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Why is a Boeing 737 NG engine not completely round in shape? – it seems to be flat at the bottom and round at the top and sides. Earlier Boeing 737 versions (100 and 200) seem to have a more rounded shape.

Boeing 737

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Thanks to all of you who took time to clearly explain the reasons for the shape of the nacelle/engine in Boeing 737NG. Srikanth –  Srikanth Jul 23 at 14:26

2 Answers 2

The engine in question is a CFM56 turbofan engine which is larger than the previous JT8D and thus has less ground clearance. This meant that they needed to flatten the bottom by moving the accessory gearbox to the side from the bottom and shrinking the fan.

To quote Wikipedia:

In the early 1980s Boeing selected the CFM56-3 to exclusively power the latest Boeing 737 variant, the 737-300. The 737 wings were closer to the ground than previous applications for the CFM56, necessitating several modifications to the engine. The fan diameter was reduced, which reduced the bypass ratio, and the engine accessory gearbox was moved from the bottom of the engine (the 6 o'clock position) to the 9 o'clock position, giving the engine nacelle its distinctive flat-bottomed shape. The overall thrust was also reduced, from 24,000 to 20,000 lbf (107 to 89 kN), mostly due to the reduction in bypass ratio.

They also mounted the engine in front of the wing rather than below to get every bit of clearance they could.

Front view Side View

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I added three pictures, if you disagree with it, please feel free to remove –  CGCampbell Jul 22 at 14:49
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FYI, the picture on the upper left is a rendering of the 737 MAX. The LEAP engine is even bigger than the CFM56 so the 737 MAX will have to try even harder to fit that under the wing than the 737 NG. –  fooot Jul 22 at 15:40
    
the LEAP's fan blades are 173cm in diameter –  shortstheory Jul 22 at 16:06
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Versus the CFM56-7 at 155cm –  fooot Jul 22 at 20:59
    
Gotcha, sorry about that. I'm editing this answer to remove a rendering of the wrong aircraft/engine. –  CGCampbell Jul 24 at 17:46

You are confusing the shape of the engine with the shape of the nacelle. Axial-compressor jet engines (standard on larger commercial jets) are invariably round and thin1. Engines with a centrifugal compressor are fatter in the middle and will occasionally be lumpy shapes depending on the internal airflow (some engines completely reverse the flow of air through them, sometimes several times2).

Positioning accessories like pumps, starters, generators, control boxes and minor things like mounting brackets are flexible. The engine manufacturer may dictate the position of some things like a gearbox, but if it attaches with a hose it can generally go anywhere the airframe designer wants to put it. The nacelle is then built to contain everything, not produce too much drag, not hit the ground, and preferably not cost too much to make. There's no reason we can't make hexagonal nacelles, we just choose not to.

  1. I consider the fan to be a bolt-on here, not part of the engine itself.
  2. See Pratt & Whitney PT-6 for an example.
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