Does a jet engines air intake count to the overall jets geometry dependent air resistance?

Taking the Saab Tunnan as an example it's overall air resistance has to be quite low because of it's huge air inlet.

Or does the inlet create air resistance as well but just not so much?

  • 2
    $\begingroup$ I'd venture to say that its supposed to create air resistance. You need to lower the speed of the air entering the engine, especially in supersonic jets to a sub-sonic speed before it goes into the compressor stage. $\endgroup$
    – Ron Beyer
    Jun 3 '16 at 17:54
  • 1
    $\begingroup$ I'm no expert, but I've heard that the shape of the intake is important. They bolt on a special intake when testing engines in a test cell, which allows the air to enter "cleanly". If the air has to turn a sharp corner coming in, it doesn't flow properly. $\endgroup$
    – Steve
    Jun 3 '16 at 17:59
  • $\begingroup$ Yeah you have to bring the air to the whole fan. Not just a part of it. If the intakes geometry isn't well designed it could happen that the air flow just hits the middle of the fan and as a result also only the middle of the compressor fans. You have to make sure to hit the whole fan with air. Otherwise its inefficient. $\endgroup$
    – user148013
    Jun 3 '16 at 18:02

This is a very tricky topic. Basically, you need a clear definition of what is part of the engine-related drag and what is counted towards airframe drag. Splitting up drag between components will always produce an arbitrary result.

An intake can generate thrust as well as drag. In case of the SR-71 at Mach 3, the intake contributed 54% of overall thrust, if you just look at the pressure distribution over the intake walls.

Generally, the job of the intake is to feed air to the engine compressor at just the right speed and as homogeneously as possible. This requires it to suck in air when at rest from all around the aircraft, and to ingest only the core of the stream of air flowing towards it at high speed. What is not ingested needs to flow around the intake lips and the engine fairing, and the drag caused by this process even has its own name, being called spill drag.

At high speed the intake needs to slow down the oncoming air and does so already ahead of the intake lip at subsonic speed. At supersonic speed this slowing down is accomplished by shocks, which adds another source of drag. This can be easily counted as part of the airframe drag, but done right this slowing down will compress the air, which in turn will increase thrust. Apart from viscous losses, this compression pays for itself and is almost drag-neutral in a good design and at ideal conditions.

Next is the flow around the intake. Just throttling the engine will change the airflow through it and thus the flow pattern around the intake. You can imagine that some configurations show marked changes in aerodynamic drag depending on the throttle setting. In case of the Eurofighter, the vicinity of the canard will even cause throttle changes to result in notable pitch moment changes.

To answer your question: Yes, the intake will produce drag, and how much it does depends on how you define drag. It is certainly wrong to think that the stream tube hitting the intake will just disappear into it and not add any drag.


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