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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

. Examples include some

aircraft propeller An aircraft propeller, also called an airscrew,Beaumont, R.A.; ''Aeronautical Engineering'', Odhams, 1942, Chapter 13, "Airscrews". converts rotary motion from an engine or other power source into a swirling slipstream which pushes the propeller ...

s, resulting in the maximum power of a single

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-t ...

turboprop A turboprop is a turbine engine that drives an aircraft propeller. A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. Fu ...

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 An epicyclic gear train (also known as a planetary gearset) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates the planet and sun gea ...

transmission. The configuration can also be used in

helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribute ...

designs termed

coaxial rotors Coaxial rotors or coax rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (contra-rotating). This rotor configuration is a feature of helicopt ...

, 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 The Boeing CH-47 Chinook is a tandem rotor helicopter developed by American rotorcraft company Vertol and manufactured by Boeing Vertol. The Chinook is a heavy-lift helicopter that is among the heaviest lifting Western helicopters. Its name, ...

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

rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

s are moderately common in

military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically authorized and maintained by a sovereign state, with its members identifiable by their distin ...

aircraft and

naval A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operations and related functions. It in ...

applications, such as

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, s ...

es, where the added maintenance cost is not a primary concern.

Aircraft propulsion and lift

While several nations experimented with contra-rotating propellers in aircraft, only the United Kingdom and Soviet Union produced them in large numbers. The U.S. worked with several prototypes, including the tail-sitting

Convair XFY The Convair XFY Pogo was an experiment in vertical takeoff and landing (VTOL) tail-sitter. The Pogo had delta wings and three-bladed contra-rotating propellers powered by a turboprop engine. It was intended to be a high-performance fighter airc ...

and

Lockheed XFV The Lockheed XFV (sometimes referred to as the "Salmon") was an American experimental tailsitter prototype aircraft built by Lockheed in the early 1950s to demonstrate the operation of a vertical takeoff and landing (VTOL) fighter for protec ...

"Pogo"

VTOL A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-w ...

fighters, but

jet engine A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typ ...

technology was advancing rapidly and the designs were deemed unnecessary. Some helicopters use contra-rotating

mounted one above the other. The H-43 Huskie helicopter uses non-coaxial

intermeshing rotors Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. T ...

turning in opposite directions. The

F-35B The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, all-weather stealth multirole combat aircraft that is intended to perform both air superiority and strike missions. It is also able to provide elect ...

variant of the

F-35 Lightning II The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, all-weather stealth multirole combat aircraft that is intended to perform both air superiority and strike missions. It is also able to provide elec ...

strike fighter uses a lift fan with contra-rotating blades.

Marine propulsion

Contra-rotating propellers have benefits when providing thrust for boats for the same reasons. ABB provided an

azimuth thruster An azimuth thruster is a configuration of marine propellers placed in pods that can be rotated to any horizontal angle (azimuth), making a rudder unnecessary. These give ships better maneuverability than a fixed propeller and rudder system. Ty ...

for ShinNihonkai Ferries in form of the CRP ''Azipod'', claiming efficiency gains from the propeller (about 10% increase) and a simpler hull design. Volvo Penta have launched the IPS ( Inboard Performance System),home : Volvo Penta
/ref> an integrated diesel, transmission and pulling contra-rotating propellers for motor yachts.

Torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, s ...

es have commonly used contra-rotating propellers to give the maximum possible speed within a limited diameter as well as counteracting the

that would otherwise tend to cause the torpedo to rotate around its own longitudinal axis.

Advantages

*The propeller-induced

heeling moment Forces on sails result from movement of air that interacts with sails and gives them motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Similar principles in a rotating fr ...

is compensated (negligible for larger

ships A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguishe ...

). *More power can be transmitted for a given propeller radius. *The propeller efficiency is increased by recovering energy from the tangential (rotational) flow from the leading propeller. Tangential flow does not contribute to thrust, conversion of tangential to axial flow increases both thrust and overall system efficiency.

Disadvantages

*The mechanical installation of coaxial contra-rotating shafts is complicated and expensive and requires more maintenance. *The hydrodynamic gains are partially reduced by mechanical losses in shafting. Contra-rotating propellers are used on

es due to the natural torque compensation and are also used in some motor boats. The cost of boring out the outer shafts and problems of mounting the inner shaft bearings are not worth pursuing in case of normal

References

External links