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For example, compare the JT9D and the PW4000 below (both from Pratt & Whitney, both on a 747):

enter image description here
(Left, Right) The PW4000 has a longer forward nacelle, and a covered exhaust cone.

On the inside—apart from optimizations—they're the same engine:

enter image description here
(Flight) Comparison from an article titled Son of JT9D takes shape. (Click image for larger size.)

From the Airbus A330 Wikipedia article:

enter image description here

All three engines above on the A330 adhere to the same noise restrictions. Apart from what it says under the CF6, all of them feature an exhaust cone that converts the annular exhaust to a circular one to reduce drag.

So, is the choice merely cosmetic/branding, or is there a trade-off of sorts?

Based on the two comparisons above, I'm leaning toward ruling out noise suppression, and internal engine design.

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  • $\begingroup$ Does the engine mfgr design the nacelle or the aircraft mfgr? $\endgroup$
    – TomMcW
    May 12, 2017 at 20:44
  • $\begingroup$ @TomMcW - I think it's co-op. $\endgroup$
    – user14897
    May 12, 2017 at 20:51
  • $\begingroup$ This was an interesting question and it deserved to be brought back. $\endgroup$
    – Pilothead
    May 31, 2018 at 17:40
  • $\begingroup$ Closely related: aviation.stackexchange.com/questions/59338/… $\endgroup$
    – Bram
    Feb 9, 2019 at 19:28

2 Answers 2

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Meeting noise regulations is the primary reason for using a long nacelle. Some engines may not need it. Since it adds weight and drag it is not used unless required. Use may be partially offset by improvement in efficiency, but this does not normally justify usage on its own.

If a particular engine design does not meet the specification, the long nacelle can block certain emissions 90deg to the engine axis which might make the installation compliant. Noise is generated where flows of dissimilar speed mix.

flow from a core mixing enter image description here

An extended nacelle encloses noise from the instabilities, shielding an observer abreast the noise source where certification measurements are made. A short nacelle provides no shielding.

mixing

The engine installation has to meet noise requirements when it is certified for use on an aircraft. Some manufacturers surpass the requirements, attempting to appeal to customers that might be more noise sensitive. Rolls claims to have the quietest installation on the 330 and they are the only one with the extended nacelle. The underlying engines differ in design; they also differ in noise output. It wouldn't surprise me if the 700 core failed to meet the reg with a mid nacelle, exceeded it with the long, and the marketers turned necessity into a feature. The Trent 7000 is developed from the 787's Trent 1000 and has a different core and its installation on the A330neo uses a mid length nacelle.

A similar but less critical situation exists for the flow from the front fan. The short nacelle on your 747 example would not provide as much noise shielding from the front fan as the mid length nacelle. The PW4000 is a derivation of the JT9D, so it is not the same engine as you state. More importantly, its certification on the 747 took place some twenty years after the JT9D under more strict noise regulations, hence the longer nacelle.

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  • $\begingroup$ Hi @Pilothead. Your answer that the difference in exhaust design is due to noise restrictions might be valid for engines designed decades apart, but how about the A330 examples in the question? Would not these all be subject to the same noise requirements? I agree it's an interesting question, and don't know the answer myself, but I can see the noise reason "flying" from the A330 example. Cheers. $\endgroup$
    – Penguin
    May 31, 2018 at 12:29
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    $\begingroup$ @Penguin The engine installation has to meet noise requirements when it is certified for use on an aircraft. Some manufacturers surpass the requirements, attempting to appeal to customers that might be more noise sensitive. Rolls claims to have the quietest installation on the 330 and they are the only one with the extended nacelle. The underlying engines differ in design; they also differ in noise output. It wouldn't surprise me if the 700 core failed to meet the reg with a mid nacelle, exceeded it with the long, and the marketers turned necessity into a feature. $\endgroup$
    – Pilothead
    May 31, 2018 at 13:45
  • $\begingroup$ OK, the philosophy of surpassing the noise requirements is a good explanation. Thanks for replying. I think your comment, which has some good extra detail, has improved the answer, as I felt it was an obvious question that many might ask, and has now been rectified. I hope you feel the same. Cheers. $\endgroup$
    – Penguin
    May 31, 2018 at 19:18
  • $\begingroup$ @Penguin Questions are great and always welcome. I agree they improve answers. In this case I think the core was just too loud and they had no choice short of a redesign. They did that with the 7000 in the neo and went back to a mid length nacelle. $\endgroup$
    – Pilothead
    May 31, 2018 at 19:20
  • $\begingroup$ "Not meeting noise regulations is the primary reason for using a long nacelle." I don't understand. Why would not meeting noise regulations ever be a goal? $\endgroup$
    – David Richerby
    Feb 8, 2019 at 11:49
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It is a trade-off. A long nacelle eg RR Trent 700 can give a performance (by mixing bypass and core flow) and acoustic suppression gain but it is heavier and produces more drag.

see "The Jet Engine" published by Rolls-Royce, p 231.(ISBN 0 902121 2 35)

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