Take that (undergrad) analysis with *liberal* amounts of salt.
It's been around for a while, and was discussed over at Keypublishing at the time. As I pointed out back then, I suspect the weight estimation method used (i.e. the weighting multipliers) is being applied outside its accurate envelope, in that it is not really applicable to 5th generation fighters. Due to internal bays, much increased fuel capacity and (other) RCS reduction measures, 5th generation aircraft are a breed apart in terms of density - I'd wager you'd get a lot less than 19.7t if you were to run the F-22 through it. Using another 5th generation fighter as the benchmark is probably considerably more accurate than a method which is calibrated based exclusively on historical trends in 3rd and 4th generation aircraft.
Do you honestly believe the J-20 will match the C919 in terms of range @ Mach 0.8 & 35000feet (slide #16)? That's precisely what an implausibly low empty weight with a reasonably accurate fuel capacity (= far too favourable fuel fraction, a key influence in the Breguet range equation) will do. And no, they're not assuming 4 external tanks - the paper predates the revelation of that load out by years!
Also, you keep repeating 4t of RAM on the F-22 and a 800kg penalty for a pair of TVC nozzles - how do you arrive at those figures? YF-22 to F-22 weight delta? That number comprises *a lot* more than just RAM (radar, EW, MAWS, data link, almost certainly beefed up structure for increased airframe life and external hardpoints, F119 was heavier & more powerful than YF119...)! While the YF-22 was not quite as much of a bare bones proof of concept as the X-35, this should give you an impression:
@trident: As a preface, any estimates are just that, an estimate, and until we get definite figures we can roughly expect at least a 10% measurement error, which, when used to create composite figures like range or wing loading, is squared. My perspective in general has not changed: we can estimate all we want, from the fanboy 15 ton claim to your high-end claim of 21 tons, to more measured claims of 19 tons and 17.5 tons. Remember, I'm the one calling it into question because even for a Western estimate, the density estimations seem too low.
That said, if you're talking about increased density; I'd argue differently. The YF-22 had working weapons bays, as did the YF-23. The YF-22 actually had a 13-ton fuel load, or difference from empty weight to loaded weight. The square nozzles were actually present on both the YF-22 and YF-23, so we can't attribute the increase in weight to the nozzles.
As to the square nozzles, check this out:
This implies a full one-ton weight increase due to the flat TVC nozzles on the F-22. However, you are free to assume that American technology is much superior to Russian technology, but the raw physics of the matter have not changed; a flat nozzle is subject to significantly greater stresses than round nozzles and materials improvements for flat nozzles can be easily carried back to round nozzle technology as well.
Lastly, about structural reinforcement, please recall that the main reason the YF-22 won over the YF-23 was its TVC. It actually stands to reason that for pure matters of maneuverability, you do want a 9G aircraft to show off its 9G-ness. With the weight gain, as well as the move from prototype to production fighter, there has to be some level of structural reinforcement, but it's unfair to assume it's as extreme as you imagine. Moreover, please do note that the YF-16 and YF-18 were also prototypes, so structural reinforcement to full flying maneuverability would be unnecessary, but the F-16 and F-18 did not see appreciable gains in weight over their prototype variants.
Regarding range, please do recall that the C919 weighs 40,000 tons, almost twice your high-estimate, and that the F-15C has a ferry range of 5,000 km with drop tanks. In practice, I see the main problem with the VTech estimate as ignoring RAM. Remember, if we assume a 7 millimeter coating of solid steel, we get 4 tons of added RAM weight. The actual RAM on the J-20 will likely be lighter than solid steel, but it makes the notion of 5 tons worth of RAM plausible.
Ultimately, Trident, I appreciate your estimate as a solid estimate of J-20 weight based on high-end volume figures and conventional construction for the J-20. But the volume figure you are producing is high-end, a relic of the period when the J-20 was a nasty surprise for American military planners and we were expecting a full 22 meters length on the J-20, and we know that the J-20 uses cutting-edge titanium construction. The importance of your weight estimate is that we know that the J-20 is unlikely to exceed 21 tons in weight and that its real weight will be somewhat lower, whether to the 17500 figure I used to prefer, the 19500 figure others here prefer, or even to the incredible 15 ton figure a Chinese media publication touted.
The weight savings, however, is ultimately a question of how much magic Chengdu's materials subcontractors have managed with regards to the structural weight. For us to know the actual empty weight, we'd need Chengdu to release their figures. Just don't be surprised if it's much lower than your high-end 21 ton estimate.
