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I am teaching aviation students theory for the ATPL Exam. And one of the subjects, instrumentation, there is question on the exam that has stumped me.

It claims that the torque motor that will drive the outer gimbal to the correct heading if it gets an error signal, is also used as the system that will erect the gyro and keep it from toppling?

But in all, but limited, information I can find, we are talking about a precession motor. That does the primary job, correcting the error.

Which begs the question: Are there two motors? A precession motor and a torque motor that is activated with a mercury tilt switch if the gyroscope starts to topple?

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  • $\begingroup$ I sense another ASE question brewing here-- "In the context of the internal workings of aircraft instruments, what is the difference between a 'precession motor' and a 'torque motor'?" $\endgroup$
    – quiet flyer
    Jan 6, 2022 at 18:57
  • $\begingroup$ A torque motor is a type of rotary actuator. Usually there is an output arm that moves like a clock hand, or just an output shaft you attach something to, and there is a resting position, and when you apply an increasing DC voltage, the arm moves off the resting position in proportion to the voltage increase. Take away the voltage and it goes back to rest. $\endgroup$
    – John K
    Jan 7, 2022 at 20:43

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It depends on the age of the instrument. The signal amplifier passes a DC current to the ‘precession motor’ to precess the gyro in azimuth back to the correct heading. Later versions use a torque motor instead. This should not be confused with the separate erection system torque motor that keeps the gyro spin axis horizontally aligned. I’m an ATPL TKI and that’s the info that I have.

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  • $\begingroup$ Thanx Nic, much appreciated answer. $\endgroup$
    – MortenHa69
    Jan 14, 2022 at 8:44

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