Months have passed since the public appearance of the brand-new Chinese combat jet. And yet there is no explicit reaction to it from the US and its armed forces, whose assets the new machine is obviously developed to oppose. In turn, the People’s Liberation Army (PLA) and its air force (PLAAF) remain surprisingly mum on an aircraft they were so willing to flaunt on a Chengdu runway in late December and the middle of January. We still don't know if the new plane is officially intended as a technology demonstrator or a developmental prototype. We also don't know what this airplane is designed to do. Is it an interceptor to replace the Shenyang J-8? Or is it a strike bomber to replace the Xian JH-7A? Or is this a multi-mode, multi-role aircraft? We even do not have official confirmation for the common reference to this airplane as the Chengdu J-20 by Internet sources and printed media.
24th Mar 2011
Months have passed since the public appearance of the brand-new Chinese combat jet. And yet there is no explicit reaction to it from the US and its armed forces, whose assets the new machine is obviously developed to oppose. In turn, the People’s Liberation Army (PLA) and its air force (PLAAF) remain surprisingly mum on an aircraft they were so willing to flaunt on a Chengdu runway in late December and the middle of January. We still don't know if the new plane is officially intended as a technology demonstrator or a developmental prototype. We also don't know what this airplane is designed to do. Is it an interceptor to replace the Shenyang J-8? Or is it a strike bomber to replace the Xian JH-7A? Or is this a multi-mode, multi-role aircraft? We even do not have official confirmation for the common reference to this airplane as the Chengdu J-20 by Internet sources and printed media. This story is our guess what the new Chinese jet is about. The new aircraft flew for the first time in the daylight of 11 January 2011. At least, this was J-20’s first flight made in public. It began and terminated at the aircraft factory aerodrome near Chengdu. This city houses a few aviation industry enterprises, including the aircraft manufacturing plant producing jetfighters and the design house developing them. A number of aviation admirers watched the J-20 fly as they took positions around the fence of the factory’s aerodrome on 11 January 2011. These people had demonstrated good situational awareness by knowing what to expect at the given date and time. Looking through dozens of J-20 images available on the Internet, one can get a good impression of the aircraft dimensions and layout. There is one thing missing, though. No-one photo available depicted doors of weapons bays. This may lead to a conclusion that these doors (along with other would-be points of interest) were either thoroughly hidden or removed from the shots by the picture-takers or their advisors before being exhibited. It is even more likely, though, that weapons bay doors we anticipate to see, are not actually fitted to the particular airframe that flew on 11 January. There is a good reason for non-fitment of those. The first operable prototype is normally dedicated to assessment of flight performance, operational envelope, power settings, checking onboard systems and proving flight control algorithms. We may see the anticipated doors later, if the first J-20 is to be followed by more operable prototypes and pre-production examples. Some of them will be purposely constructed for weapons testing and fire trials. Thanks to the Chinese authorities and their liberal attitude to the local aviation admirers and spreading of their pictures, we have sufficient information about J-20’s outward appearance. In particular, there are some shots giving a good view of the wing, canards and their relative position to the airplane’s body. The design is a relatively big tactical jet with the canards (foreplanes) and large delta wing. The length of its fuselage falls between 70ft and 80ft, wingspan between 42ft and 46ft. The J-20’s maximum takeoff weight is estimated at 90,000lb, and airframe construction weight twice less than that, approximately 45,000lb. Engines There is a common belief that the Chinese are not good at aero engines, and are dependant on Russian expertise and hardware. A popular assumption is that the J-20 is powered by two Russian engines of the Saturn AL-31F family or their Chinese clones. By now, the popular AL-31F family has grown to a dozen of distinguishable variants, with recent developments being the Item 117 (powers the Sukhoi PAKFA and Su-35), AL-31FP (Su-30MKI/MKM/MKA), AL-31FN (J-10), WS-10G and WS-10 Taihang (both Chinese derivatives for J-10 variants). Two engines at full afterburner generate a thrust between 54,000lb and 80,000lb. This should be enough for supercruise, or supersonic cruise flight at military power, the highest power setting without afterburning. With limited fuel stores and a handful of air-to-air missiles, the J-20 can fly vertically without losing speed. While J-20 operable prototypes may well rely on the AL-31F family engines, Chengdu designers may have other solutions in their minds. The AL-31F does not appear a good choice for sustained super cruise, even though the semi-experimental Items 117 already enabled the Su-35 go supersonic at military power. The Chinese may have chosen the AL-31F only for the initial part of the flight testing as well-tried and high-performance power plant. In future the J-20 may appear with other engines, better suited for the supersonic flying. China is believed to have obtained the AMNTK Soyuz R-79/79M/179 series engines developed for the Yakovlev Yak-141 vertical takeoff/landing deck fighter. Several Yak-141 examples flew, but the project was cancelled in early 1990s on the ground of high costs. Another option is the D-30F series engines powering the MiG-31 high-altitude high-speed interceptor. Center of gravity There are some shots depicting the J-20 on the Chengdu factory aerodrome, taxiing. They give a good view of the long fuselage, wing-to-body junction and undercarriage. They enable us to estimate position of the airplane’s center of gravity (COG). The undercarriage is three-point with two main wheel struts and a nose gear, fighter’s classics. We need to take the main landing gear strut and draw a vector originating in the wheel’s ground contact point. The vector goes upwards and is angled to the vertical by some 15 degrees towards the airplane’s nose. It crosses the fuselage center line somewhere near the likely COG position. The 15-degree angle is often assumed by aircraft designers to ensure the airplane does not fall back when taxiing or at landing, which may happen if COG moves too far to the rear. Thus found, the J-20’s COG appears to be somewhere near the leading edge of the wing. Rather strange, knowing that properly designed fighter aircraft have COG at 25-35% (starting at the leading edge and going to the trailing) of the wing’s mean aerodynamics chord (MAC). For instance, the Sukhoi Su-27 has its COG at 35% MAC. For this Russian aircraft the center of lift is 30% MAC at subsonic regimes and 55% MAC at high supersonic speeds. At subsonic regimes, the Flanker is slightly (5%) statically unstable. Going supersonic causes the aerodynamic focus to more rearward by 20-30% of the wing’s MAC due to changes in the pressure pattern over the wing. In the case of the J-20, the utterly forward MAC positioning is fairly strange for a maneuverable fighter. Balancing aerodynamic forces against the gravity would require relatively high deflection of the canards serving as control surfaces. Should the J20’s pilot try to execute a high-G maneuver at subsonic speeds, he may be restricted by the fact that the canards are already set at a high positive deflection angle (leading edge upwards). In other photos downloaded from the Internet the J-20 is depicted on the glide slope as it goes in for the landing with undercarriage down. Its canards are set at a rather high positive angle, while the wing’s leading edge devices deflected downwards. The trailing edge surfaces (elevons) are also deflected down, at a small angle. With the leading and training edge down, the J-20’s wing is highly curved. This enables the airplane to attain high angles of attack without stalling and without shifting the wing’s center of lift to the rear. The history of aviation knows solutions for delta-winged airplanes on the generation of more lift at landing. For instance, the canards can be placed closer to the fuselage’s nose, to achieve a greater leg, in relation to the center of gravity. With the larger leg to COG, the canards become more effective, and can balance a bigger lift of the wing. The Tupolev Tu-144 supersonic jet liner had retractable foreplanes kept inside the fuselage all the time except at landing. Chengdu designers ignored this. Rather, they positioned the canards fairly close the airplane’s COG, thereby sacrificing the airplane’s controllability at subsonic speeds for some other purpose. Purpose. Going supersonic causes the aerodynamic focus to more rearward by 20-30% of the wing’s MAC due to changes in the pressure pattern over the wing. Canards set relatively close to the wing help balance the aircraft at supersonic regimes. This adds more evidence to our assumption that the Chinese designers have developed their new jet for supersonic flying. But why is this strange, uncommon COG positioning? Are Chengdu designers unfamiliar with the classic solutions for a delta-winged, canard-equipped fighter? No, this does not seem the case in the view of their previous design, the J-10 lightweight fighter. The J-10 is a classic design with “proper” COG positioning, like in the books. A J-10B twin seat operational trainer accompanied the J-20 on 11 January 2011 as a chase plane. Two months before, at Airshow China’2010, PLAAF’s “1st August” display team demonstrated their skills and aircraft performance in solo and formation flying. The team was in Zhuhai with a J-10B and six J-10s. The J-10 was the star of the Airshow China 2008 and 2010, each time stunning the public with high performance aerobatics. It is a very maneuverable airplane, and a testimony of the Chinese designers’ skills in development of agile fighters. Possible answers First. The J-20’s air ducts run long. This seems to have been an important consideration at the design stage. The J-20’s air ducts are notably longer than the F-22A Raptor’s or Sukhoi PAKFA’s. If the J-20 and PAKFA have same family engines (AL-31F, Item 117), then why the J-20 needs a longer air ducting? Perhaps, the Chinese designers want a smoother airflow at the entry to the engine’s fan? Or do they want another engine type, better suited for sustained supersonic cruise? Second. The Chinese designers chose to make the air ducting S-shaped, in order to hide the fan blades from the enemy sensors. This measure reduces airplane’s radar and heat signature. The J-20’s air intakes are shaped similar to those on the Lockheed Martin F-35 Lightning II, under the concept of a "diverterless" supersonic inlet. This gives another piece of evidence to the fact that the J-20 is optimized for supersonic regimes and supercruise, like the F-35. Third. The J-20 has a remarkable distribution diagram for the airplane’s cross section along the fuselage centerline. Not only the fuselage itself, but also the thickness of the wing, canards and tailplanes shall be taken into account. By our reckoning, the J-20’s diagram runs exceptionally even. It comes without a distinct peak (found in so many other aircraft designs) running smoothly at approximately the same height from the tips of the air intakes all the way to the engine nacelles. Presumably, the designers at Chengdu wanted to make the airplane’s equivalent body of rotation as narrow as possible. At the same time, they needed to allocate considerable inner volumes for weapons carriage, a distinctive feature for all fifth-generation fighters. The Chinese designers have managed to shape the J-20 in a way that gives their airplane a much smoother cross section distribution diagram compared to those for the F-22A Raptor, the F-35 Lightning II and the Sukhoi PAKFA. From a designers’ viewpoint, every airplane is a compromise. Naturally more complex, supersonic aircraft are even more so. Seemingly, the Chinese designers made their choice in favor of perfecting their airplane’s transonic qualities, - and had to sacrifice other things to achieve their goals. Had the Chendgu designers committed to make the J-20 “a classic aircraft”, they would have never moved the wing to the far rear of the fuselage. Instead they took a plausible way of making the airplane’s body of rotation as narrow as possible, in the view of the necessity to accommodate customer-specified internal weapons bays, long S-shaped air ducts to the engines, the big engines and fuel stores. From the aerodynamicist’s point of view, the main peculiar feature of the J-20, and something that sets it apart from contemporary US and Russian fighter designs, is its remarkably smooth cross section distribution diagram. Other designs have “peaks” at 55-70% of their fuselage length. For instance, the Sukhoi Su-27 family aircraft have distinctive peaks at 55-60%, depending on particular model. Smooth cross section distribution diagram is important for transonic drag. Starting with the Century Fighters, supersonic aircraft have been shaped to comply with the “area ruling”, as it was known in their days. For high Mach numbers, M>2, where the supersonic airflow pattern is well set, the distribution diagram and area ruling are not that important. This leads to another interesting finding about the new Chinese airplane. Had then J-20 been developed for high supersonic speeds, its designers would have not committed to making such important compromises. The J-20 is optimized for transonic and moderate supersonic regimes, up to Mach 1.6. The compromises and sacrifices made in the process were necessary to enable the J-20 attain worthwhile performance at military power. More specifically, the J-20 is able to attain and maintain supercruise regimes at M=1.4-1.6. The J-20 can accelerate further and exceed twice the speed of sound, but this would necessitate afterburning, and the consequences that come with it: higher fuel burn, lower range and bigger heat signature. Supercruise-capable J-20 provides a high-energy launching platform for air launched weapons. High speed will help the Chinese jet evade enemy interceptors, outrun and out-zoom them. While the J-20 may prove a good interceptor, its main task seems to be anti-shipping. Flying high and fast, J-20 crews can explore kinetics of their aircraft to deploy air-to-surface missiles against hostile warships at stand-off ranges, and evade fleet-defense interceptors. If one day J-20s be used in anger, the most probable scenario would see their crews flying strike missions against valuable and well-defended targets. In today’s Chinese air power system, this role is played by the Xian JH-7A strike aircraft and Harbin H-6 bombers. Obviously, these two types are outdated and need replacing. The potent US Navy has emerged from a tiny fleet of wooden ships and ironclads. During the US civil war a famous duel took place. On 9 March 1862 the VSS Virginia (ex-Merrimack) engaged the USS Monitor, and first-ever battle of ironclads ensued. The Confederacy gunners scored numerous direct hits, but the USS Monitor, a 987-ton armored vessel with a large cannon turret on a low freeboard, seemed to have impunity to enemy shells. The North Americans built fifty monitors modeled on their namesake. For their peculiar outward appearance, these were nicknamed “cheese boxes on rafts”. After the Battle of Hampton, the US Navy never loses at sea, and its “cheese boxes” sail everywhere they please. The J-20 is China’s tool to ram them. BOX 1: Sukhoi’s cross-section distribution Proper shaping of supersonic aircraft requires a lot of effort. This can be illustrated by the story of the Sukhoi Su-27 fighter, company’s designation T-10 and NATO codename Flanker. Sukhoi understood the need for a very accurate optimization of the new fighter’s cross section distribution diagram in the early 1970s. A dedicated four-year effort on lowering the Su-27’s “wave drag” brought a meaningful 25% reduction, but not before Sukhoi designers completely redeveloped the airframe. Thus, the original Su-27 flown in a prototype form in 1977, gave way to the reworked Su-27S getting airborne in 1981. The Su-27S’ cross section distribution diagram differs in having a thicker airframe aft of the canopy (at 40-50% fuselage length) and thinner (by 0.5-1 square meters) at the rear fuselage (at 80-90%). The airplane’s thickest cross-section (at 60-65%) was narrowed from 4.2 square meters down to 4. The Su-27S’ cross section distribution diagram features a distinct peak (at 60-65%) and a forward plateau (of 2.2-2.5 square meters, at 25-50%). Reflecting the “rearward shifted” nature of the diagram, the forward half of the Su-27 generates 40% of the drag, leaving the rest of 60% to the rear half. In the Soviet Union, the related investments began to pay off in early 1980s, after powerful computing systems, such as home-grown MVS1000 with one-trillion operations per second, became available. More or less workable fluid mechanics software packages appeared in late 1980s – early 1990s. They contributed into a considerably improved aerodynamics of the advanced fourth generation aircraft, such as the Su-33 carrier-borne and Su-30MKI multirole fighter of the Indian air force. China has its own ways, but it has been learning from the west and Russia in the field of aeronautics. After Moscow and Beijing had ended their political differences and re-established strategic partnership, the Chinese aircraft manufacturers contracted certain Russian companies and teams to do some work for them. The next step was made earlier this century, when the Chinese corporations began employing foreign nationals directly, offering them well-paid jobs in the country. Today, the well-mastered practice of aircraft reverse engineering is being increasingly supplemented by copying bits of aircraft-development systems from foreign countries for use in the existing Chinese structure. In that sense, the Russian expertise in the aeronautical and military-industrial fields finds another, very interesting and intriguing application. BOX 2: Sukhoi’s carrier-killer The Soviet Russia always wanted but never succeeded in making an ultimate carrier killer. Can the Communist China prove different? Moscow always saw the US Navy carrier groups as a serious threat. In early 1960s Yakovlev, Tupolev, and Sukhoi began R&D on shaping a fast missile-carrying aircraft for anti-shipping. Hundreds aircraft layouts were considered. Sukhoi fared better and won a governmental contract for construction and flight testing of a full-scale technology demonstrator. The T-4, also known as the Sukhoi 100, flew in August 1972. The airplane featured a large delta wing blended into the dart-like fuselage and small foreplanes (destabilizers). Its airframe was largely a welded construction made of titanium and steel. The 135-tonne (nearly 300,000lb) MTOW machine was capable of Mach 3 and altitude over 20,000m (65,600ft). Flying high and fast, the T-4 represented a high-kinetic platform for stand-off deployment of two huge X-45 anti-ship missiles. The USN carriers had strong air defenses with McDonnell F-4 Phantom II and, later, Grumman F-14A deck interceptors armed with long-range air-to-air missiles. To defeat these defenses, attacking aircraft had to be very technically advanced, not only as a flying vehicle but also in terms of onboard equipment for acquisition, selection, tracking and illumination of sea-going targets. Sukhoi did not encounter much difficulty in making its new airplane fly fast, but wasted lots of time making it good at M=0.8 also, as the customer had specified. Fast and heavy, the T-4 required high-class aerodromes. This was fraught with difficulty to assure effective employment in a wartime, when availability of the good aerodromes could not be guaranteed, especially in the view of their questionable sustainment to enemy strikes. Enabling the T-4 operate out of ordinary aerodromes required intensive wind tunnel testing, using the T-112 tunnel at TsAGI, the Central AeroHydraDynamics Institute in Zhukovsky near Moscow. Careful selection of wing’s profiles and their relative thickness, camber and twist of the wing, and the use of deflectable leading edge and drooping ailerons boosted the T-4 aerodynamics efficiency (lift to drag ratio) at low speeds from 7 to 9. Sukhoi managed to complete a fraction of the T-4 flight testing before the project was shelved in 1975. Ever-growing air defense potentials of the USN carrier battle groups demanded thorough optimization of aerodynamics and overall technical performance of the intended carrier-killer. Eventually, the Soviet Union gave up this technological race due to a shortage of resources, scrapping the T-4 and its further evolution T-4M/MC, or the Sukhoi 200. Instead of the much-wanted uncompromised carrier-killer, the Soviet navy air arm was given the versatile Tu-22M (NATO Codename Backfire) swing-wing bomber developed to an Air force specification to multirole, multimode aircraft. Launched five years after the T-4, a less sophisticated Tupolev took only two years to be developed and flown, and seven more years to enter squadron service.