The South China Morning Post reported on March 11th that China’s testing of a new type of hypersonic UAV is showing remarkable performance. According to the disclosed data, its lift-to-drag ratio far exceeds that of the supersonic cruising F-22. It can deliver a 600kg payload to the United States, 8000 kilometers away, at a speed of Mach 7!
Hypersonic Lift-to-Drag Ratio: Breaking the Conventional Hypersonic Maneuvering Techniques
The report cited a paper published on March 23rd in the Journal of Mechanics regarding the aerodynamic technology of hypersonic UAVs. It suggests that China’s new generation of hypersonic UAVs can now challenge the aerodynamic performance of the most advanced U.S. fighters.
The report indicates that the F-22, relying on powerful thrust engines and excellent transonic aerodynamic design, achieves a lift-to-drag ratio of about 8.4 and can sustain cruising at 1.8 Mach. However, the supersonic lift-to-drag ratio of this fighter is not very high, dropping to around 4 at 2 Mach. This would severely affect the aircraft’s range, resulting in a relatively short endurance for the F-22, similar to the issues faced by the F-35.
However, a paper published by Chinese scientists in the Journal of Mechanics suggests that China’s new generation of hypersonic UAVs has surpassed the design limits of the F-22. While the lift-to-drag ratio of both is comparable at transonic speeds, China’s hypersonic UAV still achieves a lift-to-drag ratio greater than 4 at 6 Mach. This indicates excellent supersonic flight performance, especially under hypersonic conditions, enabling long-range endurance. This represents a significantly advanced aerodynamic technology in the era of hypersonic flight, akin to adding wings to a tiger for hypersonic aircraft.
Hypersonic Lift-to-Drag Ratio: Breaking the Conventional Understanding of Technology
The lift-to-drag ratio is easily understood as the ratio of lift to drag. In aircraft, a higher lift-to-drag ratio indicates better aerodynamic performance, which is more favorable for flight and typically results in better climb performance.
Aircraft lift comes from the angle of attack of the fuselage and wings, the lift generated by the Bernoulli principle on the wing surfaces, flaps on the wings, various aerodynamic surfaces such as canards, and the vortices generated by wingtips and strakes, among others. Drag, on the other hand, comes from two types: induced drag and parasite drag. The former is the drag produced when lift is generated by the wing, or simply put, the drag produced by the wing’s cross-sectional area. The latter is the drag produced by the relative motion between the aircraft and the air, which is roughly proportional to the square of the speed. Therefore, at high speeds, parasite drag predominates, and as speed increases, drag increases, leading to a decrease in the lift-to-drag ratio, mainly due to the influence of parasite drag. Reducing the cross-sectional area or streamlining the aircraft’s fuselage can reduce parasite drag.
This explanation might be a bit complex, but in simple terms, at a constant speed, the lift-to-drag ratio can be directly understood as the glide ratio. For instance, as mentioned earlier, the transonic lift-to-drag ratio is 8.4. This means that for every meter of descent without power, the aircraft can glide 8.4 meters horizontally. Modern gliders can have glide ratios (lift-to-drag ratios) of 50 to 60, while airliners can achieve 15 to 20. For example, the Boeing 747 has a lift-to-drag ratio of 17, but the Concorde, cruising at Mach 2, has a lift-to-drag ratio of 7.14.
So, it’s easier to understand now. The preceding explanation clarifies that the lift-to-drag ratio changes at different speeds. Therefore, it’s inevitable for the lift-to-drag ratio of hypersonic aircraft to decrease. But here’s the question: why can China’s new hypersonic UAV maintain a high lift-to-drag ratio even at hypersonic speeds? This requires understanding another concept: aspect ratio.
The early aspect ratio was the ratio of wingspan to chord length (the length of the air flowing over the wing). Later, as wing designs became more complex and didn’t fit simple ratios, it was changed to the ratio of wing span squared to wing area. Straight wings have a high aspect ratio, while delta wings, or even “wingless” aircraft, have a low aspect ratio. The former is suitable for subsonic flight, while the latter is suitable for high-speed and even hypersonic flight.
For instance, the aspect ratio of the B-52 bomber is 6.5, the U-2 reconnaissance aircraft is 10.6, and the Global Hawk UAV has an aspect ratio as high as 25. These are low-speed reconnaissance aircraft and bombers. But what about fighters? The F-16 has an aspect ratio of 3.2, the F-15 is 3, the F-18 is 3.52, the Su-27 is 3.5, the MiG-29 is 3.5, the Eurofighter Typhoon is 2.4, the F-22 is 2.3, and the J-20 is 2.2.
The lift-to-drag ratio of an aircraft with a certain aspect ratio is determined. If there are no additional accessories on the aircraft surface, then modern aircraft will have various designs to increase the lift-to-drag ratio of fixed aspect ratio aircraft. These could include canards, strakes, various aerodynamic surfaces, and even changes in the fuselage cross-sectional area to reduce drag.
For example, the indented fuselage design, which involves appropriately reducing the fuselage cross-section at the wing installation area, resembles a waist shape overall. Indented fuselage designs use transonic area rule design to achieve the purpose of reducing zero lift drag (parasite drag) when flying at transonic speeds.
An excellent fighter should ideally have a high lift-to-drag ratio at subsonic speeds and a slightly lower lift-to-drag ratio at supersonic speeds. However, the design considerations for subsonic lift-to-drag ratios often conflict with those for supersonic lift-to-drag ratios, so one has to be chosen over the other. Therefore, while the F-22 achieves a high subsonic lift-to-drag ratio of 8.4, it drops to 4 at 2 Mach because compromises must be made. However, China’s J-20 performs much better in this regard compared to the F-22.
J-20’s Lift-to-Drag Ratio: Design Excellence
Many netizens like to compare the J-20 with the F-22, and indeed, the comparison is made against the F-22. One problem that the J-20 faces compared to the F-22 is inadequate engine thrust. To achieve the same supersonic cruising speed as the F-22, it needs a smaller aspect ratio to reduce drag. According to public data, the F-22 has an aspect ratio of 2.3, while the J-20’s aspect ratio is less than 2.2.
From a normal design perspective, if the F-22 has a lift-to-drag ratio of 8.4, then the J-20 with a smaller aspect ratio will surely have a lift-to-drag ratio less than 8. However, in reality, it’s quite the opposite. The J-20’s lift-to-drag ratio exceeds 10, thanks to its canards and, more significantly, its large strakes between the canards and wings. Previously, CCTV reported that the J-20’s “lateral wing-body strake canard layout” won the Chinese Patent Design Gold Award. This indicates a deep understanding of aerodynamic design in China’s defense industry, with aerodynamic layouts that are incredibly complex:
An integrated unconventional aerodynamic layout featuring a lift body fuselage based on vortex control technology, canards, strakes, wings, lateral wing-body strakes, outward canting bifurcated ventral fins, and outward canted all-moving twin vertical tails, with controlled and controlled vortices including nose fin vortex, intake fin vortex, canard vortex, strake vortex, and wing leading-edge vortex – complex multi-vortex coupling!
Regarding the supersonic lift-to-drag ratio, the J-20 uses a modified area rule that is more compatible with the F-22. It also has canards for better automatic trimming to reduce supersonic trimming drag (shifting the center of gravity aft at supersonic speeds increases the aircraft’s angle of attack, requiring additional drag from tail surface trimming). Additionally, the all-moving tail reduces the drag from tail shock waves at altitude to a minimum, thereby overall enhancing the J-20’s supersonic lift-to-drag ratio. The J-20 achieves both a high lift-to-drag ratio at subsonic speeds and a high lift-to-drag ratio after supersonic speeds, which are almost contradictory results, yet perfectly embodied in the J-20.
A larger lift-to-drag ratio indicates higher aerodynamic efficiency, better maneuverability at higher altitudes and speeds, and superior combat performance for fighters, which remain highly responsive even at high supersonic speeds, with greater range and fuel efficiency. With the J-20 now equipped with the WS-15 engine, its overall performance will be elevated compared to its previous WS-10A engine.
With the excellent aerodynamic design of the J-20, China’s defense industry has made a leap in various aerodynamic designs following the J-20. Experience gained from the matured design of the J-20 can be applied to transonic, high-speed, and hypersonic designs, such as the newly announced hypersonic UAV mentioned in the paper, although the paper does not specify the exact model. Nevertheless, it is evident that this should be an upgraded version of the MD-22.
Upgraded Version of the MD-22: Delivering a 600kg Payload to the United States, 8000 Kilometers Away!
The paper published in the Journal of Mechanics mentions the similarities between the hypersonic UAV and the MD-22 wide-speed range hypersonic aircraft publicly disclosed in 2019. The performance of this UAV is somewhat “crazy”:
Length: 10.8 meters, wingspan: 4.5 meters, takeoff weight: approximately 4 tons;
Speed range: 0 to 7 Mach, maximum range: 8000km;
Designated as a near-space hypersonic technology test platform;
Capability to withstand 6g steady-state overload under high-speed conditions;
The most critical parameters are the speed range of 0 to 7 Mach and the overall size of the aircraft. Reaching a maximum of Mach 7 indicates that this aircraft can test flight envelopes using hydrocarbon-fueled TRRE engines. The aircraft’s size indicates that it can accommodate relatively large engines, suggesting potential future applications in manned large-scale aircraft engines.
Another aspect is its positioning as a near-space hypersonic technology test platform, indicating that the MD-22’s mission would likely include testing critical tasks such as engines. Additionally, it can undergo 6g steady-state overload testing under high-speed conditions, suggesting that the MD-22 can test its maneuvering performance under hypersonic conditions.
The MD-22 is already capable of delivering a 600kg (1,300 pound) payload to a distance of 8000 kilometers, equivalent to the distance between China and the United States! However, the data on the new hypersonic UAV mentioned in the paper indicates a length of 12 meters and a wingspan of 6 meters, which is larger than the MD-22. Furthermore, the data in the paper mentions that this UAV still achieves a lift-to-drag ratio greater than 4 at 6 Mach, indicating that the performance of the new hypersonic UAV surpasses that of the MD-22 by a considerable margin.
This also implies that the new UAV can maneuver flexibly at high altitudes. This type of aircraft can carry out maneuvering combat tasks at high hypersonic speeds, such as engaging in air combat with the next-generation NGAD of the U.S. military. These highly maneuverable hypersonic UAVs can serve as loyal wingmen for China’s sixth-generation aircraft in combat tasks or be deployed as suicide drones for ultra-long-range assault strikes. The maneuverability at hypersonic speeds also poses a more severe challenge to missile defense systems.
There’s an interesting fact: the loyal wingmen currently being researched or nearing deployment by the U.S. military are all subsonic models, such as the XQ-58A Valkyrie and MQ-28, to date, no supersonic or hypersonic models have appeared. In contrast, a significant proportion of China’s publicly disclosed UAVs are supersonic or hypersonic, such as the WZ-7, FH-97, Anjian drone, MD-22, and the newly announced hypersonic UAV in this paper.
It seems that the U.S. military has never intended for UAVs to perform supersonic combat tasks. Or perhaps, the U.S. military believes that subsonic loyal wingmen carrying out bomb truck missions are sufficient? The operational concepts of the Chinese and U.S. air forces appear to be heading in two different directions. The Chinese Air Force has been heavily influenced by the axiom “in the world of weapons, only speed matters”, while the U.S. military seems to be immersed in the illusory pleasure of future NGAD sixth-generation aircraft equipment and unable to extricate itself.
The mistakes made by the U.S. military in self-conceit are numerous, such as the Navy’s littoral combat ship and Zumwalt-class destroyer, as well as medium-voltage AC all-electric propulsion, have all gone astray. The Army’s Crusader howitzer and continuous failed hypersonic missiles, as well as the Air Force’s production stoppage of the F-22 fighter and the indecisiveness of the F-35, are all typical mistakes. The absence of supersonic or hypersonic versions in the next generation of UAVs is puzzling. It’s unclear how the U.S. military has considered this. The future confrontation between the Chinese and U.S. air forces will inevitably lead to fierce competition, and the superiority will soon be revealed.