Direct Indicating Compass (DIC)

Wg Cdr Rajagopal

Direct Indicating Compass

Direct Reading Compass

Direct reading or indicating compass is the simplest type of aircraft compass
DIC uses horizontal component of magnetic field
The maximum permissible deviation on any heading is less than 10 deg
Horizontality, Sensitivity and Aperiodicity are required in a good compass

Construction of Direct Reading Compass

Direct indicating compass consists of a liquid filled compass bowl
The magnetic assembly consists of a compass card in a pivoted assembly
A lubber line in outer casing provides the reference direction
The lubber line is aligned with longitudinal axis of aircraft

Horizontality

Horizontality is the ability to measure horizontal component of earth’s magnetic field
Magnetic field can be resolved into horizontal and vertical components
Horizontal component is the directive force to determine magnetic north
Vertical element is undesirable and induces errors
Magnetic dip is the angle between the earth’s magnetic field from horizontal
Dip angle is zero at equator, magnet is horizontal to surface of earth
Dip angle is 90 degrees at poles. magnet is vertical to earth’s surface

Pendulous Suspension

Magnetic compass uses pendulous suspension to improve horizontality
Keeps the magnet inside the compass horizontal to surface of earth
Pendulous suspension reduces the effect of vertical component
Centre of gravity is designed to counter act the vertical component
Reduction of vertical component brings the magnet back to the horizontal
This results in increasing the effect of horizontal component

Sensitivity

Sensitivity is the ability to accurately point to magnetic north
Earth’s magnetic field and pole strength of compass determines sensitivity
Sensitivity increased using multiple short magnets
Multiple magnets produce higher magnetic flux
Iridium tipped pivot in jeweled cup increases sensitivity
Reduces friction at the pivot points
Liquid filled compass cup for reduces weight, increasing sensitivity

Aperiodicity

Aperiodicity is the ability to quickly stop oscillations of compass card
Damping of oscillations in compass improve its aperiodicity
Use of multiple short magnets increase damping effect
Damping is also increased by using viscous liquid in compass bowl

Acceleration Error

Acceleration of aircraft even along a straight line causes acceleration errors
Red end is pulled towards the nearer pole due to acceleration error
Acceleration error is zero at equator due to zero dip angle
Near poles magnetic dip as well as acceleration errors are high

Acceleration Error varies with Magnetic Dip Angle

The primary cause for acceleration errors is magnetic dip angle
Magnetic moment of force passes through compass pivot point
Moment due to its reaction acts through centre of gravity
Dip angle causes these two forces to act at different points of the magnet
These two forces cause a moment resulting in acceleration error

Acceleration Errors

No errors due acceleration in northerly or southerly directions
Acceleration as well as moment are in line with each other
Maximum errors are seen while accelerating in westerly or easterly directions
Acceleration and moment are perpendicular to each other
Acceleration errors depend on four factors
Heading of the aircraft
Magnitude of acceleration
Design of magnet system
Magnetic latitude

Acceleration Errors in Northern Hemisphere

As a general rule, acceleration shows an apparent turn towards the nearer pole
Acceleration shows an apparent turn towards the north pole
Accelerating in westerly direction over indicates the heading
Accelerating in an easterly direction under indicates the heading
As a general rule, deceleration shows an apparent turn towards the equator
Deceleration can be considered to be acceleration in opposite direction
Decelerating in westerly direction under indicates the heading
Decelerating in an easterly direction over indicates the heading

Acceleration Errors in Southern Hemisphere

As a general rule, acceleration shows an apparent turn towards the nearer pole
Acceleration shows an apparent turn towards the south pole
Accelerating in westerly direction under indicates the heading
Accelerating in an easterly direction over indicates the heading
As a general rule, deceleration shows an apparent turn towards the equator
Deceleration can be considered to be acceleration in opposite direction
Decelerating in westerly direction over indicates the heading
Decelerating in an easterly direction under indicates the heading

Turning Errors

Turning errors results in error in compass indications during turns
Error is maximum in turns through magnetic north or south
Error is minimum in turns passing through east or west
Turning errors are caused due to magnetic dip resulting in a moment
The error increases with increase in magnetic latitude
At the magnetic equator, the only turning error is due to liquid swirl
Liquid swirl creates a heading lag of 5 degrees

Clockwise Turns in the Northern Hemisphere

Right hand or clockwise turns through north slows down magnet’s movement
Clockwise turn from 315 to 045 through north makes compass sluggish
By the time compass indicates 045 the actual heading is already 065
Compass under indicates hence stop turns early
Right hand or clockwise turns through south speeds up magnet’s movement
Clockwise turn from 135 to 225 through south makes compass brisk
By the time compass indicates 225 the actual heading is just 205
Compass over indicates hence stop turns late

Turning Errors Summary

Whenever the pilot turns through the nearer pole
Aircraft and compass rotate in the same direction makes compass sluggish
Pilot should undershoot the turn or roll out early
Liquid swirl will increase turning error
Whenever the pilot turns through the further pole
Aircraft and compass rotate in the opposite direction makes compass lively
Pilot should overshoot the turn or roll out late
Liquid swirl will reduce the turning error

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