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Home > Airplanes > About turbofan engines and whether the Airbus A340 have outsold the Boeing 777?
About turbofan engines and whether the Airbus A340 have outsold the Boeing 777?
Aviation - Airplanes
Monday, 07 January 2008 20:05

Dear Sir,

Thank you for your most interesting WebPages. As a non expert in the field but with some basic background knowledge, I would be interested in learning about your opinion in the three-shaft versus two-shaft turbofan question. Both, Rolls Royce and GE/PW, of course, do have their arguments for or against the respective competitors' design. However, starting from the RB211, it appears to me that the 3-shaft TRENT / RB211 family at RR "standardizedly" covers a considerably larger thrust range
than GEs or PWs engines.

Another question: According to an RR commentator - and as far as I can learn from the respective companies' WebPages, the Airbus 340-500 and 340-600 have so far outsold the 777-200LR and 777-300ER, in spite of the fact that the 777 appears to be quite efficient (however, the Airbus planes offer more cargo space than the 777 do and the 777-200LR only reaches its maximum range with optional fuel tanks, consuming space that otherwise could be used for cargo containers). How can one explain this?

Thank you in advance for your answers.

Best wishes,

Johannes Helm,
Oslo, Norway

PS: I am not an industrial or business person and do not have any affiliations whatsoever with any aircraft or related company.

Hi Johannes,

I am not an expert about aircraft engines. However, your question triggered me to do a search on the subject but I am still unable to give you an opinion as to the arguments for the three-shaft versus two-shaft turbo fan question. Perhaps one day when I talk to an Engineer who is familiar with this topic, then maybe I can give you a better answer. At the moment, I am flying the Boeing 777 which is fitted with the Rolls Royce Trent 892 three-shaft turbo fan engines and I am quite happy with them.

The reason why the Airbus A340-500 and A340-600 have outsold the Boeing 777-200LR and Boeing 777-300LR is probably because the Airbuses have 4 engines but overall, Boeing 777s (all models) have already made a sale lead over the A340, particularly in the hotly contested Asian market. Boeing has sold 615 Boeing 777s since it launched the program in 1990, while Airbus has sold 323 A340s since 1987.

So your figures about the Airbuses outselling the Boeing 777s only refers to the Long Range version of the Boeing 777s.


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3 vs 2 shafts

I am an aeronautical engineer working on large turbofans for a big company.

Companies such as Rolls believe that using 3 shafts and hence having 3 compressor stages and 3 turbines allows them to optomise each stage for the pressures and temperatures of the flow that appear in those sections therefore making them more efficient. It also allows for a more shorter and rigid design potentially reducing nacelle drag.

A company such as General Electric however believe that 3 shafts are over complicated. Particuarly having 3 concentric shafts up to 10 metres long running inside each other is very complex arrangement from a bearings point of view as it is difficult to design sufficient oil routing to the inner bearings which support the location of the shafts and all associated loads.

In my experience however, differences in each engine arrangement with regards to all aspects of the design pretty much cancel each other out and both arrangements can produce very good performance.
Matt , 10 Feb, 2009

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