J-20 5th Gen Fighter Thread VI

[latenlazy]

As I mentioned earlier, I'm already reducing J-20 empennage weight by more than deducible from the difference in area to the F-22 alone (originally out of laziness, granted ) - you can consider that a bonus for lower thickness and less beefy mountings if you like. Point being, I find it hard to justify going even lower than what went into my estimate anyway, even if I accept that caveat of yours.

I'm aware, but at the end of the day my critique is over precision. Regardless of whether you find it harder to justify going even lower than what you originally estimated, the point is there were factors you may not have originally considered in your initial arguments. With more exacting considerations we can do better, and more precise analysis will ultimately speak for itself.

None taken, but some awareness of wing design did go into it as well. I completely agree that a more accurate assessment would have to include measurements of actual t/c ratios, but the conical camber (especially pronounced on the F-22) and twist on both aircraft basically makes that impossible without an actual drawing.

Bearing in mind that, as delft correctly points out, you can make a wing only so thin (for the aspect ratios on the J-20 & F-22 I don't think lower than 3% chord, at best) and that there comes a point where thinner actually means *heavier* to take care of aeroelastic issues, assuming similar thickness absent accurate info is hard to argue against. This is the very trade-off which makes (modified) delta planforms so popular in recent fighter aircraft - the long root chord allows for a reasonable absolute thickness (= fuel volume and structural weight for stiffness against flutter & aileron reversal) at a t/c ratio suitable for low supersonic drag.

If you bisected the conical camber of the F-22, you'd get a thinner wing with a shallower conical angle. To be *very* clear, I'm not actually saying this is the case with the J-20. There's just a lot of unknowns there that I'm not comfortable with, especially since wings, by function of being thin objects with wide area, can be deceptive in how much volume they actually add.

Delft makes an excellent point here of course, but this only adds to my uncertainty about how we regard the weight contribution of wings. This really just leaves us back where we started, in the middle of nowhere on how to assess weight.

Considering, as you mention, the quality of the information at our disposal, that we should end up within about 5% of each other ("very low 20s" to low 21s) is not too divergent at all I think It's obviously very hard to actually quantify the error margin on my estimate, but I'd hazard a guess at putting it in the 500kg ballpark, which would mean Totoro's and my estimate even have a (narrow) range of overlap, if we credit him with about the same accuracy.

Totoro estimated roughly 20% greater in volume than the F-22. Assuming the same density (which he hasn't claimed to do) , that would make a weight estimate of 23.64 tonnes. I'd say that's quite a dramatic difference, all things considered, well outside of 5% of each other.

I think an error of 500 kg plus minus might be a tad overconfident...for example, if you missed average frontal cross section by say 0.25 m^2, then that would be a 5 m^2 difference over a 20 m length. That's not small, and while I haven't done the math yet I have trouble thinking that won't throw a weight estimate off by more than 500 kg.

 

[Klon]

Part 1.

Needed to split this response because it was past the character limit. Might not be the only one I have to do this for today. Sorry guys.


We know because PLAAF officers have said so in public?
#3345
#3551

Fine. There's no point discussing happiness, as it can't prove anything regarding the underlying debate. Like I said, they can be happy with many things about the J-20, including its overall capability and flying characteristics, even if it's 20% more voluminous and heavier than the F-22. Even if they aren't happy about something, I doubt the public would hear about it.

Maybe they went with a longer fuselage because they wanted a better fineness ratio for transsonic drag. Maybe the cross section isn't as large as you think it is. This is why we fuss over measurements, because without them statements like "they went for a longer plane with a larger cross section" are conjectures, not facts.

I don't know what to make of this part. That the J-20 is longer than the F-22 is a fact. I assume you agree? That it has a larger cross section has been found by the two members who have performed a measurement. Apparently you don't accept it. I could try measuring it myself, but I suspect you'd reject it just the same (if it turned out consistent with the other two). The only measurement you probably wouldn't automatically reject is your own, which you refuse to provide. Reject here means refuse to use and claim it could be significantly wrong, even if your stated position is 'we don't know / can't know.'

I didn't say the J-20 matches the F-22 in everything. I said that if the PLAAF is happy with the J-20 we probably shouldn't presume that whatever its specifications are that they are prohibitive to performance.

You said,

Of course, but my point was that if they're happy with the J-20 and they tendered for a fighter that was competitive with the F-22, that should reasonably preclude the J-20 being a 25 tonne giant insofar as being that heavy would dramatically inhibit its ability to meet those requirements.

My argument is that just because something was a requirement is not a guarantee that it was achieved and you haven't shown that a weight of 21 to 23 tonnes is unacceptable performance wise.

Hmmm, on one hand I agree that questioning people's motives can be a pretty unpleasant thing to do. On the other hand, that doesn't mean we shouldn't question the motives for participation in a discussion. Remember, when I agreed with you I threw in a "but". As I've pointed out before, the purpose of these discussions is ultimately accuracy. We can't pretend that subtextual, sometimes subconscious assumptions and biases that don't have substantive basis can sometimes colour a person's arguments. Within the confines of PLA watching, this can be just as true of people who are very bullish on China's technological progress as those who are very bearish. It saves a lot of time if people are upfront about whether they're willing to consider evidence openly, or if they are simply arguing to try to prove some preconceived conclusions for themselves. You may think this is cynical, but I do not think that makes it an invalid form of inquiry. After all, I have some trouble thinking that isn't what you're trying to do when you're asking me how believable I find some recent claims to be (such as China's EM catapults or the JF-17's AESA).

I concede the point. You've shown that impugning motives and psychologizing is the way to go.
With those three examples I was indeed trying to make a point. I was asking whether people here use the same standards for Chinese claims and claims from elsewhere, particularly when the Chinese claims seem implausible.

Just for the record, I *am* American, though I try not to stand for any views or interests in these discussions either. The point about NK wasn't meant to push for any particular views as much as to point to an example where analysis, for whatever reason, has considerably missed the mark on assessing a country's technological abilities.

Ah, here we go. I've seen you make this statement before (you're making a warning about underestimating potential enemies), and I don't believe you. This forum is totally irrelevant and you should contact your representative with your warnings, if you're being sincere. I think this is a patent cover for your desire to assume the best about Chinese military developments while not wanting to be accused of fanboyism. Wow, this really is necessary and productive.

Maybe don't rely on machine translators, especially when it is translating something that itself had to be translated from another language.

Here is the original Wang Hongzhe quote.

"“首次亮相航展的国产KLJ-7A型机载有源相控阵火控雷达的作战距离为170千米，与F-35的水平相当，而且这是在该型雷达的体积要小于F-35装备雷达的水平上实现的"

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A more precise translation of what he's saying would be that "the KLJ-7A's combat range is 170 km, equivalent/comparable in quality/performance to the F-35s. There's a bit more wiggle in the Chinese than the English translation of the French translation of the Chinese.

I will rely on machines translators, since they're very advanced and useful. In any case, there wasn't any problem with their output here. Now we're in a situation where we have to consider whose translation was more accurate, yours or Kenhmann's. You're probably right, the Chinese seems less clear.

A very good point, so we really have no basis of judging how accurately the claims are, though Chinese industry commentators *might* have some real information here because they did steal data on the F-35. It's why I said the claim is "believable", not "true". If you think it's false that's for you to decide. If you keep asking me for reasons on why it might be true I will keep giving those reasons to you. If you're not willing to consider those reasons with a willingness to believe or at least consider them then let me know so I'm not wasting my time. I've already made clear what my thoughts on this claim are.

I asked you three times because you didn't answer the first time, said there was a grey area the second time and after we both clarified it turns out there might be grey area (the third time).

I'm not obligated to provide my own estimates, especially since I don't really have the time. If I'm going to spend the time to do it I'd rather do it precisely, which I don't think we have the tools for. I've found that getting precise well adjusted length and wingspan estimates, when we have the pictures to support such exercises, and defending them eat enough of my time as it is. At the same time, I want to be clear that I respect and appreciate the work Trident did, and remain open to considering totoro's estimates if he chooses to post what he did in detail. I can, however, appreciate their work without agreeing on everything.

I don't think it's unreasonable or unfair to "nitpick" or "blow holes" (or as I call it, grading people's homework) when people present detailed work in a debate. The whole point of presenting detailed work is, after all, to have the work be transparent to critique. You don't need to provide equivalent work for a critique to be valid (though I think it should be mentioned that reading thinking about someone's arguments, and then typing a response, also requires work). The point of discussion and debate isn't to agree or admire. You also don't have to find me credible or fair for my critiques to be valid. If you have issues with my points, focus on addressing the substance of those points and not whether you think I'm being fair or not (or whatever else you think of my character).

That Trident and totoro reached such divergent results should tell you something about how well we can do estimates with the information we have on hand. I don't think I need to tell totoro that I respect his contributions to discussion greatly, but when I asked for his specific methods we ended up not getting into the weeds (which is understandable, of course). This is important, because without detailing specific method we can't grade the homework. Would you have been so willing to accept what totoro's estimates were if he said the J-20 had 20% less volume than the F-22?

Trident, if I recall correctly, uses *volume* to estimate mass. Specifically he assumes they have the same density and use estimated volume differences to determine mass differences.

If I thought you were acting in good faith, the above could make sense. However, I think that in your desire to not have the J-20 be heavier than the F-22, you also don't want it to have a larger volume and thus try to invalidate any attempt at measuring said volume. Estimating dimensions and calculating volume is not something that can only be done well by experts. If we have quality measurements of the J-20's length and wingspan, there's no reason to think we can't also get the same for the cross section.

 

[Klon]

Part 2.

Did I say it is actually 0.5 meters? No, I said it could be 0.5 meters smaller and we wouldn't know without precise measurements. Trident said that his estimates have large error bars. I detailed some of my problems with the assumptions Trident used earlier, but even without them there will be measurement error.

No, you said, "Consider also that if the J-20's fuselage cross section were say only .5 m^2 smaller than the F-22's, multiplied across that alone could neutralize half the volume difference if the J-20 shared the same cross section area as the F-22." I refused to consider that because the actual measurement said it was instead 0.6 square meters larger. I don't think there's any reason to think a mistake like getting 5.9 square meters instead of 4.8 square meters occured (5.3 square meters for F-22).

Did I say my counterpoints are somehow better? No. The point behind counter arguments is that it's just as important to consider the ways some argument or assumption could be wrong as it is to consider the ways they could be right. Trident was already making the positive case. If I disagreed with some of what he did, naturally I would be making the negative case. If the positive case is already self evident, then why would I need to address them again? This is a discussion that ultimately hinges on technical details. I don't need to frame everything around equivalencies. If you have issues with the substance of the counterpoints themselves you should address them. If you don't want to believe my points that's your prerogative. I'm not here to give you affirmation...And for what it's worth, I did actually say the error could go both ways earlier in one of my posts.

Trident stated that he used the most charitable assumptions (that he still found reasonable) for how light the J-20 can be when making his assessment, so you weren't actually arguing in the other direction. Also, the vast majority of your warnings about measurement and scenarios of what the volume could actually be were toward a lower volume before any argument began. When you know the errors could be in both directions but only make the case for one, that's not a desire for accuracy, it's motivated reasoning to protect your preferred outcome. You never said, for instance, that the J-20 could be 25% percent more voluminous, instead always presenting reasons why it could have the same volume as the F-22. In my view, this is simply a result of your bias and not some desire for only the most accurate conclusions. Similarly for your dismissal of any attempts to actually measure the volume.
As for 'incredulity,' there are countless things that all of us find not to be credible, so this isn't any kind of point.

@Tirdent Can you tell me what numbers you found for the volume of the two planes?


------------------------------------------
Summary of what I think for anyone reading:

• the J-20 is longer than the F-22
• it has a larger fuselage cross section, from all measurements that have been reported here
• it has a larger volume (Totoro say 20%, waiting for Trident's number)
  
• we can know all of the above with reasonable certainty from measurements based on pictures
• the reason latenlazy tries to invalidate all volume measurements is his refusal to accept what they show
• my view on the weight is that it's most likely proportional to the volume (the J-20 with a similar density to the F-22), which means around 21 to 23 tonnes
• it's not hard for me to think there was some weight reduction (one or two tonnes, maybe), but 6+ tonnes is in the category of extraordinary claims requiring extraordinary evidence
• I can't say much about the magazine where the claim was published except that it looks like a 'popular science' kind of publication without official affiliation, unlikely to be strong evidence by 'PLA watching' or any other standard
• my larger point is that claims from China can be wrong
• we know that Zhang Zhaozhong and Yin Zhou are regularly wrong on TV, presumably intentionally
• I posted three examples of Chinese claims for which I think that if they were made by the 'other side,' people here would be much less likely to accept them or would outright reject them (for other side you can take the U.S., with other circumstances also switched, or India or another country that is behind China in military technology)
• I can't definitively prove that these claims are false, but I also don't think anyone here can prove they're true
• the larger discussion should thus also be about standards of evidence.

 

[PiSigma]

Part 1.

Fine. There's no point discussing happiness, as it can't prove anything regarding the underlying debate. Like I said, they can be happy with many things about the J-20, including its overall capability and flying characteristics, even if it's 20% more voluminous and heavier than the F-22. Even if they aren't happy about something, I doubt the public would hear about it.

.

I said this before and I'll say it again. J20 design exceeded PLA requirements for several areas, meet most of the rest. Overall, the design surprised the military on how good it is, therefore the air force is happy.

 

[Blitzo]

I can't say much about the magazine where the claim was published except that it looks like a 'popular science' kind of publication without official affiliation, unlikely to be strong evidence by 'PLA watching' or any other standard

• my larger point is that claims from China can be wrong
• we know that Zhang Zhaozhong and Yin Zhou are regularly wrong on TV, presumably intentionally
• I posted three examples of Chinese claims for which I think that if they were made by the 'other side,' people here would be much less likely to accept them or would outright reject them (for other side you can take the U.S., with other circumstances also switched, or India or another country that is behind China in military technology)
• I can't definitively prove that these claims are false, but I also don't think anyone here can prove they're true
• the larger discussion should thus also be about standards of evidence.

I'll address this part of the reply, because I think it is most pertinent to our previous discussion.

I don't think anyone disputes that Chinese military rumours can be wrong, sometimes even deliberately so.


No one can prove those claims to be definitively true or definitively wrong, I agree. I think what we're left with is whether the 15 ton number should be kept as part of the conversation about J-20 overall in some manner, which is dependent on how we judge the credibility of the evidence.

So I do agree that the last bullet point you make is probably the biggest area of contention.
Fundamentally, I think there is enough evidence put together such that it is safer to keep the number in circulation, but with the important caveat of acknowledging skepticism towards its validity, with the understanding that we do not know what the true empty weight number actually is.

Unless further information comes to light regarding the credibility or lack of credibility of the magazine, as well as any further information or lack of regarding whatever advancements in weight reduction that AVIC has actually achieved or not (which up to this point have been alluded to but far from solid).

 

[Air Force Brat]

A navalized J-20 will be interesting... because J-20 looks kinda big... I wonder how radical of a design change would navalizing the J-20 do to it's stealth characteristics?

Quite Radical, and extremely unlikely to happen, in spite of much wishful thinking on the part of many.. the J-20 doesn't fit anybody's mold of a carrier fighter,, not saying it couldn't be done, but it would require a great many engineering and structural changes. It looks big, because it is, though its no bigger than the SU-33 or J-15, but it doesn't appear to have any design que's that would lead us to believe that is their PLANAF bird of the future.

 

[siegecrossbow]

A navalized J-20 will be interesting... because J-20 looks kinda big... I wonder how radical of a design change would navalizing the J-20 do to it's stealth characteristics?

Alternatively, go for a bigger carrier.

 

[Air Force Brat]

Alternatively, go for a bigger carrier.

which they no doubt will at some point, but the J-20 would need a LOT of internal work, and to be honest, it would be a whole new airframe, just as the F-35 Charlie is very much different that either the Alpha or the Bravo!

I just don't honestly believe that is in Chengdu's plans or should I say PLAN!

 

[kurutoga]

Let's look at the necessities of 4th gen naval jet for China

• In both East Asia and SCS, J-20 can be deployed, together with the ground network, to counter F-35 in both defense and offense situations
• 016 or 017 are not meant to counter US in Middle East or Africa

The logical conclusion is the naval 4th gen jet can be developed together with the 3rd and 4th carrier. Let's say they both reach some maturity in 2025 and can be usable around 2030, it means the first flight of the new naval jet can be around 2020-2022. This is a good news due to availability of the engine and other newer technology because it is whole 10 years later than J-20. So, why rush?

Personally, I don't believe FC-31 is for the navy either. It could be purely for export.

 

[Tirdent]

Qualitative promises for jet fighter capabilities are quite a bit more ambiguous, while strength and weight claims for parts manufacturing are quite a bit more testable.

What's qualitative about an EM scatter diagram?

"Most highly loaded part" in a plane doesn't necessarily mean strongest part possible with forging.

This is an interesting excerpt I found while trying to dig up where I read that titanium can have cracking problems in the milling process. The F-22 *does* compromise on some strength properties for others, apparently.

We've been rather generalizing "strength" so far. Doesn't specifically mention bulkheads (though fatigue is certainly a major concern), and careful control of the grain flow in a forging also helps a lot in that regard.

If they look the same to you then ¯\_(ツ)_/¯. I did question much earlier in our discussion whether either of us have the qualifications to make confident visual judgments, so there was always going to be a limit to that point.

What I meant is that even the legacy F-15 bulkhead has its rim and stiffeners machined down to a thickness which looks every bit as thin as those on the J-31 bulkhead, so I fail to see anything about the latter which would defeat the newer (single-piece) F-22 process.

Interestingly enough, in the guancha article I linked, there *is* a picture of a finished F-22 bulkhead and, for what it's worth, I do think in this picture the F-22's looks a lot thicker than the J-31s.

The rear one is the unfinished forging again, and unlike the J-31 and F-15 pictures this one is of such low resolution that I don't think we can really gauge the thickness of the final part in the foreground.

Furthermore, jobjed seems to have identified where inside the J-31 that bulkhead goes here and it lives in the rear fuselage. That F-22 bulkhead OTOH appears to be one of the forward wing attachments (it includes cut-outs for the main bays but is positioned sufficiently far aft that it lacks openings for the side bays and the inlet ducts are already close to the centreline). If that is true, it's not surprising that it might be thicker.

I think I've read before that the F-22's titanium bulkheads are about 25% of its structural weight (don't take my word for this), which probably makes them about 4-5 tonnes, but if it's 40% titanium, then that means there are about 3 tonnes of titanium that aren't bulkheads. If that's actually the case, and if we presume that 3D printed bulkheads can't save that much weight, I think that probably makes the titanium weight savings claim stronger, not weaker. Why wouldn't China try to take advantage of that technology, if they had it, at every turn? Wouldn't printing bulkheads be a pretty strong indication that they already are?

The rest would be in wing spars & tail boom structures, perhaps a couple of longerons, I think. Wing spars and longerons would be essentially like really long beams/plates, so similar considerations apply as with bulkheads (no weight benefit from 3D printing without novel geometries).

Where I could see significant savings even with conventional part structure is the tail booms, which are EB welded together out of several pieces on the F-22 - EB welding is a very high-performance process, but 3D printing out of a single piece offers interesting opportunities. Then again, this is an area where the analogies between the F-22 and J-20 unravel altogether, with the former having conventional h-stabs against the J-20's canards

I doubt it, but it doesn't need to in order for them to use the manufacturing process to save weight?

That's the central part of our disagreement, because I think it would indeed have to be for a substantial weight saving. How else is there anything about that bulkhead which could not have been done with a machined forging already for the F-22?

It's single piece, so is the F-22 part, they're both titanium and the thickness of the stiffeners/rims was apparently doable back on the F-15, grain flow may be less favourable on the 3D printed version - where's the weight saving coming from unless it's hollow with internal stiffeners? I suppose it's a long-winded way of saying that...

As I said earlier, the claim is what it is. You can take it or leave it.

I'll leave it