Lesson Eight - PA-18 Weight and Balance Mystery Solved
Weight: Simple, it is the amount in pounds an object weighs.
Arm: Not the one extending from your shoulder. It is the distance in inches measured from the Datum.
Moment: Is not a measurement of time, i.e. “Just a moment.” It is the product of multiplying the “W” (weight) “X” (times) the “Arm” = Moment.
Hence the formula: W X Arm = Moment.
Total of all Moments added together, divided by the total weight = “CG” (Center of Gravity).
Datum: Is the point in space that the measurements for the arms are located. It can be located wherever the manufacturer decides.
You are accustomed to Cessna’s and other airplanes' CG graphs that simplify this issue of weight and balance.
Piper, in its infinite wisdom, decided when designing the Super Cub (PA-18), to place the Datum 60” ahead of the “Wing Leading Edge”. Then to add confusion, the CG limits or range are also in relation to the Wing Leading Edge.
For N917VK the CG ranges on floats at max gross weight of 1760 pounds is 16.2 inches to 20 inches aft of the wing leading edge. At 1410 pounds it is 11.1 inches to 20 inches aft of the wing leading edge.
Bottom line, for you to understand the PA-18 weight and balance solutions, always, after dividing the total moments by the total weights, subtract the 60” inches from that number to determine if your aircraft is within proper CG range.
Note: The CG range varies with the gross weight of the PA-18. N917VK’s gross weight is 1760 pounds. N645DT’s gross weight is 2000 pounds. An STC from Whipaire, the manufacturer of Whipline Floats, makes this possible.