Originally Posted by john_tullamarine
Tim,
How is the DOI Dry Operating Index calculated when used in Mass & Balance calculations?
(a) start with the empty weight, CG, and IU (or moment) which follow from
EWIU = (EW * EWCG) / convenient moment value
(b) problem is that the EW configuration is not much use for routine operations as it doesn't include all that routine stuff we put in aircraft and don't want to have to recalculate every time we do a loadsheet calculation.
(c) trick is to define operating configurations which include useful stuff. This is done just the same as a normal longhand loading calculation starting with the EW data and ending up with the data relevant to whatever configuration you are considering. There is nothing sacred about what you call any particular operating configuration or what is included in it ... dry operating is just one of many and is meaningless unless you include a configuration definition with it ....
(d) the calculation is stock standard ..
EW.................EWCG................EWIU
extra bit.....at this location.....gives you this IU
add up............ignore...............add up
(replace the dots with spaces to make it look like a sum ... ) and you get ..
operating CG = (Total IU * the convenient moment) / Total weight (mass if you prefer).
If you have decided to call the operating CG the dry operating weight CG then you would call the total IU the dry operating index.
Is the DOI the same as the no of inches from the LEMAC to the CG at DOM?
No. DOI is an index unit value related to the DOM and DOCG as above.
and can it then be calculated as the % Mac - the distance of Datum to LEMAC?
%MAC is just another way of writing the CG. Note that %MAC is not the distance from the datum to the LEMAC .. rather the distance from the LEMAC to the CG expressed in the following way
%MAC = (distance from LEMAC to CG) * 100 / (length of the MAC)
As a bit of background .. why %MAC ? The OEM design aerodynamicists like to work with CG in terms of %MAC for sensible engineering reasons. They generally get to write the flight manual and run up the OEM's loading stuff so do they do it in the easiest way for the pilots ? .. of course not, they do it in the easiest way for the aerodynamicists. Sensible operators then rehash the information to simple CG values to make life easy for operations.
"If you have decided to call the operating CG the dry operating weight CG then you would call the total IU the dry operating index."
So just to check that I got this right, the moment of the CG at DOM is the DOI?
Do you divide it by something? In my studybooks we've got dry operating indexes as three digit sums for example 45,0
Perhaps something like this?
CG = X, DOM = 34000kg, DOI = 45,0 (45000?)
X = DOI/DOM (x = 45000/34000 = 1,31 which leads to CG at 31% MAC?)
Best Regards
Tim