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I have a hard time understanding the physics behind the pneumatic erecting system in an attitude indicator. According to my book and the linked image, the applied force is “precessed 90 degrees” (anti-clockwise seen from above) and thus re-aligns the gyro. I understand how the pendulous vanes change the exhaust of the air, but since this part is stationary, i.e. not rotating, how can the net force be precessed?

Erecting Mechanism https://savageplane.files.wordpress.com/2017/06/pendulous-vanes-figure-10-101-screenshot-from-2017-06-06-171632.png

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Gyroscopes have two types of motion.

The gyroscope spins within its housing (usually at a pretty high speed). The housing can change attitude within some limits.

The vanes aren't part of spinning gyroscope, but they are part of the housing (which can shift/rotate with respect to the airframe). The vanes create a torque to change the attitude of the housing.

There is a youtube video with some animations that try to show this effect.

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  • $\begingroup$ Thank you. That’s a nice video, yet I still have trouble seeing how the gyroscope moves the force 90 degrees since the housing only can tilt. It seems like the force should remain where it is. $\endgroup$ – schn Feb 3 '19 at 11:24
  • $\begingroup$ The housing can pitch inside the fork, and the fork (which is pretty light) can roll. So forces that try to pitch or roll the housing will cause it to move in either of those two axes. $\endgroup$ – BowlOfRed Feb 3 '19 at 16:26
  • $\begingroup$ Okay. I think my understanding of gyroscopic precession wasn’t up to date, so I’ve looked some more into that. $\endgroup$ – schn Feb 3 '19 at 16:34
  • $\begingroup$ Precession (roll force but yaw movement): youtu.be/XPUuF_dECVI?t=2167 $\endgroup$ – mins Feb 3 '19 at 18:37
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The pendulous vanes do move around the hinge at the top. And it stays vertically aligned due to gravity, that’s why when the gyro is moved, a vent closes and another will be opened.

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  • $\begingroup$ Okay. It kind of makes sense that they should rotate with the gyro. Thanks! $\endgroup$ – schn Feb 3 '19 at 2:13

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