【Military Second Dimension】 Author: Golden Bomber
Foreign renowned military media, "1945," reported that recently, the Chinese aerospace research team, when studying the B-21 "Raider" stealth strategic bomber's flying wing stealth configuration, used a newly developed domestic software to draw an important research result: the B-21 has four aerodynamic defects. This news has sparked widespread attention. Many people are interested in how China was able to study and draw these conclusions without access to any classified data of the B-21, and whether the experience gained from analyzing these defects will be applied to China's first strategic bomber, the H-20?
▲B-21
According to information disclosed by foreign media, the self-developed software by China is called PADJ-X High-Fidelity Computational Fluid Dynamics Simulation Platform. In the past, similar platforms were monopolized by Western media, but through continuous research, China finally has its own platform. It was this new platform that the Chinese team relied on to study the B-21. The relevant research results were also published in one of China's authoritative journals, "Acta Aeronautica et Astronautica Sinica." This shows that China is not only verifying the reliability of this software, but also conducting detailed reverse analysis of the actual aerodynamic issues of the B-21.
However, as is well known, the B-21 is the most advanced stealth strategic bomber of the United States, and its first flight time is not very long. It adopts a pure flying wing, tailless layout, with the core goal of achieving low radar cross-section in all directions and frequencies. However, many parameters of the B-21 are absolutely confidential. So where do the data fed into the analysis system by the Chinese research team come from? The answer is public video materials. As the saying goes, professionals see the techniques, while laymen just watch the spectacle. After the B-21's first flight, for ordinary military fans, it may just be seen as a sci-fi aircraft, but for professional aeronautical engineering scholars, its design features have already been fully revealed.
▲Related report on the 1945 website
Chinese research teams have reversed modeled the B-21's appearance based on these public video materials and then analyzed them using their self-developed software. Based on the basic theories of aerodynamics, they eventually identified four core issues from the B-21.
First, the B-21 has control instability issues during normal cruise conditions. Solving the balance problem would consume additional engine energy, significantly reducing the B-21's cruise efficiency and shortening its theoretical range. In simple terms, because the wingspan and fuselage height of the B-21 are more compact than those of the previous B-2 bomber, which optimizes the stealth shape but also brings negative problems. According to the simulation data from the Chinese team, the front vortex of the B-21's wing moves forward compared to the B-2, resulting in a pitch moment coefficient of about 0.07 within a certain range, leading to a decrease in aerodynamic stability. Without aerodynamic stability, the plane's maneuverability becomes an issue. To maintain balance, additional energy must be used to generate a moment to stabilize itself. But this means the B-21's range has to be reduced. From publicly available data, the B-21's range is indeed shorter than that of the B-2 according to U.S. official media, which might be the reason.
▲B-21
Second, loss of control at high angle of attack. A high angle of attack refers to a large angle between the leading edge of the wing and the airflow. Typically, this posture is adopted by strategic bombers during high-speed penetration or evading enemy missile interceptions, or when facing complex airflows. Unlike traditional tail-equipped fighter jet layouts, flying wing layouts are prone to large areas of airflow separation at the trailing edge of the wing under high angle of attack conditions.
The chaotic vortices generated within the airflow separation area no longer flow along the surface of the wing, causing a significant loss of lift. In simple terms, the B-21 may experience loss of control at high angles of attack. Therefore, to ensure stability, its angle of attack range is limited by 15%-20% compared to traditional layout strategic bombers, limiting its maneuverability. However, the B-21's main penetration method is stealth maneuvering rather than high-speed penetration, so high-angle-of-attack states are extreme cases, and their impact on overall combat effectiveness is relatively small.
▲B-21
Third, the flying wing layout imposes constraints on stealth capabilities under specific conditions. As is well known, the B-21's stealth mainly relies on smooth curved surfaces on the exterior, which can reflect incoming radar waves to non-threatening directions instead of directly back to the radar source, thus achieving stealth. However, the simulation data show that due to the need for control surfaces such as elevator ailerons and split rudders to achieve maneuverability, the RCS (Radar Cross Section) of the B-21 may increase by 40-60% under specific deflection angles. In simple terms, the B-21 has a "flash window" for radar signals. This is almost impossible to improve for the B-21.
Of course, flash windows are not exclusive to stealth strategic bombers. Even if the B-21 does not suffer from these stealth disruptions, it is almost unavoidable due to the opening of the bomb bay causing a loss of stealth. Regardless, this radar signal flicker inevitably poses a huge risk to the aircraft. Fourth, the control issues at low speeds. Simulation data show that at lower flight speeds, the B-21's flight attitude will be very difficult to control, which places great demands on the flight control system: the flight control system must calculate roll, yaw, pitch coupling variables simultaneously, and any mistake could lead to failure. Even a brief response delay of 0.5-1 seconds could result in a catastrophic accident.
▲H-20 rumors
So how should we evaluate this simulation study conducted by China? It can only be said that, from the conclusion alone, it is "half true and half false." As mentioned earlier, many specific parameters of the B-21 are impossible for China to obtain exact data. The final data fed into the analysis system is only the reverse-engineered data by the Chinese research team. Although the Chinese research team has a very professional background, there will inevitably be differences in many specific data compared to the actual situation of the B-21. This makes the final research results deviate to a large extent.
However, the above four conclusions are limited to the field of aerodynamics, following basic principles, and still have a high degree of credibility. In fact, it can be said that all flying wing layout aircraft probably have these issues, just to varying degrees and impacts. Through some means, these issues can still be avoided. Whether these issues can be resolved is also an important indicator of the capability of different countries' aviation industries. Now, what concerns foreign media even more is that China is likely to use these cutting-edge research findings to improve the H-20, allowing it to avoid falling into these "pitfalls." If the H-20 can solve the problems that the B-21 failed to resolve, then it would be no problem for it to take the top position in the world strategic bomber industry.
Original: toutiao.com/article/7618148051990954537/
Statement: This article represents the views of the author himself.