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An inlet cone, as far as I know, is a component on an engine to guide the intake air smoothly into the compressor stage of a jet turbofan engine, it performs as a fairing(aerodynamic covering)of the axis on which the intake fan is installed, to improve the aerodynamic efficiency. *The engines are all on subsonic airliners.

However, I found that on some modern high bypass turbofan engines, this cone(fairing)is in a pointed, sharp, conical design that seems to only appear on supersonic aircrafts, as a rounded design is optimized for subsonic flight. For example, in the images below: enter image description hereenter image description here

While on some turbofan engines, this cone(fairing)is in a smooth, rounded design, very much like it would have been used on the nose design of the aircraft. For example, the engines on a MD-80:enter image description here Why do those modern engines have this conical sharp intake cone(or fairing)design? Is this because of the air acceleration above the speed of sound when intaking air at cruising speeds?

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This is not an inlet cone as found on supersonic aircraft engines. This is a spinner used on subsonic turbofan engines, both the rounded and pointed one. Older engines used rounded ones; the JT8D in your question was first designed in 1963. Later engines use pointed ones. There are three purposes:

  1. The spinner provides a smooth flow path for air entering the engine. Both shapes can do this well.

  2. The shape minimizes ice accumulation so anti icing is not necessary.

  3. The conical and elliptical shape helps to deflect FOD away from the engine core. Pointed spinners are better at remaining intact after impact, as from a bird strike. FAA testing for bird ingestion gets only more rigorous, as in this proposal.

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  • $\begingroup$ Thank you for the answer. However, I found that the engines(PW-1100)on the later released A320-neo actually have rounded spinners. As in your answer, this shape is not the optimal design, and this newly-designed engine shouldn't be using such spinners. What is the purpose of still using them? $\endgroup$
    – Richard M
    Commented Oct 22, 2022 at 9:32
  • $\begingroup$ @RichardM This is a design choice. I gave you the main considerations which neither choice would fail to satisfy when done correctly. Older GE and Rolls engines like the CF6 and Avon used rounded spinners but newer engines from these same manufacturers use pointed. Pratt generally has stuck with rounded except for its v2500 IAE joint venture engine, apparently deciding that there was not enough difference to make a change. $\endgroup$
    – Pilothead
    Commented Oct 22, 2022 at 17:34
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To elaborate on Pilothead's answer's point #2:
A conical spinner accumulates ice less than a rounded spinner because it has a much smaller stagnation point (well, nearly-stagnant area). However, it also sheds more ice into the core, at least for the B737 (italics are my addition):

The CFM56-7 spinner has a unique conelliptical profile. The first 737-3/400's had a conical (sharp pointed) spinner but these tended to shed ice into the core. This was one of the reasons for the early limitation of minimum 45% N1 in icing conditions which made descent management quite difficult. They were later replaced with elliptical (round nosed) spinners which succeeded in deflecting the ice away from the core, but because of their larger stagnation point, were more prone to picking up ice in the first place. The conelliptical [pointy, then rounded farther aft] spinner of the [737-NG]'s neatly solves both problems.

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