Gyroscopic Instruments: Principle and Errors

Gyroscopic Principle and Instrument Types

A gyroscope is a device with a spinning rotor, suspended in gimbals, that maintains its orientation due to angular momentum. The two main properties are rigidity (the tendency to resist changes to its axis) and precession (the predictable movement of the axis when an external force is applied at 90° to the force direction). Aircraft gyroscopic instruments—such as the attitude indicator, directional gyro, and turn coordinator—exploit these principles to provide reliable reference information.

Methods of Driving Gyroscopes

Gyroscopes can be powered by:

• Air/Vacuum systems: Use suction or pressure to spin the rotor, common in light aircraft.
• Electrical systems: Use electric motors for higher reliability and less susceptibility to pressure changes, often found in larger or more modern aircraft.

Types of Gyroscopic Instrument Errors

Gyroscopic instrument errors can be grouped as follows:

• Real Wander: Caused by internal mechanical friction or imperfections in the instrument, leading to random drift.
• Apparent Wander: Includes errors from the Earth's rotation (earth rate error) and from the aircraft's movement over the curved surface of the Earth (transport wander). These cause the instrument's reference to slowly drift away from true orientation.
• Gimballing Errors: Occur when the gimbal rings reach their mechanical limits, especially during steep turns or unusual attitudes, leading to false indications.

Rigidity, Precession, and Drift

• Rigidity depends on rotor speed and mass distribution; more rigid gyros are less prone to error but require more force to precess.
• Precession is used deliberately in instrument design but can also introduce errors if external forces act unintentionally.
• Drift/Topple refers to the gradual misalignment of the gyro's axis, either from mechanical (real) or environmental (apparent) causes.

Practical Implications

Directional gyros must be periodically realigned with the magnetic compass due to these errors. Understanding the sources and behavior of gyroscopic drift is essential for accurate instrument interpretation and safe flight.