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For subsonic aircraft (Mach 0.7), Is boundary layer diverter a necessary requirement for jet intakes? A lot of legacy aircraft were flying without them.

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  • $\begingroup$ According to the 0th law of aerodynamics: brick flies with enough thrust, no, it's not necessary. But some subsonic jets do have them, e.g. K-8/JL-8 Karakoram, L-39 Albatros. $\endgroup$ Dec 27, 2022 at 7:30

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At the beginning of the jet era, airplanes where powered by turbojet with centrifugal compressor which, as explained here, were able to ingest (and digest) even a not perfectly clean airflow. A typical example of this kind of turbojets was the Rolls-Royce Nene which was used on many jets of the post WW2 era:

Rolls-Royce Nene

With the advent of the more efficient axial-compressor turbojets/turbofans, the cleanliness of the airflow has become important due to sensitivity of the axial-compressor to the airflow it ingests. At that point boundary layer diverters or, more generally, short intakes, have become necessary to provide the first compressor's stage of the jet engine with a clean and stable airflow.

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  • $\begingroup$ Axial compressor turbojets were used in production aircraft even before centrifugal compressor ones (the BMW 003 and Junkers Jumo 004). As far as I can find all designs that used them had pitot (round, not attached to anything) intakes. $\endgroup$
    – Jan Hudec
    Dec 29, 2022 at 9:36
  • $\begingroup$ @JanHudec: yes right, they had a straight intake from the nose. It might be that the combination radial jet plus intake without diverted was an intermediate step. I'll add your comment to my answer if you agree. $\endgroup$
    – sophit
    Dec 29, 2022 at 10:04

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