J-21/J-31 Chinese 5th Generation Stealth Fighter

[MiG-29SMT]

Thank you for polluting this thread with meaningless technical mumble jumble and white racist lies about bad Chinese "genociding" Uighurs and explanations that as a partially white person it is okay for Mongolic native Americans to be genocided from Mexico.

The larger the engine the more thrust is generated as well as the more fuel is consumed though at lower thrust specific fuel consumption TSFC.

F135 will be easier to make it have higher efficiency than RD-93

Trent engine is the most powerful engine ever.

Checkmate uses same logic, a larger engine allows a more efficient engine in terms of SFC thus use only one type 30 instead 2 RD-93s

 

[MiG-29SMT]

The larger the engine the more thrust is generated as well as the more fuel is consumed though at lower thrust specific fuel consumption TSFC


in Few words F-35 has a better TSFC in the F135 than RD-93, add J-31 has 2 engines thus the J-31 needs higher fuel burn to achieve same performance

thus J-20 is more efficient than J-31

but still J-20 is underpowered

 

[pipebomb]

there is one thing China can not compete with, it is the main concept.

Su-75 is one engined, F-35, has one engine, why?

First is producibility and price, single engine fighters are cheaper to make.

Another thing is Su-57 and Su-75 use the same engine, same is basically F-22 and F-35 since f-135 is an evolution of f-119.

the smaller engine relies more in fuel why?


Propulsive Energy is the Key
The theory put into practice with turbofans is something called propulsive efficiency. It is much more efficient to move a large volume of air at relatively slower speeds than to move a small volume of air at higher speeds.
In flight, the fan blades spin at around 3,000 RPM. Any higher and the fan tips start to run supersonically, making a huge amount of noise in the form of a piercing drone. In contrast, the low pressure shaft spins at 12,000 RPM and the high-pressure shaft at around 20,000 RPM. So, how do you slow down this rotation — going from a high RPM at the back of the engine to a lower RPM at the front?

"The Most Fascinating Machines": How Jet Engines Work - The Points Guy

The numbers are staggering, the technology is almost incredible -- and yet they power the safest form of transportation.

thepointsguy.com

. On a jet, an inch of thrust lever movement at a low rpm may be worth only 200 pounds of thrust, but at a high rpm that same inch of movement might amount to closer to 2,000 pounds

https://www.faa.gov/regulations_pol...ation/airplane_handbook/media/17_afh_ch15.pdf

. As rpm increases, mass flow, temperature, and efficiency also increase. Therefore, much more thrust is produced per increment of throttle movement near the top of the range than near the bottom.

https://www.flightliteracy.com/operating-the-jet-engine-part-two/

The basic thrust equation indicates that as forward speed Va increases it is necessary to increase either the mass flow, or exit velocity Vex , or both, in order to hold the thrust, F, constant.

In the supersonic region afterburner equipped engines enable large increase in thrust with Mach number

https://nptel.ac.in/content/storage2/courses/101101002/downloads/Lect-03.pdf

.

basically is you increase the temperature or the air mass flow, basically a bigger blade or engine diameter, smaller blades need higher RPM or more fuel burn so this increases the need for better materials.

Though i get to your point on single engine fighters, what is your opinion on the logic behind china (j-10 to j-35) & France's ( m2000 to Rafale) moved toward twin engine aircrafts.

 

[MiG-29SMT]

Though i get to your point on single engine fighters, what is your opinion on the logic behind china (j-10 to j-35) & France's ( m2000 to Rafale) moved toward twin engine aircrafts.

Is not if it is single engine or twin engine, the factor is the size of the fan blades, a bigger fan has a larger air mass flow rate, thus the thrust generated will be less dependent upon the burning of fuel.

So the larger F135 or F119 will be easier to have a lower SFC thus it is easier in terms of metallurgy and fuel than smaller engines.

Rafale supercruises with M88 because Rafale is light, the problem of J-31 is it is heavier than MiG-29, Rafale or Eurofighter, thus engines like EJ-200, M88 or RD-93 are not ideal for a larger machine in terms of weight.

F-35 has a larger diameter engine, it has a larger fan than RD-93 or EJ-200, so it is similar to a big engine like Trent 1000 it has larger fan blades so it will have more thrust generated by the fans rather than the combustion chamber, it allows a larger air mass flow rate at the intake lips.

the larger the diameter, the less fuel you need to achieve similar thrust.

in airliners you have more powerful engines than fighters, but they have a higher bypass ratio.

Rafale uses 2 engines because relatively it is a very light aircraft, so 2 tiny engines allow high performance.

J-31 despite what our Chinese posters claim, went for a twin engined aircraft because they lack an ideal engine like F135, the Turkish, the korean an Chinese fighter (J-35) are limited by engine, but not F-22 or T-50/Su-57 that have truly powerful engines, so they can use larger engines that are more economical.

To compare Rafale to J-31 you have to consider Rafale is light less than 10,000 kg empty weight, MiG-29 11,000kg empty weight, F-35 weighs 13,200kg at empty weight, J-31 is around 13500kg-14500kg at empty weight but it can not fit larger Al-41s it uses smaller RD-93, the J-31 will have it harder to get a better engine than its cousin J-20, the Chinese engineers knew that so J-20 was chosen, but J-20 is too heavy for naval use weighs more than F-14, so they had no other option than use J-31.

 

[rockdog]

Is not if it is single engine or twin engine, the factor is the size of the fan blades, a bigger fan has a larger air mass flow rate, thus the thrust generated will be less dependent upon the burning of fuel.

So the larger F135 or F119 will be easier to have a lower SFC thus it is easier in terms of metallurgy and fuel than smaller engines.

Rafale supercruises with M88 because Rafale is light, the problem of J-31 is it is heavier than MiG-29, Rafale or Eurofighter, thus engines like EJ-200, M88 or RD-93 are not ideal for a larger machine in terms of weight.

F-35 has a larger diameter engine, it has a larger fan than RD-93 or EJ-200, so it is similar to a big engine like Trent 1000 it has larger fan blades so it will have more thrust generated by the fans rather than the combustion chamber, it allows a larger air mass flow rate at the intake lips.

the larger the diameter, the less fuel you need to achieve similar thrust.

View attachment 123388

in airliners you have more powerful engines than fighters, but they have a higher bypass ratio.

Rafale uses 2 engines because relatively it is a very light aircraft, so 2 tiny engines allow high performance.

J-31 despite what our Chinese posters claim, went for a twin engined aircraft because they lack an ideal engine like F135, the Turkish, the korean an Chinese fighter (J-35) are limited by engine, but not F-22 or T-50/Su-57 that have truly powerful engines, so they can use larger engines that are more economical.

To compare Rafale to J-31 you have to consider Rafale is light less than 10,000 kg empty weight, MiG-29 11,000kg empty weight, F-35 weighs 13,200kg at empty weight, J-31 is around 13500kg-14500kg at empty weight but it can not fit larger Al-41s it uses smaller RD-93, the J-31 will have it harder to get a better engine than its cousin J-20, the Chinese engineers knew that so J-20 was chosen, but J-20 is too heavy for naval use weighs more than F-14, so they had no other option than use J-31.

Nobody really knows the weight of latest version of J31, you just assume it's big so it's heavy. But the first FC31 took off 5 yrs ago, there were lots of improvement.

Thanks to world's biggest manufacture capability, the composition material and 3D printing are reported applied on J20 and J31. Plus the weight of T/R components of AESA, are remarkblely reduce, there are lots of domestic articles talked about it.

This is the article said the 3D print tech is already applied to J20, J31:

It says reduced the 38% weight of traditional structure. I think i told you in J20 thread.

And you don't want to hear, you don't want to learn, you still stick to your old knowledge, i can't force you to accept that.

 

[MiG-29SMT]

Nobody really knows the weight of latest version of J31, you just assume it's big so it's heavy. But the first FC31 took off 5 yrs ago, there were lots of improvement.

Thanks to world's biggest manufacture capability, the composition material and 3D printing are reported applied on J20 and J31. Plus the weight of T/R components of AESA, are remarkblely reduce, there are lots of domestic articles talked about it.

This is the article said the 3D print tech is already applied to J20, J31:

View attachment 123398


View attachment 123399

View attachment 123401


It says reduced the 38% weight of traditional structure. I think i told you in J20 thread.

And you don't want to hear, you don't want to learn, you still stick to your old knowledge, i can't force you to accept that.

The reduction is not on all the fuselage, it is in some parts, and the materials continue using titanium, aluminium or carbon composite in general terms, in few words

3D printing sees Airbus reduce material weight by 15%

3D printing sees Airbus reduce material weight by 15%

Looking for a quick solution to produce spacer panels for cabin retrofits, Airbus found a new partner in Materialise’s Certified Additive Manufacturing.

www.themanufacturer.com

While in general there are reduction in weight, stealth aircraft carry more fuel and weapons internally and the reduction is not as dramatically as you think

Eric Masanet, the Morris E Fine junior professor at Northwestern University in the Robert R McCormick School of Engineering and Applied Science, now says it’s been confirmed that using 3D printed metal parts can reduce the weight of an aircraft – by up to 7% – and those are breakthrough numbers.
https://3dprint.com/71279/3d-print-aircraft-weight/


I know you like to exaggerate as typical Chinese forum poster, but if you see in general 7% translate in a stealth aircraft in few hundred kilograms.

see it says – by up to 7% – and those are breakthrough numbers.


Now additive technology is not a Chinese invention nor only a used in China, but Stealth brings increased cross section


But the FC-31 has its own weight problems. Its empty weight is believed to be as high as 38,000 pounds. This affects its maximum takeoff weight, which is estimated to be 56,000 pounds.

Higher takeoff weights require bigger engines and more fuel, reducing the amount of armaments an aircraft can carry.
By comparison, the F-35C has an empty weight of 34,800 pounds and a maximum takeoff weight of 70,000 pounds.

China's first stealth fighter for aircraft carriers is emerging, but a big problem still weighs it down

A model jet spotted on a fake aircraft carrier is the latest hint about what China's navy might have planned for its carrier air wings.

www.businessinsider.com

what you fail to realize is the bulkhead used on an F-16 are much lighter than on F-35 just by size
see both aircraft cross section

I will say it simple does not matter how much 3D printing you have on F-35 or J-31, the aircraft are much bigger than a Mirage 2000 or F-16 and the worst they carry little weapons, F-16 is much lighter than F-35 and F-35 does not carry much more weapons, now J-31 is a brick does not matter how much you present it

The picture shows F-35 is a wide as a F-15 and the F-16 is much slimmer than F-35, stealth aircraft always weight more simple because they carry bigger fuselages.

same is Rafale is thinner and much more aerodynamic, J-31 is much fatter than F-35

You still do not understand F-22 is a brick too but it is a brick with powerful engines, J-31 is a brick without powerful engines

 

[rockdog]

But the FC-31 has its own weight problems. Its empty weight is believed to be as high as 38,000 pounds. This affects its maximum takeoff weight, which is estimated to be 56,000 pounds.

Higher takeoff weights require bigger engines and more fuel, reducing the amount of armaments an aircraft can carry.
By comparison, the F-35C has an empty weight of 34,800 pounds and a maximum takeoff weight of 70,000 pounds.

China's first stealth fighter for aircraft carriers is emerging, but a big problem still weighs it down

A model jet spotted on a fake aircraft carrier is the latest hint about what China's navy might have planned for its carrier air wings.

www.businessinsider.com

Intresting, so you buy Business Insider's that article,

But never seen you bought this one:

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 may not lead the stealth-fighter pack, but it doesn't need to in order to pose a threat.

www.businessinsider.com

As i said again, the jet and engine are parallel projects, it's not fully powered dosen't mean it will never.
But the platform is always important, just like F15A and F15E/F

 

[MiG-29SMT]

Nobody really knows the weight of latest version of J31, you just assume it's big so it's heavy. But the first FC31 took off 5 yrs ago, there were lots of improvement.

Thanks to world's biggest manufacture capability, the composition material and 3D printing are reported applied on J20 and J31. Plus the weight of T/R components of AESA, are remarkblely reduce, there are lots of domestic articles talked about it.

This is the article said the 3D print tech is already applied to J20, J31:

View attachment 123398


View attachment 123399

View attachment 123401


It says reduced the 38% weight of traditional structure. I think i told you in J20 thread.

And you don't want to hear, you don't want to learn, you still stick to your old knowledge, i can't force you to accept that.

see

compared to the MiG-29; the F-35 and other stealth fighters have more mass due to internal structures and bigger and heavier bulkheads

Your fantasy is to think the structure of a 4th Generation fighter is similar to the larger and more massive aircraft like F-22 or J-20.

does not matter how much you fantasize, F-16 has smaller bulkheads and even without 3D technology are lighter than those of F-35

 

[MiG-29SMT]

Intresting, so you buy Business Insider's that article,

But never seen you bought this one:

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 may not lead the stealth-fighter pack, but it doesn't need to in order to pose a threat.

www.businessinsider.com

As i said again, the jet and engine are parallel projects, it's not fully powered dosen't mean it will never.
But the platform is always important, just like F15A and F15E/F

You fantasize because you think a stealth fighter structurally is smaller or equal to a 4th generation, first stealth aircraft have S ducts increasing the internal area (remember the shortest distance between 2 points is a line), the have to include internal weapons bays, the proof is F-35 weighs almost 2 times the weight of an F-16 and the F-22 is 50% heavier than an F-15

I will put it simple for you, MiG-29 weighs less than F-35 even using an early MiG-29A the bulkhead size of MiG-29 are smaller and you can see it in that picture only the LEX and forebody have relatively big bulkhead but those provide ample lift

The Korean fighter is easy comparable to J-31 see its huge bulkhead structure

even considering it has all the advances it supposes to have is heavier than Rafale or F-18A

 

[MiG-29SMT]

Intresting, so you buy Business Insider's that article,

But never seen you bought this one:

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 can't compete with the US's and China's 5th-gen fighter jets, but it doesn't need to

Russia's Su-57 may not lead the stealth-fighter pack, but it doesn't need to in order to pose a threat.

www.businessinsider.com

As i said again, the jet and engine are parallel projects, it's not fully powered dosen't mean it will never.
But the platform is always important, just like F15A and F15E/F

I will tell you how the USA limited the korean fighter

It carries weapons externally, if they use the space to give it internal weapons bays, fuel fraction will be reduced, since it has F414 well the engines are not very powerful for a stealth fighters.

Why? simple at dry power barely will exceed the empty weight, to achieve higher performance needs bigger size for extra fuel and internal weapons bay, in few words like J-31, the Korean fighter in under powered

 

[pipebomb]

Is not if it is single engine or twin engine, the factor is the size of the fan blades, a bigger fan has a larger air mass flow rate, thus the thrust generated will be less dependent upon the burning of fuel.

So the larger F135 or F119 will be easier to have a lower SFC thus it is easier in terms of metallurgy and fuel than smaller engines.

Rafale supercruises with M88 because Rafale is light, the problem of J-31 is it is heavier than MiG-29, Rafale or Eurofighter, thus engines like EJ-200, M88 or RD-93 are not ideal for a larger machine in terms of weight.

F-35 has a larger diameter engine, it has a larger fan than RD-93 or EJ-200, so it is similar to a big engine like Trent 1000 it has larger fan blades so it will have more thrust generated by the fans rather than the combustion chamber, it allows a larger air mass flow rate at the intake lips.

the larger the diameter, the less fuel you need to achieve similar thrust.

View attachment 123388

in airliners you have more powerful engines than fighters, but they have a higher bypass ratio.

Rafale uses 2 engines because relatively it is a very light aircraft, so 2 tiny engines allow high performance.

J-31 despite what our Chinese posters claim, went for a twin engined aircraft because they lack an ideal engine like F135, the Turkish, the korean an Chinese fighter (J-35) are limited by engine, but not F-22 or T-50/Su-57 that have truly powerful engines, so they can use larger engines that are more economical.

To compare Rafale to J-31 you have to consider Rafale is light less than 10,000 kg empty weight, MiG-29 11,000kg empty weight, F-35 weighs 13,200kg at empty weight, J-31 is around 13500kg-14500kg at empty weight but it can not fit larger Al-41s it uses smaller RD-93, the J-31 will have it harder to get a better engine than its cousin J-20, the Chinese engineers knew that so J-20 was chosen, but J-20 is too heavy for naval use weighs more than F-14, so they had no other option than use J-31.

Thanks for your reply but i think you misunderstood my point. Ofcourse bigger engine will have higher mass flow for a given sfc. And twin engine aircraft will have a weight penalty for achieving same thrust. I am not disputing this in anyway. I was wondering why would both countries specially france go this route(twin engine). My guess would be china is not confident enough to pull up a single engined manned naval fighter (which is odd). However i have no idea what are the reasons for France since they had already built and operated land and naval single engine fighters.

I have somewhat similar views on single engined fighter as well

Now I have been cooking some 'khayali pulao' which i would like serve here at DFI, interested members can indulge themselves.

If India has ambition to become a future global super power in next 3-4 decades single engine AMCA makes a ton of strategic sence and also some operational.

First let me shortlist operational gains :

i} Level of difficulty of designing and manufacturing a low bypass military jet engine increases with reduction in size. Simply because of tighter margins. Hence a bigger(f119 or izd30) single engine AMCA makes more sense.

ii} The financial input required for design & development of a smaller engine & larger one is more or less the same.

iii} Single engine aircraft has significantly lower operating and lifecycle cost.

iv} Single engine AMCA would also have higher export potential.

Now lets look at numerous strategic benefits

1] A larger engine would mean IAF can build their own heavy air superiority fighter like f-15, su-30, su-57, j-20 etc.

2] IAF could design a long range stealth bomber with four similar engines.

3] There could be a high bypass variant of this engine which would enable us to build c17 & c2 equivalent aircraft transport fleet.(then we will only need 1(c27j or il112v) instead of 4 types of foreign aircrafts in IAF fleet as we already have do228)

4] We could design Wide body civil/military aircraft(for eg a330mrtt, kc46 or an awacs)

These are all super power capabilities only in possession of us, former Soviet union & future china will possess . IAF with indigenous heavy fighter, home grown tranport fleet & a long range stealth bomber, home grown tanker and awacs fleet will secure our seat to the high table.

Also what this engine could do for our civil airline sector, to our manufacturing sector, our foreign exchange reserves.

This is the GOLDEN ticket to a truly atmanirbhar bharat.

 

[MiG-29SMT]

Thanks for your reply but i think you misunderstood my point. Ofcourse bigger engine will have higher mass flow for a given sfc. And twin engine aircraft will have a weight penalty for achieving same thrust. I am not disputing this in anyway. I was wondering why would both countries specially france go this route(twin engine). My guess would be china is not confident enough to pull up a single engined manned naval fighter (which is odd). However i have no idea what are the reasons for France since they had already built and operated land and naval single engine fighters.

I have somewhat similar views on single engined fighter as well

Single engine fighters always are easier to keep the most aerodynamic shape

the most aerodynamic shape is the sears-haack body

as you have said a single engined fighter will adapt more easy to the sears-haack body, a good example is
F-104 or F-16

Even LCA or F-35 follow that basic aerodynamic guide,

The A-11 followed the sears haack body too, however for stealth needs a round shaped cross section has the problem of enhanced creeping way

So for the sake of stealth SR-71 added Chines

the SR-71 has Chines also because they are wing LEX, so they are generating lift and that lift is ahead of the center of gravity, so when flying at supersonic speeds the center of lift will move backwards less

Lex also improve high AoA due to vortices

why I am telling you this well to understand stealth you have to understand its pros and cons

 

[MiG-29SMT]

Thanks for your reply but i think you misunderstood my point. Ofcourse bigger engine will have higher mass flow for a given sfc. And twin engine aircraft will have a weight penalty for achieving same thrust. I am not disputing this in anyway. I was wondering why would both countries specially france go this route(twin engine). My guess would be china is not confident enough to pull up a single engined manned naval fighter (which is odd). However i have no idea what are the reasons for France since they had already built and operated land and naval single engine fighters.

I have somewhat similar views on single engined fighter as well

now after we spoke of aerodynamics let us see J-31

It is a Brick, a better design is YF-23

If you look at YF-23, you discover it is highly influenced by SR-71, it has Chines, for the same reason I mean it is for stealth and the Chines also work like wing LEX as on F-16 or Su-27, It has a better Sears-Haack body resemblance

The presence of a non-zero trailing-edge sweep angle at diamond wing shapes is mainly caused by low-observability requirements for UAV configurations. As a result of the high leading-edge sweep angle, though, the aerodynamic characteristics of diamond wing planforms are quite similar to delta wing configurations. Aerodynamic potential and high maneuverability known from delta wing configurations are of particular interest and therefore generally maintained for diamond wing shape

http://mediatum.ub.tum.de/doc/1127549/868611.pdf

One of the advantages of diamond shaped wing is the center of lift is farther ahead than those of a true delta wing, so YF-23 has LEX for aerodynamic reasons and stealth.

So to answer Why they chose J-31 is mostly weight.

The F-14 has a high aspect ratio wing at landing, this improves lift, J-20 is too heavy, it has a low aspect wing, to bring it into the carrier means high landing speeds.

CL means coefficient of lift so you can see at high aspect ratio you can land on a carrier al lower AoA and with higher lift.

So J-31 has a less swept wing, so converting a lighter less heavy fighter was easier.


Now consider the engine issue, Rafale is light it weighs less than 9900 kg empty weight, but J-31 at least is in the range of 13000-14500 kg empty weight.

J-20 offers an engine that can adapt easier to higher air mass flow rates and lower SFC, however J-20 is not good for landing on a carrier, basically China has no other option.

true a twin engined is better for Survivability if one engine fail at sea, so Rafale is twin engined, but J-31 has lower TWR, so it is not good

 

[MiG-29SMT]

Thanks for your reply but i think you misunderstood my point. Ofcourse bigger engine will have higher mass flow for a given sfc. And twin engine aircraft will have a weight penalty for achieving same thrust. I am not disputing this in anyway. I was wondering why would both countries specially france go this route(twin engine). My guess would be china is not confident enough to pull up a single engined manned naval fighter (which is odd). However i have no idea what are the reasons for France since they had already built and operated land and naval single engine fighters.

I have somewhat similar views on single engined fighter as well

Johnson subsequently proposed the A-12 with the J58 engines in a mid-wing arrangement to reduce the airplane’s side profile. Chines along the forebody reduced fuselage sloping while providing additional lift and stability. The single vertical stabilizer was replaced with two all-moving vertical fins, one on top of each engine nacelle. These were canted inward for further RCS reduction. Serrations on the wing edges incorporated radar-absorbent materials. Johnson noted in his project diary, “This airplane weighs about 110,000 to 115,000 pounds and, by being optimistic on fuel consumption and drag, can do a pretty good mission.”10
https://ntrs.nasa.gov/api/citations/20090007797/downloads/20090007797.pdf

The addition of fuselage chines to the “Blackbird” family’s circular forebody transformed its cross-section into a “stealthy” twodimensional “flying saucer,” a discovery made independently by both Frank Rodgers and Ed Lovick. (Paul F. Crickmore)

“LD” MacDonald and Ed Lovick advised Dick Fuller how best to modify the A-11 design into the reduced RCS A-12. This included using chines, suggested by Frank Rodgers during the A-4 series of designs. Once these were applied a cross-section of the fuselage forebody resembled a two-dimensional flying saucer! The wings were blended into both the fuselage and engine nacelles, whilst the twin tails were made of composite material and also canted inboard at 15 degrees. Other more subtle RCS reduction innovations included incorporating Ed Lovick’s serrations that were applied to the wing leading and trailing edges as well as to the chines, which were then filled with dielectric material, as explained earlier. The A-11’s rectangular inlets were replaced with round, axisymmetric units; and the two translating spikes, used to regulate mass airflow into each inlet, not only helped to shield the front of the compressor face from incident radar energy, but also were covered in RAM.

http://pentagonus.ru/_ld/22/2227_OAV12_Lockheed_.pdf

compared to YF-23 or SR-71, J-31 is far much boxier that in order to reduce drag and enhance area rule the nozzles and engine nacelles were left rounded increasing creeping wave reflections

F-22 can use large booms with sloped sides to increase stealth

 

[MiG-29SMT]

The F-14 features engines in separate nacelles, set well apart from each other so that damage to one of them will have minimal effect on the other. The main central and rear area of the fuselage consists of two separate engine nacelles joined together by a shallow flat area known as a pancake”. At the extreme rear of the aircraft, this pancake is little more than a decking between the engine pods. This leaves a deep tunnel between the engines which imposes a small drag penalty. However, it adds to overall lift, gives an extra attachment area for weapons pylons, and provides some additional fuselage space for fuel and equipment. The rear part of the broad between- engines pancake is gently curved upwards to reduce both the supersonic trim drag and the negative zero-lift supersonic pitching moment.

Read more https://aviatia.net/f-14-tomcat-fuselage/
Many military aircraft have twin engines buried in the rear of the fuselage. This combination can lead to relatively high drag for the afterbody portion of the aircraft. This results from the generally lower afterbody fineness ratio compared with the forebody 4 fineness ratio, the steep closure slopes required to fair in the nozzles, and the greater proportional wetted area of the empennage region. In addition, the rear portion of the aircraft with control surfaces, appendages, etc., is a region of high mutual interferences between surfaces immersed in a complex flow field.

https://core.ac.uk/download/pdf/42897035.pdf

The addition of booms to the basic afterbody results in increases in the total afterbody drag for all configurations.

https://core.ac.uk/download/pdf/42897035.pdf

On the J-31, you have 2 engines quite close, with a wedge fuselage extension for stealth purposes, on F-14 and Su-57, the pancake adds lift; on F-14 the swept wing when it was swept at 68 deg, off set the extra drag it generated, but on F-15 as on J-31 the lesser swept wing angle forced engines closer to each other to keep area rule needs.

The two usually used are the S-bend which has a lower engine location and uses the engine exhaust to replace part of the fuselage boattail (saves drag) but has more inlet loss, a distortion risk, a drag from fairing out the inlet,

Engine Placement

 

[MiG-29SMT]

One of the largest contributions to airframe RCS occurs any time a relatively flat surface of the aircraft is perpendicular to the incoming radar beam.  To prevent these RCS "spikes," the designer may slope the fuselage sides, angle the vertical tails, and so on,  so that there are no flat surfaces presented towards the radar

If the aircraft is primarily designed for low detectability by a nose-on threat radar, the wings and tails can be highly swept to reduce their contribution to RCS.

https://ae.ieu.edu.tr/documents/ae_405.pdf?fbclid=IwAR13iipYd-q9QmeHXPPav9zDOSiIJsob-xuEDCCp1mhQQvVeIyPkNmi0JiI

Trapezoidal wings • In trapezoidal wings, the leading edge sweeps back but the trailing edge sweeps forward. • Some Delta wings also feature a slight sweep forward angle at the trailing edge. • Advantages • Better performance than Delta wings at transonic speeds and during transition to and from supersonic conditions. • They do not require upwards deflection of the trailing edge control surfaces for pitch control and therefore do not lose lift. • Their centre of lift lies further forward and therefore the tail must not lie too far back on the fuselage. • They are preferred for stealth applications. • Flaps can be easily used. • Disadvantages: • High wing loading. • They stall at much lower angles of attack than Delta wings. • This problem is overcome using Leading Edge (Root) eXtensions (LEX or LERX) and/or canards.

http://www.ltas-cm3.ulg.ac.be/AERO0023-1/ConceptionAeroFighter.pdf

 

[SexyChineseLady]

J-31/35 Computer Generated Graphics by Chinese fans! Look pretty accurate.

 

[MiG-29SMT]

 

[HariPrasad-1]

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The larger the engine the more thrust is generated as well as the more fuel is consumed though at lower thrust specific fuel consumption TSFC


in Few words F-35 has a better TSFC in the F135 than RD-93, add J-31 has 2 engines thus the J-31 needs higher fuel burn to achieve same performance

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thus J-20 is more efficient than J-31

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but still J-20 is underpowered

It is not even as good as a decent 4th generation fighter. This is the reason why china buys Su35 inspite of having so called 5th generation plane. If it is pitted against Rafale or even super sukhoi, it will have zero score against rafale even after losing half a dozen so called 5th generation plane lost.

 

[Super Flanker]

It is not even as good as a decent 4th generation fighter. This is the reason why china buys Su35 inspite of having so called 5th generation plane. If it is pitted against Rafale or even super sukhoi, it will have zero score against rafale even after losing half a dozen so called 5th generation plane lost.

Is it about China Buying SU-35 when China has 5th generation J-20? That logic is so flawed. If we go by this logic than we can also say that the F-35 is is not so Advanced and Hence US Airforce is buying more F-15EX.

You see mate, you can't always Operate 5th Generation Aircrafts in large numbers because it's very expensive in doing so and hence you also need 4th Generation Aircrafts to supplement and build up a bulk of your fleet using them. Also there were several other reasons for China buying these SU-35 Flankers from Russia.