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This video from a high-profile skepticism channel perports to debunk a flat-Earth nut. But, Greg’s explaination of why the artificial horizon instrument on an aircraft works properly seems to me to be horribly wrong. In fact, I think the nutjob has it right in terms of how a bare isolated gyroscope would appear: The plane, travelling far enough to go over the curve of the Earth, would turn around a gyroscope that naturally keeps its axis true relative to the “fixed stars”. And a supported gyroscope would feel presession as the framework holding it was turned.

Can someone knowledgable confirm this?

And please comment on the video too: it will take some traffic to Be noticed over the background junk comments and this is a serious teaching site normally of high quality, so it should be brought to his attention from proper authority.

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It's really hard to build an ideal gyroscope. Forces such as friction will tend to cause problems that make the unit precess. Such errors need to be corrected over time if you want to use it as a navigation instrument.

For airplanes, that means that the gyroscopes that drive the artificial horizon are not unconstrained. A simple gyroscope with full range of motion would tend to maintain orientation with respect to the stars. But this unit would also collect errors over time and wouldn't maintain that axis for a long period.

A traditional spinning airplane gyroscope has a mechanism inside that assumes the aircraft is flying level most of the time. The local gravity vector opens vanes that allow air forces to push it slightly. These forces cause the gyro to precess in exactly the direction that causes it to align with the gravity vector.

Modern ring gyroscopes don't erect in the same method, but still require compensation methods for accumulation errors over time.

The video is confusing because when it talks about gyros that are driven in a particular direction by gravity, it shows an image of a mechanism that would not be (the classic gimbaled gyroscope).

If you wanted to do the test mentioned in the video, you'd use a different gyroscope suitable for tracking all axes, not one designed for aircraft navigation with constant corrections and assumptions about local gravity.

Some info on the traditional style gyro can be found on this Aviation SE question: AI in a turn

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    $\begingroup$ Wow. I didn't know about the air vanes in mechanical aircraft gyros, thanks! It might be worth emphasizing that a "ring gyro" is an optical device working by the Sagnac effect and is an extremely accurate angular vector velocitometer that has to be integrated to an orientation. Otherwise, "ring gyro" simply sounds like a mechanical device. $\endgroup$ – Selene Routley Mar 21 '17 at 23:49

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