Drag index of a store is a shorthand way of estimating the drag of a store at a typical cruise mach number. The basic airplane has a drag index, and each piece of added equipment (pylons, tanks, bombs, pods, etc) had a drag index. When all the drag indexes (indicies??) are added, the complete configuration drag index is found.
The drag on a body is drag coefficient x dynamic pressure x reference area, or D = Cd x q X S. To get the total configuration drag, you could look up the Cd for each item (store, pylon, pod, basic airplane), look up the reference area for each item, calculate the drag on each item, than add all the drags together. A simpler way is to use the same reference area for all elements, add all the coefficients (drag indexes) together, and calculate the total drag.
There should be a list of store drag indexes somewhere in the stores TOs, but if you can't find it or if the store isn't listed, all is not lost.
A drag index for a clean F-16 is the drag coefficient at 0.8 mach using the reference area of the F-16 (300 sq. ft.). So what is a drag coefficient? Using a clean F-16 as an example, say the drag at 0.8 mach, sea level, is 8000 pounds.
D = Cd x q x S
so Cd = D / qS
D = 8000 lb
q at .8 sea level is 947 lb/ sq ft
S = 300 sq ft
so Cd = 0.0282
The basic airplane drag index would be 282, because it is more convenient to use whole numbers instead of all those decimal places.
To calculate a drag index for a store, first you have to find its drag coefficient based on it's cross section area. That information (Cd) may be difficult to find, but the manufacturer should be able to give it to you. If that fails, then use 0.15 for pointy store (Mk-82) or .20 for a blunt store (AGM-65) as a good estimate for Cd. Then calculate the Cd based on the F-16 reference area, 300 sq ft.
Cd (F-16 ref area) = Cd x (store cross section area / 300 sq ft). This value, after removing the 4 decimal places, is the store drag index.
Another way to estimate a store drag index is to find another store of similar shape, and use its drag index multiplied by the ratio of the store cross section areas. For example a Mk-84 and a Mk-82 have similar shapes. If you know the -84 drag index, (DI-84), you can estimate the DI-82 :
DI(82) = DI(84) x Area (82) / Area (84)
I apologize if I've gone into more detail than you wanted.
There is some interesting information in Table A4, F-16C. Compare the drag index for 2 - 370 tanks with 2 TERs, with 2 - CBU-87, and with a Lantirn pod. You will see that they all have about the same drag. Those tanks must really be slick.
Drag index is of course referenced to the wingspan of the aircraft on which it is being carried, so the drag index of a MK-82 on an Eagle will be different than on a Viper. This was mentioned but not emphasized.
<b>"It's time to get medieval, I'm goin' in for guns"</b> - <i>Dos Gringos</i>
Drag coefficients, like all force coefficients, are referenced to wing area. So a store drag index is also referenced to wing area. Moment coefficients are referenced to wing area x MAC. But you are correct about a store having a different drag index for different airplanes.
After thinking for a while, I remembered that stores have different drag indices at different stations on the wing. The formula above does not cover the station effect.
Also the drag in these calculations should be profile drag according to a source I read; do you agree?