COG:
~ of a system of particles: a specific point at which, system's mass behaves as if it were concentrated
~ mass: a function only of the positions and masses of the particles that comprise the system

In some helicopter simulation formulas:
~ helicopter's body mass is assumed to be constant, fixed and concentrated in the COG
~ body mass change in time is considered as an external disturbance
~ gravity is considered to depend on helicopter's altitude

External disturbance to COG:
~ payload changes; helicopter takes new sensors, for example
~ fuel in fuel tank decreases during helicopter's mission flight

catch ~>Helicopter ~>Xi ~>Formulas!