Discounting the more than likely possibility of going up against a ship with better equipment, you're running into the individual strengths of a ship.
A Viper has higher straight-line speed than a Cobra (all other things being equal, i.e. A-level thrusters), but a slower turning radius at a given speed. It does therefore require different tactics, commonly referred to as "zoom and boom". This means using your high speed to open up the distance to a point where you can turn around to face him while he's still chasing you, passing at high speed and inflicting what damage you can, etc. Repeat until BOOM. It will help to fit weapons on your Viper that work best at longer ranges, and for you to practice using them that way. If you prefer up-close turn-fighting, I'd recommend flying an Eagle or Cobra. FWIW a well-piloted Eagle may look flimsy but can be a force to be reckoned with.
The same way you shouldn't get in a turning fight with a Zero if you're flying a Corsair, play to the strengths of your craft, and the weaknesses of his. The reasons for these tactics are different from an atmospheric aircraft battle, but they are still valid in 3D space combat.
EDIT: The zoom and boom tactic should be used because as a faster ship you have a constant "energy advantage" over your opponent in the Cobra. This is the common terminology that will point you to these tactics in most flight-combat instructional guides. Energy advantage in atmospheric flight is gained with altitude over your opponent, along with aircraft performance. Removing the gravity and lift mechanics in space actually simplifies the rules and should help making tactical styles easier to choose in the heat of battle.
