How, for example, would the long inlet of the tail-engine on an MD-11 or an L-1011 affect its thrust production/performance, if at all?

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I am not well versed in fluid dynamics, so any insight on this would be helpful. I wouldn't be surprised if the effect of the aforementioned distance is negligible, as the airflow would not seem to be disrupted. Nonetheless, this piques my curiosity.


2 Answers 2


Long, winding intake ducts can increase the drag on the airflow moving through them, reducing the amount of air which the engine can ingest as opposed to a traditional diffuser design on a nacelle. Aircraft like the Harrier as well as the F-22, F-35 and F-117 have solved this problem with the use of large, suck in doors along the length of the intake duct in order to provide additional air mass flow during low speed or ground operations, where ram pressure cannot provide the additional airflow through the fan or engine core.


In subsonic flight, Long ducts will increase drag on airflow because the gap between boundary layer and duct inner surface will increase as going horizontally towards the engine.

But in another case during supersonic flight, long ducts can be of advantage because the shock wave produced by ram air will deflect and travel more through duct as shown in figure, which will increase the pressure even before it reaches the compressor.enter image description here


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