Wikipedia
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 aircraft propellers, resulting in the maximum power of a single 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. 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 non-coaxial propellers on separate shafts; one turning clockwise and the other counter-clockwise. Tandem-rotor helicopters such as the CH-47 Chinook also use a counter-rotating arrangement.
The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and rotors are moderately common in military aircraft and naval applications, such as torpedoes, where the added maintenance cost is not a primary concern.
Usage examples of "contra-rotating".
The stealthed, disc-shaped aerial combat robots were a metre and a half wide, their central section a curving mesh-grid to protect the wide-cord contra-rotating fans in the middle.
Friction from the high-altitude cells formed an intermediate pattern of contra-rotating winds in the mid-atmosphere, but at much lower velocities.
Buzzbots zipped through the air above them, the latest craze: little flying saucer-shaped aircraft with contra-rotating fans at the center, controlled by v-gloves.
The armor was non-metallic but tough enough to draw a shriek from the contra-rotating diamond saws in the bar's edge.