Armchair Generals Conclude China’s Latest J-20 Fighter Isn’t Maneuverable. Wind Tunnel Experiment Proves Them Wrong
The agility of a modern, complex aircraft cannot be simply eyeballed
When the Chinese J-20 stealth fighter was revealed nearly nine years ago, speculation was rife about its role and mission upon entering service in the People’s Liberation Army Air Force (PLAAF). In English language defense media, the most highly circulated and popular theories suggested it was a dedicated interceptor aircraft intended to destroy force multipliers like tanker aircraft or airborne early warning and control (AEW&C) aircraft, or a dedicated long-range strike aircraft akin to a modern F-111.
However, other similarly sized stealth aircraft like the U.S. F-22 and Russian Su-57 are widely accepted to be air superiority fighters with secondary interceptor and strike roles, and even aircraft like F-35 are intended to field serviceable air superiority characteristics. Some arguments backing up J-20’s interceptor/strike theory is doctrine driven, suggesting the J-20 was intended to specifically operate in the PLA’s so-called “anti-access, area denial” strategy. However, this is unconvincing, because such a doctrine would still hold a place for air superiority fighters.
Indeed, the primary argument surrounds the aircraft’s agility. Much commentary initially suggested J-20’s supposed length and its aerodynamic configuration made it poorly suited for aggressive air combat maneuvering (ACM). Therefore, J-20’s supposed limited agility and kinematic properties would place firm limits on its roles. Agility certainly does determine role; however, the aerodynamic performance of a modern, complex fighter aircraft cannot simply be eyeballed.
J-20 aircraft did make multiple appearances at the recent Zhuhai Airshow 2018. This year, it performed longer and more aggressive flight displays than at Zhuhai 2016, and high-level individuals associated with the J-20 also revealed additional information. Taken together, Zhuhai 2018 finally provided clear, official statements regarding the intended role of the J-20 as well as the agility it is capable of. This piece will examine new developments and past documents and rationale to consider the J-20’s intended level of agility.
Why Care About Agility Anyway?
The agility of fifth generation fighter aircraft such as the F-22 and Su-57 outperforms past fourth generation aircraft in many flight regimes and loadouts. The F-35, designed from the outset as a multirole strike fighter, has often been criticized for being insufficiently agile; however, in recent years even the F-35’s agility has come around to be accepted as competitive against past fourth generation fighters, especially under similar combat and fuel loads.
In other words, competitive agility is important for any fighter aircraft intended to face opposing fighters, even in a secondary role. Furthermore, there are few clear indications as to what future fifth generation versus fifth generation air battles may look like, and it would be foolhardy for any air force to dismiss agility in such confrontations.
Indeed, in stealth versus stealth engagements, one could even make the case that equal advancements in stealth, sensors, networking, electronic warfare, and weapons systems may end up cancelling out each side’s advantages and cause beyond visual range engagements to transition into within visual range combat where the importance of ACM is further elevated.
Following this, one may naturally assume that the J-20 would feature at least competitive agility. However, many proponents of the interceptor/striker position do not consider air superiority to be a primary mission that the PLAAF would have required the J-20 to fulfill.
A commonly insinuated premise is that the Chinese aerospace industry was not capable of producing a fifth generation air superiority fighter, and would have to “settle” for a less technically challenging interceptor or striker instead. Such a position has often been articulated in two parts; one reflecting the J-20’s inherent aerodynamic design, and the other in relation to the J-20’s powerplant. We will now consider both parts in closer detail.
Dr Song’s Paper and the Limits of Eyeballing
Public commentary surrounding the J-20’s aerodynamic design has been of dubious quality, at best. Past assertions primarily revolved around the J-20 being “too large” to be agile, despite being shorter than a Flanker. Occasionally, estimates of the J-20’s wing loading seek to extrapolate its agility in relation to other fighters. However, such eyeballing comparisons are simplistic and potentially dangerously flawed, as they do not provide a holistic picture of the additive effects of various design features where the whole may be greater than the parts.
The infamous Cold War misattribution of the Mig-25 as an agile fighter aircraft demonstrated how visual estimates alone were unreliable when ascertaining an aircraft’s kinematic performance.
More legitimate estimates can be made if one has experienced aerospace engineers, a wind tunnel, and accurate, representative models of the aircraft in question. Fortunately, the results of such an investigation for the J-20’s aerodynamic configuration exist, thanks to Dr. Song Wencong.
Song, who passed away in 2016, was a Chinese aerospace engineer and aircraft designer, and chief designer of the single seat J-10 aircraft, as well as mentor to Yang Wei, who was chief designer of the JF-17 and subsequently the J-20. In 2001, Song published a paper titled “Aerodynamic configuration study of a small aspect ratio, high lift aircraft,” which investigated a future fighter aircraft’s configuration intended to resolve design conflicts between stealth, high maneuverability, supercruise, transonic performance, and high angle of attack performance.
An English translation of the paper has been available online for a few years now, provided by PLA forum-goer “Siegecrossbow,” and is recommended reading for anyone interested in the J-20’s aerodynamic background. The paper describes Song’s proposed aircraft in great detail; however, we will only examine the concluding remarks:
The proposal employs lift-body LERX canard configuration. It is unstable in both the lateral and yaw directions. The proposal employs small aspect ratio wings with medium back sweep angle, relatively large dihedral canards, all moving vertical stabilizers far smaller than those on conventional fighter aircraft, and S-shaped belly intakes. According to our assessment, the proposed aircraft will have excellent supersonic drag characteristics, high AOA lift characteristics, high AOA stability and controllability, and excellent stealth characteristics.
Needless to say, this configuration almost perfectly reflects the J-20’s aerodynamic features and overall design. Considering Song’s past leading role in Chengdu Aerospace Corporation as well as his work with Yang Wei, it is overwhelmingly likely that Song’s study became the basis for what would became the J-20. Therefore, the quality and depth of analysis shown in this paper and its English translation should serve as a minimum benchmark that any opposing theories of the J-20’s aerodynamic configuration should meet.
The Engine Factor
Aside from aerodynamics, it has been suggested the J-20 lacks agility because it is underpowered. The engine factor is arguably the more sensible of the two arguments, as it is universally accepted that the J-20 is currently using a derivative of the AL-31F engine which is relatively underpowered for the aircraft of its weight class. However, interpreting the J-20’s powerplant issue and understanding the performance of the aircraft even using “underpowered” engines is not quite so simple.
Only a few nations are capable of producing high performance turbofans independently, and many such nations have had many decades of cumulative, well-funded research and development supporting their industry. Turbofans remain a relative bottleneck for the Chinese aerospace industry, and though advancements and milestones have been achieved in recent years – such as the successful mass production and induction of the WS-10 family – it is undeniable that Chinese engines lag behind other domains of the aerospace industry such as avionics, aerodynamic design, weapons systems, and systems integration. In the mid 2000s, when the first credible rumors of the J-20 (then known as “J-XX”) began to emerge, consistent caveats stated that the J-XX would initially be powered by interim engines before its intended, more capable powerplant was ready.
The J-20 is an aircraft of similar weight class to the F-22 and Su-57. Unlocking the J-20’s full potential therefore requires the intended WS-15, an engine in a similar performance class to the F119 that powers the F-22. By contrast, the AL-31 and WS-10 families are peers to the F100 and F110 engines that power various F-15 and F-16 variants. The WS-15 is currently in advanced development, and despite reports from some media outlets, it is still years away from mass production, not to mention flight tests of the WS-15 with the J-20 have not even been rumored.
When the WS-15 is integrated with the J-20 it will unlock the aircraft’s true kinematic capability. However, it is likely that the J-20 was intended to field at least serviceable agility with interim, underpowered engines.
Suggesting the J-20 is intended to be a dedicated interceptor/striker on the basis of interim engines alone is flawed, as its kinematic performance even with interim engines must be considered with its aerodynamic design, and its overall combat role must consider its stealth, sensor fusion, datalinking, and weapons. The totality of those factors would still easily make the J-20 the most capable air superiority fighter in the PLAAF’s arsenal and credibly rivaling other fifth generation fighters, even though it will not possess the kinematics of the F-22.
Official Testimonies at Last
Finally, a number of official statements and testimonies over the last year provide illuminating revelations regarding the J-20’s role and agility.
An information brochure provided by Aviation Industry Corporation of China (AVIC) at Zhuhai Airshow 2018 briefly describes the J-20. The last sentence states: “Major operational missions include: seizing and maintaining air superiority, medium & long range fast interception, escort and deep strike.” Interpretation of this sentence is consistent with assertions that the J-20 is an air superiority fighter with other secondary roles such as interception, strike, and escort. After all, an aircraft designed with characteristics for air superiority has the ability to make a respectable interceptor or striker aircraft, but an aircraft designed for dedicated interception or strike rarely makes an agile air superiority fighter.
An interview at Zhuhai including the J-20’s lead test pilot Li Gang and chief designer Yang Wei revealed additional details about the aircraft’s intended performance. As previously reported, Yang heavily hinted that an engine with thrust vector control had begun testing on the J-20, suggesting there is an interest to further enhance the aircraft’s agility and underlining the importance of agility for the aircraft. Furthermore, Li directly stated that the handling and agility of the J-20 was similar to that of the J-10. This may be the most consequential statement yet regarding the J-20’s kinematic performance, but requires careful assessment.
Naturally, Li did not specify which flight regimes, speeds, or loads the two aircraft performed “similarly” in. But it would be reasonable to think the statement was meant to reflect combat scenarios for both aircraft where agility is relevant. For the J-10, this may be a loadout with air-to-air missiles with jettisoned external fuel tanks but with operationally representative internal fuel. For the J-20, this may represent a “stealthy” loadout with only air-to-air missiles and similarly representative internal fuel.
A little remembered interview of J-20 pilots in the 2017 PLA 90th anniversary parade adds additional weight to suggest the aircraft is already quite agile. One pilot stated the J-20’s overall agility was “very formidable,” with subsonic agility described as “pretty good” and supersonic agility described as “spectacular.” This may suggest the aircraft is designed to emphasize ACM in the supersonic regime, which would be consistent with an eventual supercruise capability when powered by the WS-15. It is unknown if the J-20 is capable of a baseline supercruise with interim engines.
The Discussion Onwards
The overall body of evidence strongly suggests J-20 is intended to be a general air superiority aircraft capable of competitive ACM as demonstrated by Song’s paper, AVIC’s J-20 brochure, as well as pilot testimonies over the last few years. Furthermore, from a doctrine perspective the idea that the PLAAF would aim for a dedicated interceptor or striker aircraft rather than an air superiority fighter is not a compelling one, as a dedicated interceptor/striker would be unable to compete against large numbers of opposing F-35 and F-22 fighters operating in the region.
In the classified domain of analysis, various worldwide military forces likely would have arrived at their own better informed and better funded conclusions, which are kept private. However, in the public and nonclassified debate, the question of the J-20’s agility and role will have consequences for discussion and debate over the modeling of Chinese air power against potential opponents.
Source: The Diplomat