Chengdu J-20 5th Generation Aircraft News & Discussions

[Asoka]

OK! Let's make things simple :

- The total mass of the airflow times the difference between exhaust speed and flight speed equals thrust.

- The high bypass of a turbofan allows better fuel economy at cruising speed and those engines are less noisy.

- The response time of a low bypass is better because it moves less air : "it shifts faster" ( combat essential ).

- Since no big fan to move slow air, less cross-section, vital for drag reduction to go faster easier ( & stealthier ).

- Lower bypass ratio produce less total thrust but more of it per kg of engine A.K.A better thrust to weight ratio.

- Lower bypass ratios perform best at higher speeds; high bypass ratios are optimized for high subsonic cruise.

- To hike thrust, fighters use afterburners ( higher nozzle speed ) and airliners maximize flow with high bypass.


Cores are, as some mates mentioned, often similar since as we showed the difference lies elsewhere.

I.E. GE F-110 to CFM-56 . . .
​

View attachment 415912

Have a great day all, Tay.

"- To hike thrust, fighters use afterburners ( higher nozzle speed ) and airliners maximize flow with high bypass."

Good summary!

While it is true having a large cross section is not good for speed, it is also true that at high altitude, when the air is much thinner, this problem is not as severe. With much less air resistance at high altitude, this is much better for high speed cruising.

Most planes achieve their maximum speed at high altitude > 30,000ft, rather than at sea level.

However, at high altitude, you need a bigger air intake to scoop up the thin air. The Mig-25 is famous for having a huge air intake, for this high speed, high altitude cruising.

While we don't know for sure, I got a feeling J-20 that, with its large air instakes and adjustable DSI inlets, it could also cruise at very high altitude and very high speed for a very long time, because of its advanced aeroframe and powerful engines.

It is notable that although F-22's top speed is listed as Mach 2.2, its Max. Speed is actually classified. Some people speculated it is, at least, Mach 2.5+, which is F-15's top speed.

F-22 is much more aerodynamic than F-15, and has 40% more thrust.

Think about it, F-22 is a speed demon.

When J-20 first appeared, a lot of people believe its a high speed interceptor like the Mig-25, and/or long range striker like the F-111, because of its size.

Now, we know J-20 has excellent subsonic and supersonic maneuverability, but will it has high altitude, high speed capability, as well?

 

[cnleio]

5x J-20 fleet flight video: http://weibo.com/tv/v/FfmsMFBrM

 

[gambit]

"- To hike thrust, fighters use afterburners ( higher nozzle speed ) and airliners maximize flow with high bypass."

Good summary!

While it is true having a large cross section is not good for speed, it is also true that at high altitude, when the air is much thinner, this problem is not as severe. With much less air resistance at high altitude, this is much better for high speed cruising.

Most planes achieve their maximum speed at high altitude > 30,000ft, rather than at sea level.

However, at high altitude, you need a bigger air intake to scoop up the thin air. The Mig-25 is famous for having a huge air intake, for this high speed, high altitude cruising.

It is about the fuel/air ratio. The thinner the atmosphere, the greater the fuel to maintain the specific ratio that was designed for the jet. Then there comes a point where the engine design cannot cross because that would mean turning into a rocket, which is mostly fuel. This is why boasting about max altitude is worthless. It is very rare that any aircraft will fly to its maximum capable altitude. It is time and fuel consuming.

 

[onebyone]

Watch J-20 of the PLAAF fly in formations during recent military drills.

 

[Silicon0000]

OK! Let's make things simple :

- The total mass of the airflow times the difference between exhaust speed and flight speed equals thrust.

- The high bypass of a turbofan allows better fuel economy at cruising speed and those engines are less noisy.

- The response time of a low bypass is better because it moves less air : "it shifts faster" ( combat essential ).

- Since no big fan to move slow air, less cross-section, vital for drag reduction to go faster easier ( & stealthier ).

- Lower bypass ratio produce less total thrust but more of it per kg of engine A.K.A better thrust to weight ratio.

- Lower bypass ratios perform best at higher speeds; high bypass ratios are optimized for high subsonic cruise.

- To hike thrust, fighters use afterburners ( higher nozzle speed ) and airliners maximize flow with high bypass.


Cores are, as some mates mentioned, often similar since as we showed the difference lies elsewhere.

I.E. GE F-110 to CFM-56 . . .
​

View attachment 415912

Have a great day all, Tay.

I am not an expert so pardon me if i got things wrong, what i understand from your post is that efficency and speed depends on airflow in engine. So by that logic same engine if provided higher airflow will become efficient and when required same engine will provide speed required in a combat only by reducing airflow to engine. Am i got it right?

 

[Taygibay]

^ ^ ^
​

If you don't need high speeds, you maximize airflow and save on fuel.
If you need higher speeds, you maximize exhaust speed, save on size.

Just for fighter engines, you can have high / low specific thrust by design.
With high comes better SFC ( fuel consumption ) in reheat/ bad in dry thrust.
With low comes better SFC dry in cruising mode and in afterburners . . .
however, the reheat boost is modest with high and better with low. The first
( H ) would have a short transit range but more time in combat with ABs
while the other ( L ) would be short on dogfight time but very long ranged.

The idea with variable cycles engines researched for next year's fighters
is to eventually allow both to exist within one engine. We're still waiting!!!

Have a great evening, Tay.

 

[Mitho1980]

Can anyone clarify that which jet is more suitable for PAF needs. J31 or j20?

 

[Cookie Monster]

Can anyone clarify that which jet is more suitable for PAF needs. J31 or j20?

Only PAF can comment on that officially. Others here on this forum can only speculate.

In my opinion J20 would be a better choice. It is built for long range and possibly to use against assets such as AWACS and refueling aircrafts to cripple the enemy's Air Force. If PAF can somehow acquire J20 in decent numbers along with China's wild weasel kind of jets then together with stealth, EW capabilities of the "Wild weasel" jets, and anti-radiation missiles it can suppress and destroy Indian air defense assets. It would give PAF potent ability to strike deep in India in the event of a war.

However China has said that J20 is not for sale. So this leaves PAF the option of only J31 or some other joint project with China or Turkey.

 

[Ultima Thule]

^ ^ ^
​

If you don't need high speeds, you maximize airflow and save on fuel.
If you need higher speeds, you maximize exhaust speed, save on size.

Just for fighter engines, you can have high / low specific thrust by design.
With high comes better SFC ( fuel consumption ) in reheat/ bad in dry thrust.
With low comes better SFC dry in cruising mode and in afterburners . . .
however, the reheat boost is modest with high and better with low. The first
( H ) would have a short transit range but more time in combat with ABs
while the other ( L ) would be short on dogfight time but very long ranged.

The idea with variable cycles engines researched for next year's fighters
is to eventually allow both to exist within one engine. We're still waiting!!!

Have a great evening, Tay.

you forget to mention YF-120 and AL-41 (MIG 1.42) engines
https://en.wikipedia.org/wiki/General_Electric_YF120
https://en.wikipedia.org/wiki/Saturn_AL-41

Same thing will do J-31 with PL-15 or may be with PL-X on external hard points

Only PAF can comment on that officially. Others here on this forum can only speculate.

In my opinion J20 would be a better choice. It is built for long range and possibly to use against assets such as AWACS and refueling aircrafts to cripple the enemy's Air Force. If PAF can somehow acquire J20 in decent numbers along with China's wild weasel kind of jets then together with stealth, EW capabilities of the "Wild weasel" jets, and anti-radiation missiles it can suppress and destroy Indian air defense assets. It would give PAF potent ability to strike deep in India in the event of a war.

However China has said that J20 is not for sale. So this leaves PAF the option of only J31 or some other joint project with China or Turkey.

Same thing will do J-31 with PL-15 or may be with PL-X on external hard points

 

[Mitho1980]

Is j 31 project still active?

 

[Taygibay]

... you forget to mention YF-120 and AL-41 (MIG 1.42) engines

I forgot nothing mate, check :

We're still waiting!!!

The 120 stayed a YF as it lost to the F119 and the
41 stopped at pre-production stage so both are say
attempts.
Sure, the technologies are still being pursued for each
under a different guise, programs or names ( JTDE to
ADVENT / some tech & name to articles 117 ) but, if
we are getting better at it, a true VCE doesn't fly today.

I was alive and adult in 1982 ... when these programs
began, I entered service then. We're still waiting!

Have a great day, Tay.

 

[Daniel808]

One of the 3D Vector Nozzles under Development for an Engine Dedicated to the J-20 Stealth Fighter (WS-15 Engine?)

 

[Asoka]

One of the 3D Vector Nozzles under Development for an Engine Dedicated to the J-20 Stealth Fighter (WS-15 Engine?)

View attachment 416425
View attachment 416426
View attachment 416427

Invented by the Americans, Europeans, and Russians in the 1980s and 90's, the 3-D TVC, sure, is a wonderful development in the history of jet engine. It ranks in the same place as afterburner, and turbo fan in jet engine design, in my opinion.

However, the fact that the Chinese has freely disclosed its own design, shows they don't regard it as a particularly hi-tech or difficult technology to master.

 

[ZeEa5KPul]

It is about the fuel/air ratio. The thinner the atmosphere, the greater the fuel to maintain the specific ratio that was designed for the jet. Then there comes a point where the engine design cannot cross because that would mean turning into a rocket, which is mostly fuel. This is why boasting about max altitude is worthless. It is very rare that any aircraft will fly to its maximum capable altitude. It is time and fuel consuming.

U 'avin a laff, m8? The thinner the air, the lower the quantity of oxygen in a given volume, the less the fuel needed to maintain stoichiometric balance in the combustion chamber of the engine. And rockets are not "mostly fuel," they have as much fuel and oxidizer as they need to maintain -- once again -- stoichiometric balance.

Perhaps you ought to get such basic engineering facts right before you joke about "Chinese physics," or presume to teach anyone here about . . . well . . . anything.

 

[Ultima Thule]

Invented by the Americans, Europeans, and Russians in the 1980s and 90's, the 3-D TVC, sure, is a wonderful development in the history of jet engine. It ranks in the same place as afterburner, and turbo fan in jet engine design, in my opinion.

However, the fact that the Chinese has freely disclosed its own design, shows they don't regard it as a particularly hi-tech or difficult technology to master.

But 3D TVC tech is also very very maintenance prone technology

Is j 31 project still active?

https://defence.pk/pdf/threads/sac-...news-discussions.207796/page-217#post-9739019