Why the J 20 is not the true Frontline's aircraft's.
Chengdu's made clear they want the J-20 to evolve into a family of aircraft. The biggest problem for the J-20 to be further upgraded is the lateness of the WS-15. We've heard nothing out of the rumor mill of late, and we have so-and-so aircraft engine manufacturer declaring they'll build around 5 WS-15 in the 2025 to 2030 timeframe, which is way too slow.
One big issue is the sizing of the J-20's current inlets. The airflow under the present DSI system is adequate for an Al-31 / WS-10X classed engine, but the airflow is insufficient for a F135 / WS-15 class engine. The easy solution is simply to adjust the DSI; i.e, reduce the size of the bumps (and possibly increase their length) to accommodate the greater airflow needed, but an alternative solution would be to enlarge the inlets, creating greater body space and potential for the engines to be shifted to enlarge the weapons bay.
The measurements of the J-20 inlets are roughly the same, perhaps a bit larger, than on the Su-27 series for the Al-31 engines. It's a bit of a pity, to an extent, since larger inlets would have guaranteed better high-altitude / high-speed airflow, sufficient to allow the J-20 to have at least sustainment (requires afterburners to break Mach barrier, but can supercruise around Mach 1.3 without afterburners) supercruise. But that'd compromise low-altitude maneuverability as you'd end up with spillage drag from overflowing the inlets. Consider how much the Su-27 has evolved. The Su-30 modification added canards, necessitating new actuators, and the Su-34 platform converted the Su-27 into a light bomber.
Biggest impediment, of course, is time. The Su-27 emerged and evolved at a time when the Russian armaments industry was wheeling from the collapse of the Soviet Union, meaning that there was limited funding for completely new designs. The Chinese plan to get a sixth-generation fighter up by 2035, and we're already at 2020 with the WS-15 very very late. Then again, as suggested, the J-20B might be significantly different from the J-20A given the 25-37% increase in thrust, allowing the aircraft to carry much more munitions and payload without sacrificing maneuverability. And the TVC expected of the WS-15 could allow the aircraft to conduct a substantial redesign as the tailfins are no longer needed for yaw control.
My basic issue is that the J-20 as a design is rather compromised to deal with the PLA's conflicting requirements and the limitations of Chinese engine technology. It's not as pure a stealth design as the F-22. The weapons payload, while better than the F-22, is roughly the same as on the F-35 so is inferior for a heavyweight stealth fighter. It's not designed for extreme maneuverability like the Su-57 is. And while the aspect ratio would imply that the aircraft is designed for and optimized for extreme speed, the J-20 is limited by the need to protect its stealth coating as well as the DSI inlets on the J-20. The only unique aspect the J-20 has going for it would be its lerx long-coupled canard lerx delta wing set-up, and even that has already been presaged by Rafale.
In other words, the J-20 is good against 4th generation fighters and in numbers adequate to 5th generation fighters, but it seems to lack its own advantages against competing 5th generation fighters. Where its advantages do lie are in the respective weaknesses of specific airframes (F-22 is only just getting upgraded with IRST, does not have EODAS, has an anemic weapons bay capacity, the Su-57 does not focus that much on stealth, the F-35 has poor agility), but it doesn't have anything world-beating. On the other hand, the J-20 has strong developmental potential from the basic design of the airframe. The limitation to the J-20 is constantly the engine, and if that limitation were lifted, the design could be modified to make it not simply a par aircraft, but something that could have a decisive advantage in one area or another.
Put another way, the J-10 is arguably better than a Block 50 F-16 or even a Block 60. The J-16 is arguably more capable than the F-15E. But all of these are last generation fighters. When it comes to 5th generation equipment, the Americans can tout their stealth and sensors on the F-35, the Russians can tout their kinematics on the Su-57, but what does China have? I mean, what do you want to talk about? I have the Song Weicong documents explaining why the J-20 layout was chosen. We have my discussion on how the J-20's weapons bay is crap.
The fact of the matter is, we can compare the J-20 to an F-22 and an F-35. The J-20 isn't an LO aircraft as China-bashers want to claim, but neither is it likely to have a stealth advantage over American warplanes unless China metamaterials design matures and the J-20 is refitted with metamaterials. Then American aircraft can also be refitted with metamaterials, and the advantage disappears. We can also compare it in terms of maneuverability. It has a 75 m^2 wing area for an empty weight range in 18000-22000 kg, I prefer the lower figures given known Chinese advances in 3D materials. This roughly puts it at a wing loading comparable to the F-22, but it doesn't give it advantages over the Su-57. We are aware that the lerx long-coupled canard lerx body lift delta planeform is superior to canard deltas (in the J-10-style configuration) by 20% in terms of lift, but this doesn't necessarily give it an advantage over lerx long-coupled canard lerx planeforms like the Rafale and Eurofighter, which have far superior wing loading compared to the J-20.
In terms of thrust-to-weight, it's competitive with 4th gens at 60% fuel and with missiles, but without the WS-15 it doesn't have a decisive advantage. In terms of weapons bay capacity and strike capability, it's better than the F-22, about the same as a Su-57, but significantly worse than the F-35 Joint Strike Fighter given that its loadout has no decisive advantage over the F-35 despite greater size. In terms of supercruise, while I've tried to argue that the air intake on the J-20 permit's very limited's supercruise, it's apparent from the volume of the intake that the J-20 intakes are comparable to Su-27 intakes and no further mass can be obtained. The Vtech drag diagrams say that total drag build-up for the J-20 at Mach 1 is 9 kt (metric), at Mach 1.1 this drops to 6.8 kt, assuming 35kft or 10.68 km. The Al-31 engine outputs about 2.65 kt of force at Mach 1.0 at 10km and about 2.85 kt at Mach 1.1. The dry thrust of the Al-31 is 7.6 kN at sea level, implying a 65% thrust reduction at altitude. At Mach 1.1, you'd need about 3.4 kt per engine and to break Mach 1 you'd need 4.5 kt per engine, or roughly you'd need a 20% increase in dry thrust to achieve pseudo-supercruise and a 70% increase in dry thrust to achieve true supercruise. Even the Al-41F-1S on the Su-57 can't achieve pseudo-supercruise on a J-20 platform without tweaking to high altitude dry thrust.
In terms of max speed, the DSI and stealth coatings on the J-20 prohibit it from having a strong max speed due to the let-off in engine thrust at high speeds and the need to preserve the engines. In terms of cost, the F-22 is out of production, the F-35 has seen massive cost reduction since its LRIP versions (around 80 million a pop these days), and the Russians are claiming they've gotten the Su-57 down to ridiculously low prices that can have you losing 2 Su-57s per F-35 and not bat an eye. The rumored J-20 price is 100 million (especially with exchange rate changes over the past few years). In terms of other subsystems, the J-20 has advantages in the operation PL-15 system, but the PLAAF lacks micromissile development as even the Russians are attempting such to fit their tiny Su-57 side weapons bay. The PL-21 can't fit on the J-20, the J-20 seems to lack an interceptor missile for very long-range interception against AEW&C and tankers, the J-20 seems to lack an anti-radiation strike missile for the SEAD mission. The Su-57 has a DIRCM that the J-20, to date, lacks, although as I've pointed out, the J-20 is very pod ready given its side weapons bay.
In short, one possibility that can be brought up is that the J-20 is going to be the Chinese 6th generation fighter. When you look at the J-20's development and design, it's clearly a J-10 derivative (and a key feature is that it inherits the anhedral-dihedral canard wing layout as the J-10 has a modified inverted gull wing from the inner section of its wing being dihedral). The ability to modify the basic J-20 design for other purposes and enhanced capabilities suggests that modding the J-20 can be how the PLAAF gets a sixth generation aircraft up by 2035 or even 2030. It could, in all honesty, be a 5.5th generation aircraft, but it's close enough to 6th generation to form a stop-gap. That also allows the PLAAF to develop more aggressive 6th generation designs that would be riskier than the PLAAF could support without a stop-gap fighter like J-20 derivatives.
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I have strong objections . But dont Have much time but I would like to tell , we cannot get any clue of its DSI without actual tests . Softwares can simulate somewhat but still not much authentic . Chinese have experience in DSI JF17 , J10 are examples , in case if they have any faults they can fix it in next iteration . Super Cruise is mainly depends on the aerodynamic shaping As an example Both Tejas and Gripen NG uses same engines but Gripen can Super Cruise but Tejas can't .
