Editing Contra-rotating (section)

{{No sources|date=March 2025}}
{{Short description|Parts of a mechanism rotating in opposite directions on a common axis}}
[[Image:Ka-32-SovietUnion-1989.jpg|thumb|right|A Soviet [[Kamov Ka-32|Ka-32]] helicopter with [[Coaxial rotors|coaxial contra-rotating rotors]], in 1989]]

'''Contra-rotating''', also referred to as '''coaxial contra-rotating''', is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of [[torque]].  Examples include some [[Contra-rotating propellers|aircraft propeller]]s, resulting in the maximum power of a single [[piston engine|piston]] or [[turboprop]] engine to drive two propellers in opposite rotation. Contra-rotating propellers are also common in some marine transmission systems, in particular for large speed boats with planing hulls. Two propellers are arranged one behind the other, and power is transferred from the engine via [[planetary gear]] [[Transmission (mechanics)|transmission]]. The configuration can also be used in [[helicopter]] designs termed [[coaxial rotors]], where similar issues and principles of torque apply.

Contra-rotating propellers should not be confused with [[counter-rotating propellers]], a term which describes propellers rotating in opposite directions but sitting apart from each other on separate shafts instead of sharing a common axis. [[Tandem-rotor]] helicopters such as the [[CH-47 Chinook]] also use a counter-rotating arrangement.

The efficiency of a contra-rotating propeller is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and [[Helicopter rotor|rotor]]s are moderately common in [[military]] aircraft and [[naval]] applications, such as [[torpedo]]es, where the added maintenance cost is not a primary concern.