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Russian engineers in China is an open secret. I am not downplaying China's R&D capabilities but they were necessary to help China bridge the generation gap in aircraft development.
The engineers at Chengdu genuinely wanted to make the J-10. However many members of the military were not optimistic about it. Much of the funding went to the big brother, the SAC and during much of the 80s the CAC had to sell bikes and refrigerators to stay afloat.
There were plenty of R&D projects that China launched that ended in failure. The J-12 is one of the best examples. The J-9 didn't even make past the model phase while the J-8 almost died due to the Cultural Revolution.
If those engineers are not just rumors but indeed true, why wouldn't the Russian government pull them out? Aren't they are currently alarmed by the latest Chinese aircraft development? and even said that China stole Russian technology??
Or are those civilian people hired by China's R&D?
Of course due to lack of funding many projects did fail.. But with the current funding I think China can allocate more to R&D.
Since China is open to hiring foreigners, why don't they also hire European or American experts to help them in their R & D in other technologies, not just for military purposes?