Chengdu J-20 news and analysis Part III

[Deino]

Given how Kamov were commissioned to do preliminary design studies for the Z-10, it's not beyond the realms of possibility Mikoyan did something similar. I've not seen any evidence though.

That’s surely possible, however by all I read IMO unlikely… For sure however is, that the J-20 is not a copy, clone or even development of the MFI.

 

[Forest Green]

I think some of the earlier Chinese stealth prototypes took heavily from the the MiG-1.44 layout but the J-20 for sure departed from it, probably because the 1.44 failed. Indeed an earlier Chinese attempt began in the '90s also failed.

 

[Deino]

I think some of the earlier Chinese stealth prototypes took heavily from the the MiG-1.44 layout but the J-20 for sure departed from it, probably because the 1.44 failed. Indeed an earlier Chinese attempt began in the '90s also failed.

Care to explain, which „earlier Chinese attempt began in the '90s also failed“?

 

[siegecrossbow]

I think some of the earlier Chinese stealth prototypes took heavily from the the MiG-1.44 layout but the J-20 for sure departed from it, probably because the 1.44 failed. Indeed an earlier Chinese attempt began in the '90s also failed.

Care to explain, which „earlier Chinese attempt began in the '90s also failed“?

He might be referring to the snowy owl from SAC.

 

[Deino]

I think some of the earlier Chinese stealth prototypes took heavily from the the MiG-1.44 layout but the J-20 for sure departed from it, probably because the 1.44 failed. Indeed an earlier Chinese attempt began in the '90s also failed.

Care to explain, which „earlier Chinese attempt began in the '90s also failed“?

He might be referring to the snowy owl from SAC.

But wasn’t the Snowy Owl simply SAC‘s contender to the same requirement, which was won by CA‘s proposal?

 

[Forest Green]

Care to explain, which „earlier Chinese attempt began in the '90s also failed“?

As referenced in the KeyPubs Stealth book at the start of the J-20 section. There was an earlier attempt which failed, so they started with a clean slate.

 

[Deino]

And since when is this one a credible source? I remember the Chinese section was so much full of errors I directly laid it back …

 

[Forest Green]

Well it talks of a 'Fighter D', also referred to as 'Concept 1993'.

 

[Deino]

Well it talk of a 'Fighter D', also referred to as 'Concept 1993'.

But these were intermediate studies and not failed projects ...

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:

If I'm reading this correctly then for the first time the PLAAF took the J-20 a little more "off the leash" at the 2022 Changchun Air Show after all previous flight displays were rather tired flybys:

https://twitter.com/TrepverterA/status/1563410562020651009

here a different video of the same maneuver

https://video.twimg.com/ext_tw_video/1563410536682835968/pu/vid/1270x720/xfVTOS8jZDnLwaG8.mp4

 

[Sferrin]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

 

[Alifzero]

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

 

[Flyaway]

View attachment 683070

I wondered at first why a C-17 was refuelling Chinese aircraft.

 

[djfawcett]

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

https://www.secretprojects.co.uk/data/attachments/201/201204-30b0b56bbeb3ff2b7d9088a834c28c0c.jpg

This the first time I saw the attached drawing. I would be rather surprised if the CG resides ahead of the center of pressure (I assume that is your neutral point. This indicated a positive stability margin. Given the J-20 a fly-by-wire, artificial stability flight control system, I would be shocked if it did not have relaxed stability - i.e. - negative stability margin.

 

[paralay]

,

 

[windcalm]

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

https://www.secretprojects.co.uk/data/attachments/201/201204-30b0b56bbeb3ff2b7d9088a834c28c0c.jpg

This the first time I saw the attached drawing. I would be rather surprised if the CG resides ahead of the center of pressure (I assume that is your neutral point. This indicated a positive stability margin. Given the J-20 a fly-by-wire, artificial stability flight control system, I would be shocked if it did not have relaxed stability - i.e. - negative stability margin.

I'm not 100% sure, but the aircraft might spend fuel in the forward fuselage, seems quite big, first. Then, the aircraft might have negative stability during combat mission. Anyhow, the main landing gear position seems not conventional for relaxed static stability fighter jets.. Or, they might focus more on long range capability rather than agility.

 

[Sferrin]

the red line is the position of the wheel of the main landing gear

View attachment 683201

Why is canard area only counted on some but not others?

 

[paralay]

,

 

[Hank58]

Seems like an overly simplified estimate of what is the effective wing planform. Some accounting needs to be made for the contribution of the wing-root leading-edge extension (and maybe the inlet planform, too).

 

[djfawcett]

The point is 25% of the average aerodynamic chord of the wing. If it turns out to be behind the main landing gear, then the front horizontal tail is involved in creating lift

Your analysis is very high level resulting in data that is not necessarily very accurate. Using only MAC of the wing will yield results that won't even get you close. All surfaces have to be included as well as up wash effect on the forebody and canard, as well as down wash effects of the canard, etc. The truth of the matter, the approach you are using simply relating the 15deg landing gear to establish cg and 25% MAC on the wing for center of pressure will yield results that extremely inaccurate.

 

[paralay]

,

 

[Sferrin]

The point is 25% of the average aerodynamic chord of the wing. If it turns out to be behind the main landing gear, then the front horizontal tail is involved in creating lift

Which canard, of any of them, will make the nose go DOWN on takeoff if it's leading edge is up?

 

[djfawcett]

Convince me with calculations

I am not going to be your professor to teach you performance, stability, and control. I would highly recommend you purchase a book by Perkins and Hage titled "Airplane Performance, Stability, and Control." It is considered one of the original authorities on the subject matter. You will soon find out how far you are off base with your assumptions.

 

[paralay]

,

 

[Forest Green]

It does appear that there's a way larger unshaded area before the cockpit in your J-20 diagram than your Su-57 diagram.

 

[red admiral]

It's some interesting analysis based on design rules of thumb, but definitely isn't correct for at least some of those types so you can't make definitive statements.

The uncertainty is from:

• Subsonic aero centre isn't always at 25% MAC
• Angle between aft-most CG and main gear position isn't always 15deg. Easy variability 5-25deg. And this calculation is very sensitive to main gear height (compressed or not?) and estimated z height of CG.

 

[latenlazy]

It's a pity that you can't prove your own thesis

It's a pity you think that these things can be calculated accurately without CFD analysis.

 

[latenlazy]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

 

[Sferrin]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I didn't see the nose come anywhere near 90 degrees.

 

[Maro.Kyo]

Convince me with calculations

You need to first understand the fact, that if you are the one arguing something, you should also be the one to prove it.

Time and time again you just show how entitled you are to your own, extremely inaccurate methodologies based on crude and personal assumptions without technical basis.

 

[Forest Green]

Convince me with calculations

Dude, you need a powerful PC and CFD software to run the Navier-Stokes equations, and they can usually only be solved iteratively, not analytically, and even then you need a precise input model, so asking for such is simply ridiculous.

 

[Deino]

Guys ... please stop here! You won't reach an agreement on if this method is correct or not and if you accept the results or not,

Anyway, it has since several posts no relation to the topic, so please stop.

 

[djfawcett]

It's a pity that you can't prove your own thesis

Deino is right, this discussion makes no sense. If you choose to ignore fact, so be it. As in the past, I refuse to be baited into this type of debate.

Best Regard, Dave

 

[Alifzero]

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I believe Su-35 done that in combat approved video. But i can't find standalone video. Still, that J-20 maneuver still look good, but maneuverability is not their main purpose afaik. Launching barrage of PL-15 is seems more important than that.

 

[TomcatViP]

It's a pity that you can't prove your own thesis

It's a pity you think that these things can be calculated accurately without CFD analysis.

Actually, they do.

 

[latenlazy]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I didn't see the nose come anywhere near 90 degrees.

Look more closely

Roll #1


Roll #2

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I believe Su-35 done that in combat approved video. But i can't find standalone video. Still, that J-20 maneuver still look good, but maneuverability is not their main purpose afaik. Launching barrage of PL-15 is seems more important than that.

Yes but the Su-35 has TVC. Launching a barrage of MRAAMs is also the main purpose of any fighter that’s supposed to be good at BVR (F-22 included). The whole point of Gen 5 aerodynamics is expanded flight envelope so you don’t have to make as many tradeoffs with flight regimes and corresponding tactical options.

It's a pity that you can't prove your own thesis

It's a pity you think that these things can be calculated accurately without CFD analysis.

Actually, they do.

If your primary interest is in modeling a hobby RC plane that’s only flying at 100 mph sure. But don’t let me stop you from insisting that aerospace engineers are wasting their time doing CFD analysis with expensive equipment for their jobs

 

[TomcatViP]

This is not a conversation around CFD but you are not going to invest hours with a simulation without prior analytical research. CFD are useful to refine a design not to paint it from scratch.

 

[latenlazy]

This is not a conversation around CFD but you are not going to invest hours with a simulation without prior analytical research. CFD are useful to refine a design not to paint it from scratch.

The point is without knowing the pressure distribution of the flow field you have no way of identifying the relationship between center of gravity and aerodynamic center. The former already sits in a line between the front and back landing gears so that can only be guessed at with rules of thumb and the latter isn’t only unknowable from eyeballing when your plane’s shape is more than just all simple wing geometry or a tube and simple wing geometry, but actually shifts around in different flight envelopes. Now if you have access to wind tunnels and physical models you’re welcome to discard CFDs entirely, but I’m going to assume that none of us here have access to those tools so the only way anyone here could claim to know what the aerodynamic center is (specified in different flight conditions no less!) is if they bothered to do the legwork of building a 3D model that they then put through a CFD. The main takeaway is these rules of thumb are bullshit and not really defensible for anyone who’s actually serious about the subject matter. (And also Paralay’s J-20 diagram isn’t even accurate. Do an overlay with one of the many top view photos we now have of the plane if you don’t believe me).

 

[Sferrin]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I didn't see the nose come anywhere near 90 degrees.

Look more closely

Roll #1
View attachment 683299

Roll #2
View attachment 683298

Your pics actually support my statement. Look at the angle of the vortex coming off the wingtips. They're almost parallel to the fuselage.

 

[latenlazy]

Not to take anything away from it but there are videos out there of F-16s and Typhoons doing that as well.

Anyway, this is interesting and even if some surely will claim it is nothing in comparison to the Su-57 or F-22 it is anyway a first IMO:


View attachment 683157

It's impressive and beautiful.
On comparison to Su-35, Su-57, and F-22, what make people said Su-35/57 maneuver more impressive is that they done the upward flatspin in #5 aircraft attitude. Still, J-20 look really good at that video

What makes this display notable for me is that the vertical roll seems to have been initiated when the nose pitched past 90 degrees. Vertical rolls I've seen before, but I don't think I've ever seen it executed at such a severe pitch angle.

I didn't see the nose come anywhere near 90 degrees.

Look more closely

Roll #1
View attachment 683299

Roll #2
View attachment 683298

Your pics actually support my statement. Look at the angle of the vortex coming off the wingtips. They're almost parallel to the fuselage.

Vortex is slightly over 90 in the below image. But it’s a bit of a meaningless point because vortexes detach at stall angles, so they’re not 1:1 with the pitch angle of the nose.

 

[Deino]

Via the SDF:

@latenlazy slow mo high def version if you want to see how the all moving slabs work during the roll.

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