yaverona,
Two points,
(a) restricting the envelope in the trimsheet is only one way to get around the error problems, although often the easiest.
(b) the problem associated with increasing numbers of seats is related to zone load planning.
A .. generating a NEW trimsheet ...
(i) if the trim is calculated on individual rows, then we just increase the size of the sheet by increasing the number of trim lines to account for additional rows
(ii) if we prefer to use zones, then there will be a tendency to expand the zone to add a row or two .. this increases the error as the range of loading distributions increases for each zone as constrasted with that presumed in the IU calculations for the zone.
B .. using the same trimsheet.
If the row distribution changes, ie individual row arms change, the same sheet can be used but with a different error analysis and different adjusted limits. However, unless the designer is very clever, this approach generally is too restrictive to be of much practical use
To answer your basic question of why we cannot (or rather, should not) use the TC envelope (the "master" envelope, if you will) is that the calculated CG in practical trimsheets is invariably wrong to some extent. At the court case after the prang, you need to be able to show that you met the legal requirement, viz., the aircraft was not outside the TC envelope limits. If you use the TC limits, then you are going to have a major problem here ...
One needs to keep in mind that the trimsheet is just a clever and convenient way to simplify doing the CG sums .. the requirement is to do whatever is necessary to stay within the TC envelope limits. Except for simple, small aircraft, almost all trimsheets will have constrained envelope limits to get around the error analysis problems.
Last edited by john_tullamarine; 29th Jan 2005 at 07:05.