The thread title says it all. A J-XY based on the FC-31 is probable but not certain, that's my understanding anyway. As to whether this paper has to do with that conversion or an exercise in a clean sheet design, that's anybody's guess.
Considering it's from Shenyang, and considering the elements of the models cannot be for J-20 (e.g.: no canard in the position behind the DSI air intake lip), and the specifics regarding the "before" and "after" aspects that are described, I think the chances of this being a "before" (FC-31) and "after" (J-XY) are not low at all.
And of course, this paper would be the one that @Interstellar talked about here:
Shenyang FC-31 / J-31 Fighter Demonstrator
F-35 has no choice, there's a great huge lift fan and doors that need to open up behind the cockpit. Personally I don't think that image is high quality enough to be making these observations and I expect no change from what we've seen so far. That might be the case but KF-21 and AMCA have...
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