I built mine with an O.S. .46 FX, which is slightly lighter weight than the .46 AX. It's too heavy. I have my battery pack moved as far back into the tail section as far as I can reach through the formers, and it doesn't balance. It went from "way too tail heavy" to "significantly tail heavy." The battery is way behind the back of the servo tray, way back.

O.S. .46 AX = 17.2 oz w/ muffler - too heavy

This is not the ideal engine for the Tower Trainer .40 Mk II. It has way more power than the airframe can use, and the extra weight up front makes it very difficult to balance with a standard 600mah NiCd receiver battery unless you add a fair amount of weight to the tail. You either have to increase the wing loading to balance it, or simply fly it more nose heavy than it should be and live with hotter-than-normal landings.

An engine in the 12 ~ 13 oz range would balance much more easily on this airframe, and the drop in maximum horsepower wouldn't be missed.

Apparently I wasn't clear about this, the O.S. Max .46 AX is a wonderful engine that is too heavy and too powerful for the Tower Trainer .40

Here is my nose-heavy Tower Trainer .40 Mk II flying just a couple of weeks ago with its unbalancing O.S. Max .46 FX on the nose:



I'm not guessing about this folks, I just built one. Use a lighter engine and save the .46 AX for a nice sport plane.