ORIGINAL: FallDownGoBoom

Bigwill:

While I agree with all of the above comments regarding 2 vs 4 strokes, for a Tower Trainer MKII I'd recommend using the 2 stroke for a couple of reasons:

1) Prop clearance. The MKII was probably designed with a 40-46 2S in mind, probably turning a 10x6 prop. If you're leaning towards a 70 sized 4S, it'll want to spin a 12'' prop, so now your prop tips are going to be closer to the ground. Any sort of nose-heavy landing will probably end up trimming your prop tips or breaking them off, especially if you're flying off grass or weeds. Which means you'll need bigger wheels, or making the landing gear taller, or both.

2) Weight and CG. The 4S will probably be heavier than a 2S. Since it's hung off the front of the airplane, it'll shift the CG forward, meaning you'd probably have to add weight to the tail. So your 5-1/2lb trainer may all of a sudden end up weighing 6lbs between the extra weight of the engine and balancing lead. Using a Saito might help with this problem, as they seem to be lighter than their OS/Magnum/ThunderTiger equivalents.

If this is your first build, I'd use the 2S just to keep things simpler and ''by the book''.

Fred