try to understand this S ducts are not new, they have existed since the 1950s,
Tell me did the J-35 Drakken or F-105 were built for stealth? what about MiG-21?
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J-35 Drakken had S intakes ducts, same F-105 that even forward swept intake lips
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Same F-101 all these 1950s used S ducts to set intakes positioned in the wing root reducing fuselage cross section.
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So stop thinking S ducts were new or for stealth, the decision to set S intake ducts on F-22, J-2-, F-35 or J-35 is based upon a general arrangement configuration.
Su-57 or X-32 use radar blockers for other reasons everything depends in the designers constrains. limits, needs and design goals.
on modern fighters S ducts are for allowing hiding the engine fan blades and allow side and bottom weapons bays.
On Su-57 or X-32 they were to allow excellent air flow and engines with less curved air intake ducts for VSTOL needs in X-32 and larger weapons bays in Su-57, however radars blockers also have cons and pros, F-117 has a radar blocker grilles but externally but that jet is an attack aircraft.
the F/A-18E and Su-57 have different types of radar blocker grilles.
Everything is what the designer wants Su-75 has an U intake using the advantages IAI lavi or F-16 have or even J-10C but improved for lower drag.