[Reading Science] The Secret Behind the Beauty of Korea's First Indigenous Fighter Jet 'KF-21'

[Asia Economy Reporter Kim Bong-su] In April, the prototype of Korea's first domestically produced fighter jet, the 'KF-21 Boramae,' was unveiled, capturing global attention. Experts immediately recognized it. Even overseas, evaluations poured in saying, "That fighter jet is the 'real deal'." Although it has not yet undergone test flights, the design itself has been praised as "beautiful and sleek," and "optimized for stable yet dynamic combat flight." There are even analyses suggesting that, while not perfect, limited stealth capabilities have been implemented through the design. It is gratifying that Korea's first domestically produced fighter jet is receiving such praise.

However, the Korea Aerospace Research Institute (KARI) has attracted interest by revealing the 'secret' behind this excellent design. According to KARI, the KF-21 underwent nine detailed design modification processes during production, resulting in a flawlessly sleek fuselage as it is now. The final model name of the KF-21 Boramae prototype is C-109, which means it went through nine shape design changes starting from C-101. In other words, after the conceptual design stage revealed as C-101, the final shape was confirmed as C-103 in 2012, but even after that, six more design improvements were made during the full development process.

KF-21 Final Model C109.

During this process, KARI was responsible for verifying the fighter jet's 'physical capabilities' through wind tunnel tests starting from the C-103 model to the prototype, and for collecting data to perfect the body lines. From 2016, KARI conducted subsonic wind tunnel tests five times over a total of 2,000 hours, from the C-103 model to the prototype. Additionally, supersonic wind tunnel tests were simultaneously conducted overseas using the same model.

A wind tunnel test involves placing the manufactured aircraft model inside a device where air is artificially made to flow at high speeds, moving it around to test maneuverability, stability, and other factors. KARI placed a 1/13 scale model, faithfully replicating the actual fighter jet, on a wind tunnel balance and created airflow at about 30% of the speed of sound to conduct the experiments.

These experiments test the fighter jet's 'physical capabilities.' Specifically, they check whether the fighter can advance through the air at the desired speed without shaking and whether it can maintain its posture for an extended time when moving its body. The results are then reflected in the aircraft design. This is extremely important. The life of a fighter jet is its maneuverability; if a fighter designed not to flip over even when tilted up to 45 degrees flips at 35 degrees, it would be a serious problem. KARI generated data through wind tunnel tests to modify the shape to prevent such incidents. Although the KF-21 is a supersonic fighter jet (maximum speed Mach 1.81), subsonic tests were conducted because problems occurring at low speeds can also happen at high speeds, and major design flaws can be detected even in subsonic tests.

Besides maneuverability, another key purpose of wind tunnel tests is to check whether the aircraft becomes unstable during very abrupt maneuvers. In such cases, the problem is often with the wings. Various reviews were conducted to decide whether to enlarge the wings and redesign them, change the wing position entirely, or whether the shape of the air intake was causing issues, leading to design modification procedures.

What stands out is the unfortunate reality that supersonic tests had to be conducted overseas. Until the development of the KF-21, Korea did not have supersonic wind tunnel testing facilities. Therefore, supersonic wind tunnel tests were conducted using overseas facilities, but the available testing time was only about one minute per session, and a long wait was required to resume testing, causing significant hardship for the personnel involved. However, Korea finally established a transonic (around Mach 1.0) wind tunnel testing facility at the Agency for Defense Development in June 2018, which is now being used for mass production and improvement work, which is welcome news.