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What is N3?

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Saw the term N 3 on a video last night. What is that?Sincerely,Chris

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Chris -N3 is the RPM count for the third spool in three-spool engines. The PMDG 777 and 757 are modeled with Pratt & Whitney engines, which are two spool engines ("spool" referring to a rotating element; in the case of the P&W engines, the fan, and a turbine set to drive it, are located on one spool, while the compressor stages, along with another turbine set, called the "core", are on a separate spool co-axial with the first. There is no mechanical linkage between the two, and the spools are free to rotate more or less independently. In practice, fuel flow, airspeed and other parameters establish certain relationships between the fan speed (N1) and the core engine speed (N2).Some engines, like the Rolls-Royce RB211-535 used on some variants of the 757, have three spools: fan, low pressure compressor, and high pressure compressor. In this case, the third spool's RPM is also instrumented, hence N3. The web sites of the various engine manufacturers sometimes have good cutaway views of their products; try doing a browser search for Pratt & Whitney, Rolls-Royce, General Electric, etc. Also, check out the reference section of your local library, or any of the encyclopedia sites on the Web.Hope this answers your question.Dave Reed

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