When we perform a spin in a Cessna 172, I have seen that the Attitude Indicator gets messed up and locks itself into a disoriented reading for sometime. I have read that this usually happens when when the Pitch limit of around 85deg is reached and the gyro topples. And as for the Heading Indicator, after the spin it constantly keeps spinning? Is this also because of the gyro toppling and if so what are the limits for the Heading Indicator?
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Gyroscopes are attached to gimbals. Usually we need a 3 axis gimbals because we travel in 3 axis.
The illustration on Wikipedia (which I have trouble to insert here) helps understanding how it works: the gimbals is attached to the vehicle frame by the pins. The gyroscope will keep orientation, so the gimbals rings must freely move. So we have the basic of (mechanical) gyroscopes.
The problem is that with some huge movements, 2 rings may align, so you may lose an axis of movement of the gyroscope, so you force the gyroscope to move with your frame, and so you have incorrect attitude. We call it gimbals lock: the when 2 rings align, you lose one freedom of movement (one axis).
Note: also huge accelerations may cause problem on gyroscope (or not calibrating it regularly: the more you travel, the more you accumulate errors). But gimbals locks is particularly problematic: the data is completely wrong.
It was mostly a problem with spacecrafts or acrobatics where you may travel on all directions. As you can see from the Wikipedia figure, how do you place the rings may affect the problematic directions, so we can set the apertures to the less common attitude, but please note that Earth rotate, so at least one ring will do a complete rotation once a day.
Now we have also other types of gyroscopes (accelerometers, one per axis), so without need of a gimbals, and GPS may provide data to reset gyroscopes, as backup or cross-check.
answered Dec 20, 2023 at 10:21
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$\begingroup$ While GPS data can remove position errors from an INS system (which uses gyros internally), since it doesn't provide attitude information, it isn't clear how the "GPS may provide..." phrase of your last sentence is correct. Please clarify. Thanks! $\endgroup$– Ralph J ♦Commented Dec 20, 2023 at 14:17
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$\begingroup$ "Now we have also other types of gyroscopes (accelerometers, one per axis), so without need of a gimbals", accelerometers measure accelerations, gyroscopes measure rotations, they are not interchangeable. An INS is built from 3 gyros and 3 accelerometers. However mechanical gyros (gimbal platform) are a thing of the past, new gyros are static and optical (Sagnac effect). $\endgroup$– minsCommented Dec 20, 2023 at 19:40