ORIGINAL: ytell
Da_rock,
Even though there may be thousands of acting moments that determine CG (the entire AC) only one is changing from start to finish of the flight and that is fuel tank.
Therefore it is (i think) justified to neglect all but this moment in calculating a change in CG position.
ytell,
You cannot calculate the change of a CG position by working out one moment. Unfortunately, nobody can neglect all but one moment computation and come up with a CG location anymore than they can compare the moments of two different weights to discover the change in a CG location.
Having worked for a couple of different airlines in computer applications with experience developing Weights & Balance and Flight Planning among other things, what is involved with CG computations really is familiar to me.
You seem to be missing quite a lot of the things people are trying to tell you about your interpretation of the problem.
Do you have an example of the computations you are trying to understand? ...one that includes all the algebra you accept as adequate to predict a CG location?