Hi, and welcome to Airwarfare!
Flaps work by making the air have to move a longer distance on top of the wing compared to the bottom. You could say it makes the wing more curved. The result of this is more lift, but also more drag, making it possible to fly slower. "combat" is just one of the positions you can set the flaps at (none of the names have anything to do with function)If you put the flaps on an axis, e.g. a HOTAS rotary, you can set it to any position you want, and not only the positions available with the keystrokes In combat you don't want to use flaps unless you want to make a very tight turn. Using flaps will result in bleeding off energy, just like using airbrakes or cowl flaps do. In a dogfight energy = life. Be it kinetic (speed) or potential (altitude) In general the plane with the most energy will have the best position to win the fight (not counting pilot skill which is the most important factor) It's possibly not possible to use the joystick on the game pad to control the view and make it return to center. You could use Num5 (default) for that. In general it's not a good idea to use a gamepad for flight sims either - a good joystick is a must IMHO. Prop pitch is best left on auto if the plane has auto-pitch control. Look at prop pitch as the gears of a car. If you take off you'll want a low gear (low % prop pitch). Once you're cruising or at high alt you'll want a higher gear (higher % prop pitch
Prop pitch is a very important but complex aspect of aircraft
see this wikipedia article on aircraft propellors You could write a book on it and still not touch on all aspects Keep an eye on engine revolutions. Keep those close to red-line using a combination of throttle and prop-pitch for best performance. Remember you can also use the proppitch (low %) to slow down a plane in a steep dive.If you do a search for "prop pitch" or "complex engine management" (CEM) on any aviation/flightsim forum you should find a lot of information. Be especialy carefull in the Bf 109 - you can very easyly blow your engine if you over-rev the engine when setting too much throttle with too low prop pitch. a propellor works just like a wing. It's an airfoil, so even with 0% prop pitch it will produce a forward-force. you increase prop pitch to let the prop blades "bite" more air (just like pulling back the stick does for the wings) This will need more force from the engine, so revs will drop with the same amount of throttle. When flying high you will need to increase proppitch too because the air is less dense. If you have CEM on, you will need to reduce fuel mixture too and possibly increase supercharger stage to get more air into the engine to burn the fuel. During normal flight you will rarely need anything else but 100% prop pitch in IL-2 Best setting depends on altitude, throttle, and speed (again, keep an eye on engine revolutions)
Your plane being twitchy is the result of using a joypad. If you use a joystick (with the right profile), you can deflect the stick a little to move the control surfaces a little. with a joypad/keys the control surfaces will move a lot and control is far worse