View Full Version : 737 MAC equations?
PanicButton
29th Jul 2003, 05:51
This is regarding the 737-300.
I was wandering, if I have the TOW, the ZFW and the index units how can I calculate the % MAC. I'm looking for a formula.
:(
john_tullamarine
29th Jul 2003, 18:21
Two separate and sequential things on the go here ..
(a) from the IU and weight one can calculate CG (or fuselage station, depending on your preference). All depends on the way the loading system has been designed but a reasonably general formula for IU is
(FS - datum) * weight
--------------------- + C1 = IU
C2
from which you can calculate the CG (FS). (I have had to turn the formulae around as I have not quite worked out how to force PPRuNe to accept leading blanks ... )
where
C1 is a constant to shift the IU to give only positive numbers (we all hate minus numbers)
datum is a reference FS location chosen for reasons best known to the loading system designer but very important when it comes to accuracy in graphical loading systems
FS is the specific fuse station in question (or CG location)
(FS-datum) is the "arm" for the loading system related to the designer's chosen datum position ... be VERY careful with loading systems using a datum different to that nominated by the OEM ... not a problem .. but loads of potential for errors if you don't know what you are doing ... The principal rule is to stick to the one set of measurements based on the preferred datum .. mix it up at your peril ....
C2 is the conventional moment > IU divisor
The above might not satisfy your need as the specific answers depend very much on the particular loading system with which you are playing.
(b) When you come to calculating %MAC, it is a matter of relating the actual CG to a corresponding proportion of the MAC
(FS - LEMAC)
------------- * 100 = %MAC
MAC
where
LEMAC is the arm of the leading edge of the MAC (OEM or trim datum chosen by the loading system designer measures as appropriate ... again, mix the datum positions up at your peril ..)
MAC is the length of the MAC
Obviously you can join the two calculations together to get the final answer in one go ... but it aids understanding to see what we are trying to do.
I don't have my 737 weight and balance manuals to hand at present so I am not able to plug in the specific values for the MAC calculation.... however, the above may be of use as background.
It is important to keep in mind that different loading system designers will design things differently according to their particular preferences, risk and error assessments .. so one needs to be careful playing with formulae ....
john_tullamarine
11th Aug 2003, 14:42
TCDS gives the figures for MAC so a suitable equation is going to be
%MAC = (FS-625.59)/134.5 *100
(.. at least the Americans still use real numbers ....)
Try this (http://www.zapops.com/header_new/b737ahm50.pdf)
Its in .PDF format, the equation & explanations on page 3.
john_tullamarine
13th Aug 2003, 06:01
.. what a dreadfully long-winded loading system document.