Contra-rotating propellors

Contra-rotating

Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby propellers or fan blades mounted on a common axle rotate in opposite directions. It is used in aircraft engines, 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 twin-propellered craft with each propeller on its own axle; one turning clockwise and the other counter-clockwise.

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 is not a primary concern to government budgets.

Aircraft propulsion

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. Kaman Aircraft designed the H-43 Huskie and K-Max light utility helicopter with intermeshing contra-rotating blades.

Tandem-rotor designs such as the Boeing Vertol CH-46 Sea Knight and CH-47 Chinook use a counter-rotating arrangement to offset torque; the rotors do not share a common coaxial hub.

A new usage of contra-rotating propulsion in aircraft can be found in the F-35B variant of the new F-35 Lightning II strike fighter, which uses a lift fan with contra-rotating blades.

Marine propulsion

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 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. Torpedoes 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.

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 usually increased.

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 torpedoes 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.

References

See also

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