J-20 5th Gen Fighter Thread IV (Closed to posting)

[Deino]

Just another idea / theory regarding these strange hexagonal structures .... posted by "Jō Asakura" at the Key-Forum:

The first bleed vent (L) serves to strengthen the initial oblique shockwave, the second (R) is to deal with residual boundary layer separation due to the intensity of the initial shock wave (and ingested BL) downstream of the compression bump surface. The problem arises in the supersonic regime when total pressure flow is degraded due to the interaction of the bleed system and shock waves. However, as these two vents increase the bleed flow rate, then the severity of these low pressure gradients is reduced.

The F-35 also suffered from similar problems but LM instigated (at least one) redesign of the duct itself. Even so it is probable that the duct design will still, to some extent, inhibit the F-35's performance. If Mr. PLAwolf is trying to sell these J-20 duct modifications as some sort of highly evolved hybrid of the DSI, he's most welcome to- but his assertions would be wrong. After all the whole idea of the DSI is a simple, elegant high pressure shock solution devoid of mechanisms and gizmos.

The passive porous elements (above) employed on the JF-17 are almost certainly due to address the flow separation imparted from the forward fuselage, remember the DSI was retrofitted. One would need very accurate dimensional data on the J-20 before one can confirm the root cause of it's DSI problems, but I would suggest they are for similar reasons- namely in the absence of a 'conventional' splitter plate and BL cavity, the design and dimensions of the fuselage forebody are not conducive to a pressure gradient that efficiently deals with flow separation, not only trans & supersonic, but @ various AoA.

To admit all this is way off my technical understandings ... but maybe someone else understands what he tells...

Deino

 

[AssassinsMace]

That is a good point. Similar mesh also appeared on intake of JF-17, although they are not hexagonal shape. I suspect they could be serving the same function.

View attachment 9389

I think they're different. On the JF-17 they look like divots. Divots can smooth out air flow on surfaces like golf balls have them to make them travel further. I don't know if that's the case here but on the JF-17 they look like they're meant to affect airflow on key locations.

 

[manqiangrexue]

the matter of J-20's engine was settled a long time ago when the picture of 2001's afterburner flameholder matches AL-31 flameholder

whether 2011 uses the same Al-31 engine or not remains to be seen, but we can assume it's still AL-31 as no changes made to the engine nozzle petals, and it's still the same blue afterburner flame

Is it safe to conclude that from the afterburner color? It seems to me that many American engines also produce a blue afterburner hue. Any engine that burns in the same temperature range should produce a blue afterburn. What is the color of the WS-10A's afterburner flame?

I might be wrong but a quick research of the flameholder makes me think that it is a relatively simple structure and given that China has access to the AL-31, and China's record of borrowing designs, it does not seem conclusive that the flameholders match upon rough visual inspection. Have we seen WS-10A's flameholder to be markedly different?

Assuming that WS-10A produces 132kN, and is reliable enough to be in service with the J-11B, I don't see an AL-31 variant that is both in service and provides significantly more thrust to merit using that instead of WS-10A (not to mention the fact that the nozzles in J-20 don't match WS-10A, which indicate to me a superior WS-10 variant). AL-31FM2 with 145kN would be a good candidate but it's still in testing so I really don't believe that that's what's in the Chinese machine.

 

[paintgun]

flameholder pattern between AL-31 and WS-10 is quite different
WS-10 afterburner flame is red

 

[manqiangrexue]

Overall - I hope You got what I meant - the items found on the J-20's exhaust are identical to an AL-31F ... only the colour is wrong and even here we have images showing the silver to peel off and even a J-20 with one silver exhaust and one standard exhaust. As such - and please don't ask me why - these engines are simply a version of the AL-31FN with a silver coat.

Deino

But the picture with 1 silver exhaust and 1 standard exhaust is what makes me think that the silver exhaust is not AL-31. If both engines were AL-31, why would they change the exhaust color and petals in 1 but leave the other standard? This picture, to me, shows engine testing with 1 AL-31 and 1 engine that is not as mature as the AL-31.

 

[paintgun]

But the picture with 1 silver exhaust and 1 standard exhaust is what makes me think that the silver exhaust is not AL-31. If both engines were AL-31, why would they change the exhaust color and petals in 1 but leave the other standard? This picture, to me, shows engine testing with 1 AL-31 and 1 engine that is not as mature as the AL-31.

the silver exhaust is the same with the 'standard' exhaust, it is just paint/coating

 

[paintgun]

Just another idea / theory regarding these strange hexagonal structures .... posted by "Jō Asakura" at the Key-Forum:

To admit all this is way off my technical understandings ... but maybe someone else understands what he tells...

Deino

I'm not entirely sold on the idea that these meshes are for aerodynamic purposes, if it is, it seems as a slapped-on solution as a quick fix to an underlying problem.
Even if these bleed vents work as intended, it might complicate or compromise stealth aspects of otherwise a nice flat surface.

Does not make sense from engineering stand point.

 

[manqiangrexue]

the silver exhaust is the same with the 'standard' exhaust, it is just paint/coating

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From this picture, I do not think they look like the same petals with different paint.

1. Why would they paint 1 but not the other?

2. What are those 2 black rings on the silver nozzle? They are not there on the black nozzle.

3. Even the section that joins the petals to the engine housing has changed in color. Why?

 

[paintgun]

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From this picture, I do not think they look like the same petals with different paint.

1. Why would they paint 1 but not the other?

2. What are those 2 black rings on the silver nozzle? They are not there on the black nozzle.

3. Even the section that joins the petals to the engine housing has changed in color. Why?

They are the same if you look closer, or maybe get a higher res

 

[manqiangrexue]

They are the same if you look closer, or maybe get a higher res

I changed the pic to a higher res as it appears now in my post. I still don't know about them being the same. Why would they paint only 1 silver and what are the 2 black rings?