Refer to figure.
Centre of gravity (CG) is the point on the longitudinal axis about which an aircraft would balance if it was possible to suspend it at that point.
It is the mass centre of the aircraft or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated and acts vertically downward, parallel to the gravity vector.
The CG is not a fixed point; its location depends on the distribution of weight in the aircraft. The primary concern in balancing an aircraft is the fore and aft location of the CG along the longitudinal axis.
The CG of an aircraft can move but must remain within certain forward and aft limits, for reasons of both stability and control.
CG towards the forward limit makes an aircraft more stable, and control forces will be higher which makes it less likely that large, abrupt control movements will be made.
CG towards the aft limit decreases longitudinal static stability and reduces pitch control forces, which tends to make the aeroplane easier to stall.
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