J-20 Stealth Fighter Design Balances Speed And Agility

Unique J-20 could fit anti-access role.
Chengdu’s J-20 stealth fighter represents the pinnacle of China’s aerospace engineering, but its existence and development have posed mysteries since the unexpected appearance of the first prototype at the end of 2010, followed in May 2012 by the debut of a second, similar aircraft. The past few months, however, have seen the first flights of a pair of significantly different J-20s, identified by the serial Nos. 2011 and 2012.
The most substantial design change in the new aircraft appears to be that the fuselage aft of the main landing gear is  a bit more slender, with a deeper tunnel in the undersurface between the engines. The tailbooms that extend aft of the all-moving vertical tails are longer, and the lower fixed stabilizers are moved aft. The trailing-edge tips of the vertical tails and canards are cropped, and the leading-edge root extensions are straight rather than curved.
The top line of the outer wall of the diverterless supersonic inlet has been drooped, and the landing gear doors changed: The main doors now close after the gear has extended and the nose landing gear door has been reshaped. The F-22-style one-piece frameless canopy on the early aircraft has given way to a separate windshield and canopy, the latter with detonation cord to shatter the canopy for ejection. What appears to be a housing for an infrared search and track system has been added below the nose, and a missile-warning sensor fairing installed below the aft fuselage.
The new variant aircraft appeared slightly more than three years after the first flight of No. 2001, so some of the changes may reflect lessons from flight testing. Other changes represent a move toward a production or pre-production design. So far, there have been few indications as to when the J-20 will enter service: The Pentagon, in its latest annual report to Congress, says it is unlikely to be operational before 2018.
However, the appearance of the new aircraft tends to confirm that the design has proved sound so far; all four prototypes are now reported to be at Yanliang air base in Xian, the Chinese air force’s main test site. The question now is what role the J-20—which is not only the first Chinese stealth fighter but the largest tactical aircraft built in China—will perform in the future force.
Close examination of the J-20 shows it has no direct analogue in the West or in Russia. The dimensions can be estimated accurately from open-source satellite images, but its characteristics are sometimes mis-assessed through a focus on overall size. Details of avionics and materials remain uncertain.

Differences between the newer J-20 configuration (above) and the first of two aircraft (below) include a recontoured lower-aft fuselage and longer tailbooms, with the ventral fins moved slightly aft. The leading-edge root extension of the new aircraft has a straight edge, and the canard and vertical stabilizer tips are clipped. Electro-optical sensor housings are installed under the nose and beneath the right-hand side of the fuselage, aft of the weapon bays. Credit: Photo via Internet

 

 Credit: Photo via Internet

 

The J-20’s wing and control surface layout is very different from that of the Lockheed Martin F-22, but the body layout is quite similar, with twin main weapon bays under the belly and side bays for rail-launched air-to-air missiles (AAMs), all located under and outside the inlet ducts. On both aircraft, the main landing gear is housed in the fuselage behind the weapon bays and the engines are close together. The big difference, however, is that the J-20 is 9.5 ft. (17%) longer than the F-22, from the nose to the engine nozzles. Most of this is in the widest part of the fuselage, and since the weapon bays are similar in size, it is almost all available for fuel. It is a reasonable estimate that the J-20 could have as much as 40% more internal fuel capacity than the F-22. The longer body will also improve fineness ratio, with benefits for transonic drag. Despite the larger body, the empty weight of the J-20 may be close to that of the F-22, largely because it has less-powerful engines without the heavy two-dimensional thrust-vectoring nozzles of the F-22’s F119s. The J-20 prototypes are believed to be flying with United Engine Corp. (UEC) AL-31F engines. The thrust difference between the two designs is very large: The F-22 has almost as much power in intermediate thrust as the J-20 does in full afterburner, although newer versions of the UEC AL-31/117S/117 could close the gap in later versions of the Chinese aircraft.
The conventional circular nozzles and the aft-body shape are less conducive to stealth than the F-22, as is the case with the T-50. This is most likely a conscious decision because a fast aircraft can tolerate a higher radar cross-section in the aft quadrant. While some observers have suggested that canards are incompatible with stealth, an engineer who was active in Lockheed Martin’s early Joint Strike Fighter efforts says the final quad-tail configuration was no stealthier than the earlier canard-delta design.
A detailed Chinese technical paper published in 2001 by Song Wencong, designer of the Chengdu J-10, points to key aerodynamic features of the J-20. The paper addresses the design of a fighter with a delta wing, canards and leading-edge root extensions (Lerxes), and discusses how the three interact. The J-20, unlike the J-10, has a broad body and the canard and wing are not close-coupled. However, according to the paper, the Lerx and canard, used together and in combination with a high degree of instability, can achieve maximum lift coefficients that are as high if not higher than those from a close-coupled canard.
The paper also discusses the vertical stabilizer design of a stealth configuration with outward-canted surfaces. Fixed, canted tails are exposed to powerful crossflows at high angles of attack, because of the formation of vortices from the wings and canard. The result is that the tails can develop powerful moments, and because the tails are canted, those forces will include pitch-up moments. One way to resolve this, the paper notes, is to use smaller, all-moving surfaces. The J-20 resembles the Sukhoi T-50 in being directionally unstable, and is actively controlled with the all-moving verticals. Song’s paper also says the canard layout provides positive post-stall recovery, without the use of thrust vectoring.
The paper identifies supersonic cruise as a requirement for a next-generation fighter and often refers to the need to reduce supersonic drag. The J-20’s supercruise performance will nevertheless be strongly affected by engine technology. China may well hope to acquire or emulate the technology being developed by Russia for the Su-35S and T-50. UEC’s 117S engine, developed for the Su-35S, is more powerful than the AL-31F (32,000 lb. maximum versus 27,500 lb. for the basic AL-31F) and has a digital control system. The T-50’s 117 engine is similar to the 117S, but it is further uprated to 33,000 lb. thrust, and according to a UEC engineer, the hot-end temperature limits are increased, to allow the engine to sustain maximum non-afterburning thrust to higher speeds. However, the J-20 will not match the F-22’s thrust-to-weight ratio, even with an engine equivalent to the 117.
The J-20’s weapon arrangement is similar to the F-22, except that the ventral bays are shorter and narrower, and are apparently capable of accommodating only four weapons the size of the SD-10 AAM. However, they do appear large enough to accommodate bigger folding-wing missiles—and China is reported to be negotiating to buy the Russian Kh-58UShKE, a Mach 4 anti-radar missile that is also intended for internal carriage on the T-50.
The side missile bays differ from  those of the F-22 in that the doors can be closed after the missile rail has been extended, and have been seen with a missile—or test shape—with low-aspect-ratio wings and folding tails. So far, no gun has been seen on J-20s, nor has there been a sign of provision for one.
The J-20 design, therefore, is an air-to-air fighter with an emphasis on forward-aspect stealth, efficient high-speed aerodynamics and range, with a modest internal payload and more than adequate agility for self-defense. The aircraft has considerable potential for development, because of its currently unsophisticated engines. But it is also large and expensive, and continued development of the J-10B shows that China plans to maintain a high-low mix of fighters for a long time to come.
This concept fits very well into an anti-access/area-denial strategy given China’s regional geography and the fact that the nation’s military and geopolitical ambitions are focused on the China Sea and its surrounding island chains. The U.S. has committed its armed forces to concentrate much of their funding on tactical fighters with a combat radius of 600 mi., much less than the distance from their bases to targets on the Chinese mainland, and has persuaded its allies to do the same.
As a result, operations are almost entirely dependent on two groups of aircraft: tankers and large intelligence, surveillance and reconnaissance (ISR) aircraft with long endurance. Under the “distributed control” concept favored by U.S. Air Force commanders as a hedge against electronic warfare, including cyberattacks, the ISR aircraft also have a control-and-communications function. However, both tankers and ISR aircraft are vulnerable to attack, and maintaining a defensive combat air patrol (CAP) over them at long range is also difficult.
The J-20’s primary mission, therefore, may be to use stealth and speed to break through the CAP and threaten vital tankers and ISR platforms. Its range gives it a “long lance” advantage—if the tankers, ISR aircraft and escorts have to stay out of the J-20’s range, the tactical aircraft that they support will not have the airborne radar cover or range needed to reach their targets.
Also, an anti-radar missile would give the J-20 some capability against shipping, even with internal weapons. China’s new CM-400AKG and YJ-12 high-speed antiship missiles will not fit the J-20’s weapon bays, but could probably be carried under the wings, and future internal weapons will increase its offensive capacity.

In a very broad sense, the J-20 could turn out to be an analogue to the Soviet-era Tu-22M2/3 Backfire bomber—an efficient and practical blend of low-risk technologies that generates options for its users and difficult problems for its adversaries. 

J-20 vs. F-22
	J-20	F-22
Overall length (ft.)	66.8	62
Wingspan (ft.)	44.2	44.5
Wing area (sq. ft.)	840	840
Operating empty weight (lb.)	42,750	43,340
Internal fuel (lb.)	25,000	18,000
Normal takeoff weight (lb.)	70,750	64,840
Max. thrust (lb.)	55,000	70,000
Min. thrust (lb.)	34,250	52,000
Clean-fuel fraction, normal T/O	0.35	0.28
Max. thrust-to-weight ratio, combat weight	0.94	1.25
Military thrust-to-weight ratio, combat weight	0.59	0.93
Wing loading at combat weight, lb./sq. ft.	69	66.5
Sources: Lockheed Martin, AW&ST analysis

Podcast Aviation Week editors discuss the design characteristics of the J-20. AviationWeek.com/Check6

Source: Aviation Week & Space Technology “China’s J-20 Stealth Fighter Design Balances Speed And Agility: Unique J-20 could fit anti-access role”