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APA has released a new paper on stealth assesment on pak-fa ,but as this is a long article i am posting only few important paragraphs which are relevant, for more info plz follow the link which i have posted at the end
A Preliminary Assessment of Specular Radar Cross Section Performance in the Sukhoi T-50 Prototype
Air Power Australia Analysis 2012-03
12th November 2012
Introduction
The Russian Sukhoi/KnAAPO T-50/I-21/Article 701 PAK-FA (Перспективный Авиационный Комплекс Фронтовой Авиации
was the first manned combat aircraft design intended to possess low observable capabilities in the radar bands to be developed and publicly flown by a nation other than the United States, the first public flight shown in early 20101.
This paper extends earlier research focussed on the design of the T-50, and the Chengdu J-20, to provide a more accurate and quantitative preliminary assessment of the specular Radar Cross Section [RCS] of the T-50 design2,3.
A full and comprehensive assessment of the RCS of any Low Observable [LO / -10 to -30 dBSM, Refer Table A.1] or Very Low Observable [VLO / -30 to -40 dBSM, Refer Table A.1] aircraft design is a non-trivial task, as careful consideration needs to be given to all major and minor RCS contributors in the design, of which there can be a large number in a complex design such as a combat aircraft.
If such an assessment is to be genuinely useful, it must consider the vehicle's RCS from a range of different angular aspects encompassing azimuthal sectors and also elevation or depression angles characteristic of the surface and airborne threat systems the LO/VLO design is intended to defeat [Refer Figures A.3 and A.4].
The assessment of RCS must always be performed at the operating wavelengths typical of the surface and airborne threat systems the LO/VLO design is intended to defeat [Refer Table A.2].
Definitions of these and other terms employed in this document are summarised in Annex C. Reference data for RCS scales, radio-frequency bands, engagement geometries are summarised in Annex A. A summary of representative threat systems is available in Annex A, in an earlier publication3.
The T-50 was developed specifically to compete against the F-22 Raptor in traditional Beyond Visual Range (BVR) and Within Visual Range (WVR) air combat. As a result, the T-50 shares all of the cardinal fifth generation attributes until now unique to the F-22 - stealth, supersonic cruise, thrust vectoring, highly integrated avionics and a powerful suite of active and passive sensors.
The PAK-FA therefore firmly qualifies as a fifth generation design. In addition, it has two further attributes not introduced in the F-22 design. The first is extreme plus agility2, resulting from advanced aerodynamic design, exceptional thrust/weight ratio performance and three dimensional thrust vectoring integrated with an advanced digital flight control system. The second attribute is exceptional combat persistence, the result of an unusually large 25,000 lb internal fuel load. The former entailed some shaping compromises, at the expense of specular RCS performance.
The basic shaping design observed on prototypes of the PAK-FA will deny it the critical all-aspect stealth performance of the F-22, critical in BVR air combat and deep penetration operations. Despite this, the extreme manoeuvrability/controllability design features of the PAK-FA, which result in extreme plus agility, result in the potential for the PAK-FA to become the most lethal and survivable fighter ever built for air combat engagements.
The publicly displayed PAK-FA prototypes do not represent a production configuration of the aircraft, which is to employ a new engine design, and extensive VLO treatments which are not required on a prototype.
This assessment, like the earlier assessment performed on the J-20 design, cannot be more than preliminary for a number of important reasons:
The final airframe shaping remains unknown, and changes may arise through the development cycle, to improve aerodynamic performance, operational characteristics, and LO/VLO performance;
The state of Russian Radar Absorbent Materials (RAM), Radar Absorbent Structures (RAS) and radar absorbent coatings technology is not well understood in the West;
The state of Russian technologies for sensor aperture (radar, EO, passive RF) structural mode RCS reduction is not well understood in the West;
The state of Russian technologies for RCS flare spot reduction, in areas such as navigation/communications antennas, seals, panel joins, drain apertures, cooling vents, and fasteners is not well understood in the West.
Achievement of credible LO or VLO performance is the result of a design having intended RCS characteristics in all of these categories4.
Proper airframe shaping, as stated in Denys Overholser's famous dictum, is a necessary and essential prerequisite for good LO or VLO performance. If shaping design is deficient, no amount of credible materials application and detail flare spot reduction can overcome the RCS contributions produced by the airframe shape, and genuine VLO performance will be therefore unattainable. While this is self-evident, it is often not well appreciated.
If airframe shaping is credible, then careful and well considered application of Radar Absorbent Materials (RAM), Radar Absorbent Structures (RAS), radar absorbent coatings, aperture RCS reductions, and minor flare spot reduction techniques will yield a VLO design.
Modelling of the shape related RCS contributions of any VLO design is of very high value, as it determines not only whether the aircraft can achieve credible VLO category performance, but also where the designers will be investing effort in RAS, RAM and coating application to achieve this effect. Moreover, it exposes the angular extents within which the aircraft has poor RCS performance, and thus provides a robust basis for development of tactics and technique to defeat the design.
This paper will focus mostly on shape related RCS contributions, both due to the high value of knowing weaknesses in the design, but also due to the uncertainties inherent in estimating the performance of unknown technologies for RAS, RAM, coatings, aperture RCS reductions, and minor flare spot reduction. Where applicable, reasonable assumptions will be made as to the performance of absorbent material related RCS reduction measures.
T-50 Prototype Very Low Observable Airframe Shaping Design Features
Figure 1. The lower fuselage of the prototype displays interesting incongruities. There is an abrupt transition between the carefully sculpted faceting of the inlet nacelles, and the smoothly curved aft engine nacelles and conventional aft fuselage. The faceting strategy is similar to the F-22 design rules, with singly or doubly curved transitions between planes (C. Kopp/Sukhoi image)2.
An extensive qualitative analysis of RCS reduction shaping feature design in the T-50 aircraft was performed in 2010. That analysis yielded the following observations, cited here for convenience2:
1.The forward fuselage is closest in general configuration to the YF-23, especially in the chining, cockpit placement, and hump aft of the cockpit canopy, although the blending of the upper forward fuselage into the upper carapace is more gradual.
2.There are important differences from the YF-23. The chine curvature design rule is purely convex, like the chine design on the F-22A. The nose height is greater, to accommodate an AESA with a much larger aperture than that intended for the YF-23 or F-22A.
3.If flare spots are properly controlled by the application of materials and serrated edge treatments around the canopy, and a good bandpass radome design using a frequency selective multilayer laminate is employed, the shaping related RCS contribution of the forward fuselage in the S/X/Ku-bands will be similar to that observed with the F-22A, YF-23 or F-35.
4.The edge aligned movable LEX are readily treated with leading edge absorbers and will not present a major RCS flare spot. The treatment of the movable join will present the principal challenge in this portion of the design. The obtuse angle in the join between the LEX and forward fuselage is characteristic of good design and is very similar to the angles used in the F-22.
5.The edge aligned trapezoidal main engine inlets are similar in configuration to the F-22, but with important differences. The inlet aspect ratio is different, and the corners are truncated in a manner similar to the YF-23. If properly treated with leading edge inserts and inlet tunnel absorbent materials, the inlet design should yield similar RCS to its US counterparts.
6.The placement of the engine centrelines well above the inlet centroids, in the manner of the YF-23, results in an inlet tunnel S-bend in the vertical plane. Sukhoi have not disclosed whether an inlet blocker will be employed. The use of an S-bend in the PAK-FA would permit an increase in the number of surface bounces further increasing attenuation and reducing RCS.
7.In the S/X/Ku-bands the basic shaping of the forward fuselage will permit the attainment of genuine VLO performance with the application of mature RAS and RAM, where the centre and aft fuselage do not introduce larger RCS contributions from the forward aspect.
8.The wing design from a planform perspective is closest to the F-22A, and the upper fuselage similar to the YF-23, permitting the achievement of similar RCS performance to these US types, from respective aspects.
9.Where the PAK-FA falls well short of the F-22A and YF-23 is the shaping design of the lower fuselage and side fuselage, where the general configuration, wing/fuselage join angles, and inlet/engine nacelle join angles introduce similar intractable specular return problems as observed with the F-35 Joint Strike Fighter design. These are inherent in the current shaping design and cannot be significantly improved by materials application. .... the PAK-FA prototype design will produce a large specular return in any manoeuvre where the lower fuselage is exposed to a threat emitter, and this problem will be prominent from the Ku-band down to the L-band.
10.This problem is exacerbated by the inboard ventral wing root fairings, claimed by some Russian sources to be pods for the concealed carriage of folding fin close combat AAMs, such as the RVV-MD/R-74 series. While these fairings do not introduce large RCS contributions from fore or aft aspects, they will adversely contribute to beam aspect RCS, especially for threats well below the plane of flight of the aircraft.
11.The tailboom shaping is reminiscent of the F-22 and F-35 designs, and will not yield significant RCS contributions from the front or aft aspects.
12.In the lower hemisphere, it will suffer penalties due to the insufficiently obtuse join angles between the wings and stabilators, and outer engine nacelles.
13.The upper fuselage fairings which house the all moving vertical tail actuators are well shaped, and the join angles are well chosen.
14.The outward cant of the empennage fins is similar to United States designs, and like the YF-23 tail surfaces, these are fully articulated with the VLO benefit of removing surface impedance discontinuities at the join of a conventional rudder control surface.
15.The axi-symmetric 3D TVC nozzles present the same RCS problems observed with the fixed axi-symmetric nozzles used in the F-35 JSF, and the application of serrated shroud treatments and tailpipe blockers as used with the F-35 JSF will not overcome the inherent limitations of this canonical shaping design. Observed from the aft hemisphere in the L-band through Ku-bands, the PAK-FA prototype configuration will produce to an order of magnitude an equally poor RCS as the F-35 Joint Strike Fighter5.
16.The centre fuselage beavertail follows a similar chine design rule as the forward fuselage does, and will not present a significant RCS contribution from behind.
These observations reflected the design of the first prototype. Subsequent prototypes, publicly displayed, do not fundamentally differ in any of these design features.
The qualitative analysis yielded the conclusion that with proper application of materials technology, detail feature RCS reduction treatments, aperture structural mode RCS reduction measures, the T-50 had potential to yield viable VLO performance in the forward sector, and with a nozzle design similar to the F-22A, had potential for viable VLO performance in the aft sector. Performance in the beam sector and lower hemsiphere were identified as problematic. This conclusion was a result of several specific shaping features, specifically the lower fuselage tunnel design, and the absence of obtuse angle joins between the aft fuselage sides and wing / stabilator joins, and the obtuse join angles in the fuselage tunnel.
Quantitative RCS modelling will demonstrate that these observations were valid.
Conclusions
This study has explored the specular Radar Cross Section of the Sukhoi T-50 prototype aircraft shaping design. Simulations using a Physical Optics simulation algorithm were performed for frequencies of 150 MHz, 600 MHz, 1.2 GHz, 3.0 GHz, 6.0 GHz, 8.0 GHz, 12.0 GHz, 16.0 GHz and 28 GHz without an absorbent coating, and for frequencies of 1.2 GHz, 3.0 GHz, 6.0 GHz, 8.0 GHz, 12.0 GHz, 16.0 GHz with an absorbent coating, covering all angular aspects of the airframe.
If the production T-50 retains the axisymmetric nozzles and extant ventral fuselage design, the aircraft would still deliver robust Very Low Observable performance in the nose aspect angular sector, providing that effective RCS treatments are applied to suppress surface travelling waves, inlet and edge reflections.
If the production T-50 introduces a rectangular faceted nozzle design, and refinements to lower fuselage and side shaping, the design would present very good potential for good Very Low Observable performance in the S-band and above, for the nose and tail aspect angular sectors, with reasonable performance in the beam aspect angular sector. The extent to which this potential could be exploited would depend critically on the design of the nozzles and other shaping refinements.
In conclusion, this study has established through Physical Optics simulation across nine frequency bands, that no fundamental obstacles exist in the shaping design of the T-50 prototype, which might preclude its development into a genuine Very Low Observable design with constrained angular coverage.
IMAGE GALLERY
Figure 3. The T-50 has five major lobes in the left and right beam aspect angular sectors, for convenience labelled A through E, with the flat lower fuselage mainlobe labelled F (KnAAPO image).
Figure 4. Mapping of the five major lobes in the left and right beam aspect angular sectors at 16 GHz (KnAAPO image).
Figure 5. The T-50 from behind, showing the corner joins in the inlet tunnel, and fuselage sides (KnAAPO image).
SOURCE:
A Preliminary Assessment of Specular Radar Cross Section Performance in the Sukhoi T-50 Prototype
APA has released a new paper on stealth assesment on pak-fa ,but as this is a long article i am posting only few important paragraphs which are relevant, for more info plz follow the link which i have posted at the end
A Preliminary Assessment of Specular Radar Cross Section Performance in the Sukhoi T-50 Prototype
Air Power Australia Analysis 2012-03
12th November 2012
Introduction
The Russian Sukhoi/KnAAPO T-50/I-21/Article 701 PAK-FA (Перспективный Авиационный Комплекс Фронтовой Авиации
was the first manned combat aircraft design intended to possess low observable capabilities in the radar bands to be developed and publicly flown by a nation other than the United States, the first public flight shown in early 20101.This paper extends earlier research focussed on the design of the T-50, and the Chengdu J-20, to provide a more accurate and quantitative preliminary assessment of the specular Radar Cross Section [RCS] of the T-50 design2,3.
A full and comprehensive assessment of the RCS of any Low Observable [LO / -10 to -30 dBSM, Refer Table A.1] or Very Low Observable [VLO / -30 to -40 dBSM, Refer Table A.1] aircraft design is a non-trivial task, as careful consideration needs to be given to all major and minor RCS contributors in the design, of which there can be a large number in a complex design such as a combat aircraft.
If such an assessment is to be genuinely useful, it must consider the vehicle's RCS from a range of different angular aspects encompassing azimuthal sectors and also elevation or depression angles characteristic of the surface and airborne threat systems the LO/VLO design is intended to defeat [Refer Figures A.3 and A.4].
The assessment of RCS must always be performed at the operating wavelengths typical of the surface and airborne threat systems the LO/VLO design is intended to defeat [Refer Table A.2].
Definitions of these and other terms employed in this document are summarised in Annex C. Reference data for RCS scales, radio-frequency bands, engagement geometries are summarised in Annex A. A summary of representative threat systems is available in Annex A, in an earlier publication3.
The T-50 was developed specifically to compete against the F-22 Raptor in traditional Beyond Visual Range (BVR) and Within Visual Range (WVR) air combat. As a result, the T-50 shares all of the cardinal fifth generation attributes until now unique to the F-22 - stealth, supersonic cruise, thrust vectoring, highly integrated avionics and a powerful suite of active and passive sensors.
The PAK-FA therefore firmly qualifies as a fifth generation design. In addition, it has two further attributes not introduced in the F-22 design. The first is extreme plus agility2, resulting from advanced aerodynamic design, exceptional thrust/weight ratio performance and three dimensional thrust vectoring integrated with an advanced digital flight control system. The second attribute is exceptional combat persistence, the result of an unusually large 25,000 lb internal fuel load. The former entailed some shaping compromises, at the expense of specular RCS performance.
The basic shaping design observed on prototypes of the PAK-FA will deny it the critical all-aspect stealth performance of the F-22, critical in BVR air combat and deep penetration operations. Despite this, the extreme manoeuvrability/controllability design features of the PAK-FA, which result in extreme plus agility, result in the potential for the PAK-FA to become the most lethal and survivable fighter ever built for air combat engagements.
The publicly displayed PAK-FA prototypes do not represent a production configuration of the aircraft, which is to employ a new engine design, and extensive VLO treatments which are not required on a prototype.
This assessment, like the earlier assessment performed on the J-20 design, cannot be more than preliminary for a number of important reasons:
The final airframe shaping remains unknown, and changes may arise through the development cycle, to improve aerodynamic performance, operational characteristics, and LO/VLO performance;
The state of Russian Radar Absorbent Materials (RAM), Radar Absorbent Structures (RAS) and radar absorbent coatings technology is not well understood in the West;
The state of Russian technologies for sensor aperture (radar, EO, passive RF) structural mode RCS reduction is not well understood in the West;
The state of Russian technologies for RCS flare spot reduction, in areas such as navigation/communications antennas, seals, panel joins, drain apertures, cooling vents, and fasteners is not well understood in the West.
Achievement of credible LO or VLO performance is the result of a design having intended RCS characteristics in all of these categories4.
Proper airframe shaping, as stated in Denys Overholser's famous dictum, is a necessary and essential prerequisite for good LO or VLO performance. If shaping design is deficient, no amount of credible materials application and detail flare spot reduction can overcome the RCS contributions produced by the airframe shape, and genuine VLO performance will be therefore unattainable. While this is self-evident, it is often not well appreciated.
If airframe shaping is credible, then careful and well considered application of Radar Absorbent Materials (RAM), Radar Absorbent Structures (RAS), radar absorbent coatings, aperture RCS reductions, and minor flare spot reduction techniques will yield a VLO design.
Modelling of the shape related RCS contributions of any VLO design is of very high value, as it determines not only whether the aircraft can achieve credible VLO category performance, but also where the designers will be investing effort in RAS, RAM and coating application to achieve this effect. Moreover, it exposes the angular extents within which the aircraft has poor RCS performance, and thus provides a robust basis for development of tactics and technique to defeat the design.
This paper will focus mostly on shape related RCS contributions, both due to the high value of knowing weaknesses in the design, but also due to the uncertainties inherent in estimating the performance of unknown technologies for RAS, RAM, coatings, aperture RCS reductions, and minor flare spot reduction. Where applicable, reasonable assumptions will be made as to the performance of absorbent material related RCS reduction measures.
T-50 Prototype Very Low Observable Airframe Shaping Design Features
Figure 1. The lower fuselage of the prototype displays interesting incongruities. There is an abrupt transition between the carefully sculpted faceting of the inlet nacelles, and the smoothly curved aft engine nacelles and conventional aft fuselage. The faceting strategy is similar to the F-22 design rules, with singly or doubly curved transitions between planes (C. Kopp/Sukhoi image)2.
An extensive qualitative analysis of RCS reduction shaping feature design in the T-50 aircraft was performed in 2010. That analysis yielded the following observations, cited here for convenience2:
1.The forward fuselage is closest in general configuration to the YF-23, especially in the chining, cockpit placement, and hump aft of the cockpit canopy, although the blending of the upper forward fuselage into the upper carapace is more gradual.
2.There are important differences from the YF-23. The chine curvature design rule is purely convex, like the chine design on the F-22A. The nose height is greater, to accommodate an AESA with a much larger aperture than that intended for the YF-23 or F-22A.
3.If flare spots are properly controlled by the application of materials and serrated edge treatments around the canopy, and a good bandpass radome design using a frequency selective multilayer laminate is employed, the shaping related RCS contribution of the forward fuselage in the S/X/Ku-bands will be similar to that observed with the F-22A, YF-23 or F-35.
4.The edge aligned movable LEX are readily treated with leading edge absorbers and will not present a major RCS flare spot. The treatment of the movable join will present the principal challenge in this portion of the design. The obtuse angle in the join between the LEX and forward fuselage is characteristic of good design and is very similar to the angles used in the F-22.
5.The edge aligned trapezoidal main engine inlets are similar in configuration to the F-22, but with important differences. The inlet aspect ratio is different, and the corners are truncated in a manner similar to the YF-23. If properly treated with leading edge inserts and inlet tunnel absorbent materials, the inlet design should yield similar RCS to its US counterparts.
6.The placement of the engine centrelines well above the inlet centroids, in the manner of the YF-23, results in an inlet tunnel S-bend in the vertical plane. Sukhoi have not disclosed whether an inlet blocker will be employed. The use of an S-bend in the PAK-FA would permit an increase in the number of surface bounces further increasing attenuation and reducing RCS.
7.In the S/X/Ku-bands the basic shaping of the forward fuselage will permit the attainment of genuine VLO performance with the application of mature RAS and RAM, where the centre and aft fuselage do not introduce larger RCS contributions from the forward aspect.
8.The wing design from a planform perspective is closest to the F-22A, and the upper fuselage similar to the YF-23, permitting the achievement of similar RCS performance to these US types, from respective aspects.
9.Where the PAK-FA falls well short of the F-22A and YF-23 is the shaping design of the lower fuselage and side fuselage, where the general configuration, wing/fuselage join angles, and inlet/engine nacelle join angles introduce similar intractable specular return problems as observed with the F-35 Joint Strike Fighter design. These are inherent in the current shaping design and cannot be significantly improved by materials application. .... the PAK-FA prototype design will produce a large specular return in any manoeuvre where the lower fuselage is exposed to a threat emitter, and this problem will be prominent from the Ku-band down to the L-band.
10.This problem is exacerbated by the inboard ventral wing root fairings, claimed by some Russian sources to be pods for the concealed carriage of folding fin close combat AAMs, such as the RVV-MD/R-74 series. While these fairings do not introduce large RCS contributions from fore or aft aspects, they will adversely contribute to beam aspect RCS, especially for threats well below the plane of flight of the aircraft.
11.The tailboom shaping is reminiscent of the F-22 and F-35 designs, and will not yield significant RCS contributions from the front or aft aspects.
12.In the lower hemisphere, it will suffer penalties due to the insufficiently obtuse join angles between the wings and stabilators, and outer engine nacelles.
13.The upper fuselage fairings which house the all moving vertical tail actuators are well shaped, and the join angles are well chosen.
14.The outward cant of the empennage fins is similar to United States designs, and like the YF-23 tail surfaces, these are fully articulated with the VLO benefit of removing surface impedance discontinuities at the join of a conventional rudder control surface.
15.The axi-symmetric 3D TVC nozzles present the same RCS problems observed with the fixed axi-symmetric nozzles used in the F-35 JSF, and the application of serrated shroud treatments and tailpipe blockers as used with the F-35 JSF will not overcome the inherent limitations of this canonical shaping design. Observed from the aft hemisphere in the L-band through Ku-bands, the PAK-FA prototype configuration will produce to an order of magnitude an equally poor RCS as the F-35 Joint Strike Fighter5.
16.The centre fuselage beavertail follows a similar chine design rule as the forward fuselage does, and will not present a significant RCS contribution from behind.
These observations reflected the design of the first prototype. Subsequent prototypes, publicly displayed, do not fundamentally differ in any of these design features.
The qualitative analysis yielded the conclusion that with proper application of materials technology, detail feature RCS reduction treatments, aperture structural mode RCS reduction measures, the T-50 had potential to yield viable VLO performance in the forward sector, and with a nozzle design similar to the F-22A, had potential for viable VLO performance in the aft sector. Performance in the beam sector and lower hemsiphere were identified as problematic. This conclusion was a result of several specific shaping features, specifically the lower fuselage tunnel design, and the absence of obtuse angle joins between the aft fuselage sides and wing / stabilator joins, and the obtuse join angles in the fuselage tunnel.
Quantitative RCS modelling will demonstrate that these observations were valid.
Conclusions
This study has explored the specular Radar Cross Section of the Sukhoi T-50 prototype aircraft shaping design. Simulations using a Physical Optics simulation algorithm were performed for frequencies of 150 MHz, 600 MHz, 1.2 GHz, 3.0 GHz, 6.0 GHz, 8.0 GHz, 12.0 GHz, 16.0 GHz and 28 GHz without an absorbent coating, and for frequencies of 1.2 GHz, 3.0 GHz, 6.0 GHz, 8.0 GHz, 12.0 GHz, 16.0 GHz with an absorbent coating, covering all angular aspects of the airframe.
If the production T-50 retains the axisymmetric nozzles and extant ventral fuselage design, the aircraft would still deliver robust Very Low Observable performance in the nose aspect angular sector, providing that effective RCS treatments are applied to suppress surface travelling waves, inlet and edge reflections.
If the production T-50 introduces a rectangular faceted nozzle design, and refinements to lower fuselage and side shaping, the design would present very good potential for good Very Low Observable performance in the S-band and above, for the nose and tail aspect angular sectors, with reasonable performance in the beam aspect angular sector. The extent to which this potential could be exploited would depend critically on the design of the nozzles and other shaping refinements.
In conclusion, this study has established through Physical Optics simulation across nine frequency bands, that no fundamental obstacles exist in the shaping design of the T-50 prototype, which might preclude its development into a genuine Very Low Observable design with constrained angular coverage.
IMAGE GALLERY
Figure 3. The T-50 has five major lobes in the left and right beam aspect angular sectors, for convenience labelled A through E, with the flat lower fuselage mainlobe labelled F (KnAAPO image).
Figure 4. Mapping of the five major lobes in the left and right beam aspect angular sectors at 16 GHz (KnAAPO image).
Figure 5. The T-50 from behind, showing the corner joins in the inlet tunnel, and fuselage sides (KnAAPO image).
SOURCE:
A Preliminary Assessment of Specular Radar Cross Section Performance in the Sukhoi T-50 Prototype
