on rudder trim. How do you use rudder trim on a WW2 era fighter plane to keep straight and level flight when cruising? If at all. On the P51 for example.
Generally speaking, rudder trim is used when needed with power changes.
The vertical stabilizer on the Mustang for example is offset 1 degree to the left. This is done to aid directional control optimized at the average cruise airspeed range. The rudder trim tab is used as needed in addition to this.
For takeoff and climb where a lot of power is in play, the useful setting for the Mustang is 6 degrees right. You'll come off this as power is reduced to a cruise setting. I usually had it around 0 and left it alone playing the rudder forces around the cruise setting. For slow flight where power comes up, you generally will crank in some rudder trim to help you keep the ball centered, and on final approach, I'll run up the trim to 6 degrees right again to aid in a go around if necessary.
The rule of thumb with trim never changes. The airplane tells you what it needs and you use what it's asking you for.