Contra-rotating propellers, also referred to as coaxial contrarotating propellers, are a complex way of applying the maximum power of a single piston or turboprop aircraft engine. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear transmission.
The placement of a second propeller behind the first takes advantage of the already-disturbed airflow, and also serves to counter the torque effect felt from a conventional propeller. The efficiency of a contra-rotating prop is somewhat offset by the mechanical complexity that comes with it.
Nonetheless, contra-rotating propellers and rotors are moderately common in both the military aerospace industry and in naval armaments such as torpedoes, where the added maintenance is not a concern to government budgets.