In Centre of Gravity (CG) calculations the "datum" is…
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A
The fixed reference about which moments are taken to calculate the position of the centre of gravity.
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B
The point through which the centre of gravity acts.
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C
The fixed reference about which moments are taken to calculate the position of the centre of pressure.
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D
The horizontal reference used to calculate the helicopter's empty centre of gravity.
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 more commonly at a distance forward 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 for 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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