Because engine and prop specs are the only varying factor after the airplane is built according to the plans, the downward forces created by downthrust are also going to vary. Thats why what Rodney and Dalolyn said is very important and necesary specially on a trainer aircraft. Since the airfoil will create different amount of lift at different airspeeds if the downthrust is not compensated properly the trainer will tend to balloon on every change of airspeed. Therefore the beginner will be overcontrolling the airplane at all times.......trying to keep it straight and level. That defies the purpose of having a trainer. A little work doing the downthrust early and the rest of the time is just for enjoyment of your plane not to constantly keep trimming everytime you go flying.