PAK-FA Low Observable Design
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).
The low observable design shaping employed in the PAK-FA prototype shows an excellent grasp of the design rules employed by American designers in the development of the F-22A and YF-23 Advanced Tactical Fighter. This reflects an observation made to one the authors by a senior American design engineer some years ago "we always end up doing the really hard work learning how to build these things, making it easy for the Russians to follow with their designs".
The likely exploitation of F-22A and YF-23 Advanced Tactical Fighter low observable shaping design rules was predicted through analysis as most likely during the past decade, and subsequently published in March 2009. Sukhoi's prototype shaping validated that analytical prediction5.
As observed previously, the Russian approach to development follows an "evolutionary" design philosophy, in which risks are retired early in the development phase of a new aircraft type or variant. Where possible, the retirement of risks is achieved in earlier programs, as demonstrated repeatedly in the development of the T-10 Flanker series of aircraft.
The PAK-FA prototypes displayed in January, 2010, are clearly intended to validate the compatibility of the overall observables shaping with the aerodynamic and structural design needs and clearly so, as the expensive detail RCS flare spot treatments we are accustomed to seeing on US prototypes are absent. The rationale for this is simple - why expend valuable but scarce development resources if aerodynamic / structural load testing shows that major changes are required to shaping of important design elements? For Western contractors, where the imperative is to extract the maximum of development funding from the customer, and make early cancellation of a program difficult, the highest risk approach will nearly always be sought by senior management. An excellent case study of the latter is the extremely high level of "concurrency risk", reported by the General Accounting Office, in the F-35 Joint Strike Fighter program.
The risk minimisation oriented development strategy explains the absence of serrations on the ventral inlet blow-in doors, and the absence of a serrated nozzle on the interim engine design. Design features which are intended to be permanent, such as the ventral weapon bay doors, aerial refuelling probe doors, and large access panels, all employ edge alignment or serrations no differently than the B-2A, F-22A and YF-23 demonstrator.
It is important to note that VLO shaping design is the single most critical aspect of VLO design with contemporary basic technology. This is because once the shaping is fixed in the design, the cost of implementing changes is prohibitive downstream, impacting structural design, aerodynamic behaviour and internal packaging of systems. If VLO shaping is done poorly, early in the development cycle, with the F-35 lower and aft fuselages being the representative case study, no reasonable downstream investment in additional absorbent materials and structures can overcome the resulting signature problems, and may introduce additional problems with weight, cost and strength/stiffness of skin panels.
By aiming for the best possible VLO shaping in the PAK-FA design from the very outset, Sukhoi's designers have demonstrated that they understand this aspect of VLO design very well. This strategy also opens up the prospect of progressive improvements in VLO performance as the design matures, and better VLO materials technology becomes available.
The prototypes show the extensive use of what appears to be conventional riveting, and conventional construction. If genuine VLO capability is intended, extensive robotic surface coating treatment or appliqué laminate technology will be required, with both techniques requiring a highly conductive substrate layer to suppress the surface impedance discontinuities resulting from the construction technique used. As observed in other areas of the Russian industrial base, coating and surface treatment technologies are well understood, and world class capabilities are available.
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. 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. 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.
The Electro-Optical System (OLS) turret employed on the prototype is likely the Su-35S OLS, and is incompatible with a VLO design, as it is a broadband spherical reflector. We can expect to see a faceted VLO fairing similar to that designed for the cancelled F-22A AIRST (Advanced IRST [Image]) in a production PAK-FA configuration.
The conventional pitot-static probes currently mounted around and forward of the cockpit are like the OLS turret, incompatible with a VLO design, and we can also expect to see these replaced with VLO design ports in a production PAK-FA configuration.
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.
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.
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. Public disclosures on Su-35S inlet treatments claimed a ~15 dB reduction in X-band RCS compared to the untreated inlet tunnels on the Su-27SK. 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.
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.
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.
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. Like the F-35 Joint Strike Fighter, 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.
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.
The shaping remedy for the beam aspect signature problem lies in more obtuse join angles, which would require considerable effort in resculpting the fuselage/wing join from the main undercarriage bays to the tail, and narrowing the usable width of the lower fuselage tunnel between the nacelles. The latter is problematic. An alternative may be the use of thick RAM treatments, in effect replacing the skins of the sides of the inner forward lower fuselage tunnel with RAM panels, with some weight penalty as a result, which would not be significant relative to overall aircraft weight, given the small area to be treated.
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. In the lower hemisphere, it will suffer penalties due to the insufficiently obtuse join angles between the wings and stabilators, and outer engine nacelles. The upper fuselage fairings which house the all moving vertical tail actuators are well shaped, and the join angles are well chosen. 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.
The axi-symmetric 3D TVC nozzles present the same RCS problems observed with the fixed axi-symmetric nozzles used in the F-35 JSF [analysis/imagery], 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 Fighter10.
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.
If production PAK-FA aircraft employ the same lower and aft fuselage design as the prototype does, they will be susceptible to aft hemisphere and beam aspect threats at depressed angles, operating from the L-band through to the Ku-band, in a manner no different to the F-35 Joint Strike Fighter.
It is worth observing that the unconventional flight control capabilities of the PAK-FA do open up some possibilities, in that they permit manoeuvres such as flat turns, or even turns where the bank angle is opposite to a conventional banking turn. Such manoeuvres permit the PAK-FA to execute, without difficulty or high energy bleed, turns away from beam aspect threats without significant exposure of the problematic lower fuselage, unlike the conventional F-35 JSF which becomes unavoidably susceptible to detection, tracking and missile shots in such geometries. As the PAK-FA will provide a similar supersonic cruise capability to the F-22, its window of vulnerability is very much shorter when attempting to evade a tail aspect threat, and it has a credible capability to defeat missile shots kinematically.
Whether the current aft fuselage design of the PAK-FA is an artefact of the use of off-the-shelf Su-35S engines, or a permanent long term feature of the design, is unclear.
The general configuration of the PAK-FA aft fuselage is as compatible with the style of 2D VLO shaped TVC nozzles used in the F-22A, and integrated with the F119-PW-100 engines, as it is compatible in principle with the superb non-thrust vector aft fuselage design used in the YF-23. The latter remains the benchmark for wideband aft sector VLO fuselage design.
Producing a 3D TVC nozzle design which has similar VLO shaping performance as the F-22A 2D TVC nozzle design is not a trivial task - there is no obvious simple solution to this problem. If the Russians have solved it, it would be a major advance in VLO nozzle design.
Until Sukhoi disclose their intentions in this area, such as deployable LO shrouds for cruising flight, or provide imagery of the production PAK-FA aft fuselage design, this will remain an unresolved issue.
From an RCS engineering perspective, the shaping design of the PAK-FA is an excellent first attempt by the Russians to produce a high quality VLO design. The forward fuselage and engine inlet area shaping design is highly competitive against more recent US designs, and with mature high quality RAS and RAM application, have genuine VLO potential. The upper fuselage, wing and tail surface shaping and planform alignment are also competitive against US designs.
The problematic lower and aft fuselage designs, if retained in production aircraft, will deny the PAK-FA the kind of deep penetration capability sought in the design of the F-22A and YF-23.
The only cited RCS performance data was a recent claim by Sukhoi that the PAK-AF will have 1/40 of the RCS of the Su-35S. Unfortunately this was not qualified by threat operating band, aspect, or whether the Su-35S was clean or laden with external stores. The RCS of the Su-35S, head-on in the X-band, has not been disclosed, but given the extensive RAM treatments applied could be as low as 0.5 - 2 m2 for a clean aircraft with no stores. If the latter were true, then the PAK-FA X-band head-on RCS would be of the order of -13 to -19 dBSM. Such performance would be consistent with the shaping design, but not with the application of mature RAM and RAS to same.
Analysis of tactical options, as published in March 2009, assumed a PAK-FA forward sector X-band RCS of about -20 dBSM, which fits the outer envelope of the Sukhoi disclosure almost exactly5,6.
The Russians have claimed that the design has engine infrared signature reduction measures, but these have not been detailed. The conventional axi-symmetric nozzle design is generally ineffective, from an infrared signature perspective, as the nozzle shrouds are exposed radiators, and the cylindrical exhaust aperture radiates into a conical volume behind the aircraft.
The use of 3D TVC nozzles with high angle rates, which are fully integrated in the DFCS, would present opportunities to minimise RCS contributions resulting from aerodynamic control surface movements, by employing where possible TVC controls for primary pitch, roll and yaw control when performing stealthy penetration. Given that this flight regime entails flight in cruise configuration, and gentle turning manoeuvres to minimise bank angles, observably large deflection control inputs would be unusual and thus very infrequent. As a result the pitch, roll and yaw rates produced by the TVC system alone would be sufficient for most control inputs in the stealthy penetration regime of flight.