Gyroscopic Drift and Topple

Wing Commander Rajagopal

Gyroscopic Wander of Drift and Topple

Gyroscopic Wander can be classified as Drift and Topple. Drift is an error that results in change in Gyro North. Topple refers to the tilt of gyroscope axis from the original position.

Gyroscopic Wander

  • Gyro wander is the departure of the gyro spin axis from its original position.
  • Wander is measured with respect to the Earth’s surface.
  • Gyroscopic wander is classified into drift and topple.
  • The deviation of the spin axis in the horizontal plane is called gyro drift.
  • Gyro topple is the deviation of the spin axis in the vertical plane.

Wander in Horizontal and Vertical Axis Gyros

  • The spin axis of a horizontal-axis gyro is parallel to the Earth’s surface.
  • Horizontal-axis gyroscopes can experience both drift and topple.
  • The spin axis of a vertical-axis gyro is perpendicular to the Earth’s surface.
  • Vertical-axis gyroscopes can topple but cannot drift.

Real and Apparent Wander

Real Wander – Real Drift and Topple

  • Gyro wander is classified as real wander and apparent wander.
  • Real wander consists of real drift and real topple.
  • Real wander is caused by manufacturing imperfections or wear and tear.
  • The gyro spin axis actually deviates from its orientation relative to a fixed point in space.
  • Real wander can result from the following manufacturing imperfections:
    • Imbalance in rotor mass.
    • Imperfectly balanced gimbals.
    • Uneven rotor bearing friction.
    • Uneven gimbal friction.

Real Drift Due to Wander

  • Real drift is caused by manufacturing imperfections or wear and tear.
  • A rotor speed of 10,000 RPM typically has a drift rate of 1.6° per hour.
  • A rotor speed of 20,000 RPM typically has a drift rate of 1.2° per hour.
  • The gyro spin axis actually deviates from its orientation relative to a fixed point in space.
  • Real drift can be reduced by improving the quality and precision of the gyroscope.

Apparent Wander – Apparent Drift and Topple

  • Apparent wander is caused by the perceived change in the gyro spin axis.
  • The spin axis actually maintains its direction relative to a fixed point in space.
  • The observer perceives a change because the Earth is moving beneath the gyro.
  • Apparent wander consists of apparent drift and apparent topple.
  • Apparent drift is the perceived deviation of the spin axis in the horizontal plane.
  • Apparent topple is the perceived deviation of the spin axis in the vertical plane.

Earth Rate and Transport Wander

  • Apparent drift and topple may be caused by Earth rate or transport wander.
  • Earth rate is caused by the Earth’s rotation about its axis.
  • Transport wander is caused by the movement of the gyro in an aircraft over the Earth’s surface.

Variation of Earth Rate and Transport Wander

Apparent Drift Due to Earth Rate

  • Apparent drift due to Earth rate is caused by the rotation of the Earth.
  • A horizontal gyro aligned with true north on a meridian appears to change direction.
  • This apparent change is due to the Earth’s rotation and meridian convergence.

Variation of Apparent Drift due to Earth Rate with Latitude

  • Earth rate varies with latitude because of changes in meridian convergence.
  • At the equator, Earth rate is zero since there is no meridian convergence.
  • The gyro spin axis will not drift in the horizontal plane at the equator.
  • At the poles, Earth rate is maximum because meridian convergence is greatest.
  • The gyro spin axis appears to rotate 360° in 24 hours.
  • This is equivalent to an apparent drift of 15° per hour.

Apparent Topple Due to Earth Rate

  • Apparent topple due to Earth rate is the perceived movement of the spin axis in the vertical plane.
  • It is caused by the Earth’s rotation and meridian convergence.
  • At the poles, apparent topple is zero.
  • At the equator, apparent topple reaches its maximum value of 15° per hour.
  • Apparent topple varies with latitude because of changes in meridian convergence.
  • Apparent Topple Due to Earth Rate = 15 × cos(Latitude) degrees per hour.

Apparent Drift Due to Transport Wander

  • Apparent drift due to transport wander occurs because of aircraft movement over the Earth’s surface.
  • The gyro spin axis appears to shift away from gyro north due to transport wander.
  • Transport wander occurs during easterly or westerly flight.
  • Flight along the equator results in zero apparent drift due to transport wander.
  • Transport Wander = Easterly Component of Ground Speed (per minute) × tan(Latitude) degrees per hour.