Directional Gyro Indicator (DGI)

Directional Gyro Indicator

Introduction to Directional Gyro Indicator

• DGI provides heading using a tied gyro with two degrees of freedom
• Spin axis of gyro is maintained in the yawing plane of the aircraft
• Gyro motor is located in the inner gimbal and readings seen on outer gimbal
• Outer gimbal can rotate through 360 deg
• Rotor axis, inner and outer gimbals are at right angles to each other
• In an air driven DGI, engine driven pump provides air jet to spin the rotor
• The outer gimbal turns whereas spin axis remains at its original position
• The readings on outer gimbal displays 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 fresh air jet strikes the rotor at an angle
• This fresh air creates an additional component of force
• This force acts at 90 deg 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
• Wedge plate separates the used air jet from rotor unequally
• This causes the rotor to return to its original position

Caging Knob of DGI

• 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 synchronise the gyro with compass heading
• Limitations of an un-caged DGI with the type of power source
• Air driven gyro is limited to 55 deg pitch and roll
• Electrically driven gyro is limited to 85 deg roll and pitch

Gimballing Error

• Gimballing error is caused when aircrafts 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 in bank during climb or descent
• In a 360-degree turn, this error varies with direction
• Zero error is seen in 4 directions which are 90 degrees displaced
• Gimballing error disappears when aircraft levels out

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
• 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 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
• Earth rate varies with latitude due to variation in meridian convergence

Apparent Drift due to Earth Rate at Equator and Poles

• 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 Drift due to Earth Rate at Mid-latitudes

• Earth rate varies with latitude due to variation in meridian convergence
• Apparent drift due earth rate = 15 x sin latitude in degrees per hour
• Gyro north is the north in the original meridian where gyroscope was aligned

Earth Rate in Southern and Northern Hemisphere

• In southern hemisphere, true north increases with respect to gyro north
• True north is 070 when gyro north is 360
• True north is more than gyro north hence earth rate is termed positive
• In northern hemisphere, true north decreases with respect to gyro north
• True north is 290 when gyro north is 360
• True north is less than gyro north hence earth rate is termed negative

Latitude Nut Correction

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

Latitude Nut Movement

• Latitude nut moves out north of equator and moved in south of equator
• DGI is free of drift due to earth rate at the corrected latitude
• Latitude nut correction is inaccurate in two cases
• Rotor speeds other than the designed speed
• Latitudes other than designed latitude

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

Transport Wander in Northern and Southern Hemisphere

• In northern hemisphere:
• Transport wander will not affect flights in a northerly or southerly direction
• Drift in easterly movement of aircraft will be same as due to earth rate
• Hence, drift is designated negative like earth rate
• Drift in westerly movement of aircraft will be opposite to that of earth rate
• Hence, drift is designated positive which is opposite to earth rate
• In the southern hemisphere the rules are reversed

Total Apparent Drift

• Total apparent drift due to earth rate and transport wander can be calculated
• Apparent drift due earth rate =
• 15 x sine latitude in degrees per hour
• Apparent drift due to transport wander =
• Easterly component of ground speed per minute x tan latitude
• Expressed in degrees per hour
• Earth rate and transport wander are added if they have same sign
• Earth rate and transport wander are subtracted if they have opposite signs

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