Correct calculation of longitudinal and lateral CG positions of a helicopter and verifying that these positions are within pre-determined zones by the manufacturer is vital for the safe operation of a flight.
The reference datum is a reference plane that allows accurate, and uniform, measurements to any point on the aircraft. The location of the reference datum is established by the manufacturer and is defined in the aircraft flight manual. The horizontal reference datum is an imaginary vertical plane or point, placed along the longitudinal axis of the aircraft, from which all horizontal distances are measured for weight and balance purposes.
For helicopters, it may be located at the rotor mast, the nose of the helicopter, or even at a point in space ahead of the helicopter. While the horizontal reference datum can be anywhere the manufacturer chooses, most small training helicopters have the horizontal reference datum 100 inches forward of the main rotor shaft centerline.
The arm is the horizontal distance from the reference datum to the center of gravity (CG) of an item. The algebraic sign is plus (+) if
measured aft of the datum or to the right side of the center line when considering a lateral calculation. The algebraic sign is minus (-) if measured forward of the datum or the left side of the center line when considering a lateral calculation.
The moment is the moment of force, or torque, that results from an object's weight acting through an arc that is centered on the zero point of the reference datum distance. Moment is also referred to as the tendency of an object to rotate or pivot about a point (the zero point of the datum, in this case). The further an object is from this point, the greater the force it exerts. Moment is calculated by multiplying the weight of an object by its arm.
Station is the same as arm. For instance, Station 20 means that the item is positioned 20 units (of distance) aft of datum while Station -20 means that the item is positioned 20 units (of distance) forward of datum.
The CG calculations are made with two simple formulas:
The first step of finding the CG of an aircraft is calculating moments of all the load and equipment of the helicopter. This calculation can be done by multiplying the items weight and their distances from the datum.
The second step is dividing the sum of the moments by the total weight of the aircraft.
Since the moment calculation of all the equipment of a helicopter would take a lot of time, operators provide the moment of the empty weight.
Every aircraft is periodically weighed and its weight is listed with each structural modification order, or any configuration order which may alter the initial value given by the operator.