The allowable CG ranges will be in the rotorcraft flight manual along with the 'stations' you will use in calculating the CG.
There are 3 CGs that have to be kept in range: longitudianal, lateral and vertical.
The longitudanal being the most common issue encountered, helicopters are generally loaded with more range fore and aft than side to side. The '0' datum is usually measured FROM a specified point that actually exists in the airframe, and all stations will be indexed from that, aft, so all your CG numbers will be positive. Example- an Astar pilot seat is at 61.02", no matter where it is on the seat track.
From time to time you may have to calculate a lateral CG at which point, positive and negative numbers are used for 'buttock lines' (one mustn't confuse lateral stations, buttock lines with STATIONS, you can't just call them lateral stations, no-o-o-o.). I've only had to do that a couple times in 48 years. One appreciates the imaginary reference point used for the longitudanal STATIONS.
There is also a vertical cg range, never had to calculate that. Oh, and the proper term is waterlines for those calculations.
Edited by Wally, 09 November 2017 - 23:10.