J-20... The New Generation Fighter III

Basically every aircraft in the world with a variable nozzle design can do this. You'll see this when power is set defferentially. When ideling, the nozzle is normally open to allow the - relatively - low pressure exhaust to exit. When adding power, the nozzels contract (to a certain point) to force the exhaust gases through a smaller exit, increasing thrust.
So nozzle area is a function of power setting on that engine (and other stuff).

Scratch said:

Basically every aircraft in the world with a variable nozzle design can do this. You'll see this when power is set defferentially. When ideling, the nozzle is normally open to allow the - relatively - low pressure exhaust to exit. When adding power, the nozzels contract (to a certain point) to force the exhaust gases through a smaller exit, increasing thrust.
So nozzle area is a function of power setting on that engine (and other stuff).

Click to expand...

It is actually slightly more complicated than that. From what I've read on CJDBY what you said is true when the aircraft is traveling at subsonic speed. When it goes supersonic thrust is increased when the nozzles are fully expanded. There is a more complex fluid dynamics explanation for this but that isn't my field of study.

Yep, when air is travelling at supersonic speed a nozzle becomes a diffusor and vice versa, wich is why in that speed region the exhaust has to open up to speed up the gases.

Scratch said:

Basically every aircraft in the world with a variable nozzle design can do this. You'll see this when power is set defferentially. When ideling, the nozzle is normally open to allow the - relatively - low pressure exhaust to exit. When adding power, the nozzels contract (to a certain point) to force the exhaust gases through a smaller exit, increasing thrust.
So nozzle area is a function of power setting on that engine (and other stuff).

Click to expand...

siegecrossbow said:

It is actually slightly more complicated than that. From what I've read on CJDBY what you said is true when the aircraft is traveling at subsonic speed. When it goes supersonic thrust is increased when the nozzles are fully expanded. There is a more complex fluid dynamics explanation for this but that isn't my field of study.

Click to expand...

Scratch said:

Yep, when air is travelling at supersonic speed a nozzle becomes a diffusor and vice versa, wich is why in that speed region the exhaust has to open up to speed up the gases.

Click to expand...

The only object that I can think of comparing to that resembles similar design was the animal eyeball, which the pupils constrict and widen in similar fashion, although for different reason. Very interesting stuff, of how the nozzles were operated.

Player99 said:

... Has anyone commented on the clear difference in this one footage between the colors of the afterburn of the three aircraft? The J-20 gives that blue flame (as we were once told to be the PSed result), while at the same time the J-10B and FC-1 give that more traditional yellowish one.

Click to expand...

Going back to that AB plume color question.

Could that actually be a hint the engines have a rather high (for a fighter) bypass ratio?
Especially the Tu-22M has really distictively blue plumes, the B-1B plumes also look more blueish as do sometimes even Flanker plumes. And from the vids, I'd say that J-10s, supposedly using the Al-31 as well, also have a blueish tone. These engines all have higher BPRs.
While the FC-1 (MiG-29 - [engine family!!]), and also US & European engines are much more yellow-orange, with a really low BPR.
It seems there are exceptions, though, like the MiG-31 or Concorde.

Blueish flames normally mean more efficient burning of a combustible. And in a higher BPR engine more oxygen might by readily available for better combustion.
Or the mixture is simply leaner, meaning they abstain from max thrust in favor of more efficiant fuel use or prolonged engine life.

Scratch said:

Going back to that AB plume color question.

Blueish flames normally mean more efficient burning of a combustible. And in a higher BPR engine more oxygen might by readily available for better combustion.
Or the mixture is simply leaner, meaning they abstain from max thrust in favor of more efficiant fuel use or prolonged engine life.

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Beyond that, would the blueish flames leave a stronger IR signature to be more easily detected?

Well, if the principles of the bunsen Burner from chemistry class still apply, I'd say that a blue flame also means higher temperature. I don't know, however, how big the difference would actually be, and an AB is already pretty hot anyway.

Scratch said:

Well, if the principles of the bunsen Burner from chemistry class still apply, I'd say that a blue flame also means higher temperature. I don't know, however, how big the difference would actually be, and an AB is already pretty hot anyway.

Click to expand...

Via wiki:

In a laboratory under normal gravity conditions and with a closed oxygen valve, a Bunsen burner burns with yellow flame (also called a safety flame) at around 1,000 °C (1,800 °F). This is due to incandescence of very fine soot particles that are produced in the flame. With increasing oxygen supply, less blackbody-radiating soot is produced due to a more complete combustion and the reaction creates enough energy to excite and ionize gas molecules in the flame, leading to a blue appearance. The spectrum of a premixed (complete combustion) butane flame on the right shows that the blue color arises specifically due to emission of excited molecular radicals in the flame, which emit most of their light well below ~565 nanometers in the blue and green regions of the visible spectrum.

J-20 Mighty Dragon landing at Chengdu airfield


J-20 Mighty Dragon landing at Chengdu airfield.

[Note: Thank you to HouShanghai for the picture.]

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J-20 Mighty Dragon parked in front of Chengdu test facility.


Interior of J-20 Mighty Dragon engines are visible.

[Note: Thank you to Greyboy2 for the pictures.]

J-20 Mighty Dragon coming in for a landing


Close-up of J-20 Mighty Dragon coming in for a landing.

[Note: Thank you to HouShanghai for the picture.]