J-20 5th Gen Fighter Thread IV (Closed to posting)

[Engineer]

And basicly China will follow that trend too simply by the fact stealth limits performance.

That's not a fact, merely your own fantasy. There is no evidence that stealth impedes aerodynamic performance, whereas current stealth airframes happen to have the best aerodynamic performance. The F-22 can be flown at 60°. Even when one assumes it is a feat of thrust-vectoring, taking 10° off and it would be still an AoA that can't be flown by Flankers. The F-22 can also be flown at Mach 1.8 without afterburners, which is also something not doable with a Flanker even with 117S engines.

Just because Russian engineers aren't able to build an aircraft with stealth and excellent aerodynamics performance at the same time, that doesn't mean others can't do it.

 

[ladioussupp]

Exactly! Current stealth airframes are able to achieve higher AoA than non-stealth airframes with the same configuration. The F-22 in particular is able to fly faster without use of afterburners than any non-stealth aircraft. If there was an actual correlation between stealth and aerodynamic performances, it would be stealth enhances aerodynamics.

The fighter is built around the engine. If there is a non-stealth fighter using F119 or F135 engine, it can be used to compare with F-22/35.

 

[siegecrossbow]

The fighter is built around the engine. If there is a non-stealth fighter using F119 or F135 engine, it can be used to compare with F-22/35.

Not necessarily true. External stores could do nasty things to a fighter's aerodynamics.

 

[MiG-29]

If you continue to make the absolutely ridicoulous statement that these aircraft are hindered aerodynamically/kinemeticallly,

Brat read well what i said, my point was from a physics, Von Karman ogive and the Sears–Haack body have traits that do not necesarily are stealthy, my whole point was stealth ask features that are not always in harmony with aerodynamics, so aircraft find solutions compensating with engines, weapons or avionics.

No aircraft is good for everything, no engineering solution works for everything, you are going to the thesis F-22 and all stealth aircraft have aerodynamics not hampered by stealth, while this might be logic, the reality is not, i mentioned the Sears-Haack body and von Karman as an example, these aerodynamics of minimun drag ask for pencil like shapes basicly sharpened at both ends, stealth ask you for faceted shapes.

Contradiction in requierement ask for different set of solutions.

Now Von karman made his statement upon mathematical formulas and research, in order to generate a stealth airframe something has be to sacrificed and other solutions to be found.

If you think S ducting, weapons bays do not impact in drag or mass then you are not considering factors which in real life are taken into consideration.

My point was MiG 1.44 was designed without faceting into consideration, and when it was revealed, specialists said it was not stealthy at all, so the rumour of plasma stealth was used to say it was as stealthy as F-22.

That big difference made MiG-1.44 different to J-20.


Engineering solutions are like getting married, when a person marries a woman, he comits to her, but he relinquishes to date other women.

In few words, a solution is also a sacrifice, stealth as such needs thrust vectoring, HMS and highly offbored missiles.

A simple visible sacrifice is external weapons, in the latest picture released you have a new system in J-20 to relatively give some degree of stealth and lower drag while the weapon is outside waiting for the missile lock on to cue into the target.

these sacrifice limits payload in stealth aircraft.

A sacrifice means also an advantage, since for example stealth gives lower RCS but the disadvantages do exist too.
Then aircraft designers compromise and find alternative solutions, in F-22 those solutions are no external stores to do not increase even more the drag and TVC nozzles.

 

[ladioussupp]

Not necessarily true. External stores could do nasty things to a fighter's aerodynamics.

I come up with a comparable pair, J-31(31001) and MiG-29 both equipped with RD-93 engines. But it is off-topic. Besides, J-31 is just a prototype now and MiG-29 is a matured/aged fighter basically designed around three decades ago.

 

[vesicles]

Brat read well what i said, my point was from a physics, Von Karman ogive and the Sears–Haack body have traits that do not necesarily are stealthy, my whole point was stealth ask features that are not always in harmony with aerodynamics, so aircraft find solutions compensating with engines, weapons or avionics.

First of all, I don't know anything about aerodynamics. However, as a scientist, I know that there is always multiple solutions to the same problem. The Von Karman ogive and the Sears–Haack body may be ONE excellent example of aerodynamics, but it cannot be the only one.

Additionally and even more importantly, science evolves. What was said in the 1960's and 1970's may not be true any more in 2010's. For crying out loud, Eistein himself refused to believe quantum physics... Back then, Von Karman ogive and the Sears–Haack body might be THE very best human could come up with. But, almost half a century has elapsed. Science has progressed significantly. New solutions may have been found. Instead of holding onto an old theory, you should pay more attention to the facts and actual data collected from these new planes.

Further, in science, if your theory does not agree with actual data, it is your theory that should be modified, not actual data. Theory is only a kind of summary that people come up with to explain data. It is seriously limited by what people know at the time when the theory is derived and the general social and scientific environment at the time. That is how science works. It is NOT gold or divine. No matter how true it is at the time, it IS limited by many factors and will be bound to be modified or proven wrong later. So any time you find a disagreement between your theory and data, you should think twice about your theory, instead of holding onto it and refusing to accept data.

 

[MiG-29]

First of all, I don't know anything about aerodynamics. However, as a scientist, I know that there is always multiple solutions to the same problem. The Von Karman ogive and the Sears–Haack body may be ONE excellent example of aerodynamics, but it cannot be the only one.

Additionally and even more importantly, science evolves. What was said in the 1960's and 1970's may not be true any more in 2010's. For crying out loud, Eistein himself refused to believe quantum physics... Back then, Von Karman ogive and the Sears–Haack body might be THE very best human could come up with. But, almost half a century has elapsed. Science has progressed significantly. New solutions may have been found. Instead of holding onto an old theory, you should pay more attention to the facts and actual data collected from these new planes.

Further, in science, if your theory does not agree with actual data, it is your theory that should be modified, not actual data. Theory is only a kind of summary that people come up with to explain data. It is seriously limited by what people know at the time when the theory is derived and the general social and scientific environment at the time. That is how science works. It is NOT gold or divine. No matter how true it is at the time, it IS limited by many factors and will be bound to be modified or proven wrong later. So any time you find a disagreement between your theory and data, you should think twice about your theory, instead of holding onto it and refusing to accept data.

Von Karman ogive and Sears-Haack body, are proven correct, in fact a typical example is an aircraft external fuel tank, but most aircraft follow these aerodynamic principles, from missiles to submarines, that is the reason i mentioned the Su-27.


Stealth set another paradign in aircraft design, this paradigm can be harmonized to some degree, but with some compromise though.

You are correct, you can bend the rules depending upon what parameters the aircraft has, by bending i mean is a compromise, you can see the F-22 is a compromise of the F-117 faceted concept with the F-15 lines, thanks to better computing power, however it does not mean there has not been compromises.

J-20 has compromises and these have some set of solution sacrificing some traits but gaining in others, these choices were taken upon what the designer bureaux thought balances more the contradictory set of requierements and the reality of physics

 

[kyanges]

.

Everyone already knows that aircraft are designed with compromises. So what's the point of this discussion so far?

 

[Engineer]

The fighter is built around the engine. If there is a non-stealth fighter using F119 or F135 engine, it can be used to compare with F-22/35.

That doesn't invalidate what I said at all. Stealth aircraft still have the best aerodynamics performance when compared to non-stealth aircraft with the same configuration. For example, F-15 has a higher thrust-to-weight ratio, yet the aircraft flies slower and climb slower than the F-22. And even when F-35 is overweight, the aircraft is still able to fly at an AoA that is unmatched by most non-stealth aircraft.

Engine is important, but so is aerodynamic. If you were to fit F119 engines to a Flanker, the aircraft still won't be able to fly at 60° AoA. And if you were to design an aircraft with nothing but the body around the engine, you would end up with a rocket which can't take off horizontally and has almost no maneuverability. Aerodynamics determine the aircraft's potential. Engine is merely a tool to tap into that potential.

 

[Engineer]

Von Karman ogive and Sears-Haack body, are proven correct, in fact a typical example is an aircraft external fuel tank, but most aircraft follow these aerodynamic principles, from missiles to submarines, that is the reason i mentioned the Su-27.


Stealth set another paradign in aircraft design, this paradigm can be harmonized to some degree, but with some compromise though.

You are correct, you can bend the rules depending upon what parameters the aircraft has, by bending i mean is a compromise, you can see the F-22 is a compromise of the F-117 faceted concept with the F-15 lines, thanks to better computing power, however it does not mean there has not been compromises.

J-20 has compromises and these have some set of solution sacrificing some traits but gaining in others, these choices were taken upon what the designer bureaux thought balances more the contradictory set of requierements and the reality of physics

Rubbish. Stealth design does not contradict and in fact make use of Von Kármán and Sears-Haack body. Von Kármán never gave a restriction or claimed a nose-cone must be axis-symmetrical (have no facet). In fact, a non-circular fuselage cross section necessitates the contour between the tip of the nose and the body to vary to give the best aerodynamic performance, resulting in a chine nose that we see on F-22 and J-20. Likewise, Sears-Hacck body does not mean a fuselage cannot be stealthy. All the Sears-Hacck body does is describe the variation of cross section from the nose to the tail. Cross section does not care about the shape and doesn't have to be circular, so a stealth airframe can perfectly satisfy a Sears-Hacck body.

Compromises occur when the designers cannot satisfy two requirements in a harmonized fashion. The F-22 and J-20 are both able to achieve stealth as well as aerodynamics performance. Sukhoi is not as capable, so was forced to compromise stealth for aerodynamics. So, taking compromise between aerodynamics or stealth is only applicable to Russia, not to US or China.