I have been flying with the .40 LAs, GPs, FPs and the Tower .40 almost exclusively for more than 40 years. The best prop for a trainer or an aerobatic trainer using any of the .40 small carb, bushing engines is an 11 x 4. For the non-believes, I carry my June 1992 issue of R/C Report, to show the Brian Lee review of the "improved" .40 FP. The RPM and thrust figures show:
11,700 and 66 for a 10x5
10,200 and 76 for an 11x5
10,500 and 85 for a 12x4

We are getting 12,000 or near that in RPM with this class of engine and an 11x4 APC in our RCPRO Club 40 Sport Racing group. Folks who show up with 10x6 props take 3 times as much takeoff run as those of us with an 11x4. If they do not sustain much damage, I just hand them an 11x4 APC prop and smile.

Your Avistar was designed to fly with a light weight bushing .40. However, on grass, that nose wheel is a problem. You will not have any better luck with the WM T-34. Look at the Sky Raider Mach II from the same manufacturer/distributer. It is better for grass as it does not have a nose wheel. It also was designed to fly with a light weight .40 bushing engine. Assembled right out of the box, as per the manual, it balances perfectly and we use them for buddy-boxing newbies.

Prior to the SR2, my favorite trainer was the Avistar. The old one like you have, not the new one.

Ken Erickson