Check your c.g. carefully if you put a big engine on the Avistar... With my OS46FX I had to add some significant weight (3+ oz) at the tail to get the c.g. back to about 3.25" from the LE, near the back of the recommended 2.5-3.5 range.. Flies nice there, and the heavier and more powerful aircraft doesn't float around like the typical trainer. Fun!