J-20... The New Generation Fighter II

gambit · May 7, 2011

supercat said:
The problem with Asymptote's argument is that it is too simplistic. I doubt you can tell which one is more stealthy by just eyeballing it, except for the nozzles. If this is the case, I would say F-22 negates all its advantages in geometry by having such huge vertical tails. The Americans have run their numbers through supercomputers, so have the Chinese.

Yes we can to a considerable degree of accuracy. For example...If the vertical stabs are perpendicular to the fuselage and/or the horizontal stabs, the from this aspect angle, this aircraft is less 'stealthy' than one whose vertical stabs are NOT perpendicular. This is called the 'corner reflector'...

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Tests were made of radar corner reflectors to determine: (a) The effectiveness of a radar corner reflector on a balsa life float for location of survivors of a Naval disaster by radar on plane and surface vessel; (b) The effectiveness of a radar corner reflector on a wooden life boat for location of survivors of a Naval disaster by radar on plane and surface vessel; (c) The relative effectiveness of the type MX137A and MX138A radar corner reflectors; (d) Improvement resulting from increasing the height of corner reflector; and, (e) Method of supporting a corner reflector on a balsa life float.

A radar corner reflector is a safety item for small marine crafts. It create a much larger RF signature than the boat's actual physical size so larger vessels can see them.

Corner reflectors are used in field weapons testing, particularly useful for missiles. We use them to test the sensitivity of the seeker's radar by providing a large reflection then with decreasingly smaller reflectors to eventually no reflectors are used at all. So yes...We definitely do 'rig' these tests. There is nothing wrong with that process. Anyone who do anything else is a fool and will cost him in the long run.

But for an aircraft whose intention is to have as low radar observability as possible, then any corner reflector on its body is a negative. The vertical-horizontal stab connection is a great reflector. The corner created by the external stores pylon to the wing's underside is a smaller reflector. The fins on the missiles and bombs are smaller reflectors. Even two edges that meet to form a 90deg corner is a negative. That is why panels are saw-toothed. If the horizontal stabs are shaped in such a way that the trailing edges form 90deg corners with the fuselage, that is a no-no. That is why the F-22's horizontal stabs are shaped the way they are. That is why the F-22's vertical stabs, despite their large size, are canted so they do not form the dreaded 90deg corner reflectors with the rest of the aircraft. Corner reflectors by themselves are not bad, only 90deg ones that we must avoid.

For an experienced observer in low radar observability, a great deal of accuracy can be inferred just by looking and comparing.

gambit · May 7, 2011

Asymptote said:
Asymptote said:

The J-20's trailing edge of main wings are align DIAGONALLY to the canards. Seems kinda odd if you look at the F-22 and PAK FA when they are both aligning to the trailing edge of the aft wings and main wings ON THE SAME SIDE. So I don't see that as alignment. More of misalignment.

Click to expand...

Bltizo said:

Is there a difference if it is aligned on the same side? The radar waves will still be scattered the same direction if the trailing edges were aligned in a similar manner to the F-22 and T-50. (Unless someone who specializes in this field can elaborate on this subject, I think my conclusion is a valid one)

Click to expand...

There is a potential problem. It is based upon controlling as much as possible the behavior of a radar signal off a trailing edge because the edge is a scattering point. By having all trailing edges aligned to the same sweep degree, we reduce (not eliminate) the odds of having a signal being the one that will rise the aircraft above a certain threshold that will trigger an alert.

Blitzo · May 7, 2011

gambit said:
There is a potential problem. It is based upon controlling as much as possible the behavior of a radar signal off a trailing edge because the edge is a scattering point. By having all trailing edges aligned to the same sweep degree, we reduce (not eliminate) the odds of having a signal being the one that will rise the aircraft above a certain threshold that will trigger an alert.

I don't see what the problem is -- what you described is common to all stealth aircraft with edge alignment. My point was that there is no difference in whether the trailing edges are aligned on the same side or opposite sides so long as the alignment is at the same angles, scattering radar in the same directions if they were aligned on the same sides.

gambit · May 7, 2011

Bltizo said:
I don't see what the problem is -- what you described is common to all stealth aircraft with edge alignment. My point was that there is no difference in whether the trailing edges are aligned on the same side or opposite sides so long as the alignment is at the same angles, scattering radar in the same directions if they were aligned on the same sides.

The problem is increased unpredictability of scattering points behavior upon other flight control surfaces down the line. Keep in mind that we are talking about an object that will create highly dynamic and transitory scattering points for radar signals. By having as few trailing edges as possible, keeping their angles aligned to each other as much as possible, including their an/di-hedrals, and minimizing the length of the trailing edges, we reduce the amount of radiation the receiver will pick up. Remember, radar detection is about how much radiation OFF the object the receiver can amass over a period of time. The shorter the time the greater the uncertainty that there is an aircraft 'somewhere out there'.

$b-2_edge_diffract.jpg$

As a 'flying wing' the B-2 had to have its trailing edge broken up into several segments as seen above in order to reduce the odds of a trailing edge scattering point signal returning to source direction and that had to be balance against aerodynamic and flight control necessities. The result is the B-2's wing shape that we see today.

Blitzo · May 7, 2011

^ Very informative and interesting, but doesn't really answer my question.

I'll put it another way (not demanding you answer it of course, just disputing a point that asymptote made a few pages back).
Let's say the trailing edge of the J-20s canard were aligned with the main wing's trailing edge on the same side. Does that produce a massively different radar return than if the trailing edge were aligned diagonally?

^ Red line shows current "diagonal" alignment, blue lines show the a hypothetical modified canard with a trailing edge which aligns on the same side with the wing behind it rather than the wing opposite/behind it as is the J-20's current "diagonal" case.

A.Man · May 7, 2011

Another One

latenlazy · May 7, 2011

gambit said:
On an irregular (but symmetrical) body like an aircraft, the only way of knowing the body's true radar cross section (RCS) is via an electromagnetic (EM) anechoic chamber...

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I have been in there many years ago and it was quite an experience.

An outdoor radar range will not give you a very accurate idea of the aircraft's RCS because the aircraft itself will reflect all sorts of 'dirty' EM sources such as TV, radio, cell phone, and even cosmic background radiation. Worst is if the range is near an airport.

An indoor EM measurement facility can also contaminate the aircraft's RCS figure if the interior is not absorbent. What happens is that a portion of the radar signal, which is conical shape and not a 'beam' like widely believe, will reflect off the aircraft and back to the receiver/observer, reflect off the interior walls and back to the receiver/observer, reflect off the interior walls then off the aircraft which will return to the receiver/observer. In other words, the non-absorbent interior is even worse of a measurement than the outdoor range.

The absorber triangular cones or spikes that lined Benefield's chamber are shaped that way for the same reason that made up radar detection: reflection. Geometry says the angle of deflection is the same as angle of approach. Each time an EM signal impact the triangular absorber cone, not all of the signal will be absorbed, a portion of that will reflect off the cone's surface. That deflection angle should be towards the other cones. So the intention here is to have any radar signal that deflect off the aircraft that heads towards the walls are absorbed as much as possible, leaving only one signal that is the true and as uncontaminated as possible that came off the side that is facing the receiver/observer.

Now...Since we know that not all of the signal will return to the receiver/observer, which we pretend ourselves as the source 'threat' radar, we would be interested in how the aircraft would be seen if there are other 'threat' receivers looking at the same aircraft, keep in mind that we are still looking at an irregular but symmetrical body...

We can bombard the aircraft from a single 'threat' source direction or from multiple directions and measure the aircraft's RCS from diverse aspect angles. We can rotate the aircraft while we are bombarding the aircraft from these different 'threat' sources. Because the interior walls are lined with absorbers, each 'threat' receiver/observer will see only the signal that came off the aircraft, not signals from the walls, not signals from the walls that reflect off the aircraft, not signals that from the walls that reflect off the aircraft that reflect off the walls again before coming to the receiver/observer, etc...etc...And keep in mind that this is happening at the speed of light. This is why we need an EM anechoic chamber to know true, as in %99.999, any RCS measurement of any body.

High end, as in 'snobbery', speaker manufacturers uses audio anechoic chambers to measure their products...

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It is only after we know the %99.999 true RCS value of an aircraft that we now move to outdoor radar range measurement.

Like this from long ago...

Notice the aircraft is upside-down and without its vertical stabs.

Outdoor radar range measurement allows us to know how the aircraft's RCS increases in a 'real world' EM rich environment. That was how the F-117 was supposedly 'detected' when it was a bird that the radar operator was seeing on his scope. Benefield's interior reflectivity figure is a secret but it is rumored to be below that of cosmic background radiation. Basically, they measured an empty chamber and recorded that figure to be the baseline for an occupied chamber. It is also rumored that the F-117's frontal aspect is %1 above Benefield's interior reflectivity figure.

There is no agreed upon standard for what make a 'stealth' aircraft so China and Russia are free to call their aircrafts any label they want. But it is also true that the US have set an unofficial standard with a clean F-16 as 1m2 at 100-150km. That is the effective range for most fighter class radar systems that can give reasonably accurate target information.

And this is how we measure an aircraft's RCS.

I know

But that's why I find it bothersome when people insist that an airframe isn't stealthy because it doesn't follow so and so rules (such as edge alignment). The fact is that both the radar beam and the geometry of the plane are 3D and therefore interact 3-dimensionally, and with such complex shapes the only way you know exactly what the RCS is is if you a) test it in a simulation, and b) test it in an anechoic chamber. For all we know the rounded LERXes are a unique solution to reducing radar signatures due some unique factor of the airframe such as the combination of anhedral and dihedral wings/control surfaces. But the main thing is we don't know. All we know is that its shape was designed with help from a computer, and that it will probably undergo tests in an anechoic chamber to help confirm simulation results.

To address what gambit said earlier, the general rules tell us a bit about what are flagrant, but they're far from giving us the whole picture.

gambit · May 7, 2011

Bltizo said:
^ Very informative and interesting, but doesn't really answer my question.

I'll put it another way (not demanding you answer it of course, just disputing a point that asymptote made a few pages back).
Let's say the trailing edge of the J-20s canard were aligned with the main wing's trailing edge on the same side. Does that produce a massively different radar return than if the trailing edge were aligned diagonally?

^ Red line shows current "diagonal" alignment, blue lines show the a hypothetical modified canard with a trailing edge which aligns on the same side with the wing behind it rather than the wing opposite/behind it as is the J-20's current "diagonal" case.

But it did answer your question.

What most laymen do not understand is that for an irregular (but symmetrical) object like an aircraft, aka a 'complex body', its radar cross section (RCS) is very seldom obvious, by 'obvious' it mean as if the radar is looking directly at the top/bottom sides. In fact and reality, an aircraft's RCS is cumulative from the many scattering points upon its surfaces. The J-20's canards by themselves means nothing but when place into relationships with other flight control surfaces and a fuselage, they being conduits to a radar signal will make them contributors to the overall RCS value.

The process towards a final RCS value goes: prediction, modeling, and measurement.

Prediction is pretty much paper work, if you will. That is where we mathematically layout what we 'think' or 'guesstimate' a certain layout will look like to a radar.

Modeling is when we actually create a physical creation of that mathematical layout. We can start with smaller scales and works towards the full intentional scale.

Measurement means exactly that, which is when we introduce the model to the 'real world' situation and see if what we measure matches what we predict.

For the J-20's canards, if their trailing edge angles are the same as the wings (blue lines), that is one less item in that mathematical layout whose scattering point behavior we have to worry about. The goal during prediction is to have as uniformed scattering point behaviors as much as possible to each other. For the J-20, the canards are active flight control elements, that mean sometimes they will move the same as each other, sometimes they will split, but they will move and the radar signals off them will be vectors. The wings will radiate off into free space. The canards will radiate ON TO the fuselage and the wings. If these vectors are the same as the wings', differs only in magnitude because the canards are smaller, it make predicting behaviors upon the fuselage and other surfaces easier.

Because an aircraft is a complex body with many scattering points as contributors to the final RCS, we must attempt to minimize each scattering point's magnitude. However, there is no sense in trying to reduce the canards' role as contributors IF we have a 90deg corner reflector from the vertical-horizontal stabs connections. That said, if we eliminate those corner reflectors, the canards and the cockpit may end up as obvious as pimples on Miss Universe. Now we have devote time and efforts to reduce those contributors. So could the canards as is be 'massively' huge radar contributors? May be not, but as is they COULD BE just enough to raise the J-20 above a certain threshold at the worst time. That is why aviation experts the world over focuses and speculate on them so much.

Blitzo · May 7, 2011

Again, as interesting as all that is, it still doesn't really understand my question. I mean a simple yes no answer could've worked.

From what I'm gleaning, your answer to this "Let's say the trailing edge of the J-20s canard were aligned with the main wing's trailing edge on the same side. Does that produce a massively different radar return than if the trailing edge were aligned diagonally?" is an overall no, because the trailing edge of the canards and main wings are aligned at the same angle.

latenlazy · May 7, 2011

gambit said:
But it did answer your question.

What most laymen do not understand is that for an irregular (but symmetrical) object like an aircraft, aka a 'complex body', its radar cross section (RCS) is very seldom obvious, by 'obvious' it mean as if the radar is looking directly at the top/bottom sides. In fact and reality, an aircraft's RCS is cumulative from the many scattering points upon its surfaces. The J-20's canards by themselves means nothing but when place into relationships with other flight control surfaces and a fuselage, they being conduits to a radar signal will make them contributors to the overall RCS value.

The process towards a final RCS value goes: prediction, modeling, and measurement.

Prediction is pretty much paper work, if you will. That is where we mathematically layout what we 'think' or 'guesstimate' a certain layout will look like to a radar.

Modeling is when we actually create a physical creation of that mathematical layout. We can start with smaller scales and works towards the full intentional scale.

Measurement means exactly that, which is when we introduce the model to the 'real world' situation and see if what we measure matches what we predict.

For the J-20's canards, if their trailing edge angles are the same as the wings (blue lines), that is one less item in that mathematical layout whose scattering point behavior we have to worry about. The goal during prediction is to have as uniformed scattering point behaviors as much as possible to each other. For the J-20, the canards are active flight control elements, that mean sometimes they will move the same as each other, sometimes they will split, but they will move and the radar signals off them will be vectors. The wings will radiate off into free space. The canards will radiate ON TO the fuselage and the wings. If these vectors are the same as the wings', differs only in magnitude because the canards are smaller, it make predicting behaviors upon the fuselage and other surfaces easier.

Because an aircraft is a complex body with many scattering points as contributors to the final RCS, we must attempt to minimize each scattering point's magnitude. However, there is no sense in trying to reduce the canards' role as contributors IF we have a 90deg corner reflector from the vertical-horizontal stabs connections. That said, if we eliminate those corner reflectors, the canards and the cockpit may end up as obvious as pimples on Miss Universe. Now we have devote time and efforts to reduce those contributors. So could the canards as is be 'massively' huge radar contributors? May be not, but as is they COULD BE just enough to raise the J-20 above a certain threshold at the worst time. That is why aviation experts the world over focuses and speculate on them so much.

What role would radar absorbent and radar transparent materials play in this? For example, if the canards were made from radar transparent materials would that help make those blips more controllable?

J-20... The New Generation Fighter II

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