F-22 and F-35 RCS revealed by USAF

[hornetfinn]

3. "it might be" together with "most likely" are your favourite sentences in any analysis. What mechanism?

Because without accurate specs, we don't know for certain and really accurately but we can make decent estimates with the knowledge we have.

I don't think the coating in F-22 or F-35 canopy is a fully reflective coating but more likely works as a layer that behaves like anti-radiation paint. Why does this coating need to be reflective at all? There are anti-radiation paints that can attenuate the reflections at least 15 dB.

[mityan]

I don't think the coating in F-22 or F-35 canopy is a fully reflective coating but more likely works as a layer that behaves like anti-radiation paint. Why does this coating need to be reflective at all? There are anti-radiation paints that can attenuate the reflections at least 15 dB.

Did you read anywhere about thin film transparent paint with such a high attenuation, or it is just a dream?

[smsgtmac]

Mityan,
You appear to have some minimal understanding of mathematics for RCS estimation, but you have a woefully insufficient grasp of RF physics and LO/RCS reduction techniques. On top of his, you've gone from being amusing to irritatingly arrogant in your arrogance (Hybris). You've become tiresome.

I have some things to do today, and when I come back this evening I'm going to show you precisely where you are sadly lacking in the knowledge that you have so eagerly 'shared' here -- and I will do so without going beyond using the information you have posted and just two introductory textbooks on the fundamentals of LO/stealth.

'How' I will do this depends upon your answers to two questions:
1) How old are you?
2) Do you have access to Amazon.com?

[KamenRiderBlade]

Mityan,
You appear to have some minimal understanding of mathematics for RCS estimation, but you have a woefully insufficient grasp of RF physics and LO/RCS reduction techniques. On top of his, you've gone from being amusing to irritatingly arrogant in your arrogance (Hybris). You've become tiresome.

I have some things to do today, and when I come back this evening I'm going to show you precisely where you are sadly lacking in the knowledge that you have so eagerly 'shared' here -- and I will do so without going beyond using the information you have posted and just two introductory textbooks on the fundamentals of LO/stealth.

'How' I will do this depends upon your answers to two questions:
1) How old are you?
2) Do you have access to Amazon.com?

(Hybris) = (Hubris)

[smsgtmac]

Mityan,
You appear to have some minimal understanding of mathematics for RCS estimation, but you have a woefully insufficient grasp of RF physics and LO/RCS reduction techniques. On top of his, you've gone from being amusing to irritatingly arrogant in your arrogance (Hybris). You've become tiresome.

I have some things to do today, and when I come back this evening I'm going to show you precisely where you are sadly lacking in the knowledge that you have so eagerly 'shared' here -- and I will do so without going beyond using the information you have posted and just two introductory textbooks on the fundamentals of LO/stealth.

'How' I will do this depends upon your answers to two questions:
1) How old are you?
2) Do you have access to Amazon.com?

(Hybris) = (Hubris)

Heh. I've adopted the late, great, Professor J. Rufus Fears' convention of using the Greek phonetic spelling of 'Hybris'.
BTW: I was originally going to type "Arrogance in your Ignorance" but I typo'd it and then decided "Arrogance in your Arrogance" worked just as well. Thought for sure someone would've said something about that one

[smsgtmac]

Well Mityan, you didn’t answer the questions, so my working assumption is you are a grownup with a credit card and an Amazon.com account.
First, your approach in applying a flat-plate model of the RF behaviors of a notional F-22 canopy is completely contrived and erroneous.

...And what is the exact value of reflection? Look at this picture.

mityans-canopy-cartoon.jpg (34.93 KiB) Viewed 114327 times

For tilted flat plane here is the RCS formula:

mityans-source-formula.jpg (4.13 KiB) Viewed 114327 times

It is taken from here:
http://books.google.by/books?id=XDs04HdQ4-gC&pg=PA183&lpg=PA183&dq=tilted+flat+plate+RCS&source=bl&ots=dol7wl54FC&sig=27W3hnnBtGpPkvwbuF5zzoA2oek&hl=ru&sa=X&ei=obZhVKDtG4HkOKL0geAC&ved=0CCcQ6AEwAg#v=onepage&q=tilted%20flat%20plate%20RCS&f=false
Why flat? We just cut a plane from a frontal part of canopy - 1 m long and 0.1 m wide - such a strip I think can be well approximated by flat plane.
Then we may remove all the rest of the canopy and all the rest of the airframe (though they also have a non-zero reflections)
and put the values in the formula (it seems the incidence angle is near 60 deg. to normal, A = the square of the strip)
So, we get the value of 0.00058 which is 5.8 times greater than reported overall RCS.
I want to remind that we nullified all other reflections from the airframe (!) and took just a narrow strip from canopy.
...
My guess is that we've been reported a true minimal RCS value
...
which is fair for very narrow angle range. And a true average value is a definitely classified information and it should be much greater evidently.

No....Not Even Close.
The ‘flat plate’ equation you present is not relevant to a flat plate tilted at the angle indicated. If you had read the paragraph after that equation in your source, you would have read this:

Inappropriate-use-of-calibration-formula.jpg (51.9 KiB) Viewed 114327 times

Note the highlighted point of interest.
Now, if you had a more complete reference for estimating flat plate specular reflection at hand, say Radar Cross Section, Second Edition, by E.F. Knott (same author as your source), J.F. Shaeffer, and M.T. Tuley, you would know how to treat estimating the specular return of a tilted flat plate:

If the plate is oriented off the specular angle, but in such a way as to maintain a pair of edges perpendicular to the radar line of sight, the radar return is no longer proportional to the square of the area, but to the square of the edge length. (P. 7)

Your ".1m" edge would have a .1m^2 effective area. Guess what that does to your calculations.
This is just the tip of the iceberg you hit. At aspect angles greater than ~40 degrees, the relevance of the theory of physical optics “gives way” (Knott et al, P. 232) to edge diffraction (as indicated in the quote above) and surface travelling wave echo. The dominance of each of these effects depends upon polarity. These mechanisms, in total, are the ‘reradiation’ that I noted earlier.

So even your understanding of the flat plate specular behavior is wrong. But that’s OK, because the flat plate model doesn't even apply to a curved surface such as the canopy under consideration.

Curved surfaces require treating specular diffraction of RF energy as a return from striking a "specular point" (Knott et al, P. 549). I have no desire to spend my time explaining what this means, even if we just covered what we could find in the public domain. ‘Knott et al’ is 600 pages long, not counting index, and even they don’t cover everything.
But even if we could simply solve the RCS of an LO aircraft canopy assembly, it still wouldn’t give you an answer that you could generalize into an overall aircraft RCS. Why? You can't presume the canopy's standalone RCS value is relevant.

David Lynch, in his Introduction to RF Stealth (hyperlink fixed), provides an excellent overview of stealth as being the total of operational techniques and aircraft design, and an advanced high school student could follow most of it Aside from one unfortunate anecdote that is an urban myth, I find it excellent in presenting the whole stealth perspective. He presents the material in the framework of the ‘Great Thoughts’ of LO/Stealth (PP. 5-8):

1. Active emissions control must be coupled with platform cross-section reduction.
2. Planform alignment: concentrating all the reflections (hate that word) in a few directions can reduce the RCS in all other directions.
3. Facets and shaping: selecting shapes that present very low sidelobes towards threats
4. Edge treatment by convolution; i.e. using a mathematical function such as a Gaussian convolved on the junction of two surfaces to create a “blend” a very low RCS sidelobes
5. Impedance control: ensuring there are no discontinuities in the platform surface impedance, edge treatment by impedance matching of the platform skin to free space
6. Exploitation of the environment.

Of particular interest, because they relate to how your argumentation and knowledge base fails, are numbers 4 and 5.
“4?” You cannot treat the RCS of the canopy as an independent RCS number if it is integrated into the overall design such that the canopy is properly ‘blended’ into the rest of the aircraft.
“5?” A good LO aircraft has excellent impedance matching between its constituent components and to the airstream.

Now, go read up on this stuff.
Come back when you know as much as you thought you did.

(Unless you were just trolling. Then just go away.)

[geforcerfx]

smsgtmac....

[KamenRiderBlade]

If you tried to 3D model the radar reflections off of the F-22 / F-35 using ray tracing from every possible point in the sphere with a .1 degree spacing between each point, I'm pretty sure the RCS at each point should be pretty low given the contours of the aircraft is designed to not be seen except at very specific points.

From straight below / above, all aircraft should have the highest RCS.

But to align your radar with a moving aircraft in that manner is a next to impossible task, so the risk is negligible.

To try to calculate the RCS from straight ahead, I'm sure the scientist / engineers at Lockheed Martin planned every curve to lower the overall RCS from every angle that matters.

Lockheed Martin is one of 2 companies with real world experience in Stealth. The other company being Northrop Grumman.

If you think you can outdo the world's leading defense contractors with cursory knowledge, you must be mentally challenged.

Even with my level of knowledge of 3D vector math, geometry, and ray reflections; I can tell that you are not going to get much if any RF sources reflected back to the source unless you're directly under or above, and realistically, that's not going to happen.

[hornetfinn]

I don't think the coating in F-22 or F-35 canopy is a fully reflective coating but more likely works as a layer that behaves like anti-radiation paint. Why does this coating need to be reflective at all? There are anti-radiation paints that can attenuate the reflections at least 15 dB.

Did you read anywhere about thin film transparent paint with such a high attenuation, or it is just a dream?

Well, how about a patent about such transparent radar absorbing coating from Westinghouse Electric Corporation which was filed in 1992: http://www.google.com/patents/US5358787

Up to almost 50 dB reduction in X-band radar reflection or another design with 15 dB reduction in very wide bandwidth...

Interesting that the date of that patent would fit rather nicely with F-22 development...

[mityan]

No....Not Even Close.
The ‘flat plate’ equation you present is not relevant to a flat plate tilted at the angle indicated. If you had read the paragraph after that equation in your source, you would have read this:

formula_again.png (166.1 KiB) Viewed 114095 times

Note the highlighted point of interest.

What does it mean? Let me explain.
If we have a laboratory, a set of devices to generate and measure a signal of interest, and we are going to perform some precise measurements, we should at first calibrate our devices. It means the measurement of exactly known pattern.
Let's take a closer look to the formula - biuld a dependency on freq (6-15 GHz) and in wide angle span:

So, indeed, we can calibrate ony in very narrow angle span near normal line.
But stop now!
Why we are talking about calibration at all? Who cares of it?
Does it mean that the formula is wrong or has a limited usage (i.e. rcs of plane undergoes different laws)?
NO.
So what was that? What kind of logical fallacy -
Composition fallacy - http://www.nizkor.org/features/fallacies/composition.html
or Red Herring ? http://www.nizkor.org/features/fallacies/red-herring.html

Go further.

formula_dependence2.png (59.47 KiB) Viewed 114095 times

Your ".1m" edge would have a .1m^2 effective area. Guess what that does to your calculations.

You've posted just a part of paragraph. Please, pay attention to underlined words.
Should I replace S=a*b in formula (5.23) by b*b ? What should I do with lambda squared?
What should I do with sin(x)/x, where x is also dependent on tilted edge "a"?

I'll tell you what it REALLY means:
Independence of frequency, as for sphere, means that we are in this region:

optical.png (12.71 KiB) Viewed 114095 times

For this region RCS is equal to geometric square of object (normal to line of sight)
For shpere it is pi*R^2, and it really frequency-independent.

For tilted plate just its square S=a*b is replaced by S=b*b,
so the result RCS is 0.01 sq.m. - much greater than I calculated.
And with NO placement to (5.23) formula.

I understand that you are trying to fool me (and every forum visitor who read this).
It is just another trap for weak minds.

Curved surfaces require treating specular diffraction of RF energy as a return from striking a "specular point" (Knott et al, P. 549). I have no desire to spend my time explaining what this means, even if we just covered what we could find in the public domain.

I tell you - this is Huygens–Fresnel principle I've mentioned.

At aspect angles greater than ~40 degrees, the relevance of the theory of physical optics “gives way” (Knott et al, P. 232) to edge diffraction

“4?” You cannot treat the RCS of the canopy as an independent RCS number if it is integrated into the overall design such that the canopy is properly ‘blended’ into the rest of the aircraft.
“5?” A good LO aircraft has excellent impedance matching between its constituent components and to the airstream.

Two statements contradicting each other? I dont know how is this logic fallacy (or trap) called, maybe you know better.
Just decide first - Edge diffraction or integration into design with no discontinuities?

1. I calculated RCS without edge diffraction which of cause can be added too.
2. Do you mean this:

discontinuity.png (26.51 KiB) Viewed 114095 times

It's just ridiculous.
It is true for tooth-like edges between parts of airframe, but not for canopy.

Now, go read up on this stuff. I dont know who is troll here.

BTW, if it may help, I am 36, live in Belarus, have a family, 3-room apartment, 2 dogs, and I drive a 17-yo peugeot.

[mityan]

But to align your radar with a moving aircraft in that manner is a next to impossible task, so the risk is negligible.

This is a case when two counterpart are at same height. For long ranges the position of ground radar or position of airborne radar - down or up 10 km - will give small difference in angle.

If you think you can outdo the world's leading defense contractors with cursory knowledge, you must be mentally challenged.

smsgtmac loves logical fallacies. This is Appeal to Authority.
http://www.nizkor.org/features/fallacies/appeal-to-authority.html

BTW.

you are not going to get much if any RF sources reflected back to the source unless you're directly under or above, and realistically, that's not going to happen.

So you dont believe David Fulghum from Aviation Week, who asserted that -40 dBsm is an ALL-ASPECT RCS?

[KamenRiderBlade]

This is a case when two counterpart are at same height. For long ranges the position of ground radar or position of airborne radar - down or up 10 km - will give small difference in angle.

It would still require that the
original position of the opposing radar be in the right place. And a small difference in angle is all it takes to make the radar refraction go in the wrong direction and away from your radar dish.

smsgtmac loves logical fallacies. This is Appeal to Authority.
http://www.nizkor.org/features/fallacies/appeal-to-authority.html

The same can be said about you.

you are not going to get much if any RF sources reflected back to the source unless you're directly under or above, and realistically, that's not going to happen.

So you dont believe David Fulghum from Aviation Week, who asserted that -40 dBsm is an ALL-ASPECT RCS?[/quote]
The entire lot at Aviation Week is very suspect from the beginning. The fact that they still employ Bill Sweetman, associate with David Axe, the rest of the folks there have lost a lot of credibility IMO due to the company that they choose to keep.

David Fulghum is no exception to this rule.

[mityan]

smsgtmac loves logical fallacies. This is Appeal to Authority.
http://www.nizkor.org/features/fallacies/appeal-to-authority.html

The same can be said about you.

I'm based on literature, it's different.

[KamenRiderBlade]

smsgtmac loves logical fallacies. This is Appeal to Authority.
http://www.nizkor.org/features/fallacies/appeal-to-authority.html

The same can be said about you.

I'm based on literature, it's different. :)

I don't trust your interpretations on your literature.

[hornetfinn]

http://www.jatm.com.br/papers/vol4_n1/JATMv4n1_p25-32_Indoor_Radar_Cross_Section_Measurements_of_Simple_Targets.pdf

Figure 7 presents the RCS values in dBm of flat plate as a function of aspect angle. Figure 8 presents them in square meters. Those calculation seems to indicate that such a flat plate would have the lowest RCS value at aspect angles higher than 30 degrees with 60 degrees being rather optimal. The higher aspect angles give about 17 to 19 dBm smaller RCS than perfectly perpendicular flat plate. So that means that if we simplify the canopy to tilted flat plate, we will get about 50 to 80 times smaller radar cross section in square meters than the actual area of the plate. Give the canopy decent radar absorbing treatment and the RCS of the canopy will be extremely small.

Another source: https://www.ee.washington.edu/research/ersl/Documents/Paper/2004%20Multple_Scattering_Effects_RCS,%20WRM.pdf

This gives pretty much the same values.