Imo, i think those gaps seen in the first pictures that everybody is/ was talking about are there for a very good reason , i.e. aerodynamic and /or temperature considerents dictating the shape of the engine fairing, only that back then the shrouds that were sealing those gaps smoothly were not there ( probably the engines have just been installed). If you look at JF-17 or even J-20 you will see the same arrangement, if you'd see a picture of them without that last piece of the engine shroud between the nozzle and the fairing , you'll have the same impression that the engine/s it stands in for something else and they look like an afterthought.
Mind you, i know the J-20 engines are not definitive, but the rumoured WS-15 can't be that different size-wise compared to the present ones. As for J-31, imo those engines are either RD-93 or WS-13 (which someone said it's hard to distinguish from RD-93!) . It is a relatively small aircraft , so nothing bigger will fit there. Anyway , just look at all those CGI's around, it gives a good idea of the engine fairing shape especially towards the nozzles.
Looking forward to some good news and especially pics tomorrow. Hope all goes well.
That is all assuming that the back end of those aircraft were designed to take a specific engine.
The J10 famously had to switch from the WS10 to the AL31 during development, but there is every chance that the original design allowed to WS10(or rather WS10A now) to be retro-fitted later. The fact that there appears to be no external changes between J10Bs with AL31 and WS10As would support this theory.
Similarly, the J20 was always designed to use WS15s, which may well have slightly different dimensions to the AL31/WS10A they have installed already.
Even the JF17 has been designed with modular design from the start, which would allow for the installing of different engine options based on the requirements of the customers.
If spacing was required for some fundamental purpose, you would expect to see that quite consistently on planes of all nationalities. But most designs from other countries, many of which which use the same engines as the J10 and JF17, do not have similarly sized gaps between their engines and their nacelles.
SAC itself is no stranger to switching engines, as they have done with the AL31 and WS10A on their various flanker variants.
With all that in mind, I think it is quietly likely that the gaps we can see on many modern Chinese designs are put there because China's engine sector still lags behind the rest of it's aviation industry, and as such, the designers were forced to make do with engines that were available instead of what they wanted. However, in order to not limit the potential of their designs, they could have designed the aft of their fighters to allow for the easy installation of engines of slightly different dimensions so that their designs can meet their full potential once the suitable engines become available. As such, it is entirely possible that the J31 was designed and built with oversized engine nacelles to allow for the retrofitting of a more powerful fifth gen engine once that because available.
Leaving such a large gap in the fuselage of a plane is ever a great idea, because all sorts of foreign objects can easily get into the plane that way, and exposing the insides of the plane to the elements like that could also cause you all sorts of maintenance headaches down the line. Considering the tiny effort it would take to design and install a cover, I would be pretty surprised if they do not both to cover that gap up at some point.
