how to find zfw cg and tow cg?

[airbroker]

hi guys,

i have problem about zfw cg and tow cg. i have info but i dont know how to find them. i tried to find but, it seems wrong. something missing info below i think. with this info, i can find zfw cg and tow cg?? i can find zfw and zfw index, but i cant find zfw cg.
if anyone helps me, i will be appreciated. thanks in advance.

information: it is a cargo plane, A300-C4
The length of the Reference Chord/Mean Aerodynamic Chord is 6.60806m
H-Arm of 0% RC – 28.34779m
H-Arm of 100% RC – 34.95605m
A H-Arm position can be converted into %RC through following formula: %RC= (H.arm- 28,34779)/0,0660806
RC percentage can be similarly be converted into H-arm through following formula:
H-arm = 28.34779 +(0.0660806 x %RC)
ITEM INDEX FORMULA:

a) With the CG of the item expressed in % of RC and weight in tonnes:

If CG < 25

I = 1000 + ((CG-25) x W x 0,275)

If CG ≥ 25

I = (CG-25) x W x 0,275

b) With the CG of the item expressed in H-arm and weight in tonnes:

If CG <30

I = 1000 + ((CG-30) x W x 4,1666)

If CG ≥30

I = (CG-30) x W x 4,1666

[Ex Cargo Clown]

I may be able to help, but you need to clarify your question. Are you aiming for a target MAC of 25%

[airbroker]

Thanks for reply. yes, it is what i want to find.
is it possible to find ZFW % mac, TOW %mac with this info what i mentioned before?
thank you for helping.

[john_tullamarine]

First up, if you can calculate ZFW and ZFW IU, then calculating ZFW CG is just a matter of plugging some numbers into an equation. The IU equation can be reworked to make the CG the subject of the equation - plug in the other numbers and out falls the CG.

Looking at your post, there are several obvious errors/inconsistencies which need to be sorted out before you get to the answer you seek.

(a) the LEMAC/TEMAC and the length of the MAC don't quite agree so one or more of the numbers is incorrect

(b) the discussion on IU formula is, I'm afraid, somewhat more complex than is necessary and, in respect of the entry constant, incorrect. The original documentation you are looking at, as well, may have other errors needing resolution.

The general IU equation is of the form

IU = C1 + (weight * (FS - TD)) / C2

where

IU is index unit
C1 shifts the entry IU number for a trimsheet - usually to get rid of negative numbers
weight is the total weight of the aircraft
FS is CG as a fuselage station (ie the present CG in metres)
TD is the trim datum. This is a CG reference position chosen to give a more accurate envelope graphic
(FS-TD) is the recalculated CG arm measured from the trim datum rather than from FS 0
C2 is the usual IU constant chosen to reduce the complexity of the IU numbers

The IU appears to be based on a CG of 25% MAC. The IU formula just gets massaged a little to account for the arithmetic difference between CG as an arm and CG as a %MAC.

You have given four equations for IU when there should only be one (two if you count the CG to %MAC conversion factor). CG and %MAC are the same thing expressed a little differently.

Assuming we start with the formula for CG as an arm, the 0.275 factor provides the CG to %MAC conversion, albeit with a significant round off error.

It would appear that the problem lies in misinterpreting the original paperwork somewhere along the way. Alternatively, the original material may have some errors to resolve. Perhaps you can scan the original material and post it so that we can help you resolve the problems ?