Why does J-20 have clear serration on the trailing edge of its wings and canard but F-35 and F-22 both don't have that?
There are 2 main reasons
First reason is the location of the horizonal stabilizer/canard.
Because small parts of aircraft will generate creeping wave earlier than big part. The small fin/canard will generate creeping much sooner. On F-35 and F-22, the horizonal stabilizer is behind the main wing, so the creeping wave return if generated will be partly blocked by the main wing. On J-22, the canard is in front, so they need away to redirect the creeping wave
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The second reason is platform alignment.
On F-35 and F-22 the leading edge of the main wing have the similar direction as the leading edge of the horizontal stabilizer
the trailing edge also have similar direction as the trailing edge of the horizontal stabilizer
On J-20, the leading edge of the main wing also have the same direction as the leading edge of the canard. But the trailing edge of the canard and the trailing edge of the main wing are different
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