Air driven directional gyros

[low n' slow]

Hi all.

I'm looking at an air driven directional gyro and I'm getting things all twisted up here.

My main question is regarding the erection system that ties the gyro. Exactly what doe the erection system tie the gyro to. Local horizontal or the aircraft lateral plane. My idea is that if the nozzle pillar and erection wedge plate are bolted to the same gimbal as the rotor is spinning in, they will never misallign, and if the nozzle pillar and wedge plate move with the rolling aircraft, this will cause the gyro to precess to follow the roll. I've found no good explanations for this anywhere, only that there is an erection system in place.

Thanks for any help in clarifying this for me!
/LnS

[mono]

The gyro does precess to follow the roll and they exhibit earth rate too. That's why they must be constantly checked and reset to magnetic heading. Only when flying straight, level and not in accel or decel.

[low n' slow]

Thanks Mono for your answer. So if flying on an E/W heading and I roll the aircraft, the gyro axis will be precessed to follow the roll.

Thanks
/Sam

[mono]

Yes. There will be an error induced because of the roll. However it's not primarily due to precession (apologies for the wording in my original post) but because the gyro is earth tied (i.e. there is a pendulous element to the erection system). Therefore when the a/c banks the gyro "sees" a false level and tries to adjust.

In more modern electrical gyros the erection system is disabled via mercury 'g' switches, while the a/c banks, accelerates, etc making them more accurate.