Well, I finally got my Goldberg Cub (.40 size - clip wing) in the air with a Tower Hobbies .75 two-stroke engine (13x6 prop). All I can say is WOW!!! The power was insane. At 3 stick-clicks above idle, the tail started flying. At just above 1/4 throttle, it was airborne in about 10 feet. Flying at 1/4-3/8 throttle seemed like a regular .40 sized Cub with a .61FS, at 1/2 throttle it was like flying my .46 sport plane, and at full throttle, it was insane. I only made 1 flight since I need to get more aileron throw (roll rate is about 2-3 seconds for 90 degrees). What was great was cruising at 1/2 throttle then step on the gas. It was like injecting nitrous in a car - it really picked up speed (not to mention the right torque-roll and climb which I may throw in a mix to compensate). Landing posed a slight problem. With this much thrust (even at idle), it wouldn't come down. Only after I adjusted the throttle trim down until it started on a slow four-stroke type idle was I finally able to land it 3/4 down the field, but landed perfectly - no bounce. It rolled on 2 wheels for about 10 feet until I pulled back on the elevator to get the tail down. On the taxi back, I burped the throttle a few times to get it to jump into the air - pretty fun doing those bunny-hops. Anyway, after a few tweaks and getting a faster roll rate, I will try the unlimited vertical that I was after.
Yes, this is very much overkill of an engine for a Cub, but what the heck.