Tweaking the aircraft.cfg is easy of course, but let's put it into perspective. Then I will point you to an aircraft.cfg line by line explanation, that is the Aircraft Container SDK, and Ron Freimuth's line-by-line explanation he did back before SDK's were made available (good tweaking notes there).
First, let me say that I am not an aeronautics engineer, nor an expert of flight modeling. I do use the tools to create flight models and know a little about FS flight model relationships. After 9 years exposure, I still rely on information posted by the late Ron Freimuth, Ian, Herve Sors, Jerry Beckwith, Tom Falley and others who really specialize in the inner workings. With that said, I offer what little I can here from a user perspective. I hope that if I state anything wrong that one of the aforementioned gentlemen will correct it.
The .air file contains the necessary data to drive the flight sim's flight dynamics engine to respond the way your model should in the real world if defined correctly. Since that same FD engine must simulate a 747, a Cessna, an ATR500, and a hang glider, the .air file defines the attributes to the FD engine so proper responses to input can be made.
The aircraft.cfg file gives you and me an easy way to manipulate some parts of the .air file easily. Actually, there are many more tables of data in the .air file that can't be modified than can be modified this way.
Air files do seem to be confusing to us all. However, with research, a knack for aerodynamics and the mechanics of flight, lots of patience and interest, you can overcome the "mystery".
It may help to put .air files into some kind of visual/graphic reference to help debunk some misconceptions. The attached graphic depicts an air file with its many data cells necessary to drive the sim's flight dynamics engine. The various colors defined in the legend show why we get so confused at times.
As Flight Sim evolved, more data cells were added, some stayed behind (no longer used - like placeholders) for compatibility for the older sims, some used by all, some not at all.
When loading your aircraft, the FSim modifies these tables with data you specify in the aircraft.cfg. These memory cells are then used as input to the flight dynamics engine.
Hope this helps get a visual of the air file concept, its general makeup, and interaction with the aircraft.cfg.
There are 100's of tables of data, some multi-dimensional.
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You can download SDK's to help understand the aircraft.cfg (Aircraft Container SDK) and generating and maintaining air files (FSEdit SDK) here. These help with the basics and get you started with a template approach. Later, you may wish to get more control of your flight model.
http://support.microsoft.com/kb/555857/en-us
Hervé Sors has a great site with tools and documents, and links to help you understand and manipulate flight models more in-depth. Be sure to get AirEd, Ron's aired.ini updates,
Ron's aircraft.cfg file explanation, and Tom Goodrick's .pdf on the basics of flight dynamics. For testing, Herve's AFSD test program gives real time feedback on the flight model. This is a great site to start collecting information and tools:
http://hsors.pagesperso-orange.fr/index.html
If you truly wish to pursue flight modeling and do the research and get the tools, AirEd is a nice tool to allow changes to the .air file raw data. Here are a couple shots of the internals. You will see things that are in the aircraft.cfg and many that are not.
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These early tables have been around since early days of flight sim. Many of these early table entries can be overridden by the aircraft.cfg. For example, table 1004 is contact point data.
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You see here many of the core tables that cannot be modified by the aircraft.cfg. Also on table 1101, there are many of the key table elements that one will modify such as lift, drag, pitch, yaw, side forces, etc. Some of these table elements may be overridden by other tables further down the air file (1525-1547 tables depending on which sim), if present. Also, some of these 1101 table influences can be affected by the Flight Tuning section of the aircraft.cfg.
If you are familiar with aerodynamics terms, much of this will be familiar to you. You will also notice gaps in table numbers as table series usually represent a grouping or category of data, not necessarily old or outdated tables. Table definitions here are provided by AirEd and the AirEd.ini description file by Ron Freimuth.
Other notes:
1. Some table data is overridden by aircraft.cfg data, if specified.
2. Some tables, if present, override other tables, depending on which FS version is used.
3. Some table data, if not specified in the air or cfg files, is calculated by the sim at run time.
4. Unless you restrict it, flight sim will update some fields in the aircraft.cfg file at run time. In the aircraft.cfg, General section, Editable = 0 turns off updates
5. If invalid data is specified, it may be ignored or recomputed.
6. The lack or either an .air or .cfg file will keep your aircraft from loading.
Today, I prefer to use higher level tools to create a flight model to hit the performance numbers. Once you have performance right, the rest seems easy. One example of a great tool is Jerry "Sparks" Beckwith's "AirWrench". Simple, easy to use, and produces nice results. Jerry's website:
http://www.mudpond.org/ has the AirWrench tool and other helpful tools, a test panel, guidelines for props tables, gear and suspension setup, and the Flight Dynamics Workbook, a pre-cursor of sorts to the AirWrench tool. I like the FDW MS Excel spreadsheet because it gives you a technical look and graphics of the tables along with the lists. It's a bit harder to use (and can be frustrating at times) but the internals and relationships are there to see. You must have the Excel spreadsheet software to use this, and remember that this software is no longer supported. Still a great tool.
Another helpful tool to manipulate raw data is the ACM package (if you can find it), or its sub-system AAM for air file manipulation. It gives you graphic representation of some tables and allows for easy manipulation to smooth curves. It also gives explanations of the data elements and how some are calculated.
this simply because there is good useful info here.