It’s been nearly one month since first flight of the Chengdu J-20, but we still know so little about the aircraft. China’s air force — the PLAAF — has remained surprisingly mum on an aircraft they were so willing to flaunt on a Chengu runway for nearly three weeks. We still don’t know if the J-20 is officially intended as a one-off technology demonstrator or a developmental prototype, although many observers have assumed the latter. We also don’t know what the J-20 is designed to do. Is it an interceptor to replace the Shenyang J-8, or a strike bomber to replace the Xian JH-7? Or is it intended to be both or neither?
Frequent Flightglobal contributor Vladimir Karnozov has published a new feature in the Russian language magazine Air Fleet that attempts to fill the contextual vacuum with a thorough aerodynamic analysis. Dissecting three of the J-20′s seemingly peculiar characteristics — center of gravity in relation to the mean aerodyanmic chord, canards in relation to the nose and overly long air intakes — Karnozov concludes the J-20 is no subsonic dogfighter. In other words, in an acrobatic game of chicken within an F-22 within visual range, the J-20 is dog-meat.
But Karnozov believes the Chinese are no aerodynamic dummies. Rather, he concludes the J-20 has been optimized for a very specific role: a high-speed, intercept-evading, aircraft carrier-killer. In an English translation of the article that Karnozov kindly contributed to The DEW Liine, he writes:
“In my view the Chinese designers optimized their new jet for M=1.4-1.6. Here comes the clue: the J-20 is a missile launching platform able to evade enemy interceptors by means of a high cruise speed. The J-20 may prove a good interceptor — very possibly. But its main task seems to be anti-shipping: firing missiles at enemy warships while denying their air defense cover.”
Read the translated version of Karnozov’s full article on the jump.
China’s tool to ram “cheese boxes on rafts”.
(author’s translation of the original text in Russian, published on 12 January by www.aex.ru)
On 11 January 2011 the new Chinese combat jet flew for the first time (in public, at least), taking off at 12:50 local time. The new airplane is referred to as the Chengdu J-20. Chengdu is the name of the city which houses a few aviation industry enterprises, including the aircraft manufacturing plant producing jetfighters and the design house developing them. A few outsiders watched the J-20 fly, as they happened “by chance” to be around the fence of Chengdu factory’s aerodrome on that day. The flight itself was uneventful. It took place in the conditions of clear skies allowing photographers to make some good shots.
Before touching down, the pilot made several passes over the runway so as to expose his airplane to the cameras of “aviation admirers” all round the place. Those took photos of the aircraft from different angles and depicted everything they wanted except for doors of internal weapons bays.
These doors were either thoroughly hidden or removed from the shots by the picture takers on the insistence of very competitive advisers. But it is even more likely that these doors were not actually fitted to the J-20 first operable prototype. They are not needed on the very first operable aircraft dedicated to assessment of flight performance, flight envelope, various engine settings, functioning of the essential onboard systems, proving flight control algorithms. As a rule, third or even later prototypes are devoted to weapons testing, but these are yet to be constructed and outfitted.
The J-20 first public flight occurred just in time when US defense secretary Robert Gates was in Beijing on an official visit. Once there, he was trying to calm down the Chinese leaders who were much worried about pending deliveries of modern US-made weapons to Taiwan. Beijing considers this island an essential part of China.
A lot of pictures appeared on the Internet on the memorable day of 11 January. These shots gave more information on the new airplane. In particular, they reveal the shape of the wing and its positioning in relation to fuselage. This makes it possible to make some preliminary conclusions about the aerodynamics layout and technical characteristics of the J-20, and make guesses as to the main task the new jet shall be solving after entering squadron service.
The J-20 represents a relatively large tactical jet with the canards (foreplanes) and large delta wing. The fuselage length is somewhere between 22 and 24 meters, wingspan between 13 and 14 meters. By our estimation the maximum takeoff weight shall be in the region of 40 tons, and operating empty weight twice less than that.
Many aviation experts believe that the J-20 relies on a pair of Russian engines or their Chinese copies. In other words, the J-20′s engines are picked out among members of the big family uniting the Item 117, AL-31F, WS-10G and WS-10 Taihang. Two engines together develop in between 30 and 40 tons of thrust. If that is so, then the capability of the propulsion system is enough for supercruise, or supersonic cruise flight at military power (highest power setting without afterburning). We may also expect that the J-20 with restricted fuel and combat load (for instance, when flying air-to-air mission) can fly vertical without losing speed at subsonic regimes and low altitudes.
The viewpoint that the AL-31F family engines power the J-20 is now prevailing. It could well be that J-20 operable prototypes have them in real life. If so, it is a good solution for initial testing, to rely on these well-tried and high-performance power plants. But in future the J-20 may appear with other types of engines. There were reports about China having got hold of the AMNTK Soyuz R-79/79M/179 series engines developed for the Yakovlev Yak-141 and other advanced fighter designs, – these were in development but cancelled for various reason. Ideally, the J-20 should have engines optimized for sustained supersonic flight, and the AL-31F is not such.
When in-flight photos appeared, the J-20 became the hottest topic for discussion among aviation enthusiasts round the world. But as it appeared, the enthusiasts, and even world-famous western journalists, had difficulty in classification of the new Chinese warplane. Is it a superiority fighter? Is it a supersonic bomber? Or, perhaps, it is a multirole, multimode airplane? Even columnists and experts with world’s leading aviation magazines have hesitated to give their clear answer to these questions, – that in the view of them having good sources in the US and European intelligence bodies, defense ministries and the industry. It seems that not only journalists, but the professionals were in a state of shock after seeing the new Chinese bird.
First of all, let’s determine J-20′s center of gravity position. There are some photos available of the J-20 taxiing, in which we can clearly see its long fuselage, wing-to-fuselage connection and landing gears. The J-20 undercarriage is fighter’s classics: three-point with a nose gear. And so it makes it easy to determine center of gravity position. To do that we take the main landing gear strut, and attach a line to it starting at the wheel’s ground contact point. The line goes up with at an angle of, say, 15 degrees, leaning towards the nose of the airplane. The point where it crosses the fuselage center line is the most likely position for the airplane’s center of gravity.
Here comes the first surprise: the likely center of gravity position rests… too far from the mean aerodynamics chord (MAC) of the wing. As a first iteration for aircraft designers, the center of gravity must be somewhere 25-35% of the wing’s MAC, – like so is prescribed in the classic aircraft design books.
But the Chinese airplane appears to have the center of gravity position somewhere at MAC’s edge. It is fairly strange for a maneuverable fighter, since balancing of the aerodynamic forces and the gravity will require relatively high deflection of the control surfaces – canards in the J-20′s case. Should this airplane try to execute high-G maneuvers at subsonic speeds, the deflection of the canards could be a limitation. All this is rather strange for a maneuverable fighter… But not for the J-20, which does not appear to be one of those!
Let’s take a look at other available photos, in which the J-20 goes in for landing with landing gear down. Apparently, the canards are set at a rather high positive angle (leading edge upwards), while the wing has its leading edge deflected downwards. The trailing edge surfaces are also deflected down, at rather a small angle. Obviously, at the approach for landing configuration, the wing’s center line is highly curved by means of the leading and training edges down, which increases lift (achieved through altering the camber of the wing). But not so much as in the case of the classical flaps.
All this is, again, fighter classics for the delta winged aircraft with foreplanes. And here lies their limitation: the pilot cannot set the trailing edge further down, since the resulting lift force that builds up on the trailing edge will be hard to balance with the canards, in the view of their limited deflection scope (canards may stall at higher deflection angles).
It is well known from the aviation history how to enable delta-winged airplanes to generate more of the lift force at landing. For that purpose the canards are placed as close to the fuselage’s nose as possible, to make a larger distance to the center of gravity. For instance, the Tupolev Tu-144 supersonic jet liner had foreplanes that were retracted into fuselage all the time except landing. But Chengdu designers did not do this. Rather, they positioned the canards fairly close to the center of gravity position, and thus sacrificed their effectiveness at landing for some other purposes.
What purposes? Firstly, for non-retractable foreplanes it is important to have them within the supersonic cone as it sets on the top of the airplane’s nose at Mach numbers exceeding 1.0. This leads to a conclusion what the Chinese must have been purposely shaping the J-20 for supersonic flying.
Why did the Chinese shape the J-20 that way? Perhaps, they are unfamiliar with the classic solutions for a delta-winged, canard-equipped fighter? No, this is not the case knowing that Chengdu’s previous design was the J-10 light weight fighter, now in PLAAF service. On its first public flight, the J-20 was escorted by a J-10B twin seater, the operational trainer version of the baseline J-10 single seat fighter. The J-10 was the star of the Airshow China 2008 and 2010, when it flew superbly with the PLAAF display team pilots at the controls. The J-10 is a very maneuverable airplane. This fact is the testimony of the Chinese designers’ skills in development of maneuverable fighter aircraft.
The J-10 is a classic design with “proper” positioning of the center of gravity, like prescribed in the books. This is clear to tell looking at the main landing gear struts attached to the fuselage somewhere near 15-30% of the wing’s MAC. So, let us ask ourselves the same question again: why did the Chinese designers shape the J-20 that way?
Here are some suggestions.
First, to achieve smooth airflow with desirable parameters at the entry to the engine’s fan, the J-20′s designers have to make the air intakes rather long – much longer than those on the F-22A Raptor or Sukhoi T-50 (PAK FA or FGFA). As it was put before, the J-20 prototype is likely to be flying on the AL-31s, but in future it may get a new engine type optimized for sustained supersonic cruise, – and that other engine is likely to be longer and require longer air channels. In any case, long air intakes were an important consideration at the design stage.
Second, the designers also needed to make air channels S-shaped, so as to hide the fan blades from the radio waves emitted by enemy radars. This is needed for a lower visibility of the airplane. It is worth to notice that the J-20′s air intakes resemble those on the Lockheed Martin F-35 Lightning II (“diverterless” supersonic inlet, DSI). This gives move ground to assert that the J-20 is optimized for supersonic regimes and supercruise, much like the F-35.
Third, let us make distribution diagram for the airplane’s cross section along the J-20′s fuselage centerline. We need to take into account the thickness of the wing, canards and empennage. The diagram appears to be very smooth, – exceptionally smooth! It comes without a peak, running smoothly at approximately the same height from the tips of the air intakes all the way to the engine nozzles.
This seems to be the main thing about the J-20. Apparently, the Chengdu designers wanted to make the airplane’s equivalent body of rotation as narrow as possible. And they needed to make provision for internal carriage of weapons, which is a characteristic feature of all fifth generation fighters. In actual fact, the J-20 appears to have much smoother cross section distribution diagram than the F-22A Raptor, the F-35 Lightning II and the Sukhoi T-50. Apparently, it required quite an effort from the Chengdu designers and so made them go for compromises on other things.
Should the Chendgu designers have made it “classic”, they would not have moved the wing all the way towards the engine nozzles. But they did because it was the only effective way to make the airplane’s body of rotation as narrow as possible, with the need for big air intakes, air-supply channels and internal weapons bays.
Again, this is the main thing about the J-20 design, and it sets it apart from all other known fifth-generation fighters. Other designs have “peaks” some 55-70% down the way from the fuselage nose tip to the engine nozzles (for the Su-27 family aircraft the respective figure is 55-60%, depending on version).
A smooth cross section distribution diagram is important for transonic drag. Supersonic aircraft are being designed in accordance with so-called “area ruling”. For high Mach numbers (M>2) the distribution diagram is not that important as for the transonic regimes, M=1…1.5. It seems the Chinese designers optimized their new jet for transonic regimes and moderate supersonic speeds.
A huge effort on proper shaping of the Su-27 was made by Sukhoi designers in cooperation with TsAGI in order to improve its transonic performance. Three TsAGI’s big wind tunnels, the T-112, T-108 and T-109 were used in the process. As a result of very accurate optimization of the fighter’s cross section distribution diagram, the Su-27′s “wave drag” was reduced by 25% – not before the designers virtually redeveloped the airplane (the initial aircraft was referred to the Su-27, the improved version became known as the Su-27S).
My impression from the J-20 is that it is an uncompromised airplane for supercruise, for flying at moderate supersonic speeds corresponding to Mach M=1.4-1.6. Such speeds can be achieved without afterburning. Surely, the J-20 can accelerate to M=2 and faster, but this would require engaging afterburners. In turn, the fuel burn will go high, lowering operational range of the aircraft and enlarging its heat signature.
In my view the Chinese designers optimized their new jet for M=1.4-1.6. Here comes the clue: the J-20 is a missile launching platform able to evade enemy interceptors by means of a high cruise speed. The J-20 may prove a good interceptor, – very possibly. But its main task seems to be anti-shipping: firing missiles at enemy warships while denying their air defense cover.
One day it may happen that the new Chinese jets would be used in anger. On such a day People’s Liberation Army would order its pilots to attack enemy warships off the coast of a freedom-loving island not far from the mainland China. In that sense the J-20 is a likely replacement for the JH-7A strike aircraft.
The history of the powerful US Navy can be traced back to the famous duel of the USS Monitor and VSS Virginia (Merrimack) on 9 March 1862, the first-ever battle of ironclads. The USS Monitor, a 987-ton armored turret gunboat, was built at New York, with a large single cannon turret on a low freeboard. Although the Confederacy gunners scored hundreds of direct hits, shells bounced off her armor: the Monitor seemed to have impunity to enemy shellfire. After the battle, the North Americans constructed fifty monitors modeled on their namesake and made them the backbone of their navy. For their rather strange looks, these ships were called “cheese boxes on rafts”. Since the memorable Battle of Hampton, the North Americans never lose at sea, and now their cheese boxes sail when and where they want. China prepares a tool to ram them.
J-20: China’s ultimate aircraft carrier-killer?
By Stephen Trimble on 9 February, 2011 in Uncategorised
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