Noob 101 right here

[JCofDI]

Trim is trim - it doesn't mean anything different based on the period the aircraft was built or the propulsion system.

The way the trim button works in this game is that you need to go ahead and put pressure on the sticks in order to make your plane fly relatively level. Once you have achieved this (doesn't need to be exact at this point), simply keep the sticks held in place and press the Trim button. It will automatically set the positions your elevators/ailerons/rudder were at when you pressed it. So now, when the sticks are centered, the plane flies like you had it held when the trim button was pressed. Simply repeat this process as needed and soon you'll be flying level and aiming much more easily.