Gyroscopic Instruments and Inertial Systems
Rigidity and precession
A gyroscope with a spinning rotor resists change to its spin axis (rigidity in space) and, when a torque is applied, precesses at 90° to the applied force in the direction of rotation. Precession rate rises with torque and falls as rotor angular momentum increases. Real drift comes from friction and imbalance; apparent drift comes from the Earth's rotation and from the aircraft's movement over the Earth (transport wander).
Earth rate and transport wander
- Earth rate about the local vertical is 15°/h × sin latitude.
- A horizontal-spin-axis gyro, such as a directional indicator, therefore appears to drift at about 15°/h × sin latitude; the direction reverses between hemispheres.
- Transport wander (°/h) = (east–west groundspeed in knots ÷ 60) × tan latitude. It reverses with hemisphere and with east/west direction, and grows at high latitude.
Gyrocompass and inertial systems
A gyrocompass uses gravity erection plus the Earth-rate signal to seek north, and becomes unreliable at high latitude where that signal is small. Inertial systems align while stationary (typically 5–10 minutes), are Schuler-tuned to an 84.4-minute period, and drift slowly; ring-laser gyros and MEMS accelerometers replaced the older spinning-mass platforms.
Worked example
A directional indicator is compared with the magnetic compass at 1200 and again at 1315. It has gained 15° in 1.25 hours, a drift of 12°/h. That is consistent with uncompensated Earth-rate drift at roughly 53° latitude, since 15 × sin 53° ≈ 12.
Traps
Mixing up real and apparent drift; ignoring transport wander on long east–west sectors; expecting a gyrocompass to work near the poles; and forgetting that an IRS needs a stationary, level aircraft and an accurate latitude before alignment.