One thing important to remember in space flight sims is that there is no down. Or if you want to think of some direction as "down", you should pick direction of enemy capital ship or group, because that would be a direction you want to avoid (similar to how in flight simulators you always want to be higher than your adversaries). Thinking that way you would perhaps have a better tendency to stay close to friendly capital ships which is a major help especially in higher difficulties. Always orienting your ship so that the FRED "plane" works as "ground level" is not beneficial at all and can hinder your situational awareness because subconsciously you're still thinking in terms of up and down when you do that.
First reason why this affects the combat is the lack of gravity. Lack of gravitational acceleration and hard deck to crash to both frees and limits tactical opportunities. First of all, energy tactics are largely negated. Ship speed is still important, but all ships perform the same at all locations, as opposed to high altitude fighters vs. low altitude fighters - say, Focke-Wulf FW-190 D-9 vs. Lavochkin La-7 fight would have vastly different outcome depending on starting positions and altitude. the FW-190 excels at high altitude and energy tactics, so it's pilot needs to fly it high and fast, dive on the opponents and pull back up after each firing run to convert kinetic energy back to potential energy. On the other hand, La-7 pilot has superior performance at below 3000 metres and also superior turn rate, so if he can jump the FW-190 pilot with "pants down" (ie. with low airspeed and altitude), the fight will be a short one to La-7's advantage.
No such things to consider in FreeSpace2 and other space simulators. Combined with the lack of accurate physics to speak of (conservation of momentum), FS2 tactics are not energy oriented at all. In games with more accurate physics modeling you could still utilize energy tactics if you have more acceleration than your target, but in FS2 the maximum speed of ships is what matters more. And in most cases the differences in that speed are not big enough to make "hit and run" tactics viable at all. That means extending (getting some distance between you and the enemy) usually doesn't work unless your enemy is willing to do it as well - however he would likely be a fool to turn his tail at a fleeing ship only slightly faster than his own! As a result - unless you're in a Horus against Ares or Ursa, running is pretty much equal to suicide in vast majority of occasions. And it opens the opportunity for your enemy to use missiles against you...
Another thing that lack of "down" changes is the perspective and flight dynamics which many people fail to acknowledge. Airplanes are designed to be turned so that you first roll to the direction you want to turn to (bank angle depending on the type of the turn), then pitch up to compensate for the loss of lift in order to stay at constant altitude, and use rudder to stay straight in relation to the air stream. Space ships in reality have to use other techniques to change their vector altogether, but in FS2, ships just turn where their nose is pointed and that opens a lot of options on how you can maneuver your craft. Regardless of this, vast majority of people that engage in turning dogfight tend to pitch up to follow the target, when they could just as easily pitch down (considering the lack of viewable cockpits) or yaw left or right. This tendency can make your flying easy to predict. Flying in more erratic and unpredictable manner will confuse your enemies and make it much harder for them to predict your flight path and stay on your tail, never mind landing shots at you. Remember to use the roll as well even though you don't technically need it for aiming and flying; it's much harder to aim at a target doing even a slow barrel roll than just turning.
Another thing you want to be able to do in a furball is to control your speed so as to not overshoot your opponent or collide with him... collisions in multiplayer are lethal. Personally I usually match the speed of the target, then map W to afterburner and S to rearward thrusters (the ones that slow your ship down).
Otherwise, I still find the Dicta Boelcke very much applicable to FS2 combat:
1. Try to secure the upper hand before attacking. If possible, keep the sun behind you.
As discussed, there are less things that can secure an advantage before engagement in FS2 than in real life aerial combat. The concept of energy in form of altitude is negated, so advantages would be the following:
-surprise
-performance
-numerical superiority
Surprise is basically exploiting someone's lack of situational awareness. This is difficult in FS2 because the targeting computer can target someone who's merely targeting you. That's why it's good to learn to use just the radar dots to engage enemy targets; you won't get to use aspect missiles, and you won't have primary lead indicator either, but you will not give any warning to the enemy before you're on top of him. Hopefully.
Performance includes all the stats and characteristics of your craft. Using your ship's aspects that have superior performance is key to using any ship effectively. Knowing what your ship can do better than the enemy ship is important, so study the stats and characteristics of each ship. Manueverability, speed, armour, weapons systems, energy system... knowing these can and will make a difference in your decisions on how to engage certain kinds of ships depending on what ship you are using.
Numerical superiority - d'oh. In combat of any sort, 2 >> 1+1 as long as you can work as a team instead of two individuals. You would be surprised at what a difference having a good wing man makes in IL-2 Sturmovik - I would imagine with some practice same would hold true for FS2 as well.
Position in the field of engagement can be a powerful advantage; using cruisers and corvettes for cover is very effective. Also, hiding in the sun is actually possible as long as you try to use surprise tactics by not targeting your target...
2. Always continue with an attack you have begun.
As discussed, turning your tail and running is usually not a viable option due to lack of usable energy tactics and small speed differences between the ships. When engaged in a furball, it's usually a fight to the end unless either participant has support coming.
3. Only fire at close range, and then only when the opponent is properly in your sights.
Holds true, but less than in the WWI and WWII aerial combat, as ammunition regenerates in form of energy. Still, you do not want to be missing a lot with double Kaysers. You'll end up running out of energy surprisingly soon that way. Also, spraying and praying will still possibly alert your target to your presence when you are attempting to sneak up on them.
On the other hand, FS2 damage modeling makes individual hits much less important than in reality and games such as Red Baron or IL-2 Sturmovik. If you score a few hits in the enemy in reality, there's a relatively high chance that it will destroy something important and the enemy will likely attempt to withdraw from the engagement as soon as possible and return to base to assess the damage. Subsystem damage modeling in FS2 is sort of futile attempt compared to, say, IL-2 damage modeling, and the most important factor remains health points (or structural integrity, whatever). And you need to score quite a few hits to the enemy before they go down, so in FS2 you can be somewhat more trigger-happy than if you were using ballistic ammo.
Interestingly, BtRL combat did in fact more closely resemble WW2 combat than FS2 combat - largely due to ballistic primaries.
4. You should always try to keep your eye on your opponent, and never let yourself be deceived by ruses.
Situational awareness...
5. In any type of attack, it is essential to assail your opponent from behind.
Potential for crude puns aside, this should be pretty simple - the best firing position is at six o'clock position. And you should plan your flight path so that you end up there, instead of the opposite. Lead pursuit, lag pursuit and true pursuit are fascinating theory but they are also more oriented for aerial combat. In FS2, when you're approacing an enemy from the side you most likely will take slight lead pursuit path by placing your targeting reticle on the lead indicator. This is very close to true pursuit though, due to the fact that primary weapons move fast. If you want to, say, intercept a group of bombers, you might want to aim a lot ahead of the group. You will need to estimate and adjust your vector so that your flight path will intercept the enemy's flight path at the same time, which will get you close to them in the fastest way possible - and time is usually of the essence in intercept missions. But I'm getting side tracked here - the point is, plan a bit ahead, but not too much, and keep adapting to the improving situation. Your main goal in a fur ball is always to get behind your opponent and then stay there until he's toast.
6. If your opponent dives on you, do not try to get around his attack, but fly to meet it.
Again, with lack of altitude differences, this can be simplified to head-on approaches; however, while still not in a furball you should consider whether or not you should offer combat with the enemy flying towards you at all. You could head for reinforcements if you are in inferior ship, or if in equal or better maneuverable ship, commit to the head-on pass and following fur ball. If the enemy is already in gun range, running away is not usually a viable option though.
7. When over the enemy's lines, never forget your own line of retreat.
In FreeSpace combat with no significant front lines this is not really that significant, especially as you usually don't finish missions by landing at home base. Nevertheless, situational awareness of the general locations of friendly forces is beneficial because you can flee there in some cases. Slaying Ravana is a very good example of this; the Sobek gives your bomber wing a very good cover that you can use to regroup and get rid of enemies at your tail. Provided you can find it in the mess and you haven't positioned the Ravana between yourself and it... which again demonstrates the importance of situational awareness.
8. Tip for Squadrons: In principle, it is better to attack in groups of four or six. Avoid two aircraft attacking the same opponent.
This was a bit outdated by the second world war, and in fact the Finnish Air Force tactics utilized pairs of planes forming larger flights. Having a wing man to cover your ass is very beneficial, as you can concentrate on offense and your wingman will cover you and keep you aware of enemies trying to get to your tail, so you know when to disengage.
The underlying message still holds true though; it's better always to have company than fly alone.
Not shooting at same target at same time should be obvious, but some people remain oblivious to the fact that there is no such thing as friendly fire - it all kills you just the same.
In short, shoot at the ship at your wingman's six, not the one at his twelve. Shoulder shooting is one of the most annoying things to encounter in multiplayer flight combat. Granted, it's not quite as lethal in FS2_Open as it can be in IL-2 Sturmovik - what with mid-air collisions being pretty much certain death in that game - but still annoying and waste of effort that could be spent in better team work.
