I'll put an update out to my trim article soon.
Basically, combat trim is a very rough auto-trim for speed, but for all speeds.

It's pretty rough, and is basically set to trim presets for different speeds. It's not exact trim like the auto trim modes, but it gets you pretty close. The beauty of combat trim is that you can use it when you like, and the second you touch the manual trim key, it kicks off. It will stay off until you use any of the other auto-trim modes, which will re-engage CT.
What is CT good for? CT is good for situations where speed is rapidly changing so long as you are in a normal configuration and not at either extreme of your speed range. That means that if you pop flaps, divebreaks, or whatever, CT won't trim you for that and you should disable it at that point. It also means that if you are either going really fast in a dive, or you are in a slow turnfight, CT isn't likely the best choice. In some situations, you may not have full elevator authority with CT enabled, especially if you are using flaps and at low speed. When dropping into a turnfight with flaps or below 200 Mph, you may be better off to dial in some manual nose-up trim which will disable CT.
Here is how I use CT. I leave it enabled, I take off, climb, cruise with CT on. I'll get in BnZ and fast E fights and leave it enabled. When you are running a BnZ and changing between 200 and 400 Mph frequently, CT is the best way to manage that. I'll disengage CT if I get in a tighter slower battle or use a notch of flaps. Then it's tike to go manual with some nose up trim. If I execute a high speed dive, I'll disable CT by tapping either nose down or nose up trim depending on the situation. By disabling CT, when the plane really gets going and I get into compression, it will be out of trim nose high and pull me out of the dive. With CT on, I wouldn't have the elevator authority to pull out, and it would keep me trimmed in the dive so the plane wouldn't fly itself out of the dive as normal.
Now... if you are a veteran flight simmer who is comfortable with manual trim and have a good control setup, you may want to disable CT and rely on manual trim and the auto modes. That will work great. If you have a more limited control setup or are not comfortable with trimming manually, use CT when and where it is appropriate. Honestly, it won't limit you much even if you leave it on all the time. Most times I forget to disable it when I should, but I'm not normally an easy kill because of it.

It's great for climb out and cruising, because you can let go of the stick and type for a bit without drifting too much.
Use CT in level bombers. There really isn't any reason not to. You should disable CT when lining up for a landing, since your flaps are not factored into the CT equasion. Use manual and auto-trim modes instead.
You may not want to use CT for A2G. Many pilots will rely on the inherant nose-up trim condition you get from diving and increasing your speed. CT can work against you in a high speed dive, and make it harder to pull out. Play with it though, the constant trimming of CT may make your weapons delivery dive a bit smoother, so you may want to use it. Don't forget though, sometimes a good way to do that is to use auto-trim for angle, which will hold you in place and trim perfectly for the speed change.
Oh, and there is one more reason to turn off combat trim. Many folks find that CT removes the "feel" of the flight model. Because the changes in speed don't really affect your trim condition as much, the plane doesn't react as you expect sometimes. If you want a realistic feel and value that, disable CT and don't use it. You can do just as well with manual trim combined with the three auto trim modes.
For more info on the auto-trim modes, I have an article here:
http://lephturn.webhop.net/trim.htm It's not updated for CT yet, but will be soon.

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Sean "Lephturn" Conrad - Aces High Chief Trainer
A proud member of the mighty Flying Pigs
http://www.flyingpigs.com Check out
Lephturn's Aerodrome for AH articles and training info!
[This message has been edited by Lephturn (edited 05-04-2001).]