RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
The problem with fitting a Contra Drive to an outrunner motor is that it would be much heavier than a similar system that uses an inrunner motor. This is because inrunners already use a planetary reduction gearbox, and this gearbox can just be swapped out which minimizes the weight increase.
If weight is not a consideration then it could be done. Also, if an outrunner was spefically designed for the application, then maybe then it would be more practical.