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

[Engineer]

Sears-haack bodies are bodies of revolution, if you understand that, then you can see why aircraft without stealth requirements have round or oval cross sections in example F-18, Concorde, Su-27 and so on.

Stealth requieres other types of non bodies of revolutions, J-20 like F-22 needs economical engines, in few words no external clutter like external stores or weapons and high thrust at dry power.

You see, you are either misinformed or you are trying to misinform. A Sears-Haack body is never about cross sections. It is about cross sectional area distribution. Sears-Haack body is simply a 3D plot of cross sectional area along the aircraft's longitudinal axis, similar to the Whitcomb's Area Rule. Any body having the same cross sectional area distribution as a Sears-Haack body has the same wave drag as a Sears-Haack body. The body can have a star shaped cross section and it will not matter. There is absolutely no requirement that the actual shape of an aircraft's cross section has to be round.

Aircraft have round cross section because it is structurally simple and more efficient. It has nothing to do with Sears-Haack body. And ironically, by your very own statement, the Su-27 does not have a Sears-Haack body. A Sears-Haack body is perfectly circular, and does not have extrusions such as wing or engine pod. Su-27 does not match any of the criteria.

Can J-20 supercruise with Al-31? maybe, do i think it does? no i do not, concorde also uses afterburner from transonic speeds to Mach 1.7, but here lies a secret you have not think, variable geometry intakes and speed actually add thrust as air is compressed like in SR-71.

The Concorde can accelerate to Mach 1.7 without engaging afterburner at all but just chose not to operationally do so, so your reference to the use of afterburner is absolutely pointless. Furthermore, all supersonic intakes add "thrust". It is not a unique trait of variable-geometry inlet. Having a variable-geometry inlet does not guarantee supercruise. Early Tu-144 cannot supercruise, and many of today's fighters cannot supercruise despite having variable-geometry inlet.

J-20 has a fixed intake and only a body of revolution on the jet nozzles, the main cross section is diamond shaped or semi-trapezoidal and has no Von karman ogive radome.

J-20 does not have a fixed intake. What J-20 has is called Diverterless Supersonic Inlet. A Von Kármán nosecone merely define the lateral geometry of the nose cone. Unlike your claim, it is never about the cross sectional shape.

F-35 has been modifided not because they do not know stealth requieres flat surfaces, but because that new shape is more aerodynamic

Not really. The F-35 exterior is modified because there is not enough internal space to fit certain systems.

F-22 has also engines that have higher thrust at dry power than Al-31 ad 4 tonnes more of thrust than Al-31.

Does it supercruise? who knows but in reallity is highly unlikely it does with Al-31.

As the J-20 is designed to supercruise, the aircraft will have less supersonic drag than an aircraft that is not designed to supercruise. So, the Flanker being unable to supercruise with Al-31 engines does not imply the J-20 cannot do such thing.

 

[Engineer]

Sears-haack bodies are bodies of revolution, you have not got me in any contradiction.

Basicly a AIM-120 is a body of revolution, a better example is an aircraft external fuel tank or Ohio class Submarine.

Su-27 like any fighter that had little or none stealth treatment has those shapes.

You can see that in the fact Su-27 main fuselage is a boulbous ogive and has a tail sting ending like a ogive, plus to podded engine nozzles with similar shape.

J-20 has other requierements, basicly flat platforms to reflect radar signatures at some specific angles.
F-35, T-50, F-22, F-117, J-20, J-31 and B-2 are huge radar mirrors.

F-22 needs a very powerful engine at dry power and no external clutter such as external stores.
The F119 needs extremely high dry power to allow F-22 to supercruise.

J-20 needs something better than Al-31s for sure, is Su-27 a perfect body of revolution of course not, but is an aircraft based upon such shape.

J-20 is based upon the pyramid shape with a diamond cross section to reduce RCS at different angles.

Nope. You are repeating the same flawed concept. Sears-Haack is about distribution, not cross sectional shape.

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is a good source explaining the purpose of Sears-Haack body, and you can see that shape is never part of the consideration.

Whitcomb's findings are related to a more theoretical concept called the Sears-Haack body. This shape yields the lowest possible wave drag for a given length and volume. The variation in cross-sectional area for a Sears-Haack body, illustrated in the following figure, tells us that wave drag is minimized when the curvature of the volume distribution is minimized. The closer the volume distribution of an aircraft or other high-speed vehicle comes to the ideal Sears-Haack body, the lower its wave drag will be.

Su-27 is an aircraft designed for aerodynamics, F-22 and J-20 are for stealth with some trade offs between aerodynamics and stealth, as such they need good engines.

Trade-off implies taking from one parameter to add to another. The fact that F-22 excels in aerodynamics performance in addition to possessing stealth doesn't sound like a trade-off. A more suitable description would be "best of both worlds".

WS-15 will be needed, Al-31 is basicly a test engine to test the aircraft basic concept, in the same way EAP did for Eurofighter.

Of course WS-15 will be needed for J-20 to fully meet design goals. However, that does not exclude the possibility of J-20 supercruising with AL-31 engines. Supercruise means cruising at supersonic speed, and J-20 merely has to fly at Mach 1.1 without afterburner to satisfy that criteria.

 

[MiG-29]

Nope. You are repeating the same flawed concept. Sears-Haack is about distribution, not cross sectional shape.

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is a good source explaining the purpose of Sears-Haack body, and you can see that shape is never part of the consideration.




Trade-off implies taking from one parameter to add to another. The fact that F-22 excels in aerodynamics performance in addition to possessing stealth doesn't sound like a trade-off. A more suitable description would be "best of both worlds".


Of course WS-15 will be needed for J-20 to fully meet design goals. However, that does not exclude the possibility of J-20 supercruising with AL-31 engines. Supercruise means cruising at supersonic speed, and J-20 merely has to fly at Mach 1.1 without afterburner to satisfy that criteria.

the Sears-Haack equation has an R that makes it a body of revolution when you understand that you will understand what is the sears haack body, the symetry of the round cross section is why it is in ICBMs bullets and aircraft

 

[latenlazy]

the Sears-Haack equation has an R that makes it a body of revolution when you understand that you will understand what is the sears haack body, the symetry of the round cross section is why it is in ICBMs bullets and aircraft

*headdesk* How many times does it need to be said. The shape itself doesn't matter, it's the distribution of cross sectional area. Here just read the damn wiki. It's not that hard to get, they even draw a picture demonstrating what they mean.

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I'll even quote the darn diagram.

"Cross-sectional area distribution along the body determines wave drag, largely independent of the actual shape. Although not congruent, the blue and green shapes are roughly equal in area, being equidistant from the longitudinal centre."

 

[Engineer]

the Sears-Haack equation has an R that makes it a body of revolution when you understand that you will understand what is the sears haack body, the symetry of the round cross section is why it is in ICBMs bullets and aircraft

Nope. You do not fully understand what a Sears-Haack body is for. The left hand side of the Sears-Haack equation is area A(x), which is independent of shape. A circle can have the same area as a square, a pentagon, or an irregular object. It does not matter. In short, the equation only defines the area as a function of longitudinal position, not shape.

Round body is used on missiles and aircraft because it is cheaper and easier to make. That has nothing to do with Sears-Haack body. Any object having the same cross sectional area variation as a Sears-Haack body has the same wave drag.

The area rule says that an airplane designed with the same cross-sectional area distribution in the longitudinal direction as the Sears-Haack body generates the same wave drag as this body, largely independent of the actual shape.

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[MiG-29]

Nope. You do not fully understand what a Sears-Haack body is for.

The cross sectional area can be calculated as follows:

[A(x) = \frac {16V}{3L\pi}[4x-4x^2]^{3/2} = \pi R_{max}^2[4x-4x^2]^{3/2}

The volume of a Sears–Haack Body is:

V = \frac {3\pi^2R_{max}^2L}{16}

The radius of a Sears-Haack Body is:

r(x) = Rmax(4x − 4×2)3 / 4The derivative (slope) is:

r’(x) = 3Rmax(4x − 4×2) − 1 / 4(1 − 2x)

The 2nd derivative is:

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Next time give a better read, going to wikipedia and paste does not mean you understand the theory, stealth modifies the cross section of aircraft for a reason the hopeless diamond was called hopeless and the F-117 faceted body was a brick flying.

See you continue dreaming J-20 has the ideal sears-haack body and with Al-31 supercruises better than Su-35BM or F-22

 

[latenlazy]

The cross sectional area can be calculated as follows:

[A(x) = \frac {16V}{3L\pi}[4x-4x^2]^{3/2} = \pi R_{max}^2[4x-4x^2]^{3/2}

The volume of a Sears–Haack Body is:

V = \frac {3\pi^2R_{max}^2L}{16}

The radius of a Sears-Haack Body is:

r(x) = Rmax(4x − 4×2)3 / 4The derivative (slope) is:

r’(x) = 3Rmax(4x − 4×2) − 1 / 4(1 − 2x)

The 2nd derivative is:

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Next time give a better read, going to wikipedia and paste does not mean you understand the theory, stealth modifies the cross section of aircraft for a reason the hopeless diamond was called hopeless and the F-117 faceted body was a brick flying.

See you continue dreaming J-20 has the ideal sears-haack body and with Al-31 supercruises better than Su-35BM or F-22

Facepalm. Throwing around equations proves nothing except that you don't know how to separate theory from application. No plane is actually an ideal sears-haack shape (su-27 or j-20). That's simply the simplest shape that demonstrates the concept. No one argued that the J-20 has an ideal sears-haack body either (reading comprehension much?). The su-27 itself is not an ideal sears--haack body. If you're going to claim that one is a more ideal sears-haack shape you better actually do the work showing it, which is to compare how both shapes distributes their cross sectional area. Otherwise your claim is worthless.

Also using Wikipedia is no worse or better than using any other webpage. Which webpage you quote doesn't change the definition and concept of a sears-haack shape, which is clearly stated on multiple pages, not just Wikipedia, that it's not the shape itself that matters, but the cross sectional area distribution.

Your f-117 example confuses wave drag with aerodynamic lift and drag. The ideal Sears- haack shape is the best shape to minimize wave drag, and has nothing to do with the other shape requirements for flight. In fact the Sears-haack cigar shape would be horrible for flight. That's why you don't see wingless flying cigars.

 

[kwaigonegin]

Facepalm. In fact the Sears-haack cigar shape would be horrible for flight. That's why you don't see wingless flying cigars.

what a flying cigar may look like.

 

[no_name]

No plane has an ideal sears-haack shape. SH shape is used to demonstrate the concept of minimum drag. Aircraft need to be designed with more criteria than that in mind.

Good luck trying to maneuver a perfect sears-haack shaped object using aerodynamic forces.

I don't want to hear no more about the words sears-haack body. We should use it's more useful derivative when talking about aircraft designs, which is the area rule.

 

[MiG-29]

Facepalm. Throwing around equations proves nothing except that you don't know how to separate theory from application. . In fact the Sears-haack cigar shape would be horrible for flight. That's why you don't see wingless flying cigars.

Look, you ae simply intelectually dishonest, in fact you simply can not even accept the sears-haack body is used.

I will give you 3 simple examples.

1-V-2, yes the german V weapons uses that shape.

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2-Most of modern external fuel tanks

3-smart weapons
open the link

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Su-27 is a modification of that shape, take the wings, tailplanes and vertical fins off and you find the Su-27 central fuselage is a modification of the sears-haack body.

J-20 like F-22 are closer to the hopeless diamond, and they need to be, since the hopeless diamond is based upon a pyramid shape.

The su-27 has area rule, treatment, if you look at the change in diameter of the radome and tail sting in the air brake junction .


Your basic theory is Al-31 will propel the J-20 to supercruise but will keep the Su-27 subsonic, which is unlikely since the only advantage of J-20 is a lower ratio wing, the rest of the fuselage is farther than Su-27 to an ideal sears-haack body and it weighs more