Helicopter Rotation Conventions

Is it true that the main rotors of American helicopters rotate counter-clockwise while European helicopters rotate clockwise? If so, why? I have been told this stems from a German twin rotor helicopter being halved and distributed between the Allies at the end of World War II.

- question from Glenn Dawkins, Steve & Tom

We confirmed these trends by comparing the rotor designs of a number of different helicopters from around the world. Overhead drawings are especially useful since the way in which the rotor blades are attached to the central hub makes it immediately obvious where the leading edge (LE) of a rotor blade is. Knowing which edge of the blade is the leading edge determines the direction in which the blades must rotate to generate lift. Based on the photos and drawings that we could locate, we found that rotors almost always rotate in the same direction depending on the nation the helicopter comes from. Examples are shown in the figure below.



Comparison of helicopter main rotor rotation direction

American helicopters manufactured by companies like Sikorsky, Boeing Vertol, Bell, and MD Helicopters all appear to rotate counter-clockwise like the SH-60 Seahawk illustrated above. The opposite convention is used in all French helicopters built by Aérospatiale, such as the Eurocopter Tigre, as well as all Russian helicopters built by Mil, like the Mi-8. However, the manufacturers in many other European nations use the same convention as the Americans. The helicopters built by Westland in the United Kingdom and Italy's Agusta all rotate counter-clockwise, as exemplified by the EH-101 Merlin and A109. German manufacturers like MBB and the Japanese companies Kawasaki and Mitsubishi also follow the American convention.

We've attempted to investigate how these two opposing philosophies emerged but have yet to find an answer. However, the story about a German twin rotor helicopter being split among the allies seems pretty unlikely. The American design layout with counter-clockwise rotor motion can be traced at least as far back as Igor Sikorsky's R-4 helicopter developed during the early 1940s, several years before the end of World War II. Other American helicopter pioneers like Larry Bell, Stanley Hiller, and Frank Piasecki subsequently employed this convention. Westland in the UK and Agusta in Italy as well as the Japanese manufacturers Fuji, Mitsubishi, and Kawasaki all entered the helicopter market by making licensing agreements to build American designs, so this sharing of technology may also explain how those nations came to adopt the American counter-clockwise rotation.

As for France and Russia, we have been unable to determine how manufacturers in those nations came to use a different convention. The same trends are also followed in countries like Poland, China, and India, probably because firms in these nations have also license-built helicopters from Russia and/or France. Whether the decision to adopt an opposite direction of rotation was made for technical reasons or was simply a matter of preference is unclear. If any readers are aware of the answer, please contact us.

- answer by Joe Yoon, 23 January 2005

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