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Aircraft like Airbus A320neo/A330neo/A350 series have a visible exhaust cone, which is not visible in the older versions of Boeing 747, Airbus A330/A321/A319 etc. Why is this design preferred over the other?

Image: Exhaust Cone not exposed
Image: Exhaust Cone exposed

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  • $\begingroup$ Note that on A320CEO the CFM56 engines have outer and inner nozzle and the exhaust cone just barely visible, but the IAE V2500 engines have common nozzle and the exhaust cone is fully inside. $\endgroup$
    – Jan Hudec
    Dec 21, 2020 at 20:13
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    $\begingroup$ An image of 'visible' and 'not visible' would be very helpful. $\endgroup$
    – skipper44
    Dec 21, 2020 at 20:16
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    $\begingroup$ Welcome to aviation.SE! Does this question help? $\endgroup$
    – Pondlife
    Dec 21, 2020 at 22:56
  • $\begingroup$ @skipper44 I've updated the question with Image URL, Thanks for pointing out. $\endgroup$
    – lousycoder
    Dec 22, 2020 at 19:12
  • $\begingroup$ @Pondlife Thank you! I can't say exactly how, mainly because I'm not an experienced aviation person. I've updated the question with the image URL and when I compare those images, it seems that the engine with exposed/visible exhaust cone may have better bypass ratio maybe? $\endgroup$
    – lousycoder
    Dec 22, 2020 at 19:24

2 Answers 2

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The « cone exposition » is a result from nacelle engine design, and the choice to mix flows or not. The two main strategies are called:

  • Mixed exhaust engine resulting to long nacelle hiding exhaust nozzle and plug.

enter image description here

  • Separate exhaust engine which is shorter and then allows a view of nozzle component.

enter image description here

The reason to choose one type of exhaust is made by considering overall engine performance (mainly efficiency and fuel consumption), noise but also visual pleasing (for business jet for instance).

Exhaust mixer is generally more efficient (secondary flow is slightly accelerate producing more thrust) and less noisy (primary flow is slow down). They might be also an advantage in material resistance in cooling hot primary flow. See this article for details about advantage of mixed exhaust.

Nevertheless, long nacelle has the overcome to increase weight and drag of the engine and this is worst as the engine is bigger. Note that it is related to turbofan by-pass ratio (BPR). More the BPR is high and bigger the engine diameter is. For commercial aircraft, the trends is to increase more and more the BPR parameter because it is directly correlated to propulsive efficiency. Therefore, there is a kind of « BPR limits » where mixed nozzle benefits do not balance its drawback.

That's why, modern turbofan (high BPR and high diameter) have likely to integrate a separate exhaust nacelle.

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It depends on the airframe/engine combination rather than than newer and older airplanes.

Very generally, P&W have mostly had visible exhaust cones when compared to GE and RR, atleast on widebodies.

See this image of the P&W JT9D on the wing of a 747-200., the RR RB 211 on the wing of a 747-400), and the GE CF6 on the wing of a 747-400

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