Thanks David.

I'm not sure if the term flat-spin is used correctly!

I've flow several fun-fly (ARF) planes that does not roll when full rudder is applied in straight upright flight. Instead, it slows turns around (laterally). By holding the rudder long enough, I may get the plane to make a u-turn.

Several 3D models in FMS do this too!

Any idea what makes the plane does that instead of rolling?

Thanks.