A study of physics shows that a body free to rotate will always turn about its CG. In aerodynamic terms, the mathematical measure of an aircraft’s tendency to rotate about its CG is called a “moment.” A moment is equal to the product of the force applied and the distance at which the force is applied. (A moment arm is the distance from a datum—reference point or line—to the applied force.) For weight-and-balance computations, moments are expressed as the distance of the arm times the aircraft’s weight, or simply inch-pounds.

Aircraft designers locate the fore-and-aft position of the CG as nearly as possible to the 20 percent point of the mean aerodynamic chord (MAC). If the thrust line passes horizontally through the CG, it will not cause the aircraft to pitch when power is changed. Designers have the greatest control over the size and location of the tail; the objective is to make the moments (due to thrust, drag, and lift) as small as possible and, by proper location of the tail, to balance the aircraft longitudinally for any condition of flight.

The pilot has no direct control over the location of forces acting on the aircraft in flight, except for controlling the center of lift by changing the AOA—but such a change immediately involves changes in other forces.

Some aircraft are subject to changes in CG location with variations of load; trimming devices (elevator trim tabs and adjustable horizontal stabilizers) counteract the forces set up by fuel burnoff and by loading or off-loading of passengers or cargo.