gambit
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If you like F1 racing, then you have at least a cursory understanding of the aerodynamics involved in those cars. Me ? Bikes. Motorcycles are aerodynamic disasters.As a Mechanical engineer and a lifelong Formula 1 fan I do have many questions regarding changing area of an Inlet of an engine (let alone a DSI one on a stealth aircraft where you require a very good engine efficiency to keep IR signature low too) and I enjoy your posts on these things too as they are very informative. Just saying we are never going to get the answers of all these questions
Let us assume that the claim of a variable DSI bump is real and installed in the jet.One thing more about variable DSI. What happens to RCS of J-20 as its DSI changes shape. I mean we talk all the time how RCS suddenly increases when the weapon bay is opened and Serbs even claim the change in RCS was just what they needed to bring F-111 down. Can similar thing happens with a variable DSI ??
If you alter the physical characteristics of a structure, the behaviors of the signal waves changes. This have been mathematically formalized by Petr Ufimtsev whose textbook I have on my shelf. I doubt that the changes produces by the shapeshifting DSI bump is as dramatic as the opening of a weapons bay, but signals leaving the DSI bump as it changes its shape and dimension would produce different behaviors when interacts with nearby structures, which would affect final RCS.
It is not " Can similar thing happens with a variable DSI ?? " But similar things WILL happen with a variable DSI bump system. These are the laws of physics that not even China can violate.
Will final RCS increase ? Yes, it will. How much to raise the J-20 above the minimum threshold ? This is where EM anechoic chamber testing is needed and just like the US with our 'stealth' platforms, China is not going release any EM anechoic chamber testing data of the J-20. But shapeshifting the DSI bump will influence final RCS.
This is why it is so entertaining debating the J-20's supporters on this forum. They are clueless on how their claims for the J-20 is consistently -- inconsistent. They consistently do not perform even basic research.
They claim that the J-20's all moving rudders -- vertical stabilators -- are superior to the F-22's rudders. Never mind that the US have the SR-71 with all moving vertical stabs, which should at least hint that the F-22 do not NEED all moving vertical stabs. All moving vertical stabs affects final RCS because you essentially have giant moving reflecting plates. The J-20's engineers are not stupid. They know. But the J-20 needs all moving vertical stabs. So the J-20 supporters essentially changed the laws of physics to say that giant moving plates do not affect Ufimtsev's maths.
Now the J-20's supporters claims that while the F-22's conventional inlet diverter plates are negative to RCS, the J-20's shapeshifting DSI bumps have no effects at all. This conclusion is done with merely using one's eyeballs.