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The datum (also known as station 0) is the reference point where all moment balance arms are measured. The exact position is given in the helicopter’s control and loading manual and it is located:

  • A
    At a convenient point which may not physically be on the aircraft.
  • B
    At or near the forward limit of the centre of gravity only.
  • C
    In any case at or near the natural balance point of the empty aircraft.
  • D
    Always at or near the focal point of the aircraft axis system.

Refer to figure.
The datum is an imaginary vertical plane or line from which all measurements of the arm are taken.

The datum, also known as station 0, is established by the manufacturer. This point can be located in the helicopter or at a distance from the nose of the helicopter such as small training helicopters datum. Therefore , the datum point may not physically be on the aircraft

Once the datum has been selected, all moment arms and the location of the Centre of Gravity (CG) range are measured from this point.

The distance from the datum to any component part or any object loaded on the aircraft is called the arm. Objects or components can be located in forward or aft of the datum line.

  • A load placed forward of the datum will have a negative arm and negative moment.
  • A load placed aft of the datum will have a positive arm and positive moment.

NOTE:
While the reference datum can be anywhere the manufacturer chooses, most small training helicopters have the datum 100 inches forward of the main rotor shaft centerline. This is to keep all the computed values positive.

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