Engine replacement: A Model B engine weighing 175 lb was replaced by a Model D engine weighing 185 lb at -62 in station. Previous empty weight was 998 lb and empty weight CG was 13.48 in. What is the new empty weight CG?

Engine replacement: A Model B engine weighing 175 lb was replaced by a Model D engine weighing 185 lb at -62 in station. Previous empty weight was 998 lb and empty weight CG was 13.48 in. What is the new empty weight CG?

• A. 13.00 inches
• B. 12.50 inches
• C. 12.73 inches
• D. 12.90 inches

Answer: (C)

Weight and balance use the total moment divided by the total weight to get the empty weight CG. When you replace an engine, you adjust both weight and moment by removing the old engine’s contribution and adding the new engine’s at its location. Start with the original: weight 998 lb and CG 13.48 in give a moment M0 = 998 × 13.48 = 13453.04 in-lb. The old engine contributes a moment of 175 × (-62) = -10850 in-lb; the new engine contributes -11470 in-lb. New total weight = 998 - 175 + 185 = 1008 lb. New total moment = M0 - (-10850) + (-11470) = 13453.04 + 10850 - 11470 = 12833.04 in-lb. New CG = 12833.04 / 1008 ≈ 12.73 inches. The new empty weight CG is about 12.73 inches.

Weight and balance use the total moment divided by the total weight to get the empty weight CG. When you replace an engine, you adjust both weight and moment by removing the old engine’s contribution and adding the new engine’s at its location.

Start with the original: weight 998 lb and CG 13.48 in give a moment M0 = 998 × 13.48 = 13453.04 in-lb.

The old engine contributes a moment of 175 × (-62) = -10850 in-lb; the new engine contributes -11470 in-lb.

New total weight = 998 - 175 + 185 = 1008 lb.

New total moment = M0 - (-10850) + (-11470) = 13453.04 + 10850 - 11470 = 12833.04 in-lb.

New CG = 12833.04 / 1008 ≈ 12.73 inches.

The new empty weight CG is about 12.73 inches.