J-20 5th Generation Fighter VII

latenlazy · Sep 1, 2022

stannislas said:
i'm sure about that, this paper was much ealier than the J-XX program, probabaly even ealier than the conceptual design phase of the J-XX program, otherwise in this paper won't estimate the program initiation date to be 06-07...

the paper is likely wrote sometime around 2002, as the price was estimate in the money value of that time

Yes that’s about where I peg the date for this document too.

ZeEa5KPul said:
There's definitely a new coupling mechanism going on. Stage 4 depicts an arcing region of low pressure from the canard to the main wing + LERX. This is a new low pressure region forming directly behind the canard instead of arcing above it. You can see the condensation region actually expanding outward from the LERX.

That’s not how *I* defined stage 4. What you’re describing with the “arcing” is the low pressure flow field fro the canard downwash. That is coveted in my description of stage 3.

ZeEa5KPul · Sep 1, 2022

latenlazy said:
That’s not how *I* defined stage 4. What you’re describing with the “arcing” is the low pressure flow field fro the canard downwash. That is coveted in my description of stage 3.

You didn't define these stages clearly. It's ambiguous whether what you're calling "primary", "secondary", and "tertiary" in the first paragraph correspond to stages 1, 2, and 3 (in which case, where's "quaternary"?) The only things clearly labelled as stages are the pictures, so I took them to mean stages. The arcing appears to start at what you labelled stage 3 and reaches its zenith in the stage 4 picture.

Whatever you meant, the manoeuvre in the video I depicted clearly shows a coupling mode distinct from the downwash you depicted in stages 3 and 4 that wasn't considered in your discussion or shown in your pictures. I think it merits mention, whether you want to call it a "stage" or not is personal preference.

latenlazy · Sep 1, 2022

stannislas said:
agree and disagree, bypass ratio is definately one of the major factor for trust boost and fuel efficiency, just think of CFM56 and F110 and F119 and F135, a larger bypass ratio definately helps,

In terms of generation changes, from F110 to F119, of course most spec will increase, but that is largerly due to better material is able to support more complicated design of the heat section. For the cold section, i.e. the fans and the bypass ratio, still most the same, so how much can the improvement of the heat section compensate the cold section over the fuel efficiency, i'm not sure, we need to have some numbers to verify

For the same high pressure compressors higher bypass ratio is definitely more efficient per unit of thrust. Between two different generations of HPCs that is not necessarily the case. More advanced HPCs are more fuel efficient.

One of the *main* changes from the F110 to F119 is an greatly improved high pressure compressor. Some of the new materials help play enabler there too, not just in the hot section. The “cold section” is not just the fans and LPC. The HPC does most of the work compressor the air for combustion in the hot section. The amount of compression is the first order factor in dictating the efficiency of the burn in the hot section. Heat is the secondary efficiency factor. There’s a reason why the HPC is typically called the “core”.

Jj888 · Sep 1, 2022

delete

latenlazy · Sep 1, 2022

ZeEa5KPul said:
You didn't define these stages clearly. It's ambiguous whether what you're calling "primary", "secondary", and "tertiary" in the first paragraph correspond to stages 1, 2, and 3 (in which case, where's "quaternary"?) The only things clearly labelled as stages are the pictures, so I took them to mean stages. The arcing appears to start at what you labelled stage 3 and reaches its zenith in the stage 4 picture.

Whatever you meant, the manoeuvre in the video I depicted clearly shows a coupling mode distinct from the downwash you depicted in stages 3 and 4 that wasn't considered in your discussion or shown in your pictures. I think it merits mention, whether you want to call it a "stage" or not is personal preference.

¯\_(ツ)_/ These states aren’t a rigorous typology on my part. They’re just observations I’m making about the overall behavior of the flow field at different flight conditions. And yes, based on how I’m categorizing the stages here the arcing would start at 3 and hit its zenith at 4, consistent with the canards contributing low pressure downwash onto the vortex system already engaged on the wing from 2, and that downwash being augmented by the low pressure field generated from the over-body camber. What you’re described as arcing *was* characterized in my description. It’s the low pressure field from the canard downwash.

“The canards are probably the last vortex contributor before the over-body camber is engaged because they’re long coupled, which means that when operating by themselves their downwash effect onto the wings should be smaller or more diffuse. They’re probably most impactful in enhancing the low pressure flow field downstream when the other vortex generators are already engaged.”

luosifen · Sep 1, 2022

Just read that Chengdu went into COVID lockdown, I guess J-20 production will get disrupted for a bit until things get under control.

ZeEa5KPul · Sep 1, 2022

latenlazy said:
¯\_(ツ)_/ These states aren’t a rigorous typology on my part. They’re just observations I’m making about the overall behavior of the flow field at different flight conditions. And yes, based on how I’m categorizing the stages here the arcing would start at 3 and hit its zenith at 4, consistent with the canards contributing low pressure downwash onto the vortex system already engaged on the wing from 2, and that downwash being augmented by the low pressure field generated from the over-body camber. What you’re described as arcing *was* characterized in my description. It’s the low pressure field from the canard downwash.

“The canards are probably the last vortex contributor before the over-body camber is engaged because they’re long coupled, which means that when operating by themselves their downwash effect onto the wings should be smaller or more diffuse. They’re probably most impactful in enhancing the low pressure flow field downstream when the other vortex generators are already engaged.”

I suspect you're not seeing the phenomenon I'm referring to. Just so we're all on the same page, I made some images of my own to show what I mean. Note that in all pictures, the J-20 is moving almost horizontally.

This corresponds to your stage 4 with canard downwash. The angle of attack is fairly shallow.

Look at the region I've circled. Unfortunately, the overcast background gets in the way, but you can see a new low pressure region forming at the trailing edge of the canard. The angle of attack is much higher than the previous picture at the J-20 turns into its climb.

The region from the trailing edge has extended downward and coupled with a low pressure region growing out of the main wing and LERX. This is why I'm deeply interested in this phenomenon - normally, low pressure regions form above physical surfaces and their boundaries don't extend beyond them. This system is forming in the empty space behind the canard and above the main wing. If you look at the clip, it looks like the low pressure region at the main wing is being pulled upward.

The J-20 here is near its greatest angle of attack and is about to accelerate vertically, which rapidly drops AoA and destroys these systems.

I don't think it's valid to dismiss this as just ho-hum downwash.

ChinaShill · Sep 1, 2022

latenlazy said:
Based on something one USN pilot on Reddit said, it seems the US’s own internal assessment from their test models suggests based on the J-20’s shape (and probably some general assumptions about RAM performance) the J-20’s RCS is comparable to early block F-35s. But who knows.

But FWIW while I think the J-20’s aerodynamic design is quite advanced the F-22’s is no slouch either, and both are probably more comparable than is being suggested in this discussion.

Can you link the Reddit comment? I'm curious to hear what the pilot said word for word.

latenlazy · Sep 1, 2022

ZeEa5KPul said:
I suspect you're not seeing the phenomenon I'm referring to. Just so we're all on the same page, I made some images of my own to show what I mean. Note that in all pictures, the J-20 is moving almost horizontally.
View attachment 96668
This corresponds to your stage 4 with canard downwash. The angle of attack is fairly shallow.
View attachment 96669
Look at the region I've circled. Unfortunately, the overcast background gets in the way, but you can see a new low pressure region forming at the trailing edge of the canard. The angle of attack is much higher than the previous picture at the J-20 turns into its climb.
View attachment 96670
The region from the trailing edge has extended downward and coupled with a low pressure region growing out of the main wing and LERX. This is why I'm deeply interested in this phenomenon - normally, low pressure regions form above physical surfaces and their boundaries don't extend beyond them. This system is forming in the empty space behind the canard and above the main wing. If you look at the clip, it looks like the low pressure region at the main wing is being pulled upward.

The J-20 here is near its greatest angle of attack and is about to accelerate vertically, which rapidly drops AoA and destroys these systems.

I don't think it's valid to dismiss this as just ho-hum downwash.

I am seeing the phenomena you’re seeing. In my book that’s stage 3 or stage 4 depending on whether the over-body low pressure flow field is engaged. The *point* of stage 3 is the canard downwash interacting with the rest of the vortex system. It’s inevitably the case that the entire area downstream of the canard is going to be a low pressure area contiguous to the wing. That’s what downwash is. Whether that forms condensation or not depends on whether the strength of the low pressure zone is enough to cause water vapor to form and what local moisture levels look like.

ZeEa5KPul · Sep 1, 2022

latenlazy said:
I am seeing the phenomena you’re seeing. In my book that’s stage 3 or stage 4 depending on whether the over-body low pressure flow field is engaged. The *point* of stage 3 is the canard downwash interacting with the rest of the vortex system. It’s inevitably the case that the entire area downstream of the canard is going to be a low pressure area contiguous to the wing. That’s what downwash is. Whether that forms condensation or not depends on whether the strength of the low pressure zone is enough to cause water vapor to form and what local moisture levels look like.

We're splitting hairs at this point. I think it's fair to say and we can both agree that it's an extension of the normal canard coupling mechanism best visualized with the arcing condensation. I consider it remarkable enough to merit a new label, you don't. Let's leave it at that.

J-20 5th Generation Fighter VII

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