If contra-props are so efficient then why didn't everyone use them? I would need some convincing. IMHO they are chosen because for a given diameter they can absorb more engine horsepower (EHP). Prop diameter is usually limited by either tip speed or ground clearance, though bending stress in long blades also has to be taken into account.
The Bear is the only long range aircraft I know of that uses contra-props, and they have hugely powerful engines. Anyone know the EHP? 7000? More? That is a lot of power for a single disc.
Dick W