Unlike the U.S.'s vintage SR-71, world's first stealthy aircraft the F-117 and the hugely impressive B-2, all developed under the utmost secrecy, the J-20 is having a particularly open development process (even more unusual when you consider China's normal habitual secrecy). Last week it was having taxi tests, and now it looks like it's flown for the first time:
You can bet your bottom dollar that American, European, Japanese, Russian, and who-knows-else security services have been training every space-borne radar, infra-red, and optical imaging system they can on the flight test zone, trying to learn how well the J-20 works.
But if you too are curious about how its stealth tech may (or may not) function, and how it compares to the F-22 Raptor, then read on. You'll be surprised, or possibly not, by how similar the aircraft are.
No flat surfaces
Flat surfaces are very good at reflecting radar--if you're using a dish far away to look at lots of flat surfaces, then lots of radar waves get sent back to you and you can tell where the target is. The original F-117 used lots of flat surfaces in its design, but each was carefully arranged so that the radar waves were bounced off in different directions. The new F-22 (like the B-2) avoids large flat surfaces, and uses a more aerodynamic shape, where blended curved surfaces bounce radar off harmlessly in directions away from the detecting dish. The J-20 looks like it uses very similar thinking--and the nose of the aircraft looks very familiar indeed.
Doors, hatches, edges and even the cockpit edge on the F-117, B-2 and F-22 have complex "notches" cut into them, so that when they're open or closed there are no big give-away seams or straight edges that reflect radar well--similar to the avoidance of large flat structures on the plane. The J-20 seems not to incorporate this tech. But it is an early prototype.
Canted tail fins
A technique used on the SR-71 and F-117 to hide the large flat surfaces of the aircraft tails (vital for aerodynamics, control and combat maneuverability) was to cant them at an odd angle so that they don't reflect radar in directions that would give away the aircraft. The J-20 does this trick too.
Angular air intakes
The air intakes on the F-117 and B-2 are carefully shielded since their large scoop-like shape tends to create what radar scientists call corner reflectors--shapes that can reflect radar much more strongly than a flat plane. Hiding the air intake in the structure works to obscure it from enemy radar, but the F-22 uses a more advanced trick where the intakes are, like the tail fins, arranged at peculiar angles that avoid forming corner reflectors. The J-20's intakes look a lot like the F-22's.
Metal is better at sending back radar waves than carbon fiber, plastic or fiberglass. That's why the F-117, B-2 and F-22 (and the European Typhoon) are made of large amounts of composite materials. As well as making them structurally strong, this lowers their radar signature--and can even affect its infra-red signature too. We have no idea what the J-20 is made of, but some of the larger box-like shapes of its body, and its all-moving tails (which need super-strength) make us think it's also made of advanced materials.
Infra-red signature, super-cruise engines
The F-117 and B-2 hid their engine exhaust from infra-red sensors (on missiles and other surveillance systems) by venting the jet plume through louvers and over the body of the aircraft. The F-22 can't do this trick, as its more powerful supersonic-capable engines make the effect harder. Instead it uses a slightly masked exhausts, and a technique called supercruise, which means it can break the sound barrier without needing the infra-red give away of reheat. This lowers its infra red signature a lot. The J-20 has been spotted  running a couple of different engine variants--and all of them would seem capable of supercruise. But its big cylindrical engines are more of an infra-red give-away.
This isn't a stealth tech per se, but is a mark of modern fifth-generation aircraft: They can lens and flex the engine exhaust petals to send the jet exhaust in different directions--meaning the aircraft can perform impossible "non aerodynamic" maneuvers. This trick can throw off following surveillance systems or possibly even fast-flying missiles. The F-22 uses door-like vectoring wedges, but the Chinese-made WS10-A and Russian AL-31F engines that seem to be  in the J-20 also have thrust vectored addons in development.
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