Compass

Magnetic Compass Properties

    1. Horizontality – Compass should remain horizontal at all times. It is achieved by having a pendulous suspension system. Centre of Gravity of compass is kept below the pivot point. It is suspended from the Top.

 

    1. Sensitivity – Ability to detect the direction of magnetic meridian. With increasing in latitudes H decreases and sensitivity decreases, sensitivity is increased by increasing the pole strength of magnet.
      1. This is done by using many small magnets.
      2. By reducing pivot friction (by having jewelled pivots.)
      3. By reducing weight of the magnets. (by using light weight magnets)

 

  1. Apperiodicity – It should not have periodic motion. It should oscillate less. This is achieved by decreasing the length of magnets, but then the pole strength decreases, to prevent this many small magnets are used. Many small magnets increase sensitivity and small magnets increases Apperiodicity.

Damping wires are attached to a magnet and magnet is dipped in the liquid, to dampen Oscillations.

Liquid is primarily used for Apperiodicity. But it indirectly increases sensitivity as well by:

  • Reducing weight due to buoyancy
  • Lubricates pivot and decreases pivot friction

Properties required of liquid

  • Colorless Transparent
  • High boiling point
  • Low freezing point
  • Low Viscosity (High viscosity decreases Sensitivity)
  • Low co-efficient of Expansion
  • Non Corrosive

Volume of liquid is controlled by expansion ballows.

Errors:

  • Acceleration Error: Occurs on Easterly or Westerly headings. In Northern Hemisphere if a/c accelerates it causes compass to point in North direction and if a/c decelerates it causes compass to point in the South direction. In Southern Hemisphere if a/c accelerates it causes compass to point in South direction and if a/c decelerates it causes compass to point in the North direction.

The effect of Acceleration and Deceleration is nil (O) on North and South headings.

  •  Turning Errors – Occurs on Northerly and Southerly headings.

In Northern Hemisphere

A/c turning from North to Easterly or Westerly heading, compass Under reads- Undershoot the turn.

A/c turning from South to Easterly or Westerly heading, compass Over reads- Overshoot the turn.

In Southern Hemisphere

A/c turning from North to Easterly or Westerly heading, compass Over reads- Overshoot the turn.

A/c turning from South to Easterly or Westerly heading, compass under reads- Undershoot the turn.

On North heading compass is sticky, under reads the turn.
On South heading, compass is lively, over reads the turn.
No Errors on EAST and WEST headings.

Liquid Swirl Errors – Liquid Swirl Error causes the compass heading to Under read on all headings.
The effect of Acceleration and Deceleration is nil (O) on North and South headings.

Magnetic to True– Datum True for Variation Calculations.

Magnetic to Compass– Datum is Magnetic for Deviation calculations.

Gyrosyn Compass has flux valve located at the wing tip.
It does not suffer from any compass errors, therefore it has no Acceleration or Deceleration errors and no Turning errors.
Whenever there is a change of Magnetic flux through a coil, EMF is induced – Electro Magnetic Induction of EMF.
Liquid swirl error in a compass causes it to show a turn, which is less than actual turn (under read) on all headings.