Counter-rotating propellers

Counter-rotating propellers, also referred to as CRP, are propellers which turn in opposite directions to each other.[1] They are used on some twin- and multi-engine propeller-driven aircraft.

For opposite-rotating propellers that share a common axis and drive mechanism, see contra-rotating propellers.
Counter-rotating propellers
World War I Linke-Hofmann R.I German heavy bomber (1917) with counter-rotating propellers
He 177A Greif with counter-rotating propellers

The propellers on most conventional twin-engined aircraft turn clockwise (as viewed from behind the engine). Counter-rotating propellers generally turn clockwise on the left engine and counterclockwise on the right. The advantage of such designs is that counter-rotating propellers balance the effects of torque and P-factor, meaning that such aircraft do not have a critical engine in the case of engine failure.

Drawbacks of counter-rotating propellers come from the fact that, in order to reverse the rotation of one propeller, either one propeller must have an additional reversing gearbox, or the engines themselves must be adapted to turn in opposite directions. (Meaning that there are essentially two engine designs, one with left-turning and the other with right-turning parts, which complicates manufacture and maintenance.)

History

Counter-rotating propellers have been used since the earliest days of aviation, in order to avoid the aircraft tipping sideways from the torque reaction against propellers turning in a single direction. They were fitted to the very first controlled powered aeroplane, the Wright Flyer, and to other subsequent types such as the Dunne D.1 of 1907 and the more successful Dunne D.5 of 1910.

In designing the Lockheed P-38 Lightning, the decision was made to reverse the counter-rotation such that the tops of the propeller arcs move outwards (counterclockwise on the left and clockwise on the right), away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration.

The counter-rotating powerplants of the German World War II Junkers Ju 288 prototype series (as the Bomber B contract winning design), the Gotha Go 244 light transport, Henschel Hs 129 ground attack aircraft, Heinkel He 177A heavy bomber and Messerschmitt Me 323 transport used the same rotational "sense" as the production P-38 did – this has also been done for the modern American Bell Boeing V-22 Osprey tiltrotor VTOL military aircraft design. The following German World War II aviation engines were designed as opposing-rotation pairs for counter-rotation needs:

The aerodynamics of a propeller on one side of an aircraft change according to which way it turns, as it affects the P-factor. This can in turn affect performance under extreme conditions and therefore flight safety certification. Some modern types, such as the Airbus A400M, have counter-rotating propellers in order to meet air safety requirements under engine-out conditions.

List of aircraft with counter-rotating propellers
TypeCountryDateNotes
Airbus A400M AtlasEU2009Four engines
Beechcraft 76 DuchessUS1974Twin engines
Cessna T303 CrusaderUS1978Twin engines
de Havilland HornetUK1944Twin engines
Dunne D.1UK1907twin engines in fuselage on a common driveshaft
Dunne D.4UK1908Single central engine
Dunne D.5UK1910Single central engine
Fairey F.2UK1917Twin engines
Gotha Go 244GermanyTwin engines
Heinkel He 177A Greif (fourth prototype onwards)GermanyTwin engines
Henschel Hs 129GermanyTwin engines
Junkers Ju 288GermanyTwin engines
Linke-Hofmann R.IGermany1917four engines in fuselage
Lockheed P-38 LightningUS1939Twin engines
Messerschmitt Me 323 GigantGermanySix engines
North American P-82 Twin MustangUSTwin engines
North American Rockwell OV-10 BroncoUSTwin engines
Piaggio P.180 AvantiItaly1986Twin engines
Piper PA-31 Navajo (some variants)USTwin engines
Piper PA-34 SenecaUSTwin engines
Piper PA-39 Twin Comanche C/RUSTwin engines
Piper PA-40 ArapahoUSTwin engines
Piper PA-44 SeminoleUSTwin engines
Vought V-173 Flying PancakeUS1942Twin engines
Vought XF5U Flying PancakeUS1947Twin engines
Wright Flyer and most other Wright models to 1916US1903Single central engine.

See also

References
Notes
  1. Counter-rotating propellant analysis using a frequency domain panel method
Bibliography
  • Gunston, Bill. Jane's Aerospace Dictionary. London, England. Jane's Publishing Company Ltd, 1980. ISBN 0 531 03702 9
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