weasel1962
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.
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.