Is there an advantage to the oval/elliptical shape of the Tu-154's S-duct intake? It appears to be taller and an overall different shape than the side-mounted cowlings.


This doesn't appear to be a feature common to trijets or even on those with S-ducts.

L-1011 Dassault F900

EDIT: Upon further research, it appears the 727-100 had an oblong intake (Thanks Terry!), and the Hawker Siddeley Trident had an entirely unique shape. So my original premise of the Tu-154 being "unique" no longer holds. I am still curious about the implications of the different designs.


2 Answers 2


This doesn't fully answer the question, but the B727-100 also had an oval engine intake.

AIRLINERS magazine Sept/Oct 99 edition:

"The oval shape inlet on the 727-100 series was changed to a circular design on the stretched 727-200. This was a direct result of the new aircraft's longer fuselage. In flight, the increased lengths from the nose to the center engine inlet increased the thickness of the "boundry layer" of air on the fuselage forward of the inlet. This would have caused aerodynamic "blanking," or disruption of airflow into the center engine, without changes to the design. The circular inlet allowed a smooth flow of air into the engine, as it elevated the bottom of the inlet away from the fuselage boundry layer and into the air stream."

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  • 1
    $\begingroup$ That's definitely a step in the right direction! On an unrelated note, I love that VP-BAP is one of the few privately owned 727s still operating in the US. $\endgroup$
    – zymhan
    May 16, 2019 at 19:05

The vertical fin is tall and thin, so it makes sense that an inlet/duct mounted in the fin and configured for optimum streamlining might not be perfectly round.


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