Gyro Erection System 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?


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