RE: looking to slow my trainner down without a new engine?
I agree with many here. You should look at a 12x4 so that you dont over-rev the engine, provided of-course that you have the prop clearance.
However, I believe, its better to fly a trainer with a medium pitch prop. It will prevent you from slowing down too much and stalling at landing. I learnt on a trainer with a 55ax and 12x6 prop. I broke the prop after 6 months and by then I wanted to leanr aerobatics. So we put in a 11x6 then and it flew aerobatics much better (it had a 65" span semi-symmetric wing). On my second airplane, Tiger-40, I went all the way upto 11x9 before I settled on a 12x4 mainly because it isint that stable at landing (Its a Phoenix Tiger not CG). I now use 12x4 or 13x4 on my Reactor because it remains controllable at a speed lower than a trainer.
Better would be learn on the 11x6. To slow down the airplane, you just need to go to idle a bit earlier
Ameyam