Which expression correctly defines CG?

• A. CG is the ratio of total weight to total moment
• B. CG is the sum of all weights
• C. CG is the ratio of total moment to total weight
• D. CG equals the total moment

Answer: (C)

The concept is that the center of gravity is the balance point along the aircraft’s longitudinal axis, found by dividing the total moment by the total weight. Moment is the weight of each item multiplied by its distance from the datum, and you sum those to get the total moment. The CG location is then the total moment divided by the total weight, giving a distance in inches (or the chosen units) from the datum.

This is the best expression because it directly converts the combined effect of all weights (moments) into a single point along the span where the airplane would balance. If you inverted the ratio (total weight divided by total moment), you’d get units of distance per pound, not a usable location. If you just summed weights, you’d only know the total weight, not where the weight is balanced. If you equated CG to the total moment, you’d have moment units, not a location.

Example: if the total moment is 120,000 in-lb and the total weight is 24,000 lb, the CG is 120,000 / 24,000 = 5 inches from the datum.