Much of this will make more sense when you are all installed, but there have been so many additions to
IL2 through the BAT supermod that you'll want to make sure (I couldn't even say about your DE packs you mentioned earlier) that what you want to install isn't already in BAT. But if it isn't, there should be good instructions in the accompanying readme files about a couple of entries you'd need to add if you are installing new planes. What it may
not say is where to find those files. In your root game folder you'll find a folder called #WAW3. In it, you can find a folder called STD (yeah, makes me laugh a little too), and in it a folder called "com". In it another folder called "maddox". In it another folder called "IL2". And inside that one, another folder called "objects". The files you will want to alter will be in there. Yeah, I know that I could have just written STD/com/maddox/IL2/objects . . . but I'm sort of talky.
AND . . . Do not try to add planes until you are not only installed, but are all patched up to where you want to be.
AND . . . When new updates arrive for BAT, you will probably need to undo changes to the air.ini. (i.e. remove your added planes.)
The rule is (for me), keep the mods for BAT simple. I mod BAT as little as possible. The stock game, patched up to the latest release 4.15, is only about 10 gigs in size. BAT is well over 100 gigabytes. (!) You may find yourself so overwhelmed with the amount of content that you might forget all about any extra mods.
And I cannot emphasize this enough: Do
not skip reading
everything, no matter what you are attempting. Most problems can be avoided if all of the instructions are followed.
Bird
PS: Good luck, and I'm sure GenX and others will help you along your way with advice I've missed.