I would argue that extensive avionics upgrades are even more drastic than re-engining a plane. There are far more complex and inter-dependent systems involved. (Though one of the things that will probably need to be adjusted for a re-engined J-20 will be the inlet).
J-20 5th Gen Fighter Thread IV (Closed to posting)
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I would argue that extensive avionics upgrades are even more drastic than re-engining a plane. There are far more complex and inter-dependent systems involved. (Though one of the things that will probably need to be adjusted for a re-engined J-20 will be the inlet).
Verum, we better informed bunch here do not doubt, we are quite sure, that J-20 will not be equipped with WS-15 when it enters service as early as 2017 (some even say 2016). But our resident big shrimps here, like their counterpart on those Chinese forums, seem to have decided to delegate all the worry-about-WS-15-and-J-20 work to others concerned.
And as some of them have noted, most of us here have faith in that CAC knows what they are doing a LOT better than anyone else, and that the good old WS-15 is doing OK and will come out gracefully in, say, about 5 years. 
plawolf
Lieutenant General
The problems with re-engining planes comes up mainly because the original airframe was never designed and built with the new engine in mind, so you end up backing to make major structural redesigns to allow the old engine fairings to accept a different engine.
Its a completely different kettle of fish when the airplane was designed from the ground up to have the final engine in place, and the current one is just a stopgap measure.
The J20's engine bays would have been designed from the start to fit the diameter of the WS15. All the connection ports and load baring structures would also have been tailor made for the WS15.
Its the current AL31s that would have been jury rigged to fit the bigger bays.
When the WS15 is ready, they would need to undo the custome piping and structural mountings to allow the AL31 to fit, but that should be considerably easier than putting all that in place in the first place.
As for the lack of thrust limiting test flight. Well any limitations could be made all but irrelevant using clever testing.
The easiest thing would be to limit fuel load. They could simulate the thrust weight ratio the J20 would be with the WS15 and full combat load by starting the manuevers with only a fraction of the full fuel load.
A lot of the fuel dumping we have seen in the past few years could easily be J20s rapidly shedding weight to reach a pre-determined weight before starting certain maneuvers.
The other options asside from reducing weight is to increase thrust.
Again, we have seen a lot of pictures of J20s in afterburner. They could easily do the test fight in afterburner to get the data, but once the WS15 is ready, the J20 would be able to perform the same maneuvers in military power.
Obviously, there would still need to be test flights with the new engine once its ready, but the number of test flight and the potential impact the results might have on the final design could be minimised with measures like those above and with computer modelling to make those test flights more for validation of results rather than discovery.
Its a completely different kettle of fish when the airplane was designed from the ground up to have the final engine in place, and the current one is just a stopgap measure.
The J20's engine bays would have been designed from the start to fit the diameter of the WS15. All the connection ports and load baring structures would also have been tailor made for the WS15.
Its the current AL31s that would have been jury rigged to fit the bigger bays.
When the WS15 is ready, they would need to undo the custome piping and structural mountings to allow the AL31 to fit, but that should be considerably easier than putting all that in place in the first place.
As for the lack of thrust limiting test flight. Well any limitations could be made all but irrelevant using clever testing.
The easiest thing would be to limit fuel load. They could simulate the thrust weight ratio the J20 would be with the WS15 and full combat load by starting the manuevers with only a fraction of the full fuel load.
A lot of the fuel dumping we have seen in the past few years could easily be J20s rapidly shedding weight to reach a pre-determined weight before starting certain maneuvers.
The other options asside from reducing weight is to increase thrust.
Again, we have seen a lot of pictures of J20s in afterburner. They could easily do the test fight in afterburner to get the data, but once the WS15 is ready, the J20 would be able to perform the same maneuvers in military power.
Obviously, there would still need to be test flights with the new engine once its ready, but the number of test flight and the potential impact the results might have on the final design could be minimised with measures like those above and with computer modelling to make those test flights more for validation of results rather than discovery.
The one important matter that is more difficult to test with different engines is the air intake. J-20's air intakes are probably designed for the WS-15 engine and a lower performance due to intake mismatch will be accepted and corrected for when considering the test results.
There ya go! the right answer, these airplanes are basically being "scratch built", and by that I mean one at a time they are fabricated like the early Raptors,,,,, one of the Raptors problems was that each airframe was "hand built", and many parts would not interchange between tail numbers. The engines may be a little different in size, but the engine is contained within its individual "case", and installed in mounts, it would be no big deal to fly an AL-31 on one side, and a WS-15 on the other side, of course "inlets and ducting would have to changed to properly "fit" each engine??? no big deal fellas! its done all the time!
and they will likely do just that when they begin to flight test the WS-15??
plawolf
Lieutenant General
True enough, but I think that can be greatly mitigated by the use of computer modelling and scale or even full sized model tests in wind tunnels
I think they will modify the intake design for the "B" model once WS-15 is ready.
Personally, I believe that any engine change on J-20 before 2017-2018 is wishful thinking. From an engineering standpoint, you want to mitigate developmental risks during the design and testing phase as much and possible. A good way to do that is to eliminate as many "variables" as possible. Introducing another engine (whether it be WS-10 or WS-15) this late in the stage presents a huge "variable" to the development process.
Well, it seems pretty clear that they're not waiting on the engine. We can probably assume that however quickly the WS-15 completes development they're going to go ahead with Al-31s for the first and maybe second batches.
You don't need full scale wind tunnel tests anymore. Some wind tunnels ( very expensive ones; perhaps we can start a thread on China's wind tunnels and other test infrastructure ) can vary the temperature as well as the air speed so that for a scale model both Mach Number and Reynolds Number can be equal to that of the full scale aircraft in flight. But for cruising flight of transport type aircraft and perhaps by now for more difficult cases computer simulation of the flow field around the aircraft can be very good indeed.
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