Chengdu J-20 news and analysis Part III

Inst

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Yes, this is a convergent ejector nozzle of the kind that was popular up to the 1960s and is still used by the M88, the RD-33 has a full con/di design (by the looks of it with independent throat/exit control, actually!).

Didn't the WS-10 on the J-11/16 switch to convergent/divergent only recently, well after we started seeing J-20s powered by the engine? Otherwise, one hitherto incidental benefit of the ejector nozzle might make it attractive for use on a LO airframe: the sheath of cool secondary air around the engine exhaust decreases IR signature. It's probably nowhere near as effective a measure for this purpose as a 2D nozzle or something, but it might just be good enough to warrant its retention on the J-20.
I'd actually see the main benefit as being stealth and maintainability; i.e, concentrating the hot airflow in the center means that coatings on the exterior, which absorbs traveling waves from the main aircraft body, are subjected either to less heat or less total heat stress. In the former case, you can use less temperature tolerant coatings. In the latter case, maintenance requirements are reduced.

As for the IR claim:

1200px-Wiens_law.svg.png


It seems that IR emissions vary as:


e^T (with an initial displacing constant). This implies that total IR emissions, depending on other variables might be higher for a small, hot object compared to a larger mass containing the same amount of heat energy.

The localization of the heat energy from the exhaust, of course, makes it easier for the aircraft to hide the high-temperature zone, but that might not be a sufficient trade-off compared to having the hot air from the engine turbine mix with the bypass air.
 

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Partial cockpit shot. Looks like all displays are off though.

Another one ...

View attachment 665366
I find it interesting that rivets are plainly visible around the fuselage/canopy area. That cannot help with its LO RCS.
If they're below the surface the effect is probably minimal.
The Chinese hacked the F-35's subcontractors. Surely they got a hold of the magical -70 dBsm rivets?
 

Trident

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I find it interesting that rivets are plainly visible around the fuselage/canopy area. That cannot help with its LO RCS.

Well, this implies other (more particularly, US) aircraft commonly considered to be stealthy would be substantially different in this regard. I can't say I'm able to discern such a difference, calling into question the assumption on which the hypothesis is premised. This kind of elementary sanity check would save a lot of back and forth.
 

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overscan (PaulMM)

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I find it interesting that rivets are plainly visible around the fuselage/canopy area. That cannot help with its LO RCS.

Well, this implies other (more particularly, US) aircraft commonly considered to be stealthy would be substantially different in this regard. I can't say I'm able to discern such a difference, calling into question the assumption on which the hypothesis is premised. This kind of elementary sanity check would save a lot of back and forth.

Indeed. For a start, any panel that needs to be opened has to be secured with some kind of fasteners. e.g. F-22.
 

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sferrin

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Partial cockpit shot. Looks like all displays are off though.

Another one ...

View attachment 665366
I find it interesting that rivets are plainly visible around the fuselage/canopy area. That cannot help with its LO RCS.
If they're below the surface the effect is probably minimal.
The Chinese hacked the F-35's subcontractors. Surely they got a hold of the magical -70 dBsm rivets?
No such thing.
 

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Closeup WS-10 from Zhuhai 2021
Does anyone know the function of the “fingers” hanging down from upper half of the inner primary nozzle? I see them retracted into the segments across the bottom of the nozzle, not sure if they are free floating and are just hanging down by gravity.

I know that these type of ejector nozzles use the secondary airflow to form an aerodynamic “divergent” area for supersonic expansion, but the outer metal petals are really short. This may imply the engines EPR is not high enough for the specific thrust needed for true supercruise.
 

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Maybe reduce thrust at idle? ISTR the first F-16s had an issue with too much thrust at idle. Had to ride the brakes when taxiing. Would be interesting to know for sure though.
 

overscan (PaulMM)

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D-30F6 engine variant in Su-47 seems technically very close.
 

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F119Doctor

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I really like the white plastic exhaust plugs

These photos seem to confirm that these petals are free floating, hanging down by gravity under static conditions.

Free floating implies to me a system that moves under pressure balance loads. Possibly under certain conditions the cooling air under the AB liner is significantly higher pressure than the central flow, and these mini petals are opening to prevent throttling this airflow into the gas path.
 

overscan (PaulMM)

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Note that China has been buying lots of D-30KP-2 engines from D-30F6 producer Perm Motors. Is it worth considering if some technical assistance occurred on WS-10 nozzle?
 

icyplanetnhc (Steve)

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I find it interesting that rivets are plainly visible around the fuselage/canopy area. That cannot help with its LO RCS.
Hardly; as others have mentioned, American stealth fighters in operation can be seen with exposed fastener heads as well. This includes the F-35. I would imagine that in war time, some kind of expedient conductive cover would be placed over them.
 

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overscan (PaulMM)

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This gives a excellent view of WS-10 exhaust and those blow in panels.

It seems clear to me the original shot on the J-20 has the inner petals in a non-divergent position. There seem to be darker patches on them corresponding to the blow in panels.
 

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overscan (PaulMM)

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Its technically an ejector nozzle as used on J-79 or the Concorde's Olympus, right? Outer flap design is quite similar to J-79. That was as good as it got prior to development of the iris nozzle.



Bearing in mind vintage WS-10 was originally derived from the GE CFM56 core, with no afterburner, it makes sense that they had no model for an iris nozzle prior to getting hold of AL-31F, while they possibly had a J-79 from Vietnam - and didn't they start copying a Soviet R-29-300 engine from Egypt?

The latest WS-10 on J-16 has apparently different nozzles, while the J-10B TVC demonstrator is different again.
 

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overscan (PaulMM)

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SAE 680295 Variable-Geometry Exhaust Nozzles and Their Effects on Airplane Performance by R. C. Ammer and W. F. Punch AEG Engineering, General Electric Co


This is a late-model J-79 variable flap ejector nozzle from an informative article on J-79 nozzle design evolution.
 

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Inst

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
Yeah so the lines about the J-20's DSI screwing it for high-speed are probably right; the Chinese kept on screwing with the DSI to provide high-speed, and their earlier experience with the J-10's DSI and the JF-17's DSI probably helped, but they're still limited to (an unclassified?) Mach 2, so it seems.

As for TVC, look at the nozzles closely. They're not parallel. It's completely weird.

===

I'm also not sure about the provenance of the specs; the title on the bottom is 模型比例, which lists 5:1. 模型比例 -> scale model, although that's implying 4.24 meters length for the scale model, which I don't think would be attractive to hobbyists.

===

Another "are you sure it's not TVC" picture from by78 on SDF:

51541204915_a6f5bdb529_h.jpg
 
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overscan (PaulMM)

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
Yeah so the lines about the J-20's DSI screwing it for high-speed are probably right; the Chinese kept on screwing with the DSI to provide high-speed, and their earlier experience with the J-10's DSI and the JF-17's DSI probably helped, but they're still limited to (an unclassified?) Mach 2, so it seems.

As for TVC, look at the nozzles closely. They're not parallel. It's completely weird.

===

I'm also not sure about the provenance of the specs; the title on the bottom is 模型比例, which lists 5:1. 模型比例 -> scale model, although that's implying 4.24 meters length for the scale model, which I don't think would be attractive to hobbyists.
The info panel was below the 1:5 J-20 scale model in the big hall. What's odd about this?

Also don't see an issue with Mach 2.0.

I'll take a look at the nozzle angles but bear in mind engines are often mounted with offset thrust lines. Su-27 from memory has them slightly toed out?
 
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sferrin

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
So almost 70 feet. Hopefully it finally puts to rest the notion that the J-20 isn't much larger than the F-22.
 
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Inst

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
Yeah so the lines about the J-20's DSI screwing it for high-speed are probably right; the Chinese kept on screwing with the DSI to provide high-speed, and their earlier experience with the J-10's DSI and the JF-17's DSI probably helped, but they're still limited to (an unclassified?) Mach 2, so it seems.

As for TVC, look at the nozzles closely. They're not parallel. It's completely weird.

===

I'm also not sure about the provenance of the specs; the title on the bottom is 模型比例, which lists 5:1. 模型比例 -> scale model, although that's implying 4.24 meters length for the scale model, which I don't think would be attractive to hobbyists.
The info panel was below the 1:5 J-20 scale model in the big hall. What's odd about this?

Also don't see an issue with Mach 2.0.
I just didn't get provenance, but Henri Kahnemann reported it on the 28th:

View: https://twitter.com/HenriKenhmann/status/1442806431934140419


So it should be authentic.

As others have mentioned, it's a shame we still don't have empty weight or fuel capacity.

Mach 2.0 is bad for the same reason the F-35 being limited to Mach 1.6 is bad; if you have to run, you have to run, and being too slow to run away means you lose the airframe.

===

FAH_Rx3VEAQmbaY


From OedoSoldier's Twitter.

Seriously, why does the J-20 with WS-10 keep on looking like it has TVC?
 
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overscan (PaulMM)

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
So almost 70 feet. Hopefully it finally puts to rest the notion that the J-20 isn't much larger than the F-22.
And span is 1m less. Nobody ever said it wasn't longer. Its about 11.5% longer and 7% less span.
 

overscan (PaulMM)

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You have to be careful as engine thrust line usually has an offset angle. So the thrust line may be several degrees down and also toed out from the centreline. Establishing the angle of the fuselage is complicated by local curvature, camera angle etc.

There may be something there but you need more rigorous analysis.
 
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Blitzo

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Finally, some basic specs for J-20.

Length: 21.2m
Span: 13.01m
Height: 4.69m
Max Speed: Mach 2.0

I don't see any TVC in the picture personally?
So almost 70 feet. Hopefully it finally puts to rest the notion that the J-20 isn't much larger than the F-22.

J-20's length has already largely been established in the past.

The below estimate I believe was done by Deino a few years ago.
Getting within 33cm of the aircraft's true length is pretty damned accurate I'd say.

j-20 compare.jpg
 

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You have to be careful as engine thrust line usually has an offset angle. So the thrust line may be several degrees down and also toed out from the centreline. Establishing the angle of the fuselage is complicated by local curvature, camera angle etc.

There may be something there but you need more rigorous analysis.

This is interesting.View attachment 665510

That seems like a pretty high res photo that it was taken from. What's the full photo angle?
 

overscan (PaulMM)

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Engine Angles.JPG
I've tried to figure out the angles here, but while you can establish the nozzle angles, establishing fuselage angle is tricky. It would be easier with a wider shot.
 
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overscan (PaulMM)

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Mach 2.0 is bad for the same reason the F-35 being limited to Mach 1.6 is bad; if you have to run, you have to run, and being too slow to run away means you lose the airframe.

I don't agree at all. How many hours have any modern fighters outside the MiG-25/31 spent above Mach 2? Practically none.
 

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Bigger question is not top speed itself, but ability to reach it in adequate time. How much longer it will take for J-20 to go from 0.8 to 1.8 compared to F-15/T-10?
 

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It's probably sluggish. I have no doubts that China would otherwise have demonstrated it doing aggressive aerobatics like they marvelously did with that J-10.
However the minimal wing, forward large canard and elongated fuselage with internal weapons bays could make it a proper ride when it comes to acceleration once a proper engine is ultimately fitted as compared to other similar designs.
 

overscan (PaulMM)

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so, looks like it's 21.2x13.01? @paralay And I'm off by 32 cm on length and 6 cm on wingspan. 76-77 meters wing area?
The length of the aircraft causes distrust
the wing area is 73 m2 the area of the front horizontal tail is 2 x 3 m2
total area 73 m2 + 6 m2 = 79 m2
Your drawing proportions aren't correct, its longer than you show, but dimensions still seem a bit off. Will try to quantify.
 

overscan (PaulMM)

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Span is unambiguous. Therefore assuming span is definitely 13.01m, does 21.2m length make sense?

Ratio Length/width = 1.6295

Using the overhead satellite view I get closest to the right aspect ratio (1.627) if the length is measured to the end of the tail boom rather than the tip of the tail.

I get 21.37m for length to tip of vertical tail, scaled from 13.01m for wingspan.


However its very low res. Anyone got a good top or bottom view?

The high res pic is good but seems to be foreshortened.
 

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