Centre of Gravity.

[stormcloud]

In a modern jet, do you want the C of G forward of or to the rear of datum and why?

[john_tullamarine]

Stormcloud,

The simple answer is that the CG position in relation to the datum is irrelevant other than in regard to the actual calculation itself. The following may be useful in explaining this statement.

Some background information, applicable to any aircraft -

(a) a manufacturer will publish station data based on convenient datum locations chosen for the convenience of the design and manufacturing organisations. The convenience of the operator is of little or no concern at this stage.

(b) there is no intrinsic value in any particular datum locations and they can be chosen on a basis no more rigorous than the whim and/or preference of the design organisation, except that lateral arms normally are measured from the aircraft centreline. In effect, the various datum locations can be viewed as being no more than nails hammered into the aircraft from which one can hang a tape measure to measure distances to any loading location of interest .. no more, no less.

(c) so far as the longitudinal arms are concerned (and this is what you are considering), the datum can be located anywhere along the aircraft from somewhere out in front of the nose through to somewhere out behind the tail, although I can't think of any aircraft where the manufacturer has chosen a datum aft of the wing.

(d) when it comes to doing pilot type loading calculations, there are two principal considerations

(i) if the CG calculation is to be done by doing a bunch of sums (either longhand or within a computerised reservation and loading system) it doesn't really matter what the datum location is (albeit that, for longhand calculations, the choice of datum can materially affect the ease of manual longhand calculations)

(ii) if the calculation is to be done manually using a graphical loading system (usually a trimsheet of some sort or other) then the errors implicit in the design and operation of the system will be minimised if the datum chosen FOR THE SHEET (and this datum can be quite different to that used by the manufacturer for design purposes) is located somewhere near or inside the CG envelope.

For light aircraft, the best location typically is the aft limit and for heavy aircraft the preferred location will depend on the shape of the envelope. Typically, in this latter case, it will end up somewhere inside the envelope.

For larger aircraft, the manufacturer often nominates a sensible second datum for trimsheet use, typically called the trim datum or some similar term. There is no particular need for the operator to use the manufacturer's nominated datum and, in many cases, the operator's trimsheet will be based on yet another datum chosen for the operator's particular reasons. Apart from a consideration of errors in the design and execution of a trimsheet, there is no reason why one datum position is better than another.

. .So it follows that the CG location for flight purposes has no significance with respect to the datum other than to the extent that the calculated position has to be within the relevant limits specified either in the AFM or, more usually, the constricted limits published in the trimsheet.

I suspect what your question is trying to determine relates more to the desirable CG for flight. If this is the case, then the main loading emphasis ought to be directed toward achieving a CG which gives minimum trim drag for a significant portion of the cruise.

The manufacturer usually provides some guidance in the loading documentation or, at the very least, in information provided for the use of the operations engineering people. Typically this turns out to be somewhere toward the rear CG envelope limit.

For longer range aircraft, attention to trim drag in load planning can add up to quite a significant cost variable. For instance, I am aware that one 744 operator which used the aircraft on payload limited sectors looked at a revised fuel use schedule to see if an improved CG control in flight and its effect on trim drag could be converted into sensible dollars. The end result was something like a sector fuel saving in the region of 150 kg (ie an extra paying customer or two) if my recollection is correct. On an annualised basis this represented a potential profit increase in the order of USD 1.0M ... not to be sneezed at, I would have thought. Even if my recollection of the specific numbers is a little flawed, the principle remains valid.

[Captain Airclues]

stormcloud

For fuel efficiency an aft c of g is preferable as it requires a lower stabiliser setting. The closer the stabiliser setting is to zero, the lower the drag.. .For stability, a forward c of g is preferable as it gives a longer moment arm between the c of g and the tailplane.

Airclues

[ 03 February 2002: Message edited by: Captain Airclues ]</p>

[stormcloud]

Many thanks.

[SentryIP]

stormcloud,

What do you mean by "datum?" On all Boeing airplanes I've flown, the "datum" when talking about the center of gravity (CG) is a point slightly ahead of the nose of the airplane from which all calculations for the location of the CG are made. Are you refering to the locations of the CG and center of pressure (CP)? Regards.. . <img src="cool.gif" border="0">

[Willit Run]

The Datum can be located anywhere. Having the datum located in front of the nose just gives the ease of using all positive numbers when doing the calculations. Many smaller planes have the datum located somewhere within the length of the fuselage, but now you have to do your math with negative and positive numbers and that makes mistakes more prevelant, so the engineers have mostly made big planes' datums slightly infront of the nose.. .Airclues is right, having an aft loaded CG is preferable for fuel consumption, especially on long flights, it can mean a significant savings in fuel.

Hope all this helps!

[fantom]

studiously avoiding a contretemps with J-T...in lay terms:. .an aft CG (never mind the datum,we are talking 'relative' here)makes the tail want to lift up - to keep the thing balanced. tails do not provide positive lift (except in eurofighter) but they can be encouraged to donate less down-lift. aft cg will do this.. .a fwd cg will require the tail to provide 'down-lift'. this increases the Apparent All Up Weight. the wings have no idea what is going on, they just provide the rqd lift, and so the total drag increases. hence more fuel rqd. to hand fly an aicraft with fwd cg is very pleasurable as it is more stable (think of throwing a dart - big weight at the front=good:big weight at the back=bad).my 330 shoves fuel backwards and forwards as it sees fit to maintain the cg in fuel-economy-trip-mode.no idea how it works though....... .QED <img src="tongue.gif" border="0">

[john_tullamarine]

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>

[Checkboard]

The reason for choosing a datum forward of the aircraft is for numerical calculations, all of the moments are positive, eliminating a possible sign error. (The moment arm forward of the datum is negative, so if the datum is forward, even located forward of the nose, then all moments are positive, as you can't load forward of that datum.)

For graphical load sheets, as indicated by John, you can get a better graphical representation by choosing a different datum, and on these sheets you will see different directions of movement for, say the forward locker as compared with the rear locker. It's a pay-off between the ease and acuracy of the graphical sheet, compared to the risk of error in moving the wrong way on the "loading bar".

[fantom]

see? it can be found.....a gracious ozzie. <img src="wink.gif" border="0">