China's fifth generation fighter project began during the same era when Fourth generation fighters supplanted Third and Second generation fighters as the Chinese Air Force's main battle gears. PLAAF’s positive experience with Fourth generation fighters’ superior maneuverability led to a reevaluation of PLAAF air doctrine based on maneuverability and super maneuverability. As a result of this, Fifth generation characteristics such as unconventional maneuverability, supersonic maneuverability, and supercruise became uncompromisable requirements for China's Fifth Generation Fighter. However, the two types of engines immediately available for China's Fifth generation fighter project are the Russian AL-31 and the indigenous engine still under development, the WS-10 engine. Both types of engines are conventional Fourth generation engines and could only reliably achieve thrust to weight ratios of around 7.5.
The issue of engine designs lagging behind aircraft designs is an old problem in China's aviation industry. Many aircrafts fail to leave the drawing board due to China's inability to develop suitable engines. Take the famous J-10 fighter, for example. If China did not purchase the Russian Al-31 engine in time it would have taken a lot longer for the plane to enter service.
Due to historical precedence of failed engine designs, the Chinese Air Force and Aircraft Manufacturers are afraid to build their Fifth generation fighter project on engines that are still on the drawing board. With China's technological capabilities and funds the Chinese could only produce engines with a thrust to weight ratio of around 9.5 (or a thrust to weight ratio of 10, if you round up.). Chinese engines are still far behind their American counterparts (which have thrust to weight ratios of around 11). As a result of this a major problem facing the Chinese designers is how to produce a fighter plane with the same maneuverability and supersonic performance as American Fifth generation fighters (or, at the very least, have no major disparities in performances).
Design requirements for transonic maneuverability always contradicted requirements for supersonic performance. The former demand wings with larger aspect ratio, smaller swept angle and greater thickness relative to its cord; the latter require wings with a smaller aspect ratio, larger swept angle and smaller relative wing thickness.
Since Fifth generation airplanes emphasize supercruise (the ability of an aircraft to travel at Mach 1.5 for 30 minutes at maximum engine output), supersonic aerodynamic shaping is even more important than those of previous planes. In the area of supersonic aerodynamic shaping, primary constraints include the aspect ratio of the wings, back sweep angle, relative wing thickness and cross sectional shape. Optimizing these parameters for supercruise has significant conflicts with the airplane’s requirement for max lift coefficient at low speeds.
The US leveraged its immense advantage in engine technologies and used a conventional aerodynamic configuration with a 40 degree back swept angle, a small aspect ratio of 2.35, and leading wing slats. This allowed it to successfully solve the problem of optimizing aerodynamic layouts for supersonic, transonic and subsonic speeds. However, because China's engine technology is 30 years behind that of the US, it is impossible for China to use conventional aerodynamic designs to solve the problem of optimizing subsonic maneuverability while maintaining good supercruise capabilities and lower supersonic drag.
Subsonic lift to drag ratio determines an aircraft's maximum range and turn performance. As a result, the Chinese fifth gen. fighter's demand for subsonic lift to drag ratio will not be lower than those of Fourth gen. fighters. Unlike most conventional Fourth gen. fighters, Fifth gen. fighters need to supercruise (the ability of an aircraft to maintain a cruising speed of M1.5 without engaging the afterburners). This means that research on supercruise drag characteristics is pivotal to the aerodynamic design of China's Fifth gen. fighter. In order to satisfy the Air Force demand for supercruise (at least achieve a supercruise speed of 1.XM), the Chinese Fifth gen. fighter must make some sacrifices in subsonic lift to drag ratio. The wings of China's fifth gen. fighter are swept backward at 50 degree angles and have a smaller aspect ratio than those of the F-22A (small aspect ratio wings swept at large angles usually have good supersonic drag characteristics but have poor low speed lift and transonic drag characteristics). This is the design threshold of a fifth generation fighter with supercruise capabilities given the restrictions imposed by China's engine technology. Yet the sacrifices made to improve supersonic drag performances did not convince the Chinese military to lower the Chinese Fifth Gen. Fighters subsonic lift to drag ratio requirements. This seemingly irreconcilable contradiction indicates that it is impossible for China to follow American design logic on her indigenous fighter. This forces China's Fifth Gen. fighter designers to give up America's proven conventional aerodynamic design (since Russia's engine technology was superior to that of China’s the Russians went back to using a conventional aerodynamic layout after many experiments. While it is true that the Russians added numerous innovative features on the T-50 the main aerodynamic layout show significant American influence) and pursue new solutions to this problem.
Due to china's weakness in the area of jet engine development, a new aerodynamic configuration (canard configuration) was chosen to resolve the conflicting requirements of her Fourth generation fighter for transonic maneuverability and supersonic performance. Designs involving both the canards and the leading edge slats already utilized the aerodynamic efficiencies of the airfoils to the maximum. As a result the aerodynamic design of J-10 cannot satisfy PLAAF's requirements for her Fifth generation fighter.
The CAC research institute decided to further relax the longitudinal static stability factor to increase the maximum lift coefficient. Data from the CAC shows that relaxing the longitudinal stability factor from 3% (Fourth generation fighter jets) to 10% results in significant improvements in lift to drag characteristics. Both transonic and supersonic lift to drag characteristics and the maximum lift coefficient value under low speed were improved. The improvements came at the expense of difficult pitch-up problems during high angle of attack maneuvers and a more complex flight control design. After weighing the pros and cons, it was decided that relaxing longitudinal static stability alone would not be enough to satisfy the requirements of a Fifth generation aircraft where transonic lift to drag ratio is concerned.
Because of this, the CAC institute decided to focus on improving the canard configuration with innovative new features.
International aviation technology indicates that conventional aircrafts employing liftbody configuration achieved excellent results in lift enhancement. However, no canard-configuration fighters employed the liftbody design. This is not because no one recognized the advantage of the liftbody configuration but the result of canard placement on canard-configuration aircrafts. Canard-configuration fighter designs generally place the canards above the wings to allow the downwash generated by the canards to interact with the wings. This allows the aircraft to use the interaction of the vortices to produce beneficial couplings that will enhance the lift coefficient. It is difficult for liftbody configurations to satisfy this condition (liftbody design requires the canards to be level with the wings).
Pursuit for supersonic cruise drag characteristics forced the CAC to tinker with the canard liftbody configuration to open a new path in its pursuit for a Fifth gen. design.
CAC discovered during experiments that although adopting the lift body canard configuration reduces the lift contributions from the canards, its overall lift performance is better than that of a non lift body canard aircraft as long as the canards, LERX, and wings were placed at proper distances and angles with respect to one another. The designers were thrilled by this discovery
Further studies indicate that canard configuration aircrafts employing liftbody and LERX derive lift not only from the longitudinal coupling between the canards and forward portion of the LERX with the wings’ shed vortices but also the benign interferences between left and right shed vortices. The latter adds significant lift to the aircraft and greatly contributes to the improvement of lift characteristics.
Even more encouragingly, aircrafts employing the liftbody LERX canard configuration could select smaller aspect ratios. This will, without a doubt, reduce pressure on engine performance. The CAC discovered after numerous experiments that candard configuration planes employing liftbody LERX could, under high AOA conditions, concentrate the lift on the plane's body and inner portions of the wings. After properly reducing the wings’ aspect ratio the highest lift coefficient actually increased instead of decreasing as predicted. This is an amazing phenomenon.
Under conventional aerodynamic configuration, supersonic drag, maximum lift under low speed, and transonic lift to drag ratio suffer from contradictory design requirements. Aircraft wing designs have the most significant effect on supersonic drag. Wings with mall aspect ratio and large sweep angles offer lower drag at supersonic speeds but are detrimental to the other two requirements. The Mig-21 is a good example of this since its wings, with a sweep angle of 57 degrees and an aspect ratio of 2.22, offers very good supersonic performance but worse performances at lower speed.
Under a lift body LERX canard configuration, however, these two traditional contradictions of aerodynamic design became, to a certain degree, reconcilable! The new discover of using liftbody LERX canard configuration allows the aircraft to select smaller aspect ratios than its conventional counterpart (very beneficial for raising the design threshold for low speed characteristics) while maintaining better low speed characteristics than conventional configuration aircrafts. This major discovery allow nations that are comparatively backwards in engine technology to use their available technology to build low cost Fifth gen. aircrafts while maintaining the said aircrafts’ supersonic and low speed high AOA capabilities.
The discovery CAC made in flight aerodynamics resulted not only in a firm technical base on which China's Fifth generation fighter project can build upon but also greatly contributed to the world wide aeronautic industry. This marks the first time that the Chinese aerospace industry moved from being a imitator of aerospace technology to an innovator and pioneer.
After solving issues related to transonic and supersonic drag to lift performance, the CAC must then solve the problem of maintaining aircraft control under low speed and high angle of attack. The solution involves the plane’s non-conventional maneuverability capabilities.
F-22's controllability at high AOA and post stall maneuverability are primarily accomplished by thrust-vectored engines. The CAC, however, has even higher standards in this area and proposed that the Chinese Fifth Gen. fighter should maintain control at high AOA even when the thrust-vectoring nozzles fail. This will allow the plane to recover safely within post-stall AOA parameters (the reliability of Chinese thrust-vectored engine was a major consideration). As a result they included unconventional aerodynamic control devices for high AOA flight in their research project.
Traditionally people believe that the post stall AOA for a canard-configuration aircraft is 35 degrees. The Israelis were the first to propose this and their proposal was taken seriously by many other countries. The French restricted Rafale's highest AOA at 28 degrees while the Chinese set the J-10’s AOA at 26 degrees. As a result the aviation community generally believes that canard configuration fighters are inferior to conventional configuration fighters in terms of high AOA capabilities since the canards’ post stall AOA restrictions severely limited the high AOA capabilities of canard configuration fighters.
Yet Chinese test pilots noticed something completely different during post stall flight. They discovered that J-10's high AOA control was far superior to that of the Su-27 (the J-10 achieved higher angles than the Su-27 during the cobra maneuver). This information was first leaked by test pilot Lei Qiang but widely questioned by military fans.
CAC's research confirms Lei Qiang's claims. Their research reports indicate that there are two types of negative pitch moment control surfaces depending on the positioning of the elevator with respect to the aircraft's center of mass. The first are the "load enhancing" control surfaces. They are control surfaces placed behind the aircraft's center of mass. Examples of this include horizontal stabilizers and trailing flaps which generate negative pitch moment by increasing lift. The second are the "load reducing" control surfaces. They are control surfaces placed in front of an aircraft's center of mass. "Load reducing" control surfaces include the canards, which generate negative pitch moment by decreasing lift. Under high AOA conditions the lift coefficient generated by the wings approach saturation and as a result the negative pitch moment of "load enhancing" control surfaces approach saturation as well. This problem, which is unsolvable by conventional configuration aircrafts at high AOA, could be effectively solved by "load reducing" control surfaces (canards). The unconventional (canard) configuration of China's Fifth gen fighter gives the Chinese fighter a "natural born" advantage at high AOA control.
Taking into consideration the needs of overall lift performance and better negative pitch control of the Fifth generation aircraft design, the areas of the canards are increased by xx%, and their largest deviation angles were increased to xx degrees. This design allows J-20 to have better high angle of attack aerodynamic performance than J-10. It is also superior to the T-50 and F-22 in terms of its high angle of attack unconventional aerodynamics control.
Having solved the issue of maneuverability, China’s Fifth Gen. Fighter must integrate RCS reduction measures into its aerodynamic design. I will only cover some prominent examples here.
Due to requirement for sideway stealth, the planes’ vertical stabilizers need to be canted either inwards or outwards to deflect horizontal radar waves in the other directions. This means that a twin-tail configuration is needed. However, a twin-tail design can reduce the maximum lift coefficient by as much as a factor of 0.4. This is very bad news for the designers whose focus is to increase the J-20’s lift capacity.
Since the negative impact vertical stabilizers have on stealth is offset by its benefit of lift improvement it is difficult to root out this problem. Ordinarily an aircraft designer could lower the negative impacts of the vertical stabilizers by adjusting the area, placement, tilt, and position of the said stabilizers. Yet modifications of the tilt and placement angles are effected by optimal RCS reduction and must comply with stealth considerations. As a result, it is more practical to alter the size and position of the vertical stabilizers. CAC's studies show that plans which decrease the size of the vertical stabilizers or eliminate them all together deserve further attention. Since there are many unresolved technical issues with the stabilizerless design, the CAC ended up picking the method which reduces the sizes of the vertical stabilizers.
Due to the aircraft's need to maintain directional stability it isn't possible for the design team to shrink the areas of the vertical stabilizers till they are within the required specs. The only way to go around this is to employ all moving vertical stabilizers, which allows the vertical stabilizers to half their areas. Vertical stabilizers that are too small, however, will negatively affect an aircraft's directional stability especially when the plane is flying at high Mach speeds or maneuvering at high AOA. In order to maintain the aircraft's directional stability there is usually a limit to the relative sizes of all moving vertical stabilizers. It is not possible to shrink them to infinitesimally small sizes.
CAC's research indicates that improved versions of twin vertical stabilizers decreases the negative impact to the max lift coefficient to the 0.1 level and, at the same time, reduces the structural weight of the vertical stabilizers (decreasing the structural weight of the vertical stabilizers by over 40%).
CAC's obsessive pursuit for its Fifth gen. plane's max lift coefficient and stringent attention to design details helped China's fifth gen. aircraft gain the best transonic maneuverability.
During the design process the CAC not only emphasized the aircraft's sub and transonic capabilities but also focused on improving its supersonic drag characteristics. Aside from choosing the wing shape with attributes such as large backward sweep angle, small aspect ratio, and relatively thin thickness that are beneficial to the aircraft's supersonic drag characteristics, the CAC also incorporated supersonic drag reducing measures on other parts of the plane. Examples of this include the elongation of the plane's body (at the expense of thrust to weight ratios due to the extra structural weight), the incorporation of all moving vertical stabilizers, and the implementation of DSI intakes (measures that lower pressure on the engines by enhancing the thrust to weight ratio via structural weight reduction). Unconfirmed information indicates that China's Fifth gen. fighter used what is known as an "adjustable DSI intake" which will, without a doubt, further enhance the aircraft's supersonic capabilities. Incorporations of such devices testify to CAC's innovative design.
As we happily examine this completely unique Fifth gen. fighter today, just how many of us actually know the dedication and sacrifices the CAC designers made, under backward technological conditions, to reach the peak of aerodynamic design? Their toils were not in vain and as a result of their hard work China's Fifth gen. fighter is now a worthy fighter capable of holding its own in the realm of fighter aircrafts.
