Contra-rotating propellers
Contra-rotating propellers, also referred to as coaxial contra-rotating propellers, apply the maximum power of a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are common in some marine transmission systems, in particular for medium to large size planing leisure crafts. Two propellers are arranged one behind the other, and power is transferred from the engine via a 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 twin-engined aircraft with the airscrew on one engine turning clockwise and the other counter-clockwise.
When airspeed is low the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow.
If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the asymmetrical torque effect of a conventional propeller. Some contra-rotating systems were designed to be used at take off for maximum power and efficiency, and allowing one of the propellers to be disabled during cruise to extend flight time.
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 concern to government budgets.
Significant aircraft
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 also use a contra-rotating arrangement to offset torque, although the rotors do not share a common coaxial hub.
United Kingdom
Some of the more successful British aircraft with contra-rotating propellers are the Avro Shackleton MR Mk 3, powered by the Rolls-Royce Griffon engine, and the Fairey Gannet, which used the Double Mamba Mk.101 engine. Later variants of the Supermarine Spitfire and Seafire used the Griffon with contra-rotating props as well. In the Spitfire/Seafire and Shackleton's case the primary reason for using contra-rotating propellers was so as to increase the propeller blade-area, and hence absorb greater engine power, within a propeller diameter limited by the height of the aircraft's undercarriage. Whilst this also applied to the Gannet, in addition this aircraft's engine had two separate power-sections, each driving one propeller.
USSR
Soviet and Russian design bureaus produced a large number of coaxial rotor helicopters and aircraft. The Yakovlev Yak EG, Kamov Ka-25, Ka-32, and Ka-50 Blackshark are particularly prominent examples. Helicopters have no need for a tail rotor when equipped with contra-rotating rotors, as the two rotors cancel out each others' angular momentum. This aids maneuverability and allows the design to be more compact. The ability to operate without a tail rotor is a large advantage because a tail rotor can take up to 30% of the power from the engine, and destroying the tail rotor is one of the easiest ways to down a helicopter in combat. The Kamov Design Bureau is virtually trademarked by contra-rotating rotor configurations, as it appears in the majority of the firm's designs.
An especially noteworthy Soviet design was the Tupolev Tu-95 Bear, the only contra-rotating turboprop bomber to enter service, as well as one of the fastest propeller aircraft. The Tu-114, an airliner derivative of the Bear, holds the world speed record for propeller aircraft.[1] The Bear was also the first Soviet bomber to have intercontinental range, allowing it to strike North American targets. (The Soviet Tu-4 was capable of reaching American targets, but only on a one-way mission). As of 2005 the Ukrainian design bureau Antonov is nearing certification of the An-70 heavy transport, powered by contra-rotating propfans.
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,[2] claiming efficiency gains from the propeller itself and the more simple hull design. Volvo Penta have launched the IPS (Inboard Performance System),[3] 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.
Dis-advantages
- The mechanical installation of coaxial contra-rotating shafts is complicated, expensive and requires more maintenance.
- The hydrodynamic gains are partially compensated by mechanical losses in shafting.
Contra-rotating propellers are used on torpedos due to the natural torque compensation. It is 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.