J-31 - Design
A scale model of the FC-31 was displayed by the Aviation Industry Corporation of China (AVIC) state-owned enterprise [SOE] stand at the 53rd Paris Air Show, which opened 01 June 2019. The model on display appeared to have undergone many changes compared to the aircraft's past designs. The area behind the cockpit and the areas where the two engines are housed have been enlarged. The upper part of the engine compartment is clearly arched, which is different from the prototype. The optimized aerodynamic design further reduces aerodynamic drag. According to photos, the main change direction of this model is the tilting of the tail, the distance between the two has been significantly changed, and the improved vertical tail has further strengthened the stealth performance.
These changes could mean the aircraft has been made more agile and is capable of carrying more fuel, which will give it a larger operational range. It might also carry additional electronic devices for communication or satellite links. The upgraded FC-31 might even feature a pair of new engines. The nozzles of the engines on the FC-31 model displayed in Paris are very different in structure and shape than the ones previously used.
On the whole, FC-31, as a foreign trade model, frequently appears overseas in the form of a model. It seems that the PLA Navy and Air Force have not officially adopted this Shenfei-designed fighter. However, in recent years, there has been no shortage of Chinese foreign trade weapons, and the precedent adopted by the PLA.
This type of medium-sized fighter aircraft, the engine thrust can not be compared with the large fighter aircraft, must also be equipped with an internal stealth bomb bay that significantly increases the cross-sectional area, resulting in a significant increase in drag, so it is necessary to deepen the aerodynamic potential.
The performance of the J-31 flight show at the 2014 China International Air Show was disappointing, and the flight capacity seemed to be quite ordinary due to insufficient design capabilities. According to many reports, the aircraft has been redesigned to include a new form of wing plane, more like the J-20 or F-35. A new aircraft carrier fighter is obviously very difficult, and stealth further increases the difficulty. In addition, the entire Chinese aviation industry is striving to break through the difficulties of advanced jet engine design, so this is also a key to the success of the J-31 or FC-31. The turbofan-15 developed for the J-20 is not suitable for the medium-sized fighter FC-31, so it may be necessary to temporarily continue to use the Russian RD-93 series turbofan engine adopted by the J-31 prototype.
Initially the J-31 was a technology demonstrator or prototype, and lacked many features such as electro-optical and missile launch warning sensors. The first airframes probably used a preexisting radar, possibly from the J-10. Other pre-existing systems included the [really smokey] Russian RD-93 engines [existing Chinese engines are even worse] found on the FC-1 fighter, as well as having an older two piece canopy over the cockpit.
The J-31 differs from the F-35 in that it has two engines, which in turn reduced its area ruling, making for more efficient supersonic flight, including future supercruise once the J-31 obtains more powerful engines.
The "31001" prototype at the Zhuhai 2012 airshow was reported to have a combat weight of 17.6 tons; as with the transition from the X-35 to the F-35, later fully equipped production models are likely to be heavier than the demonstrator.
A scale model of the FC-31 at Zhuhai 2014 showed several important upgrades from "31001", including the installation of a Electro Optical Targeting Sensor (EOTS) pod under the nose (shiny gold fairing). The updated FC-31 design has several radar cross section reduction measures, such as clipped wing edges, and revised vertical stabilizers, to reduce reflected radar signals.
Scale models displayed at the November 2014 Zhuhai Airshow revealed several planned differences from the 31001 flying prototype. The differences include a stealthier cockpit, a next-generation helmet mounted sight, holographic cockpit displays, EOTS, aerodynamic revisions and more powerful engines. While "31001" flew crowd pleasing maneuvers at Zhuhai, a new and updated display scale model on of the J-31 / FC-31 showed several design refinements that are probable in the final plane. The new model featured redesigned wings, clipped at the corners to enhance stealth, and all new vertical stabilizers, enhanced for stealth. The new model also had an Infrared Search and Track sensor installed on the underside of its nose, to enable the J-31 to track the heat signatures of enemy aircraft.
It also had redesigned, stealth optimized engine nozzles, which suggest that a Chinese 10- to 11-ton thrust engine will ultimately replace the Russian RD-93. Notably, such future engines could allow the J-31 to supercruise, a feature that the F-35 does not have. The new stealthy engine nozzles on the FC-31 model probabbly indicate plans to replace the legacy RD-93 turbofans. The replacement turbofan engines will each generate about 10-11 tons of afterburning thrust, which could enable the J-31 to supercruise.
A flight simulator/cockpit avionics display at Zhuhai 2014 showed state of the art pilot/cockpit interface technology, including a holographic Heads Up Display (HUD), a built in helmet wide display, and multi-functional display screens that would show and fuse sensor data, managed communications, as well as firing and controlling weapons.
The J-31 inlet design features diverterless supersonic inlet [DSI] similar in concept to that used on the American F-35. The F-35's diverterless inlet lightens the overall weight of the aircraft. Traditional aircraft inlets were comprised of many moving parts and are much heavier than newer diverterless inlets. The diverterless inlet also eliminates all moving parts. DSI moved from concept to reality when it was flown on a Block 30 F-16 in a highly successful demonstration program consisting of twelve flights flown in nine days in December 1996.
The aircraft must demonstrate controllable flight at higher Mach numbers, which the aircraft may experience during high speed bursts and/or dives. At these conditions, the airflow passing adjacent to the wing (and particularly, the airflow passing adjacent to the upper surface of the wing) may accelerate to supersonic velocities before passing through a normal shock and decelerating to subsonic velocities upstream of the wing trailing edge.
One problem associated with the occurrence of adjacent supersonic and subsonic regions of the flow on the wing upper surface is that the static pressure in subsonic regions can be significantly higher than the static pressure in supersonic regions. Accordingly, the wing may be subject to atypical pressure gradients, which can stretch the capability of the control system. A further drawback is that the position of the shock wave separating supersonic flow from subsonic flow can shift significantly over the chord length of the wing, depending on factors including the aircraft angle of attack, freestream Mach number, sideslip angle and control surface deflection. The position of these shock waves is not always advantageous, and can limit design options to meet certification requirements.
One approach for addressing the foregoing drawbacks is to install a "shock bump" on the wing upper surface. The shock bump can include a generally smooth, faired projection that reduces the Mach number at which the shock wave occurs (thereby reducing shock losses), and can also provide a single location at which the shock tends to anchor, even when the freestream Mach number and/or angle of attack change. While the shock bump can improve the controllability of the airfoil at near sonic flight conditions, the level of increased stability and control that it provides may not be sufficient for all wing designs and/or may not be optimal, particularly for airfoils designed for cruise at Mach numbers close to 1.0.
The second, improved prototype first flew on December 26, 2016. The new J-31 prototype is three tons heavier and about 20 inches longer than the original technology demonstrator; it also had key improvements like an IRST sensor, stealthier wings, cleaner burning engines, and an improved radar. In addition to avionics and datalinks that enable sensor fusion, SAC officials state that the production J-31s (which could appear soon as 2019) could have supercruise capability, giving them a leg up over current F-35 fighters. Its WS-13 engines would be replaced by domestic WS-13E or WS-17 turbofan engines to give it that advantage in speed. The combination of the J-31's high speed performance, and suggested payload of 6 PL-12 or 4 PL-21 long range air to air missiles suggests that the J-31 has been optimized as an air superiority fighter, though it can be fitted with a wide array of Chinese precision guided munitions like the LS smart bombs.
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