In arm calculations, distances in front of the datum are negative; behind are positive.

In arm calculations, distances in front of the datum are negative; behind are positive.

• A. Distances in front of the datum are positive; behind are negative.
• B. Distances in front of the datum are negative; behind are positive.
• C. Distances above the datum are negative; below are positive.
• D. Distances left of the datum are negative; right are positive.

Answer: (B)

In arm calculations, the datum is a fixed reference point along the aircraft. Distances measured in front of that point are assigned negative values, while distances measured behind it are positive. This sign convention makes the moment created by each weight easy to sum: multiply the weight by its arm (with the arm’s sign) to get a signed moment about the datum. We then sum all these signed moments and divide by the total weight to find the CG location relative to the datum. Why this helps: moving weight aft of the datum increases the positive moment, shifting the CG aft; moving weight forward increases the negative moment, shifting the CG forward. The datum being arbitrary means you can choose a convenient reference point, and using this sign convention keeps calculations consistent and straightforward.

In arm calculations, the datum is a fixed reference point along the aircraft. Distances measured in front of that point are assigned negative values, while distances measured behind it are positive. This sign convention makes the moment created by each weight easy to sum: multiply the weight by its arm (with the arm’s sign) to get a signed moment about the datum. We then sum all these signed moments and divide by the total weight to find the CG location relative to the datum.

Why this helps: moving weight aft of the datum increases the positive moment, shifting the CG aft; moving weight forward increases the negative moment, shifting the CG forward. The datum being arbitrary means you can choose a convenient reference point, and using this sign convention keeps calculations consistent and straightforward.