I never really understood how the special manouvers work.
I have a certain velocity and I have to spend 1 thrust point for every hex my aerospace fighter moves. It moves 1 hex for every point of velocity I have.
let's say I have a velocity of 8. I should cover 8 hexes and spend 8 thrust points. What if I want to do a special manouver during the movement? The hexes the manouver includes must be computed in the overall maximum movement?
A lot of your confusion seems to stem from the misconception that a unit's velocity always equals the amount of thrust it spends in a turn. Because aerospace units keep half of their previous turn's velocity(Total War, p. 84) every turn, once a unit reaches velocity 8, it only needs to spend 4 thrust at the beginning of each turn to maintain that velocity.
It's actually extremely rare for a unit's velocity in a turn and the amount of thrust it spent in that turn to be exactly the same.
If I do a looping manouver it should be considered a two hexes movement? So, if I have a velocity of 8, I still have 6 hexes to go?
Close. Because the rules for Loops say that the first 4 points of velocity are spent in the loop, if you loop at velocity 8 you would have 4 hexes left to go.
If I fail the pilot check for the special manouver, I can try that again?
Sorry, unless the maneuver in question is a side-slip, you cannot. Total War p. 84 says "With the exception of a side-slip, which can be executed at any time during movement, special maneuvers occur at the start of the Movement Phase(Aerospace)." If you attempt a maneuver, fail, and move forward the requisite number of hexes, you are no longer at the beginning of your movement and thus can no longer attempt the maneuver. Units with average or poor piloting skills should keep this in mind, as failure might leave them in a vulnerable position during the Weapons Phase.
And anyway, the hexes I move with a special manouver can be computed in the needed total for a simple hex face change? Or they must ever be hexes of "straight movement" as the low-altitude turn table says?
This question has three and a half answers.
1: The hex moved during a side-slip does count towards a unit's minimum straight movement.
2: The hexes moved forward after a failed maneuver do count towards a unit's minimum straight movement.
3: None of the other maneuvers actually involve moving into a new hex as part of the maneuver, so none of them have any effect on a unit's minimum straight movement requirements. Their effect on the unit's velocity may change its minimum straight movement requirements, however.
3a: As part of the above answer, a unit that VIFFs has its velocity reduced to 0 and thus qualifies for a free facing change if it chooses to, assuming it did not make such a change in that hex at the end of the previous turn's movement. Do note that it still has not moved into a new hex, and thus can never perform a powered facing change, which can never be made in the first hex of movement.