A moment is the product of a weight multiplied by its arm. The moment for a piece of equipment is in fact a torque value, measured in units of inch-pounds (in-lb). To obtain the moment of an item with respect to the datum, multiply the weight of the item by its horizontal distance from the datum. Likewise, the moment of an item with respect to the center of gravity (CG) of an aircraft can be computed by multiplying its weight by the horizontal distance from the CG.
A 5 lb radio located 80″ from the datum would have a moment of 400 inch-pounds (in-lb) (5 lb × 8″). Whether the value of 400 in-lb is preceded by a positive (+) or negative (−) sign depends on whether the moment is the result of a weight being removed or added and its location in relation to the datum. This situation is shown in Figure 4-2, where the moment ends up being a positive number because the weight and arm are both positive. The algebraic sign of the moment, based on the datum location and whether weight is being installed or removed, would be as follows:
- Weight being added aft of the datum produces a positive moment (+ weight, + arm).
- Weight being added forward of the datum produces a negative moment (+ weight, − arm).
- Weight being removed aft of the datum produces a negative moment (− weight, + arm).
- Weight being removed forward of the datum produces a positive moment (− weight, − arm)
When dealing with positive and negative numbers, remember that the product of like signs produces a positive answer and the product of unlike signs produces a negative answer.