
>my head and I thought I'd throw it out for the flying gurus to take a
>stab at if I can explain my question.

I can't give you a full answer, but the start of that answer is something
like "pendulous vanes". The AI is corrected to local horizontal.


--
Paul Tomblin > http://xcski.com/blogs/pt/
"Mission Control clears SMS-1 to Canaveral via thrust vectors, Up, Hold
Earth, right turns, expect further clearance in ten days."
http://www.avweb.com/news/usedacft/181561-1.html

[Peter Duniho]

"Matt Young" > wrote in message
ink.net...
> [...]
> The AI works on the principle that a spinning gyro will remain in the
> plane in which it spins, right? If taking a long flight, ie
> transatlantic, why does flying around the curvature of the Earth not
> affect this.

As Paul said, the AI has a mechanism in it that pulls the AI straight up and
down relative to gravity.

This mechanism is always active, even in turns, and so you can theoretically
remain in a coordinated turn long enough to create a significant error in
the AI. In practice, you almost never remain in a coordinated turn long
enough for it to be a problem.

A picture would be better to explain how it works, but basically there are
air ports at the bottom of the indicator that are normally covered by a
little "door" that hangs from above the port. The same vacuum that runs the
gyro pulls air through the ports.

Whenever the indicator is not straight up and down with respect to gravity,
the doors on each side open to the extent that the indicator is tilted. The
ports are configured so that a port that opens pushes the indicator in the
correct direction to right the indicator back to straight up and down (this
includes that they are positioned to account for gyroscopic precession with
respect to the force they create).

This mechanism is the same thing that brings the AI to vertical after you
start the airplane. Look at the AI before you start the engine; you'll
almost always find that it's showing a bank and possibly also pitch. After
you start the engine and the vacuum pump is working, the righting mechanism
brings the AI to the correct position.

Pete

[G.R. Patterson III]

Peter Duniho wrote:
>
> This mechanism is always active, even in turns, and so you can theoretically
> remain in a coordinated turn long enough to create a significant error in
> the AI.

According to my Jepp manual, any bank over 5 degrees disconnects the erection
mechanism, which is why CFIIs recommend steeper banks for instrument work.

George Patterson
None of us is as dumb as all of us.

[Peter Duniho]

"G.R. Patterson III" > wrote in message
...
> According to my Jepp manual, any bank over 5 degrees disconnects the
erection
> mechanism

By cutting off the airflow to the ports? Makes sense. I would bet that not
all AIs are the same, but that's good to know. Thanks.