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Thursday, November 15, 2012

J-31 Preliminary Report Part I

Author's Note: Unsurprisingly a great deal of information about China's new stealth fighter is unknown. Specific details such as radar cross section figures are nonexistent at this point. Estimates and theories comprise the bulk of public "knowledge" about the J-31 and are thus subject to change and scrutiny. As a side note, I'm sorry for the late post but being a full time student and finding the time to write these is pretty hard. 

It has been more than a month since China's second stealth fighter made its public debut. A cloud of speculation still shrouds China's second stealth fighter. Even its exact designation has not been confirmed hence the use of the J-21, J-31, and F60 designations by many sources (they all refer to the same aircraft). The primary purpose of this article is to list what few details are  known about jet. Following this, the article will discuss the educated guesses by aviation experts as to its origin, purpose, capabilities, and impact on the region.  

Image 1: Among the first images of the J-31 released. The aircraft shown is designated 310001 hence the theory that the aircraft is designated as the J-31. Furthermore the writing on the tail, 鹘鹰, is Chinese for “Falcon Eagle” (Cenciotti, 2012). Hence, the J-31 designation will be used for the remainder of the article. 

What is Known 
  • First images released by Chinese military forums prior to Secretary of Defense Leon Panetta's visit to China on September 19th.
  • Images taken outside a Shenyang Aircraft Corporation airfield 
  • Weapons bays underneath fuselage, no side weapon bays. 
  • The aircraft is approximately the size of an F-35 (estimated by using objects in image as reference) 
  • Twin power plant with non thrust vectoring engine nozzles 
Origin and Purpose

As stated before there are virtually no details as to the circumstances of development or resources devoted to the J-31 project. The J-20, China's first stealth fighter, is being developed and built by Chengdu aircraft corporation/conglomerate. However, the J-31 made its debt outside of a Shenyang corporation airfield. Shenyang the chief competitor to Chengdu. 

"The various plants of the Avic group, such as Chengdu Aircraft and Shenyang Aircraft, have a long tradition of rivalry. To overcome that, the group began bundling them together from 2008 into specialist subsidiaries in which they were supposed to work together. But the defense ministry opposed tight integration of the defense subsidiary—including Chengdu and Shenyang—in order to maintain closer control and probably to retain and foster competition among them."  - Bill Sweetman 

Some theorize is possible that the J-31 lost to the J-20 in a fighter aircraft competition. However, the design differences between the J-31 and J-20 suggest they were designed with completely different roles in mind. The J-20 bears many of the characteristics of heavy fighter or strike aircraft vs the lighter build of the J-31. It is unlikely that the two aircraft competed for the same role. Traditionally the PLAA has a history of procuring a light and heavy fighter.  

“In traditional PLA thinking, there has always been a necessity for ‘light’ plus ‘heavy’ in terms of equipment.” - Gary Li, 2012

The most plausible theory put forward is that the J-31 will serve as a light fighter supplementing the heavier J-20. Both aircraft are in their prototype stages. However, it is also possible that the J-31 is merely a test aircraft and not a prototype with a finalized version to enter production. The United States built several prototype stealth aircraft that never entered service e.g. the Lockheed HAVE BLUE demonstrator aircraft.


Shenyang took a conservative approach in designing the J-31 as it extensively borrowed from features from the Lockheed F-22 and F-35. The design similarities between the J-31 and the Lockheed developed stealth fighters leads many experts to believe in industrial espionage on the behalf of Shenyang occurred. (Cenciotti, 2012) Both Lockheed Martin and the Pentagon have been the target of multiple cyber attacks from China. It is plausible that classified design details were compromised during these attacks.

The shaping of the intakes on the J-31 bear remarkable similarities to those on the F-35. The tail design and chined nose emulate the F-22 closely (However J-31 design features conventional tail nozzles).

Of particular interest to some observers is the use of twin wheels on the front landing gear. Aircraft featuring twin wheeled landing gear are typically carrier based aircraft. However, the implementation of a twin wheeled landing gear system is hardly grounds to suggest the J-31 is destined for carrier operations. Building a stealth fighter is difficult. Building a stealth fighter for carrier based operations is extremely difficult. Its unlikely that the second stealth fighter ever produced by China would be carrier capable. Carrier based aircraft must include fold-able wings, tail hooks, strengthened heavy landing gear, and an extremely durable airframe that can take the structural strain of carrier landings. The current J-31 design only features ONE of the listed criterion, possibly two. Furthermore, the J-15 is already been developed for carrier use on the Liaoning.

Bill Sweetman believes the J-31 is equipped with twin Russian RD-93 engines. China's domestic jet engine designs continue to prove problematic and demonstrate poor performance and reliability. The J-31 is further hampered by China's lagging domestic fighter radar designs. For example, the domestically produced radar used in the 4.5 generation J-10 can track 10 targets while engaging 4 targets vs the F-15's AN/APG-63 (V) 1 radar built in the 1990s can track 14 and engage 6 targets. Without sufficiently capable internal systems, the J-31 will look externally impressive but will be outclassed by other 5th generation fighters. 

Part II will contain sources and uncovered information mentioned in outline. 


  1. Why do you think Shenyang went so far as to copy the "V" paint job from the F-35 intakes? It's one thing to follow the engineering directions the F-35 took, but to blatantly copy it down to an aesthetic detail like that makes me wonder if it really is aesthetic after all.

    Thanks for the article!

  2. To add insult to injury? I can't think of a plausible reason why. Its one thing to copy stealth coatings but specific markings have no impact on performance.

  3. Early generation AESA are quite heavy as is their cooling array. This alone could dictate J-31 NLG configuration, as could limited ability (due to wheelbase vs. total length) ability to aerobrake, requiring sturdy NLG to accept sudden nose-low loads as the MLG brakes are applied.

    AESA processing tech is not necessarily as crucial to A2A fights as it is to ground mapping. After all, if you are -successfully- signature reduced, you can mostly push-pull your F-Pole as you like.

    More important will be total ERPs as the real benefit of digital radar technology is to launch from here (then pump or grind out of the fight) while guiding from waaaaay over there. Maximizing illuminator vs. shooter standoffs gives you all kinds of kinematic and trajectory control over the shots.

    This doesn't mean that ECCM and other elements of design performance will not be important but simply the ability to guide against X-many targets is only of relative importance to how wide a scan volume and PRF interleave you can maintain and flashlighting the whole sky is not something you want to do from missile ranges anyway.

    Remember, if the enemy is in range, so are you.

    It's interesting to see the engine choice adopted. After the J-10 which essentially is using an AL-31 as installed on the Su-27 (the original PW1120 used on the IAI Lavi is only about 2/3rds as powerful) it would certainly not be unreasonable for a similarly powerful (AL-37FU/AL-41FS) to have been adopted for the J-31. The difference is likely one of TSFC and mass-flow. RD-93 is likely in the .7-.8 regime of it's RD-33 predecessor but if thrust moves up to 20,000lbf as with the RD-133, we are talking about something like 200lbs/sec mass flow.

    The F135 has around 310lbs/sec equivalent which means that the Chinese are either not getting all they could from the inlet system, or more likely not getting sufficient internal stage rise and combustor temps to make up the difference.

    With a straight across weapons bay (no STOVL module nonsense) the jet's ruling is likely closer to an F-15 or F-22 so why not take advantage of a simpler configuration, leaving some reserve for growth while installing two engines which are roughly half the weight of the single F135 while supplying around 90% of the thrust?

    The result will be a lot less boat tail drag and burn through problems with the more widely spaced taiils while trim on the more conventionally forward set wing and widely spaced tails will also likely benefit, allowing for easier integration of weapons and high alpha maneuver options.

    If the scaling assumptions are correct, this is an F-135 with a late F-18C (F404-GE-402) or F-18E (F414-GE-100) propulsion tray but beyond the fuel vs. radius issue, (.7lb/lb/hr X2 vs. .91lb/lb/hr X1) the twin approach may prove to be an excellent choice as power increments similar to those intended for the F414 EEP and EJ-200 growth engines will bring the jet to 50,000lbf installed quite quickly whereas the F-35 has little or no inlet margin on mass flow and so can only improve through AVET core temp insertions.

    The JF-17 is essentially obsolete on entry but there is no reason why the majority of it's systems could not be transferred directly to an updated LO configured airframe with an added selling point of being -far- easier to CPFH maintain than the F-35 is shaping up to be.