J-20 - physical parameters and other overflow from main thread

kwaigonegin

Colonel
Is that a fact? If so, there ought to be mass firings at Chengdu if they thought it was acceptable to sink billions into a project they knew would result in a crippled fighter. Happily for the employees at Chengdu, they thought no such thing.

The current iteration of the J-20 might have some problems with radar returns from the rear because the engines haven't been shaped and treated for RCS reduction. This is a problem that is being partially addressed by the WS-10 variant being tested, and will be fully addressed by the WS-15.

The J-20 was designed to be, and shall be, an all-aspect air-superiority fighter.

Let's not be so harsh, we all make mistakes.

Your own statement agrees with my observation... the J20 is certainly NOT an all aspect stealth at this point in time which is what the subject is about in regards to the article. As you said RCS from rear will most likely be picked up. I believe SK's air defence net will see the blip.
Perhaps if over flown over Myanmar or Sudan it might get lucky ... I'm merely saying it can and will be seen on radar if overflight over SK... .
 

jobjed

Captain
J20 is not an all aspect stealth because it was not designed to be. It just has enough stealth to make it effective at A2A engagement mostly at stand off distances.

If it had overflown SK on some sort of recon run, I'm 100% sure SK's air defence umbrella would've seen it.

Stealth aircraft always get detected by low frequency radars like the Type 517 on the rear of a 052D, F-22 included. I don't understand why people make a big fuss over something that is a given. Detection of stealth aircraft is easy, engagement is not. AFAIK, there is not a single radar-guided missile in service that can reliably engage stealth aircraft.
 

kwaigonegin

Colonel
this is why thread gets off topic, derailed and goes into a wild spin. I was alluding to the fact that if a J20 was flying over SK a few das ago, it will most likely get detected to further highlight the ignorance of the reporter. Nothing more nothing less.

Nothing to do with engagement, F22 etc.. LOL.
 

Quickie

Colonel
That's a somewhat unflattering F-22 picture though - because its fuselage sides slope at a shallower angle and are more blended toward the rear than on the J-20, there is less contrast at the ventral edge which can give the impression that little waisting occurs in the rear fuselage.

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While that's perhaps still a bit less pronounced than on the J-20, the F-22 rear fuselage seems to taper slightly more in the vertical plane:

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Doesn't seem like a huge source of error, all in all.

The trough between the engines could work both ways - by reducing the depth of the structure in that area they may actually weaken it and require reinforcements (a case of "holes being heavy", similar to the weapons bays).

To do away with lens distortion, it's better to select pictures that are taken from a distance, with maybe a telephoto lens. Close distance wide angle shots may be clear and impressive but it tends to distort the actual dimensions of the object due to perspective, or lens, distortion. The picture of the F-22 firing a missile is almost for sure a long distance shot for safety reasons.

The trough between the engines could work both ways - by reducing the depth of the structure in that area they may actually weaken it and require reinforcements (a case of "holes being heavy", similar to the weapons bay.

Actually, reducing the overall mass of a certain section will reduce the structural reinforcement required to handle the g-forces related to the mass of that section.

The reduced mass of that part of the reinforcement structure together with the reduced mass of the section to begin with, will contribute to a lower overall mass/weight of the aircraft.
 

Tirdent

Junior Member
Registered Member
To do away with lens distortion, it's better to select pictures that are taken from a distance, with maybe a telephoto lens. Close distance wide angle shots may be clear and impressive but it tends to distort the actual dimensions of the object due to perspective, or lens, distortion. The picture of the F-22 firing a missile is almost for sure a long distance shot for safety reasons.

Doesn't matter in this case - taken from the ground like the J-20 photo probably was:

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Quickie

Colonel
Doesn't matter in this case - taken from the ground like the J-20 photo probably was:

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The thing is the geometry of cross section of the 2 aircraft fuselages are very different.

From the air inlet to engine fairing, for the case of the J-20, it starts with a bigger rectangular section and ends with a much smaller rectangular section at the engine fairing.

For the F-22, it starts with a rectangular shaped section (as it's the case with the J-20) but ends with a trapezium shaped section at the engine fairing, with the top of the rectangular section (at the front) and trapezoid shaped section (at the aft) remaining about the same.
 
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Inst

Captain
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:

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@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:
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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.
 

Inst

Captain
The SK claim is not credible for the J-20, unless you believe the Chinese deliberately wanted to show the Americans the RCS characteristics of the J-20. It is not believable that the J-20 managed to avoid detection for the simple reason that the Americans had E-2Ds in the area, which works on UHF band, and that the J-20's canards, ventral strakes, and tailfins would light up the E-2D's radar due to resonance effects. Moreover, in an actual use mission, the J-20 would have to drop its frontal-aspect stealth relative to the American-Korean training exercise, and reveal its back corners to avoid detection.

Since the J-20 could not credibly evade detection by American surveillance, the United States has a vested interest in stating that it managed to detect the J-20, and there were no American claims, we can reasonably conclude that no J-20 was sent out, or if it was sent out, it was sent out far away from American detection zones.

It is way more likely that the Chinese sent out a reconnaissance drone to attempt to capture real RCS figures on the F-35. I'm shocked that no one has pointed out that the American training exercise is basically a grand opportunity for the Chinese and Russians to run their latest counter-stealth radars against the latest American stealth fighters, and represents a sort of Trumpian misfire.

As to other possibilities, this may represent an attempted overflight by the J-20 over the Second Island Chain in a test of its range capabilities. But I definitely doubt the Chinese would have deliberately compromised the J-20's RCS in a Korean overflight.
 
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