Instrumentation

11
Gyroscopes
The Relationship Between Precession and Rigidity
The rigidity of a gyro depends on two properties:
• Moment of Inertia (a combination of the mass and the effective radius at which the mass
operates)
• Rotor rpm
Moment of Inertia. Moment of inertia is a measure of how big and how heavy the gyro is.
A gyro with a greater radius will have a larger moment of inertia than a smaller one with the
same mass. A gyro with a greater mass will have a larger moment of inertia than one of the
same radius but less mass.
In order to minimize weight (mass) but get a greater moment of inertia, the mass is often
concentrated at the rim of a gyro. A bicycle wheel, which uses spokes, is a clear example.
11 Gyroscopes
Rotor rpm. The faster the rotor spins, the greater the gyro’s rigidity.
The rigidity of a gyroscope is increased if the mass is increased, the effective radius at which
the mass operates is increased, or if the rotor rpm is increased.
The rate of precession is directly proportional to the applied torque but inversely proportional
to the moment of inertia and the rotor rpm rate.
Effectively, this means that precession and rigidity are opposite characteristics. If a gyro has a
lot of rigidity, it will not precess very much. If it precesses a lot, it cannot be very rigid.
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