To understand what kind of aircraft it is you have to see History, China bots will tell a bunch of lies that the aircraft came from a failed project J-9 and so so.
In reality the aircraft is a new design, not related to any old project.
Is a 5th generation fighter? the answer is not at this moment.
If you look before the 1990s most aircraft carried their air to air missiles on external hard point, why? the external weapons bay reduce the size of the aircraft and reduce the cross section
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If you compare a Mirage III and a F-106 you will see the american aircraft is much bigger and heavier due to weapons bay
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So while they say it is a 5th generation fighter it is a big aircraft thus it is much heavier than even Su-27/30 at empty weight.
So the use of internal weapons bays are only useful if you increase engine thrust in order to take the advantage of a cleaner aircraft aerodynamically
The Chinese thus need a very powerful engine and since they did not have it they opted for canards, these by themselves do not equal better agility, a F-16 or SU-27 can beat aircraft like Viggen or Kfir.
What makes an aircraft agile is a high Thrust to weight ratio and a low wing loading a feature that eurofighters typhoon, rafale or Gripen do have.
Canards increase wing loading at different AoA thus they chose them because they have a delta wing.
Thrust vectoring does not increase instantaneous turn rate, it only increases sustained turn rate.
Therefore the increase only is in the range of 10% at the most because let is remember by vectoring the nozzle thrust you reduce the thrust used to move forward aircraft in a straight direction and potentially it can create spin on its own axis as in post stall maneouvring.
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Thus while the J-20 follows basic aerodynamic features already seen on F-22, MiG-1.44 or F-35 the jet by using canards or thrust vectoring does not make it superior.
There is no way you can make an aircraft invisible to radar simply because aircraft are made with many materials and some are highly reflective.
Stealth is only reduction in the radar range and this is limited by radar power density and wave length.