Sukhoi PAK FA: Technical Analysis Part 2By Sergio ConiglioAirframe
The aerodynamic configuration of the PAK-FA maintains a vague reference to the Su-27 as regards the fuselage and the location of the engines, which are installed in widely separated nacelles forming a tunnel with the flat bottom of the fuselage. The general planform is a tailed delta, similar to the F-22, with the all-moving horizontal tailplanes close-coupled and on the same plane to the wing without any gap. The twin vertical surfaces, canted outward by perhaps 25°, are also all-moving. This solution as been used rarely in recent times; in particular the ill-fated Northrop YF-23 had a pair of all-moving butterfly tailplanes. The all-moving verticals however had been fairly used in supersonic designs dating back to the late 1950s or 60s, in particular the SR-71 which used a pair of all-moving verticals canted inward to reduce the induced roll moment when the surfaces were rotated, and most of the North American design of the period - the RA-5C VIGILANTE, its contemporary YF-107 and the unique XB-70 - as well as the British BAC TSR 2 used a similar solution. In the PAK FA design, their reason d’être arguably consists in enabling the smallest possible vertical surfaces for the sake of reduced radar signature and supercruise drag, while at the same time also maintaining (in combination with the 3D TVC nozzles) excellent manoeuvrability.
The underfuselage tunnel between the engine nacelles contributes significantly to the overall aerodynamic lift generation, just as in the Su-27 and MiG-29 as well as in the F-14 - arguably the real originator of the “centreplane lift” concept. This lift is added to that provided by the large wing and should enable excellent manoeuvrability even at high altitude - a potential advantage of the F-22 and now the PAK FA over all their rivals. The widely separated engines also offer much better survivability in the event of battle damage or accidental fire/explosion.
The fuselage sides have marked “chines”, again like the F-22 and its unfortunate competitor, the YF-23. This shaping can be assumed both to contribute toward reducing radar reflectivity and to develop, at high angles of attack, favourable lift-enhancing vortexes flowing above the inner wing upper surface just above the engine nacelles. The wing has dropping leading edges providing for a variable camber airfoil and separate flaps and ailerons, these latter contributing towards enhanced TO/landing performance (this should anyway be very good, given the huge lift generated by the aircraft configuration as a whole). The inner part of the wing leading edges is stepped longitudinally with a much longer chord which blends forming, in part, the engine nacelles’ upper “lips” and then merging into the fuselage to enhance the lift generating characteristics of the overall aircraft configuration, somewhat akin to a lifting body. Possibly for this reason, but also to ease a smooth airflow into the engines at very high angle of attack, the upper intake projecting false “lips” appear to be hinged parallel to the sweep real intake lips, thus providing a variable camber like the wing leading edge. In this way, the upper surface of the air intake contributes to overall lift generation. It is also possible that the movements of these peculiar elements, when linked to the full authority digital flight control system, could contribute in some way to the aircraft’s longitudinal control, acting like a third control surface (in line with the Sukhoi tradition as exemplified in the three-surfaces Su-30MKI). It seem however clear that the “lips” cannot move as fully independent control surfaces, due to their primary role in ensuring a correct airflow to the engines.
The possible rationale behind the fuselage “chines” and wing strakes could be to generate two vortexes over each wing upper surface, thus enhancing lift (via more diffused vortex lift) at high angle of attack (AoA). In particular, the two inner vortexes (those generated by the fuselage “chines”) would energise the airflow over the inner wing upper surface blending with the fuselage above the engine nacelles. The two outer vortexes (those generated from the wing strakes outboard the intakes lips) would transfer their kinematic energy to the upper outer panel wing airflow. Furthermore, given the expected path of such latter vortexes, they would also interact with the upper airflow over the all-moving horizontal tailplanes - thus replicating the superior longitudinal control provided in the Su-27 by its peculiarly located slab tailplanes.*
Summing up, lift appears to be generated by following elements, working in a synergic way:
• Wing outer panels (outside the engine nacelles) with dropping leading edges (variable camber airfoil);
• Engine nacelles upper surface blended with outer wing panels and fuselage with dropping intake upper false lips/leading edges (variable camber);
• Fuselage tunnel between the engine nacelles;
• Vortexes generated from the front fuselage “chines“, enhancing the engine nacelles upper surface lift and possibly the all-moving verticals’ control authority at very high AoA;
• Vortexes generated by the wing strakes outboard the engine nacelles, enhancing the outer wing panels lift and possibly the all-moving horizontal tailplanes control authority at very high AoA.
The fuselage has the already mentioned flat bottom and a straight tapered upper part ending in a flat and somewhat smaller “sting” between the engine exhausts. The installation of a braking parachute in a bay in the upper part of the sting makes room for the rational introduction in the extreme tailcone of a wide-scanning ECM antenna or perhaps a rear hemisphere surveillance/tracking radar (experiments were carried out a few years ago on a modified Su-32FN). The second prototype, which was used for taxi trials on 23 January appears to have a different tail cone, for unclear reasons.
The rear fuselage beavertail appears wider than in the Su-27/-30 albeit with a similar layout, and should offer more freedom of movement to the multi-axis thrust vectoring control (TVC) exhaust nozzles which will most certainly be fitted to the engines of the T-50 (although their current presence on the PAK FA is not certain). This configuration with the widely exposed round engine exhaust nozzles is however detrimental in terms of rear-emisphere IR and radar signature.
The PAK FA is claimed by Sukhoi to offer “unprecedented small signatures in the radar, optical and infrared range”, and this is certainly true as regards Russian combat aircraft and quite possibly all existing non-American designs. At the same time, it is evident that the PAK FA has been designed with a close attention to stealth characteristics, but is not intended to be an uncompromising stealth aircraft à la F-22. When certain design features detrimental to low observability were deemed to be all-important, these were adopted nonetheless. It would be extremely interesting to watch the eventual results of this approach in terms of maintainability and operational availability, particularly in the light of the in-service experience so far with the F-22.
An element which maintains some similarity to the Su-27 family is the landing gear. All the members retract forward, easing the emergency extension which in this way can be accomplished simply by gravity and air pressure. The main tyres, again like the previous Sukhoi design, when retracted lays flat in bays partially above the air intakes and partially inside the thick wing root fairing born out from the air intake upper part and as a continuation of the sweep surface linking the fuselage side to the outer wing, running above the upper air intake lip.
The PAK FA appears to be built with a significant percentage of composites, including most of the wing, horizontal tailplanes and dropping intake lips skin, centre-forward engine nacelles, most of the fuselage skin and the doors of the weapons bays and landing gear bays. Metal parts seem to include the dropping wing and intake lips leading edges (with the exception of the inner sections where the conformal aerials are expected to be installed, and which should thus be built of dielectric material), the engine intakes and the wide fairings blending the outer wing panels to the fuselage. Press reports suggest a 75% (being weight) being made of titanium alloys and 20% by composites, which sounds plausible.Powerplant
The planned engine for the T-50 is understood to be the new Saturn AL-41F, expected to offer about 17.5 tons of thrust in full afterburning mode and somewhere in the range of 12 tons in dry mode. The latter figure would comfortably enable supercruising (i.e., supersonic cruise flight without afterburner) at around Mach 1.5, thus in the same class as the F-22. The prototype/technology demonstrator now flying was expected to be powered by the Saturn 117S, a much improved version of the AL-31F intended for the Su-35 but still less powerful at 14.5 tons in full afterburning than the AL-41F. There however are some indications to suggest that the aircraft already has the new engines.
The engines are fed by two-dimensional raked air intakes with the upper lip generating an oblique shock wave favourable to dynamic pressure recovery in the supersonic regime, which for the PAK FA could approach Mach 2.3÷2.5. While in appearance of fixed geometry, it is possible that a variable-position upper ramp, to generate multiple oblique shocks is part of the system for a further better dynamic pressure recovery in the high supersonic speed regime.
The tight shape of the engine nacelles and the position of the ventral “venetian blind” auxiliary intakes seem to suggest that the PAK FA does not feature a serpentine air duct to the engine compressors, as typically adopted for low-RCS aircraft. It is possible that the Sukhoi designers have preferred to limit the compressors’ strong radar reflection by inserting a grill in front of them, while optimising the air intakes for higher max. speed and supercruise performance.
The engines are mounted with a slight forward convergence (some 3°). This, in twin-engine aircraft with conventional exhaust nozzles, would typically reduce thrust asymmetry in the event of an engine flame-out - although with the drawback of reduced controllability. Given however the installation of TVC nozzles, the choice of converging axis built into the nacelles could be the outcome of an aerodynamic local airflow optimisation due to interaction of all the aircraft elements.
A large fuel capacity in line with the previous Sukhoi fighters is certainly provided, let’s say in the order of 12,000 litres. A fully-retractable in-flight refuelling probe is installed on the left side of the fuselage in front of the windscreen.Armament
The standard air-to-air armament is carried internally in two identical tandem weapon bays, which can be estimated at about 5m x 1.2-1.3m. The bays’ position inside the tunnel between the engine nacelles ensures a discrete opening of their doors at weapons launch, otherwise a drawback for a stealth aircraft. In addition, the doors have saw tooth-shaped edges to further reduce radar signature. The size of the bays can be assumed to allow internal carriage of eight R-77-class radar-guided AAMs with folding wings, i.e. the same figure as for the F-22.
Similar to American 5th generation types, for the “second/subsequent” days of war operations, four additional underwing hardpoints can be installed under the outer wing panels. However no wingtip store positions appear to have been foreseen. A dark area to the right side of the upper front fuselage under the cockpit betrays the installation, similarly to the Su-27, of a single cannon (a 30mm GSh-30-1?) for close combat engagements.Avionics
The combat avionics of the T-50 has been under development for some time, and some elements will almost certainly be installed in the Su-35 interim fighter. The main sensor will be a Tikhomorov NIIP X-band radar with active AESA antenna, which was unveiled at the latest MAKS Air Show in August 2009. The 1m-dia. antenna contains some 1,500 solid stat transmit/receive modules by NPP Pulsar, which places it in the same class as the F-22’s APG-77. Tikhomorov claims an exceptional range of ~400km against a 1m² equivalent radar surface target. The radar entered bench testing in November 2008, and a flyable operational prototype will be completed by mid-2010.
In a very innovative development, the main X-band antenna will be supplemented by auxiliary L-band antennas installed in the wing inboard leading edges. In addition to the obvious IFF/SSR functions, this arrangement (which is also being offered for retrofit on the Su-27/-30 family as well as the Su-35), has a very clear anti-stealth search function. Most current stealth or semi-stealth designs - and most particularly the F-35 JSF, although not the F-22 - are optimised to reduce radar signature against X-band fire control radars as the main threat, and their low-observability features and shapings do not work as well against L-band radars. Of course, the lower the frequency the higher the wavelength the poorer the accuracy of distance and angular measurements, and thus even apart from excessive volume, weight, power and cooling requirements a fighter aircraft could not possibly rely on a main L-band system alone. However, the presence of the additional L-band antennas will provide an important early warning function against at least some low-observable targets, and it may also enable a “mini-AWACS” role. It is additionally conceivable that these antennas could also be used for the detection and disruption of sensors and digital communications systems operating in L-band, including e.g. the all-important JTIDS/MIDS/Link-16.
While the PAK FA has no functioning radar yet, it already sports the protruding head of an electro-optic IRST system in front and to the right of the cockpit’s windscreen. This will maintain the excellent mixed solution (radar/IRST) used in all modern Russian fighters, event tough the IRST seeker’s “ball” is at odds with the search for a reduced radar signature in the front emisphere. The decision to add the L-band antennas while maintaining the IRST reinforces the perception of the T-50 being mainly intended for air defence roles against intruding low-observable strike aircraft.
The Indian Factor
Back in early 2007, Russia and India reached an agreement to cooperate on a Fifth Generation Fighter Aircraft (FGFA) based on the PAK FA for the Indian Air Force. The programme is officially described as involving a 50-50% split as regards both financing and R&D activities, but it is nearly universally understood to rather cover a scheme, under which India will fund a substantial portion of the PAK FA’s development bill in exchange for access to the relevant technologies.
The Indian Air Force’s requirements do differ rather substantially from the Russian Air Force’s, and are reported to demand a twin-seat configuration as well as possibly a different wing and control surfaces. Hindustan Aeronautics Ltd is expected to become responsible for some 25% of the total development workload for the FGFA programme, involving modifying the PAK FA single-seater airframe to a twin-seater configuration as well as the mission computer, navigation system, cockpit displays and ECM dispensers. HAL will of course also take care of eventual series production of a tentatively planned total of some 200-250 aircraft.
Indian sources have ventured into suggesting that the FGFA could be in service by 2015, but this is quite obviously not feasible given that development has not yet started. A logical date would be well into the 2020s.
As a first tentative assessment and on the basis of the basis of the scarce information as currently available, the PAK FA (T-50-1?) looks like a mix of well-proven solutions from previous Sukhoi designs married to several new ideas, in particular as regards the still superior quality of Russian aerodynamic research.
It is also possible that the significant delay suffered in developing a Russian counterpart to the F-22 could have turned into a blessing in disguise, giving Sukhoi designers a period of reflexion to generate a well balanced design. This would relate in particular to the decision not to push for extreme low observability characteristics at the expense of everything else, including not only flight performance but also acquisition costs and most importantly maintenance requirements and thus operational availability.