Why China’s J-20 and FC-31 Aren’t So Stealthy?

A J-20 fighter jet is fitted with WS-10B engine for flight trials. photo by CCTV.

The J-20 is one of two fighters being simultaneously developed in China.  The other aircraft is the Shenyang FC-31, a smaller multirole fighter that is being developed by the Shenyang Aircraft Corporation and could potentially be commercially exported to other countries.

On September 28, 2017, a Chinese Ministry of Defense spokesperson stated that the J-20 “has been officially commissioned into military service.” It remain unclear, however, if the J-20 has achieved initial operational capability. Reports suggest that the J-20 has been assigned to a testing facility and is unlikely to be fully operational until 2018 or 2019.

Using a Physical Optics simulation algorithm, co-founders of the Air Power Australia think-tank Dr. Michael Pelosi and Dr. Carlos Kopp determined that the J-20, like the F-22, has also achieved some Low Observable design goals for enhanced stealth. Such a design allows the J-20 to bypass radar and electronic countermeasures with low to zero visibility. The Global Times confirmed in May 2018 that Radar Cross-Section testing was conducted when developing the stealth capabilities of the J-20. However, some aspects of the aircraft, such as the round nozzle of earlier models  (the WS-15 may have a stealthier design) may work against its stealth capabilities.

China lacks composite industries

Compared with the United States and Japan, China’s military composite material industry started late, and early military product technology developed slowly due to foreign blockades.

Hengshen CoLtd and Zhongfu Shenying are the leading manufacturers of carbon fiber devices in China. Chinese composite industries are mostly based on Automotive Carbon Fiber and composites.

Many car companies, such as BMW, Audi, GM, Honda, and Polestar, have established agreements with carbon fiber material producers for mass production. They are investing in their processes to support low-cost carbon fiber manufacturing.

China learned composites manufacturing from automotive industries, but Chinese capability to manufacture military grade composites yet to reach maturity phase and achieve mass production capability.

In recent years, China’s military-civilian integration in the field of science and technology for national defense has made substantial progress and phased results, and “civilian participation in the military” has reached a new level. The quality and technical level of “Civilian Army” has also been continuously improved, and the product level provided has been upgraded from general supporting products to integrated products and sub-system products. This is main reason China is developing Comac 919 to gain dual-purpose technology and later repurpose them for military use.

Until China achieve enough capability to build composite structures using baked-in and mesh for aviation industries, Chinese J-20 and FC-31 will be using steel, aluminium and titanium structure which are by design good electromagnetic deflector.

American Lockheed Martin and Northrop Grumman pioneered Microwave curing of composites. Microwave curing of composites can offer substantial reductions in the time and energy required to manufacture carbon fibre reinforced plastics (CFRP). This improvement in processing times and energy consumption derived from a composite materials ability to volumetrically absorb electromagnetic (EM) energy within the GHz region.

Engine Properties

China may be working to incrementally improve the J-20’s stealth capabilities. Advanced versions of the WS-10C engine are reported to feature sawtooth serrations around its edges that are designed to redirect radar away from the nozzles. The WS-10C engine is in working state, but far from mature state. In contrast, the F-22’s Pratt & Whitney F119 engines have square nozzles, which greatly improves stealth.

FC-31 with Russian RD-93 engine.

Two variants of the J-20 model were tested and one then employed. One model used axisymmetric exhaust nozzles fully open, and the other used axisymmetric exhaust nozzles fully closed. Nozzle RCS from the forward and aft aspects varies weakly with nozzle position. Therefore all simulations presented are for a closed nozzle, which is the most frequent case in operational use of such aircraft, and thus of most interest.

In May 2018, Indian Air Chief Marshal B.S. Dhanoa told a press conference that the radars on India’s Su-30MKI fighters were “good enough” and could detect a J-20 from “several kilometers away”, while answering a question on whether the J-20 posed a threat to India.

Design of the aircraft

In a production design, the radome seam / join to the fuselage can produce significant RCS contributions if poorly implemented.

The photographic imagery of the J-20 prototypes was not of sufficient quality to incorporate any useful detail of panel join boundaries, door boundaries, and other surface features which produce RCS contributions due to surface travelling waves coupled to the aircraft skin. Even were such detail available, there is no guarantee production aircraft would retain the prototype configuration, reducing the value of any such results.

Visual physical configuration has been claimed to be influenced by foreign aircraft, including the F-22, F-35, Mig 1.44, Rafale and Eurofighter Typhoon.

The position of the canards,  delta wing leading and trailing edge surfaces, and fully moving tail surfaces was set to neutral, reflecting an optimal cruise configuration at nominal supercruise altitudes and airspeeds. Large deflections by these control surfaces in flight would produce large but transient increases in specular backscatter.

The nose aspect sector has excellent potential for achieving Very Low Observable performance due to the absence of any major specular scatterers;

The tail aspect sector is largely degraded in RCS performance by the use of axi-symmetric nozzles which introduce strong specular and diffraction returns; the nozzles destroy the otherwise very reasonable behaviour of the rest of the airframe in this angular sector; the tail surface geometry introduces a further degradation in performance, but constrained to narrow lobes;

RAM Coating

Chinese and Russian are known to have immature technology on composites and RAM. Chinese technology is based on mostly Russian technology. Russian themselves is yet to master RAM technology and import composites materials from Italy and Germany which Russia has no longer access due to sanction.

The RAM must provide some measure of impedance matching to a high permittivity and low impedance carbon-fibre or other composite skin; the airframe designers lose freedom in choosing skin panel materials for mechanical properties alone; and finally improvements in available RAM can only be accommodated by replacing most or all of the aircraft skin panels, rather than stripping and reapplying coatings during periodic depot maintenance cycles.

If we assume that combat attrition is a serious consideration in PLA-AF planning and design definition, then the two most obvious choices in optimisation are thus:

  1. L-band through S-band – most suited for a design intended to penetrate deep into an opposing IADS, the intent of the RAM being to defeat early warning and acquisition radars;
  2. X-band through Ku-band – most suited for a design intended to fight inside its own supporting IADS, the intent of the RAM being to defeat X-band fighter radars, and Ku-band missile seekers.

If we look at the Chinese Comac 919 project, China is developing in material science and is heavily dependent on European and Russian suppliers to assist China develops composite structures. China may have used paint coating rather than actual RAM coating on top of the composite structure.

Chinese doctrine

Opinions vary about the J-20’s comparative strengths as an air superiority (air-to-air) fighter or a strike (air-to-ground) aircraft. Some analysts believe that the J-20’s emphasis on frontal stealth makes it an effective interceptor, meant for mid-air engagements not suited for penetrating enemy air defenses and damaging critical infrastructure on the ground such as high-value targets would include airfields, command bases, and other military installations.


China is still developing avionics and radar systems. Chinese are known to borrow ku-band technology from Russia and produces X-band radar for J-10C and J-16. The PLA-AF is developing a strategy for air warfare and the capability of carrying deep strike missiles for J-20 is unavailable. Type 1475 is a passive electronically scanned array. It is very comparable with Russian systems such as Irbis-E APAR/PESA. When it comes to Type 1475A (KLJ-5A), it is already an AESA (which though has mechanically steered arrays). It is for sure not as powerful and sophisticated as the industry leaders like APG-79/81) – but is definitely a step up to the original PESA module.

Equipped with AESA, China pitted its J-10C and J-11A/B against Thai Gripen-C, Chinese AESA radar proved useless than the Gripen-C’s outdated PESA PS-05/A Mk.4, hence China ended with disastrous results in WVR and BVR combat. The PLAAF vs ThAF drills where J-10A/B suffered a disastrous 40:0 spanking by the Thai Gripen.

Basing the J-20 further current avionics, electronics, radar and design means the J-20 can conduct air-to-air missions using PL-12 and PL-15 (145km range) within the relative safety of China’s Air Defense System. To save Chinese media image, it’s highly likely that the J-20 will never go against Indian Rafale or Taiwanese F-16V.  

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