J-20 5th Gen Fighter Thread VI

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latenlazy

Brigadier
I'm iffy on the cross sectional area, tbh. The F-22's verts are huge, it has larger and thicker tailplanes compared to the J-20's canards, and its wings seem to be thicker too. These differences will add up.
To follow up on this point and show what I mean, I did a *very* rough exercise with some *very* loose assumptions. I took a quick estimate of the average wing thickness of the F-22, getting about .25 meters, and multiplied it by the wing area of the F-22 minus the roughly rectangular area that is the fuselage in between the wings (because wing area usually includes the fuselage area between the two wings), 78-27=51, and got about 12.5~13 m^3. Now for the sake of this exercise, let's say the J-20's wings are half as thick. They seem thinner than the F-22's, though maybe not half as thick, but indulge me here. Assuming the J-20's wing area is about the same as the F-22's, that would mean on differences in wings alone the J-20 would be about 6 m^3 less voluminous, all else held equal. My point here is that thin but wide features are not volumetrically trivial, and by the nature of their thinness we sometimes miss that while an absolute difference in thinness between two features isn't significant their proportional differences can have sizable effects on dimension.

I estimated that the width of the F-22's fuselage is about 3.9 meters, and its height is about 1.6 meters. Obviously, the F-22's fuselage isn't a rectangle, but again, for the sake of simplicity and this exercise, let's treat it as such and let's also say that the cross section area of the fuselage is consistent across its length. Let's also assume the J-20's fuselage has identical cross sectional dimensions as the F-22, again for the sake of simplicity and this exercise. We know for a fact that the J-20 is longer than the F-22, let's say 20.5 meters to the F-22's 18.8. With these very loose assumptions, the F-22's fuselage volume would be 3.9*1.6*18.8. The J-20's would be 3.9*1.6*20.5. In other words, the difference in volume between the two fuselage's, if they were rectangular prisms, would be (20.5-18.8)*1.6*3.9, which is about 10.7 m^2. This is assuming that their cross sections are rectangles. Under the assumptions of this exercise, the J-20 would only be 4 m^2 larger than the F-22. However, as both cross sections are rather more close to trapezoidal or hexagonal, the base of difference between two fuselages with the same cross section but different lengths would be even smaller than if they were rectangular, and thus holding the assumptions over wing thickness the same you would get an even smaller difference.

Of course, this is where people will quibble with me and point out the F-22's fuselage is actually shorter than 18.8 m, and that its last stretch of length is all tail booms and tail control surfaces, *but* that is also true for the J-20, though perhaps to a lesser extent. Furthermore, I would argue at least some of that will be compensated by the fact that the J-20 has both smaller and thinner vertical tails, and by the fact that the J-20 has no horizontal tails. As I've emphasized in the previous paragraph, the volume contributions of wings and control surfaces should not be regarded as trivial, and in the F-22's 4 tails are not tiny by any margin. They are, in fact, *significantly* larger than their counterparts on the J-20, and not simply in area but also in thickness.

More to the point though, if the J-20's fuselage were even .1 meters smaller in both height and width (3.8x1.5), then its cross section in this exercise would be about a half meter smaller than the F-22's, and you would get a J-20 with a fuselage that is less voluminous than the F-22's despite being longer (3.8*1.5*20.5=116.85 vs 3.9*1.5*18.8=117.31). As I said earlier, these small differences can add up. A decimeter here or there can multiply into much great disparities. Measurement variance alone should compel a very long pause for anyone insisting that rough assumptions about comparative dimensions should tell us anything about extrapolated estimates (like the argument that the F-22 and J-22 look like they share "roughly" the same fuselage cross sections, so the J-20, by having a much longer fuselage, should definitely be greater in volume, which seems to be the most common justification for conclusions about the J-20's volume).

Are any of these figures accurate? H*LL no. I'll be the first to admit the entire exercise is built on a set of conceits, but the point of this exercise wasn't to present a . ccurate figures. It was to demonstrate visual size disparities in the fuselage of two planes may not reflect the presumption of great gulfs in volume that a judicious eyeballing might suggest. Even small changes in a number of parameters could defy the exaggerated expectations that may come from extrapolating a difference in length, so we ought to be very *careful* about presuming without *very* precise and *thorough* measurements. This is why I keep insisting, over and over again, that images can be misleading, and that planes are complex geometric shapes. It doesn't matter how incredulous someone thinks the idea that the J-20 may not be volumetrically bigger than the F-22 is. It may be, or it may not be, but we shouldn't be using visual impressions to pass confident judgements. We invented measuring tools for a reason. Eyes by their nature do deceive.
 
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plawolf

Lieutenant General
The J-20 also has larger fuel volumes than the F-22, which imply that despite being greater in volume, more of this volume is empty space.

What makes this figure really weird, though, is that it seems to be implying that the J-20, with WS-15, can hit around 1.36 T/W fully loaded and in combat weight sport 1.67 T/W. That's less of an aircraft and more of a missile. Aircraft are typically limited by 9G human tolerance, so is the objective to achieve 9G at mid-supersonic velocities?

I suspect the current J20s have a lot of designed for, but not yet built in optional add-ons that could be activated when they get their WS15s, so the TWR of WS15 J20s won’t be so eye popping.

For all we know, to achieve that 15t figure, the current J20s may have fuel tanks installed that are 1/4 smaller than the room available to save weight; or have a tiny gun magazine; or have some systems removed or have smaller/lighter versions of the equipment like radar installed than what the plane was designed to have etc.

Just look at all the weight saving measures seen on the Z10 as an example of the kind of temporary sacrifices the PLA is prepared to make to have a bird fly as it was intended rather than have all the bells and whistles installed in the event of short to medium term sub-optimal engines options.
 

delft

Brigadier
Estimating the weight of an aircraft whether one existing and from the outside or when designing one is a very complex matter. One important point is that a thicker wing has a larger volume but a lower weight. A delta wing is considerable lighter than a comparable trapezium wing.
In the matter under consideration: The brilliant shape of J-20 has wings that will be considerably lighter as a fraction of the aircraft gross weight than those of F-22 when build with the same level of technology and a similar specification. Then there is the improvements in the hardware and software available to calculate aerodynamics and structural stresses in the near twenty years between the projects and the improvements in structural materials and production technology. It all adds up to an improvement in structural weight of much more than 3 or 4%. I don't believe in smaller fuel tanks but leaving out the gun for the time being is a possibility.
A matter that has not been considered in the last ten pages is the T/W of the aircraft when flying at some high Mach number in the stratosphere. That depends on the air density, Mach number, intake shape, tail pipe shape and of course the engine, its pressure ratio, by-pass ratio and maximum cycle temperature. This is a very complex matter of which I know not enough by a large margin.
 
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Inst

Captain
The reference was to the titanium structural components of the F-22, which according to some sources, make up 40% of the F-22's weight. A 40% reduction in 40% comes out to about 16% of weight reduced

@Air Force Brat: you are underestimating the advances in 3D printing, which allow for the construction of hollow titanium pieces, reducing weight while preserving strength in desired directions. I am, of course, doubtful that the J-20 is as light as claimed, but it's not wholly impossible..
 

siegecrossbow

General
Staff member
Super Moderator
We have word from AVIC? I don't care; my friend seigecrossbow here is a better source on J-20 than AVIC cus he agrees with me." That sounds like denial to me, Brat. Unless your bit about the G-force was somehow supposed to be reasoning? In which case, you'd need to show calculations for why 15 tonnes with NEW construction and engineering methods couldn't possibly be strong enough to sustain whatever G-limit you think J-20 has (cus I'm pretty sure that's not public yet either LOL). I hope you know that just because someone achieved something beyond what your training in physics would allow you to comprehend, that does not mean that s/he BROKE the laws of physics LOL. If you were born 200 years ago, you'd likely say that the whole concept of a flying machine that moves faster than sound itself violates the laws of physics.

Since when did AVIC directly claim that the J-20 is within the 15 tonne weight class? The only source I could find was from Dr. Cao, who gave a breakdown on the amount of titanium and composite materials used but didn't release any weight related info. Where is the original link sourced by this article?
 

manqiangrexue

Brigadier
Since when did AVIC directly claim that the J-20 is within the 15 tonne weight class? The only source I could find was from Dr. Cao, who gave a breakdown on the amount of titanium and composite materials used but didn't release any weight related info. Where is the original link sourced by this article?
Gonna refer you to this:
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And this: https://www.sinodefenceforum.com/j-20-5th-gen-fighter-thread-vi.t8169/page-39#post-477426

So basically, this isn't a primary source from AVIC, but it is from a secondary source who wrote his article based on his interactions with AVIC. That's why I have noted that the reliability is not indisputable, which opens the possibility up for discussion. To be honest, I thought the number was far-fetched when I saw it, but then, after reading all the compelling reasons for how that might have been achieved, I was convinced that it's possible, even by layman calculations not to mention at the hands of a master engineering team. I haven't heard any compelling reasons at all for why it's not possible, only what amounts to disbelief that a 1997 American effort could be outdone by so much...
 

Air Force Brat

Brigadier
Super Moderator
Since when did AVIC directly claim that the J-20 is within the 15 tonne weight class? The only source I could find was from Dr. Cao, who gave a breakdown on the amount of titanium and composite materials used but didn't release any weight related info. Where is the original link sourced by this article?

AVIC is not going to tarnish their sterling reputation with some "internet nonsense", no doubt LockMart and Boeing have already incorporated and are incorporating these processes into their current production aircraft,, and it does indeed allow fractionally lighter aircraft, which is a big deal....

but really, no one would dream of claiming the F-35 could be 25% lighter with these new processes,,,, the F-35 is already using state of the art construction processes, the whole center fuse titanium tub is welded robotically,,, giving a very dramatic weight savings and increased strength, along with production efficiencies....

but hey?? these are the interwebs???
 

Air Force Brat

Brigadier
Super Moderator
Gonna refer you to this:

And this: https://www.sinodefenceforum.com/j-20-5th-gen-fighter-thread-vi.t8169/page-39#post-477426

So basically, this isn't a primary source from AVIC, but it is from a secondary source who wrote his article based on his interactions with AVIC. That's why I have noted that the reliability is not indisputable, which opens the possibility up for discussion. To be honest, I thought the number was far-fetched when I saw it, but then,

I would have stuck with your gut, and been honest, but hey, that's just me?? but you did state that AVIC was your source,,, my work is done here for the day!
 
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