RCS for dummies

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linkomart

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Unread post22 May 2019, 07:39

Hi, I got a lot of PM:s from the "Gripen vs F-35 thread" asking about canard and stealth. PM:s isn't the best so I'll guess a new thread is in order. A few caveats:
* This thread will not contain any classified information, just basic physics.
* I'm not an expert, nor a teacher, but since many ask me questions, I'll try my best.
* Feel free to correct or come with own information, but stay on topic and no classified information.
* It will be a lot of simplifications, there are always caveats and exceptions but this is general, from me at least.
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loke

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Unread post22 May 2019, 18:39

Thanks for taking the initiative -- I am looking forward to this!

An introduction is found here: http://aess.cs.unh.edu/Radar%202010%20P ... on%201.pdf

The math is making my head spin! Slide 11 shows some typical RCS values of various objects.
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sprstdlyscottsmn

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Unread post22 May 2019, 18:47

Hmm, they say Large Fighter is 6, when it has been quite established that the F-15 is a 25 and most medium fighters with no RAM are ~5. Only very small fighters are naturally 1 (F-5 small).
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viper12

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Unread post22 May 2019, 21:39

The maths isn't too hard ; it's similar to what I suppose most students in scientific fields see in the first 2 years of college (medical students don't qualify based on my experience...).
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garrya

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Unread post23 May 2019, 02:58

i would like to contribute some radar scattering charts
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eloise

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Unread post23 May 2019, 08:56

linkomart wrote:Hi, I got a lot of PM:s from the "Gripen vs F-35 thread" asking about canard and stealth. PM:s isn't the best so I'll guess a new thread is in order. A few caveats:
* This thread will not contain any classified information, just basic physics.
* I'm not an expert, nor a teacher, but since many ask me questions, I'll try my best.
* Feel free to correct or come with own information, but stay on topic and no classified information.
* It will be a lot of simplifications, there are always caveats and exceptions but this is general, from me at least.

Is there any fighter fly at the moment equipped with frequency selective surface?
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hornetfinn

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Unread post23 May 2019, 09:12

sprstdlyscottsmn wrote:Hmm, they say Large Fighter is 6, when it has been quite established that the F-15 is a 25 and most medium fighters with no RAM are ~5. Only very small fighters are naturally 1 (F-5 small).


Those nice radar scattering charts show that RCS varies a lot by aspect angle. For example the RCS of that business jet can vary between 0.001 to over 100 square meters. So it might have peak RCS of 100 square meters, average RCS of something like 1 square meter and rather low minimum RCS. In real life RCS varies a lot which makes using single value for RCS a bit unrealistic. Of course in most cases (like simulations) some average or mean RCS value is close enough in most cases. Of course for military aircraft the forward sector RCS is usually the most important and is usually those values are used.
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swiss

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Unread post23 May 2019, 15:03

Interesting Thread. :D

And here the frontal RCS from the Su-27. Roughly 22 m2. Dotted line is very likely the the Su-35.

https://translate.google.ch/translate?h ... STELLS.HTM

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sprstdlyscottsmn

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Unread post23 May 2019, 16:14

hornetfinn wrote:Those nice radar scattering charts show that RCS varies a lot by aspect angle. For example the RCS of that business jet can vary between 0.001 to over 100 square meters. So it might have peak RCS of 100 square meters, average RCS of something like 1 square meter and rather low minimum RCS. In real life RCS varies a lot which makes using single value for RCS a bit unrealistic. Of course in most cases (like simulations) some average or mean RCS value is close enough in most cases. Of course for military aircraft the forward sector RCS is usually the most important and is usually those values are used.

Right, I tend to think of "posted" RCS values as being a 30 degree frontal average. I never think of them as minimum or maximum.
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ricnunes

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Unread post23 May 2019, 19:05

hornetfinn wrote:In real life RCS varies a lot which makes using single value for RCS a bit unrealistic.


First of all, great thread indeed.

Secondly and commentating the above, I would say yes and no.
Well, of course what you say is fully correct hornetfinn but by looking at the RCS simulations/diagrams above I can't stop to think that the behavior looks very, very similar for all aircraft (they produce a "similar image" on the diagram, if you will) and this even for very different aircraft like the Mig-29, the Su-27/35, the F-117 and even the F-35 model simulation.
So while being true that when we say that for example the F-35 has a RCS of 0.001 m2 (supposedly as a front aspect average) but in some angles the RCS will be quite higher than that, the exact same thing also applies to any other aircraft as well that has a different RCS (also in the same front aspect average) of lets say, 0.1 m2 or 1 m2 and/or so on and all of this proportionally so.
This may be a gross oversimplification but imagine that the F-35 RCS in some angles reaches 1 m2. Then I'm pretty sure that the RCS an 'X' aircraft which would have a frontal average RCS of 0.1 m2 would have for the same angle which for the F-35 would be 1 m2 a much, much higher value than 1 m2.

Bottom line (and IMO), average values are definitely not "pin-point precision" values (afterall they are averages) but IMO allows us to perform comparisons in terms of RCS between different aircraft with a quite good (I believe) level of accuracy.

Well, my 2 cents anyway :wink:
A 4th/4.5th gen fighter aircraft stands about as much chance against a F-35 as a guns-only Sabre has against a Viper.
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swiss

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Unread post23 May 2019, 21:40

ricnunes wrote:
hornetfinn wrote:In real life RCS varies a lot which makes using single value for RCS a bit unrealistic.


First of all, great thread indeed.

Secondly and commentating the above, I would say yes and no.
Well, of course what you say is fully correct hornetfinn but by looking at the RCS simulations/diagrams above I can't stop to think that the behavior looks very, very similar for all aircraft (they produce a "similar image" on the diagram, if you will) and this even for very different aircraft like the Mig-29, the Su-27/35, the F-117 and even the F-35 model simulation.
So while being true that when we say that for example the F-35 has a RCS of 0.001 m2 (supposedly as a front aspect average) but in some angles the RCS will be quite higher than that, the exact same thing also applies to any other aircraft as well that has a different RCS (also in the same front aspect average) of lets say, 0.1 m2 or 1 m2 and/or so on and all of this proportionally so.
This may be a gross oversimplification but imagine that the F-35 RCS in some angles reaches 1 m2. Then I'm pretty sure that the RCS an 'X' aircraft which would have a frontal average RCS of 0.1 m2 would have for the same angle which for the F-35 would be 1 m2 a much, much higher value than 1 m2.

Bottom line (and IMO), average values are definitely not "pin-point precision" values (afterall they are averages) but IMO allows us to perform comparisons in terms of RCS between different aircraft with a quite good (I believe) level of accuracy.

Well, my 2 cents anyway :wink:


Fully agree with your opinion Ric. As we can see, the RCS of all fighters and also EFT and missiles is dramatically increasing directly from the side. Its also interesting when we compare the F-35 model with 4 gen such as Mig-29 or Su-27. The Stealth Fighter seems to be much better optimized for the frontal RCS (+/- 30 °). So even for a modern stealth aircraft, it is therefore very important to have good mission planning to keep the RCS as low as possible against high end threats.
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sprstdlyscottsmn

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Unread post23 May 2019, 22:06

swiss wrote: The Stealth Fighter seems to be much better optimized for the frontal RCS (+/- 30 °). So even for a modern stealth aircraft, it is therefore very important to have good mission planning to keep the RCS as low as possible against high end threats.

Good thing the F-35 knows this and has multiple display options to help the pilot avoid detection zones.
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eloise

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Unread post24 May 2019, 03:17

swiss wrote:Interesting Thread. :D

And here the frontal RCS from the Su-27. Roughly 22 m2. Dotted line is very likely the the Su-35.

https://translate.google.ch/translate?h ... STELLS.HTM

Image

When it is written like that, the RCS value is calculated in dBsm instead of m2
If you see the ring as 0-> 1-> 100 then the RCS value is in m2
If you see the ring as -10 -> 0 -> 10 ->20 then the RCS value is in dBsm
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eloise

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Unread post24 May 2019, 03:22

garrya wrote:Image

There is one simulation from Hellenic airforce with rather different result
source.PNG

F-35 model.PNG

f-35 scattering chart.PNG

F-35 mean rcs.PNG


They simulated F-16 as well
f-16 scattering chart.PNG
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garrya

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Unread post24 May 2019, 09:49

eloise wrote:There is one simulation from Hellenic airforce with rather different result

The differences came from the way they made the model
Zikidis's study doesn't simulate air intake and they use a transparent radar cone, the pitch angle is also different
Jiang's study uses a very simple air inlet model.
The study is in Chinese, this is a rough translation
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