RE: Coaxial contrarotating propellers
I may be sticking my you-know-what out a little with this but, I believe the term to be used is contra-rotating. Counter-rotation applies to planes like lightnings and Piper Seminoles - planes where the propellers of the opposing engines turn in opposite directions. To put it simply, the second propeller straightens the airstream in an attempt to recover the energy in the swirling stream of air behind a normal propeller. Airplanes with incredible amounts of excess horsepower (Bear Bomber, etc) use them so prop diameter doesn't get too large (tip speeds too high). The Pogo and the Salmon used such props to counter the torque effects of swinging a large prop at near-zero airspeeds.