[brendn]

OK. I'm no expert on this myself, but for someone who claims to have just figured out how an attitude indicator works, the author certainly doesn't explain it very well. I was left with the impression that the author is uncertain of the distinction between an accelerometer and a gyroscope. An accelerometer, like he says, can be likened to a hanging weight or pendulum. Accelerometers can measure the direction of gravity of a stationary object because the only force acting on the pendulum is acceleration due to gravity. However, as soon as the object is moved, other acceleration vectors are added to the already-present gravity vector.

(Imagine an accelerometer hanging from a rope. If you swing that rope in a circle around your head, the accelerometer will report that the force vector points to its bottom, away from your head, and not down to your feet as gravity alone would indicate.)

Which gets me to the OP. Because the article does not make a single mention of angular momentum (among other reasons), I would guess that the author doesn't understand how gyroscopes work. A gyroscope is essentially a spinning disc-shaped mass that resists change to its axis of rotation. If you built a housing that allowed it freedom of motion in three dimensions (called a gimbal (http://en.wikipedia.org/wiki/Gimbal)), it would remain on its original axis no matter how you tilted its enclosure (barring the inevitable energy loss due to friction, etc.)

So, in the case of an airplane, you would set the disc spinning on level ground. Then, while you're flying, any changes to your pitch or roll would be around the gyroscope (which, remember, is allowed to move freely relative to the cabin). The gyroscope wants to preserve its angular momentum, so it will still be spinning on the same axis relative to level ground. Any difference between the gyro and the cabin therefore indicates the orientation of the plane relative to the ground.

You'd probably get a more concise explanation from the Wikipedia page on Gyroscopes (http://en.wikipedia.org/wiki/Gyroscope). The Wikipedia page on attitude indicators (the visual display for a cockpit gyro) is also quite coherent and less prone to rambling (http://en.wikipedia.org/wiki/Attitude_indicator).

[EDIT: Toned down some grumpiness. I must be tired.]

[quux]

Private pilot here. Mechanical attitude indicators don't need to be reset on level ground, when they first start spinning they're almost always tilted but once they spin up they level out on their own. Because they're driven by vacuum from the engine, sometimes they don't fully spin up and level out until the engine's being run up or takeoff.

I've always wondered how the attitude indicator self levels, and the OP's explanation sounds plausible. Next time I go flying I'll have to spend a few minutes in a constant rate turn to see if I can make the attitude indicator drift like he says it should.

Edit to add, here's a link to a youtube video showing the inner working of an AI, including the pendulous vanes, neat:

http://www.youtube.com/watch?v=KUSklh3MKtA

[brendn]

Yeah, I kind of glossed over the vacuum aspect of the attitude indicator because I don't understand all its intricacies. The video pretty much confirms my understanding of how the gyro is kept spinning. The original author's description of air puffs sounds a lot like how the filtered air is ducted in to the housing and vented off in quadrants. I suspect that has more to do with maintaining the gyroscopes precession (http://en.wikipedia.org/wiki/Precession) than leveling the gyro in mid-flight.

I've heard that during prolonged banks or pitches, the AI can eventually find a different "level" than the true horizon. I'm trying to find more info into that phenomenon though.

In the mean time, I've found this article that seems to be talking about the same accuracy and drift issues as the original post, but explains the issues with more clarity and also includes sensor graphs and code samples: http://myahrs.wordpress.com/2012/04/24/turning-errors-contin...

[blake8086]

The system you're describing (which is a real thing) would allow you to measure your absolute orientation.

The gyroscopes in iDevices only measure rotational acceleration, which is only the second derivative of your orientation, so you need to do a bit more work to extract an (slightly error-prone) orientation out of it.

[tgb]

I haven't specifically used an iDevice, but all the gyroscopes I'm familiar with provide the first derivative, not second.

I'm also curious why he doesn't mention any use of magnetometers, which can give absolute orientation (not just the derivatives). Are the changes in position/elevation too large for airplanes to rely upon the local magnetic field of the earth?

[caf]

Any difference between the gyro and the cabin therefore indicates the orientation of the plane relative to the ground.

Unless you flew far enough that local level ground is no longer approximately parallel with level ground where you took off.

[kolinko]

There is a difference between electronic gyros and mechanical gyros.

[brendn]

Could you enlighten us? I've read about fiber optic gyros, but I assume they haven't gotten small enough to fit in a mobile device. (Since they require kilometers of fiber.) I really don't know enough about how the solid state (?) gyros work.