Aircraft radar cross section


New Member
I have decided to do this lay-manised article from my limited understanding on radar cross section (RCS) after reading quite a bit of disinformation on the RCS especially that of the F-15 without understanding what it means. The article also attempts to bridge the disconnect between Boeing’s RCS reduction claims of the F-15SE and public perception of the F-15’s high RCS.

Firstly, the basics….

i) RCS numbers is not consistent throughout an aircraft.

If one does a polar plot which is a RCS map of a typical aircraft, the RCS numbers will look like a "+". That means RCS will peak from the front and sides.

ii) RCS is higher from the sides than the front.

RCS is dependent on how perpendicular the facing is to the tracking radar. If one has a flat face facing the radar, then the radar returns are stronger as compared to a curved face.

The simple reason that the surface area of the sides is greater than the front means that RCS from the sides can be assumed to be higher than the front.

iii) Inlets affect RCS.

Some people have the perception that engine inlet/intakes are empty holes. Others have the conception that intakes generate super high RCS.

The reality is that inlets are not empty tubes. Inside are fan/turbines etc which will create radar returns when radar waves hit. An aircraft design that manages to reduce RCS from a frontal angle will have to consider reducing RCS for the intakes otherwise the intakes will still contribute to the RCS. However, intakes will not increase RCS from the sides.

The reason why intakes are an issue is because fan blades are made of metals/alloys that are highly radar reflective. One can’t change the metals on the blades or coat it with RAM easily.

iv) RCS has blind spots in certain areas.

If one looks at an aircraft from a 30-60 deg angle or 300 to 330 deg angle, it is clear that the aircraft surface area is not facing the onlooker. Hence RCS is lower. It can drop to -10 db or under 0.25 m2 even for a non-stealth aircraft.

v) Pylons/external munitions also affect RCS

This is logical. Pylons increase the reflective surface area. So do external munitions/fuel tanks. However, this is greater at the sides than the front. A2A missiles have smaller frontal surface area but large bombs have greater surface area. That’s why a weapons bay is a pre-requisite with lower RCS fighters.

vi) The wings and tails contribute to RCS too

Yup, those contribute to RCS too. The point about new designs such as the F-22 and F-35 is that their wings and tails although looking flat are not really flat but slightly curved. That’s probably why the F-15SE has a proposed canted tail. The current F-15 tail (like the F-18) is ~90 deg perpendicular to the side which increases its RCS. So are its wings in relation to the top.

Canting the tail reduces RCS from the side but increases RCS in a different direction (where there is less RCS). This evens out the RCS.

One can probably cant the tails of dual-engined fighters but canting the single tail of an F-16 logically can’t happen. Duh.

vii) In summary, 5 elements affect RCS

Primarily, RCS can be managed by 4 elements by the fighter aircraft designer.
- type of surface (eg ram coating)
- type of design (eg angle vs flat)
- passive cancelling
- active cancelling

The last 2 are ECM techniques. Additionally, different wavelengths affect radar returns but the use of the wavelength is determined by the radar emitter.

viii) The front RCS is especially important for a fighter

RCS from the front is important because a fighter needs to approach its target and the only way it can do that is forward. It can’t generally fly sideways or backwards. Missiles still fire from the front (even though some can go high angle after launch). Reducing RCS allows the fighter to get to its target before detection faster.

As a guide, cutting RCS by a factor of 10 reduces detection range by 44%.

ix) So is the F-15 RCS really 405m2?

Probably correct for earlier model F-15s but that would be from the side. As a comparison, an F2H banshee would emit a 400+ m2 RCS from the side.

If one look at pics of early model F-15s, those can be seen to have flat sides.

The CFTs fitted to the –E model onwards would reduce side RCS as these curves the surface and cover a large proportion of the previously flat fuselage. The newly designed F-15SE appears to take the RCS reduction further.

The RCS from the front is probably under 10 m2 (10 db) – my estimate. Applying RAM coating to the nose and other front facing surface area and tackling the engine intake returns with blockers should be able to reduce RCS further.

If an F-15 pilot picks up fighter radar on the RWR emitting from the F-15’s sides, it won’t take a genius to guess what the pilot should do…

Have omitted citations and references as this is not meant as a research paper but I have relied on a few.

Hope this helps the lay person in understanding RCS. Feel free to correct any errors.


New Member
For a "from a layman for laymans" description it's not bad. Of course there are details missing and it's all a bit simplified, but for the start...

I would like to add that RCS values are largely dynamic variables as already insisted by you and that the RCS also depends on the wavelength used by the radar.

One of the reasons why engine compressor blades "like" to reflect radar energy is owed to the fact that they are a relative flat surface and the rotation causes irregularities. Shielding the compressor face by what ever means is already helpful to cut the frontal RCS value by a fair margin.

With regards to measures taken to reduce RCS I would list them as shape, materials and treatment. It is not just about RAMs, RAS and shape only, the finish of the aircraft, tight manufacturing tolerances, treatments to fill potential gapes and avoiding irregularities throughout the airframe are very important. That's the reason why you don't see any antennas on the F-22, they are all embedded into the airframe to avoid such irregularities.

Rounded shapes aren't necessarily the best choice, certainly better than a flat 90° angled surface, but flat angles ~45° are usually better to redirect radar energy away from its source. Sharp edges can help to split the energy and direct it into multiple directions further reducing the probability of detection.

I've thought about writting something similar about LPI, but haven't found the time and nerves to do so yet.


Banned Member
Good read.

As an add on.
The radar emits an electromagnet field. An electromagnetic field is a "thing" that affects charged particles (exert a force on them) So when an object is moved into the electromagnetic field of the radar, small charged particles (electrons) are acted upon and they begin to oscillate (in tandem with the EM field). Now oscillating electrons: that's how an classic antenna works. And in truth our object becomes an antenna, which emits a signal induced by the original EM-field of the radar.
So while the job of the radar guy is to make an antenna that can create particular strong signals, the job of the stealth-guy is to make an antenna that's just a bad antenna.

Think of our airplane as being bathed in an (induced) EM field, with lots of small ribbles or waves travelling back and forth on the surface. (The wavelenght of these ribbles are given by the original field. via the frequvency of that field) Now imagine that there is some structual feature on the surface that allows two (or more) waves to travel back and forth on the same feature. If the conditions are right these waves can make constructive interference or even resonance, producing a very strong wave (high amplitude), this is bad if we want to make a bad antenna (stealth) because the high amplitude signal is exactly the thing that the enemy radar-reciever will be looking for/able to pick up,
so hence surfaces with sharp angles and lots of features are bad (since these can participate in constructive interference/resonance), while f.ex. curved surfaces are good since they are feature less (one can imagine the waves travelling aimlessly around the featureless surface) .


Banned Member
I posted this some time back in another thread, maybe it has interest

he Radar equation, which, if we simplify a lot , is


Pr is power recived, Pt is power transmitted, RCS is the radar cross section of the object, R is the range/distance to the object. We will assume that transmitter and reciever has the same location.
In reality the above constant is not a constant and is depending on the radar, enviroment and other stuff which we, for the sake of simplicity, will just collect in a single constant.
Notice that the R^4 dependence is due to the signal first travelling to the object, which then emits a new signal that has to travel back (to the antenna).

Let's imagine the following scenario: We have a radar that's viewing two objects, O1 and O2, these objects have different RCS (respectively RCS1 and RCS2) and different range to the radar. Let's for fun sake say that O2 has a RCS in the interval [0.001*RCS1, 0.01*RCS1].

Let's say that the extreame range at which our radar can detect object O1 is R1. We can then use the above to deduce that this radar should be able to detect, everything else equal, object O2 in the range interval [0.18*R1,0.32*R1].. in words; object O2 can stay undetected within 18-32% of the range that O1 is detected at.

From what I have been able to dig out; The Eurofighter is reported to have an RCS of about 1 square meter. The F117 is reported to have an RCS in the interval 0.001 to 0.01 square meters.
The above will thus suggest that if you have a radar that can detect the EF at, say 100 Km, this radar will, everything else equal, detect the F117 at a range between 18 and 32 Km.