L-1011 Weight and Balance: Arm Calculation

To facilitate calculation in the development of loading system the moment is reduced to a more workable magnitude by the following index equation:

Weight x (Arm - 1216.4)
--------------------------------
10,000 lb. in. or kg. in.

What we have done in this case is to change our frame of reference from the leading edge Mean Aerodynamic Cord (MAC) to the quarter cord, 25% of the Mean Aerodynamic Cord. All movement forward of the quarter cord will be negative numbers; all movement aft of the quarter cord will be positive numbers. The reason for using the quarter cord is that it is the approximate center of pressure on the wing. The aircraft will normally be operated at the center of gravity very close to the quarter cord.

Additionally, it is by convention, that is most if not all manuals are manufactured and indexed so that they refer to the quarter cord. By using the index system we simply add and subtract Index units, taking the and resolute and ensuring that it is within the operating envelope of the aircraft.