Like other PLAAF fighter aircraft, the FC-31 is powered by Russian-made jet engines, in this case two Klimov RD-93 models, which are a specialized variant of the same engine that powers the famous Mikoyan "MiG-29." There have been rumors that a team of renegade designers from the Mikoyan bureau in Moscow assisted Shenyang in the development of the aircraft, but a senior MiG official stated "no, as far as I know they [the Chinese] completed this design themselves, and they seem to have done a good job on their own."
The MiG official may be correct. An aircraft of this type designed with Russian assistance would probably perform better.
The FC-31's flight routine shows that it "bleeds" too much energy -- so when it enters into a turn it begins to lose altitude. Even during straight and level flight the pilot has to engage the engine's afterburners in order to keep the aircraft from sinking to a lower altitude. These are defects in the aircraft's aerodynamic design that a Russian design team would not have made.
Western aerospace analysts point out that the FC-31 flown at Zhuhai is a "clean" jet in that it is not armed, which means that an aircraft configured for a real mission and fitted with weapons would be even heavier and would perform even worse.
Отсюда
Если кратко - дизайн сам по себе ущербный, самолет плохо летает, постоянно используя двигатели для коррекции. Когда FC-31 поворачивает, он теряет высоту! Пилот вынужден использовать движок, что б приподнять машину. Самолет теряет слишком много энергии на маневрах. Ни у Рапторов, ни у Т-50 такого нет. Китаецы такие китаецы