The problem is weight. They are too heavy. A .61 tends to be heavy in a .46 size plane. It takes planning and modification to build one without adding tail weight. The .60-.90 size planes tend to fly a lot better with one of the new .91s that are built in the .61 case. You might say they are too heavy compared to the .91 and produce less power. I have both the OS .91FX and the Magnum .91XLS. Both are the same weight as the .61s and produce great power. The planes tend to be sized for the larger .91 engine.

The problem can be worked on a .46 plane by moving the elevator and rudder servos to the rear and moving the battery back. Use a wood prop and a light spinner.

On the .61-.91 planes, you have to go on an extreme diet to get 3D performance with a .61. Light servos, light battery, light landing gear, light spinner, light everything. Pull-pull is lighter. It can be done, but it's work.