I'm new at 3D and just bought a U-Can Do 3D 60. I have an OS 61 fx and an OS 91 fx at home, which should I use best and with what prop?

the .61

In my opinnion. I have tried both 2 stroke and 4 stroke motors in the U cando. For 3d the 4-stroke shines. I have an A.S.P. 91 $169. and it flies it perfect. on a previous UCD 60 I had a O.S. 91- 4 pumped at $325.. and i would have to say the A.S.P. flies it just as good. both are on Omega 30% Heli fuel. The 91 also balances perfect with just a need to move the battery around to obtain desired C.G. The .91 has unlimited vertical and hovers at 1/2 to 3/4 throttle with reserve to pullout.

I would go with the OS 91 fx, no reason to go with the 61. The two motors weight the same, although the muffler for the 91 weighs a bit more. The only time I would go with the 61 is if the plane couldn't handle the power of a 91, the U-Can-Do is no problem with the 91.

I use a 15x4 APC prop on my U-Can-Do 60 with OS 91fx. It turns around 10,500 peak, which I back off to around 10,300. Lots of power - when my 91 is running right it pulls out very strongly from a hover. Lots of people report problems with the 91, I didn't have any problems for the first 100+ flights, but I am now starting to have problems, although it may just need some tuning (the muffler was coming loose, which I didn't realise, so I messed around with the low & hight speed settings).

I think you will love this plane. It is my first 3D plane. Very good for hovering. Other manuvers I have been trying include inverted flat spins (very good), knife edge spins (these look really impressive to me, especially with a fair bit of power added).

Malcolm