Contra-rotating propellers

From Freepedia

Contra-rotating propellers, also referred to as coaxial contrarotating propellers, are a complex way of applying the maximum power of a single piston or turboprop aircraft engine. 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.

When airspeed is low the mass of the air going back 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 already-disturbed airflow.

If it is well designed a contra-rotating propeller will have no rotational air flow, pushing 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 felt from a conventional propeller. Some contra-rotating systems were designed to be used at take off, for maximum power and efficiency, allowing one of the twin propellers to be disabled during cruise to extend flight patrol time.

The efficiency of a contra-rotating prop is somewhat offset by the mechanical complexity that comes with its design however.

Nonetheless, contra-rotating propellers and rotors are moderately common in both the military aerospace industry and in naval armaments such as torpedoes, where the added maintenance is not a concern to government budgets.

Image:Fairey Gannet AEW3 folded.jpg

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 vertical takeoff and landing Convair XFY and Lockheed XFV "Pogo" fighters, but jet engine technology was advancing rapidly and the designs were deemed unnecessary.

The British felt otherwise, however; 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 Griffin with contra-rotating props as well.

Soviet and Russian design bureaus also produced a large number of contra-rotating helicopters and aircraft. The 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; this aids maneuverability and allows the design to be much more compact. An especially noteworthy Soviet design was the Tupolev Tu-95 Bear, the only contra-rotating turboprop bomber to enter service in the world. It was also the first Soviet bomber to have intercontinental range, allowing it to strike North American targets.