J-20 5th Generation Fighter VII

taxiya said:

@siegecrossbow,

As I promised to post the source of J-20's lift coefficient value, the source is 顾诵芬《我国战斗机发展战略研究》
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4th gen fighter utilize high lift, low drag, high stealthy design. According to pre-study result, the maximum lift coefficient value of fourth gen aircraft is targeted at 2, 30% higher than 3rd gen fighters.

Just note, this is probably referring to pre-prototype J-20 (with underbelly intake) discussed in the paper that I quoted in earlier posts. The final figures of J-20 may be different. However, we have heard of figure more than 2.

I am really curious about F-22 and F-16's figures.

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stannislas said:

yeah, that's why i read a lot of people was worried that after switching to WS-15, despite J-20 may able to supercruise in a higher speed, but the combat range will decrease

but the issue for this 顾诵芬《我国战斗机发展战略研究》, was that it was pretty old and way before the allegedly major changes of the WS-15 design spec news just few years ago, so i'm not sure how much will these specs still hold

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stannislas said:

i'm not even sure this is for J-20, more like Gu's idea on what PLAAF is expecting on the 4th gen fighter back then, more likely to be the design spec estimation from SAC, i.e. the 雪鸮, or even not that, as this book was pretty old, should be even ealier than the project 718...

you can see it clear on the RCS spec requirement, and that's why SAC lost on the project 718 bid

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latenlazy said:

I actually think it’s the opposite. The primary vortex generators on the J-20 seem to be the LERXes, the secondary seems to be vortex coupling between the the LERXes and the dogtooth that forms from the deflection of the leading edge flaps, and then the tertiary seems to be the canards. The strongest coupling happens at higher AoA where the over-body camber that starts at the inlets generates a much more powerful low pressure field over the top of the whole fuselage, enhancing the vortices being engaged by the other vortex generators.

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.

In fact, this arrangement is very similar to the Eurofighter’s, which also employs long coupled canards. We see that the canards on the EF are only contributing to vortex generation over the main wings once they’re coupled with strakes that sit midway between the canards and the wings. In the EF it’s actually the strakes seeding the vortex over the wings, while the canard downwash is used to enhance the vortex generated from the strakes.

The J-20’s vortex coupling dynamics actually seem to have four distinct stages, and I think studying those four stages can tell us a lot about the particulars of how the J-20 employs vortex lift for various flight maneuvers. See the attached images to get a sense of what I’m talking about.

Stage 1
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Stage 2
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Stage 3
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Stage 4
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ZeEa5KPul said:

I see your stage 4 and raise you a stage 5:

Slow the video down if you don't see what I'm talking about.

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latenlazy said:

Your stage 5 is imo just another view of stage 4. But this is a great video to demonstrate what I mean.

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ZeEa5KPul said:

Nope, there's more going on than what stage 4 depicts. Slow the video down and look carefully at the trailing edge of the canards.

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latenlazy said:

As I understand it this document wasn’t just someone’s idea of what to expect. It was the actual requirements tender for the J-XX program.

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latenlazy said:

I think that’s just part of the canard shedding onto the wing. Stage 3 or stage 4 depending on how engaged the over-body low pressure zone is.

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latenlazy said:

Well if the WS-15 is flying at max military thrust then yeah its range will go down. But it doesn’t need to fly at max military thrust since that’s like 30%-50% more than current engines. One of the main points of more advanced engines is that they are actually more efficient per unit of thrust. So for the same bypass ratio an engine based on the WS-15 core is probably more efficient than one based on the WS-10 core. Which means you can spend that budget to maintain the same efficiency but with lower bypass.

The F-22’s short legs are not really a problem with the engine’s bypass ratio, but the fuel fraction of the airframe. And of course if you choose to supercruise the entire way your range is going to drop like a stone regardless of your fuel fraction or your bypass ratio.

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