Willit Run,
Not really, I suspect. If you check a few large aircraft (manufacturer's weight and balance manuals) I suspect that you will find that the manufacturer recommends, and uses for the embodied loading systems, a second datum located in or near the envelope. The usual term is "trim datum" or something similar. Can you indicate one which doesn't for my interest, please ?
fantom,
Would we ever be at loggerheads ? ... and you have the basics covered there ..
The question of datum selection (mainly for GRAPHICAL loading systems) is VERY important and has a material implication for loading safety.
For example, many years ago I designed a trimsheet for a particular corporate jet operator, chosing a datum at a suitable mid station position. Their previous system (which was along the lines of the standard GAMA style commonly seen in the usual US light aircraft POH) had used a quite inappropriate datum for the purpose.
The first occasion on which the operator trimmed the traditional ballast requirements for local proficiency training with my sheet, I received a call to the effect that the sheet had them outside the CG limits. It took a little time to convince them that this only indicated that, for some years previously, they had been operating on such flights outside the envelope - due to the comparatively poor execution accuracy of the previous system.
The previous system's problem was due principally to the selection of an inappropriate datum and the consequences of that choice on the characteristics of the system. The system itself was adequately designed in other respects - it was just a bit inaccurate in the execution.
As a tip for pilot punters ... if the CG envelope, AS IT APPEARS DRAWN IN THE TRIMSHEET or other graphical system, has a more or less rectangular sort of boxy appearance, then the datum is reasonable. If the envelope is slanted severely then the accuracy of the calculation is compromised considerably when one contemplates the way in which the sheet is completed by the pilot, load controller, or despatcher.
It comes down to two main considerations -
(a) ignoring the fact that the designer usually compresses the permissible AFM CG limits for error analysis reasons, the physical dimension on the sheet (at a given weight) between the forward and aft limit (as drawn) represents (as a moment change) the allowable CG range at that weight.
If the selected datum is FAR FORWARD of the envelope, then the envelope, as drawn, leans over from the lower left hand corner to the upper right hand corner. As the selected datum MOVES AFT, the envelope, as drawn, rotates anticlockwise, becoming more erect (upright). If the datum moves further aft to a position FAR AFT of the envelope, then the envelope, as drawn, becomes something like a mirror image of the far forward case - it leans over from the lower right hand corner to the upper left hand corner.
The physical dimension between the forward and aft limits, which represents the permissible CG, can be drawn WIDER on the paper if the datum is located within or near the envelope.
Considering the way the system is used, this means that the error in plotting the loading solution can be minimised or, if you prefer, it is much more accurate, presuming a modicum of care is taken in the execution of the sheet.
(b) a centrally located datum means that individual trim lines can be drawn for SMALLER load increments, for much the same reasons as apply in (a). This means that the moment change calculated by individual trim lines can be plotted more accurately.
. .The end result, other things being equal, is that a centrally located datum gives you a trimsheet which has the potential to be near as accurate as a longhand calculation for all practical considerations.
Based on relevant error analyses I did as a young chap when I was teaching myself how to design trimsheets (and I have designed a great many over the years), a good trimsheet has the potential to give a CG calculation accurate to better than 0.1 inch. Considering the inaccuracies inherent in loading calculations generally, and there are many, this level of accuracy exceeds the expected physical, if not the numerical, accuracy of a longhand calculation. It is very important to distinguish between these two aspects of accuracy - what the calculator display indicates really has not much to do with the real world.
[ 07 February 2002: Message edited by: john_tullamarine ]</p>