RE: .52 or .61 engine for Cox Tailwind Trainer 40??
Agree with glowplug. A plainbearing .40/46 will be acceptable, a good BB 40 will be more than acceptable, & a good BB .46 or 51 will be much more than necessary for training. A .61 will be too much -- 60 size engines are notably larger & heavier than 46's.