J-20 Radar scattering simulation

Introduction

J-20 is the world’s third operational fifth-generation stealth fighter aircraft after the F-22 and F-35. It made China the second country in the world after the USA to operate a fleet of stealth fighter. By the time this article is published, it is estimate that there are around 208 J-20 produced.

Goals of the simulation:

The aim of our simulation is to evaluate the mean and median radar cross section as well as the radar scattering pattern of the J-20 at 4 frequencies:

• VHF – 150 MHz
• L Band – 1150 MHz
• S Band – 3150 MHz
• X Band – 8150 MHz

.Because stealth aircraft tend to deflect radar signal away from the source, so a change in total arcs can have a big impact on median and mean RCS. So the mean and median RCS of J-20 will be simulated and calculated for 2 cases with different frontal arcs value.

• Case 1 simulates situation when the stealth aircraft have to face multiple radars located far from each others, thus they can illuminate the fighter from directions where the strong reflection lobes located
• Case 2 simulates situation when aircraft face small number of radars located close together so the stealth fighter can turn its nose toward targets

The details are in the following table:

Note: The contour map results for Case 2 is actually a part of Case 1 contour map, but it will be shown separately so that viewer can have an easier time visualize the radar lobes arrangement and how direction affect stealth fighter RCS.

Explanation of some terms:

Median RCS is the middle RCS value of all RCS data. Which mean 50% of RCS spike inside the arcs will be higher than the median value, and the others 50% of RCS spike inside the arcs will be smaller than the median value

Mean RCS is the average of all RCS data.
It is very important to remember that both Median and Mean RCS does not tell the whole story, two object with the same Median and Mean RCS does not necessary equally visible to radar when viewed from the same direction ,thus we also need the contour map to see the arrangement of the reflection lobes.

RCS value illustrated inside the contour map will be measured in dBsm. To convert RCS between dBsm and m2, we can use the following guidance.

CONVERTING RADAR CROSS SECTION IN SQUARE METERS TO DECIBELS

dBsm = 10 x log₁₀(RCS/1m²)

CONVERTING RADAR CROSS SECTION IN DECIBELS TO SQUARE METERS

RCS = 10^{(Decibels/10)}

Basics Rules:

Because not every information about J-20 is available in the public sectors, we will follow several rules in the simulation.

• In real life, J-20 skin has a layer of radar absorbing material (RAM) to absorbing radar energy. However, in this simulation, all external surface including the canopy will be treated as perfect electrical conductor (meaning they will reflect radio wave like metal)
• Stealth fighter will have trailing edge and leading edge treatment on its wings, horizontal and vertical stabilizers as well as inlet lips to reduce the magnitude of radar return from edge scattering and creeping wave return effect. But those effect were not added in our previous simulation for F-35 and Su-57 because there wasn’t enough information about their leading and trailing edge treatment. However, this time, the trailing edge treatment on the canard and main wing of J-20 in the form of edge serration will be simulated because they are very easily to identify visually
• J-20 radome is a FSS radome, so it will be treated as perfect electrical conductor for all frequencies except the bandpass frequency
• Luneburg lens which often used to increase stealth aircraft’s RCS will not be included in this simulation
• All cooling vents will be modeled
• The inlet , the radar blocker and the first 2 fan stages of 2 × WS-10C engine will be modeled
• Even though there is no current data on the absorbing capability of the RAM used by J-20. The inner surface of the inlet duct from the start of DSI bump leading to the engine stages and the radar blocker will be coated with a layer of MnZn ferrite RAM. This rule is made due to the fact that it is not possible to evaluate the RCS reduction effect of S-duct inlet without a layer of RAM added. The absorbing capability of the MnZn RAM will be based on actual real world measurement data. The current absorbing capability of the RAM used in our simulation is around -8 dB at 8.15 GHz and -4 dBsm at 3.15 GHz. Keep in mind though that actual RAM used on stealth aircraft may have absorbing rating of -15 to -25 dB depend on the actual frequency.

Software used:

• Model of the J-20 is created using SolidWorks software, the model of the Infrared pod (Chinese EOTS) is created by Autodesk FBX. They are then converted to the same format to be added together. 
• In most studies, radar scattering simulations are often done using POFACETS because it can run on most commercially available computer and provides convenient run–times. Unfortunately because POFACETS only rely on Physical Optics method to predict the RCS of target. As a result, it cannot take into account phenomena such as creeping waves return, thus lead to underestimation of target RCS value. Hence this radar scattering simulation will be done using Ansys HFSS. HFSS allows edge scattering, creeping wave, cavity return to be simulated accurately, thus it will be our chosen software.

Model photos:

OUTSIDE:

Top view

Side view

Bottom View

Head on view

Tail on view

SMALL PARTS AND REFERENCE IMAGES

EOTS window and pitot tube

 

 

Canard root hump and antenna hump 

Wing actuator humps and small humps near ventral fins

Humps on LERX, and belly DAS sensor

Trailing edge serration treatment on main wing and canards

INTERNAL PARTS – S-duct Inlet and engine

Contour Map Result

• VHF band result of clean J-20 in Case 1 (-60° to +60° horizontal arc, -22° to +22° vertical arc)

• VHF band result of clean J-20 in Case 2 (-20° to +20° horizontal arc, -20° to +20° vertical arc)

• L-band result of clean J-20 in Case 1 (-60° to +60° horizontal arc, -22° to +22° vertical arc)

• L-band result of clean J-20 in Case 2 (-20° to +20° horizontal arc, -20° to +20° vertical arc)

• S-band result of clean J-20 in Case 1 (-60° to +60° horizontal arc, -22° to +22° vertical arc)

• S-band result of clean J-20 in Case 2 (-20° to +20° horizontal arc, -20° to +20° vertical arc)

• X-band result of clean J-20 in Case 1 (-60° to +60° horizontal arc, -22° to +22° vertical arc)

• X-band result of clean J-20 in Case 2 (-20° to +20° horizontal arc, -20° to +20° vertical arc)

3D Maps Result:

• VHF band result of clean J-20

• L-band result of clean J-20

• S-band result of clean J-20

• X-band result of clean J-20

SUMMARY:

From the simulation results, it is easy to note that J-20 has good signature characteristic. Even though, J-20 RCS is higher than F-35A RCS through the frequency range (approximately 3.5 times in X-band, 1.5 times in VHF band), it still has much better RCS characteristic when compared to Su-57. The canard return blended very well with the main wing return, and due to their high swept angle, the first high RCS spike of J-20 actually located at around 50 degrees boresight. This mean it is very easy for J-20 pilots to keep enemy adversary within their stealthy sector. Add to the fact that J-20 has very big antenna aperture, it can be highly lethal in BVR combat.

References

Big thanks to: Stealthflankers, Akela Freedom, Eloise for helping me materialize this project.