Gyroscopic Drift and Topple

Drift and Topple of Gyroscopes

Gyroscopic Wander

• Gyro wander is the departure of the gyro spin axis from its original position
• Wander is calculated with respect to surface of earth
• Gyroscopic wander is classified as drift and topple
• Deviation of spin axis in the horizontal plane is called Gyro drift
• Gyro topple is the deviation of spin axis in a vertical plane

Wander in Horizontal and Vertical Axis Gyro

• Spin axis of horizontal axis gyros are horizontal to the earth’s surface
• Horizontal axis gyroscopes can drift as well as topple
• Spin axis of vertical axis gyros are vertical to the earth’s surface
• Vertical axis gyros can only topple but cannot drift

Real Wander

• Gyro wander is classified as real and apparent wander
• Real wander consists of real drift and real topple
• Real wander is caused due to manufacturing imperfections or wear and tear
• The gyro spin axis actually deviates from its orientation to fixed point in space
• Real wander is caused because of four types of manufacturing imperfections
• Imbalance in rotor mass
• Imperfectly balanced gimbals
• Uneven rotor bearing friction
• Uneven gimbal friction

Real Drift due to Wander

• Real wander is caused due to manufacturing imperfections or wear and tear
• Rotor speed of 10 000 rpm has a drift rate of 1.6° per hour
• Rotor speed of 20 000 rpm has a drift rate of 1.2° per hour
• The gyro spin axis actually deviates from its orientation to fixed point in space
• This can be improved by improving quality of gyroscope

Apparent Drift and Topple

• Apparent wander is caused due to perceived change in spin axis of gyroscope
• Spin axis actually maintains its direction with respect to a point in space
• Observer perceives a change in spin axis with reference to earth’s surface
• Apparent wander consists of apparent drift and apparent topple
• Apparent drift is the perceived deviation in horizontal plane
• Apparent topple is the perceived deviation in vertical plane

Earth Rate and Transport Wander

• Apparent drift and topple may be caused due to
• Earth rate is caused due to rotation of earth around its axis
• Transport wander is due to movement of gyro in an aircraft

Apparent Drift due to Earth Rate

• Apparent drift due to earth rate is caused due to rotation of earth
• Horizontal gyro aligned to true north at a meridian appears to change direction
• Due to rotation of earth and meridian convergence

Variation of Apparent Drift with Latitude

• Earth rate varies with latitude due to variation in meridian convergence
• Earth rate is zero at equator since meridian convergence is zero
• Gyro spin axis will not deviate in its horizontal plane
• Earth rate is maximum at the poles since meridians convergence is maximum
• Gyro spin axis will deviate by 360 degrees in 24 hours
• So, the spin axis will deviate by 15 degrees per hour

Apparent Topple due to Earth Rate

• Apparent topple due to earth rate appears to move spin axis in a vertical plane
• Apparent topple is caused by meridian convergence and rotation of earth
• At poles apparent topple is nil or the spin axis will not deviate in vertical plane
• At equator apparent topple is at its maximum value of 15 deg per hour
• Apparent topple varies with latitude due to change in meridian convergence
• Apparent topple due earth rate = 15 x cosine latitude in degrees per hour

Apparent Drift Due Transport Wander

• Apparent drift due to transport wander occurs due to movement of aircraft
• Spin axis of gyro appears to shift from its gyro north due to transport wander
• Transport wander occurs during in easterly or westerly movement
• Flight along the equator has zero apparent drift due to transport wander
• Transport wander =
• Easterly component of ground speed per minute x tan lat in deg per hour

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