I generally don't like to reference Key Pubs forums due to the amount of noise there, but occasionally there are interesting finds and discussions. In particular, regarding the PAK FA, a certain user believes that advances in composite materials can make the engine's compressor face or the inlet guide vanes stealthy, rendering S-ducts unnecessary and therefore obsolete. He did link several studies in the public domain, like this one. I'm wondering if members of this forum can pitch in on this concept: can this concept be implemented right now? Because if this really is possible, then more efficient straight inlets may return for 5.5 or 6th generation aircraft.