Directional Gyro Indicator (DGI)

Wing Commander Rajagopal

Direction measurement using Directional Gyro Indicator (DGI)

A DGI (Directional Gyro Indicator) is an aircraft instrument that shows the aircraft’s heading using a gyroscope, independent of magnetic compass errors. It provides stable directional reference, helping pilots maintain accurate heading during turns, turbulence, and acceleration.

Introduction to Directional Gyro Indicator

  • DGI provides heading using a tied gyro with two degrees of freedom.
  • Spin axis of the gyro is maintained in the yawing plane of the aircraft.
  • Gyro motor is located in the inner gimbal, while readings are seen on the outer gimbal.
  • Outer gimbal can rotate through 360°.
  • Rotor axis, inner gimbal, and outer gimbal are at right angles to each other.
  • In an air-driven DGI, an engine-driven pump provides an air jet to spin the rotor.
  • The outer gimbal turns whereas the spin axis remains at its original position.
  • The readings on the outer gimbal display the heading on a lubber line.

Self Adjustment of DGI

  • A self-adjustment mechanism maintains the spin axis in the yawing plane.
  • The self-adjustment is carried out in two distinct steps.
  • Coarse adjustment takes place when a fresh air jet strikes the rotor at an angle.
  • This fresh air creates an additional component of force.
  • This force acts at 90° to the point of application.
  • As a result, the spin axis is brought back to its original position.
  • Fine adjustment is done by using a wedge plate.
  • The wedge plate separates the used air jet from the rotor unequally.
  • This causes the rotor to return to its original position.

Caging Knob of DGI

  • A spring-loaded caging knob is provided to prevent topple locking mechanism.
  • This spring-loaded switch can also re-erect a toppled gyro.
  • The button can manually synchronize the gyro with the compass heading.

Limitations of an Un-caged DGI

  • Air-driven gyro is limited to 55° pitch and roll.
  • Electrically driven gyro is limited to 85° pitch and roll.

Gimballing Error

  • Gimballing error is caused when the aircraft applies bank to turn.
  • Error is caused if the outer gimbal moves to keep the spin axis steady.
  • Pitching movement during bank aggravates this error.
  • Maximum error is seen during bank while climbing or descending.
  • In a 360° turn, this error varies with direction.
  • Zero error is seen in four directions which are 90° apart.
  • Gimballing error disappears when the aircraft levels out.

Calculation of Real and Apparent Drift while using Directional Gyro Indicator

Real Drift due to Mechanical Imperfections

  • Real wander is caused by manufacturing imperfections or wear and tear.
  • A rotor speed of 10,000 rpm has a drift rate of 1.6° per hour.
  • A rotor speed of 20,000 rpm 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 wander is caused because of four types of manufacturing imperfections:
    • Imbalance in rotor mass.
    • Imperfectly balanced gimbals.
    • Uneven rotor bearing friction.
    • Uneven gimbal friction.

Apparent Drift due to Earth Rate

  • Apparent drift due to earth rate is caused by the rotation of the Earth.
  • A horizontal gyro aligned to true north at a meridian appears to change direction.
  • This occurs due to Earth’s rotation and meridian convergence.
  • Earth rate varies with latitude because of variation in meridian convergence.

Apparent Drift due to Earth Rate at Equator and Poles

  • Earth rate is zero at the equator since meridian convergence is zero.
  • The gyro spin axis will not deviate in its horizontal plane.
  • Earth rate is maximum at the poles since meridian convergence is maximum.
  • The gyro spin axis will deviate by 360° in 24 hours.
  • Therefore, the spin axis deviates by 15° per hour.

Apparent Drift due to Earth Rate at Mid-Latitudes

  • Earth rate varies with latitude because of variation in meridian convergence.
  • Apparent drift due to earth rate = 15 × sin(latitude) (°/hour).
  • Gyro north is the north in the original meridian where the gyroscope was aligned.

Earth Rate in Southern and Northern Hemisphere

  • Southern Hemisphere:
    • True north increases with respect to gyro north.
    • Example: True north is 070 when gyro north is 360.
    • Earth rate is considered positive.
  • Northern Hemisphere:
    • True north decreases with respect to gyro north.
    • Example: True north is 290 when gyro north is 360.
    • Earth rate is considered negative.

Latitude Nut Correction

  • The latitude nut is used to correct apparent drift due to earth rate.
  • It creates real wander by varying rotor speed to correct earth rate.
  • The real wander is equal and opposite to the error caused by earth rate.
  • The setting is correct only at a particular latitude.
  • The latitude nut moves out north of the equator and in south of the equator.

Latitude Nut Movement

  • The DGI is free of earth-rate drift at the corrected latitude.
  • Latitude nut correction is inaccurate in the following cases:
    • Rotor speeds other than the designed speed.
    • Latitudes other than the designed latitude.

Apparent Drift due to Transport Wander

  • Apparent drift due to transport wander occurs because of aircraft movement.
  • The gyro spin axis appears to shift from gyro north due to transport wander.
  • Transport wander occurs during easterly or westerly movement.
  • Flight along the equator has zero transport wander.
  • Transport Wander Formula:
    Easterly component of ground speed (NM/min) × tan(latitude) = Drift (°/hour)

Transport Wander in Northern and Southern Hemisphere

  • Northern Hemisphere:
    • No effect during northerly or southerly flight.
    • Easterly drift is the same as earth-rate drift (negative).
    • Westerly drift is opposite to earth-rate drift (positive).
  • Southern Hemisphere:
    • The above rules are reversed.

Total Apparent Drift

  • Total apparent drift is the sum of earth rate and transport wander.
  • Earth Rate: 15 × sin(latitude) (°/hour)
  • Transport Wander: Easterly component of ground speed (NM/min) × tan(latitude) (°/hour)
  • Add both values if they have the same sign.
  • Subtract them if they have opposite signs.

Direction of Drift and Topple

Refer to the drift and topple direction diagram for the Directional Gyro Indicator (DGI).

Total Drift Calculation of a DGI

Use the earth rate and transport wander formulas above to calculate the total drift of a Directional Gyro Indicator.