Contra-rotating
Encyclopedia
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 propeller
s, 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, 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 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.
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
and Lockheed XFV
"Pogo" VTOL fighters, but jet engine
technology was advancing rapidly and the designs were deemed unnecessary.
Some helicopters use contra-rotating coaxial rotors mounted one above the other. The H-43 Huskie helicopter uses non-coaxial intermeshing rotors
turning in opposite directions.
The F-35B variant of the new F-35 Lightning II
strike fighter uses a lift fan with contra-rotating blades.
Contra-rotating propellers have benefits in providing thrust for boats for the same reasons. ABB
have provided an azimuth thruster
to ShinNihonkai Ferries in form of the CRP Azipod
, claiming efficiency gains from the propeller itself (about 10% increase) and the more simple hull design. Volvo Penta
have launched the IPS (Inboard Performance System), an integrated diesel, transmission and pulling contra-rotating propellers for motor yachts.
Torpedo
es have commonly used contra-rotating propellers to give the maximum possible speed within a limited diameter as well as counteracting the torque
that would otherwise tend to cause the torpedo to rotate around its own longitudinal axis.
Contra-rotating propellers are used on torpedo
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 ships.
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
. Examples include some aircraft propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
s, resulting in the maximum power of a single piston or turboprop
Turboprop
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
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
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
. The configuration can also be used in helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
designs, where similar issues and principles of torque apply.
Contra-rotating propellers should not be confused with counter-rotating propellers
Counter-rotating propellers
Counter-rotating propellers, found on twin- and multi-engine propeller-driven aircraft, spin in directions opposite one another.The propellers on both engines of most conventional twin-engined aircraft spin clockwise . Counter-rotating propellers generally spin clockwise on the left engine and...
, 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
CH-47 Chinook
The Boeing CH-47 Chinook is an American twin-engine, tandem rotor heavy-lift helicopter. Its top speed of 170 knots is faster than contemporary utility and attack helicopters of the 1960s...
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 rotor
Helicopter rotor
A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight...
s are moderately common in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
aircraft and naval applications, such as torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
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
Tailsitter
A tailsitter is a type of VTOL aircraft that launches and lands on its tail. One of the most famous examples of this type of aircraft is the Ryan X-13 Vertijet. Among the propeller-driven versions were the Lockheed XFV, and the Convair XFY Pogo. Studies and wind tunnel models were made of a...
Convair XFY
Convair XFY
|-See also:-References:NotesCitationsBibliography* Allen, Francis J. "Bolt Upright: Convair's and Lockheed's VTOL fighters". Air Enthusiast Volume 127, January/February 2007, pp. 13–20. ISSN 0143-5450....
and Lockheed XFV
Lockheed XFV
-See also:-References:NotesCitationsBibliography* Allen, Francis J. "Bolt upright: Convair's and Lockheed's VTOL fighters". Air Enthusiast , Volume 127, January/February 2007, pp. 13–20. ISSN 0143-5450....
"Pogo" VTOL fighters, but jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
technology was advancing rapidly and the designs were deemed unnecessary.
Some helicopters use contra-rotating coaxial rotors mounted one above the other. The H-43 Huskie helicopter uses non-coaxial intermeshing rotors
Intermeshing rotors
Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted on the helicopter with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. The arrangement allows the helicopter to...
turning in opposite directions.
The F-35B variant of the new F-35 Lightning II
F-35 Lightning II
The Lockheed Martin F-35 Lightning II is a family of single-seat, single-engine, fifth generation multirole fighters under development to perform ground attack, reconnaissance, and air defense missions with stealth capability...
strike fighter uses a lift fan with contra-rotating blades.
Marine propulsion
Asea Brown Boveri
ABB is a Swiss-Swedish multinational corporation headquartered in Zürich, Switzerland, and best known for its robotics. ABB operates mainly in the power and automation technology areas. It ranked 143rd in Forbes Ranking ....
have provided an azimuth thruster
Azimuth thruster
An azimuth thruster is a configuration of ship propellers placed in pods that can be rotated in any horizontal direction, making a rudder unnecessary...
to ShinNihonkai Ferries in form of the CRP Azipod
Azipod
Azipod is the registered brand name of the ABB Group for their azimuth thruster. Originally developed in Finland jointly by Kvaerner Masa-Yards dockyards and ABB, these are marine propulsion units consisting of electrically driven propellers mounted on a steerable pod.The pod's propeller usually...
, claiming efficiency gains from the propeller itself (about 10% increase) and the more simple hull design. Volvo Penta
Volvo Penta
Volvo Penta was founded in 1907 in conjunction with the production of the first marine engine, B1. The Penta company soon became an established internal combustion engine manufacturer, which in 1927 delivered the engine to Volvo's first passenger car....
have launched the IPS (Inboard Performance System), an integrated diesel, transmission and pulling contra-rotating propellers for motor yachts.
Torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
es have commonly used contra-rotating propellers to give the maximum possible speed within a limited diameter as well as counteracting the torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
that would otherwise tend to cause the torpedo to rotate around its own longitudinal axis.
Advantages
- The propeller-induced heeling moment is compensated (negligible for larger ships).
- 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, expensive and requires more maintenance.
- The hydrodynamic gains are partially reduced by mechanical losses in shafting.
Contra-rotating propellers are used on torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
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 ships.