Actually, you can trim a flat-bottom plane to have less pitch trim changes with throttle and airspeed (differnet issues) I've never tried to make it 0 change, but you can reduce it.

You apply a combination of these three things. how much of each is another story, and frankly, MinnFlyer is right, time for a new plane. That said here's what you do:

- Shim up the TE of the wing to reduce wing incindence.
- Put in some down-thrust on the engine.
- Reflex the ailerons, which is basically changes the airfoil shape.