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The soviets were big on low observation aircraft technologies long ago, and much of the groundwork was actually utilised by the US to design later influential aircraft that incorporated such ideas;
>Lockheed incorporated into its bid a text written by the Soviet/Russian physicist Pyotr Ufimtsev from 1962, titled Method of Edge Waves in the Physical Theory of Diffraction, Soviet Radio, Moscow, 1962. In 1971 this book was translated into English with the same title by U.S. Air Force, Foreign Technology Division. The theory played a critical role in the design of American stealth-aircraft F-117 and B-2. Equations outlined in the paper quantified how a plane’s shape would affect its detectability by radar, its radar cross-section (RCS). This was applied by Lockheed in computer simulation to design a novel shape they called the “Hopeless Diamond”, a wordplay on the Hope Diamond, securing contractual rights to produce the F-117 Nighthawk starting in 1975. In 1977 Lockheed produced two 60% scale models under the Have Blue contract. The Have Blue program was a stealth technology demonstrator that lasted from 1976 to 1979. The success of Have Blue led the Air Force to create the Senior Trend program which developed the F-117.
One key technology in this field is plasma stealth, using ionizing radiation to shield the craft from incoming radio waves. This has been incorporated into the SU-27 after 2000’s. No doubt the pak Fa features this also.