Saturday, December 17, 2011

Fifth Generation Fighter Aircraft- Sukhoi PAK FA

The emergence of the Russian Sukhoi PAK-FA marks the end of the United States' quarter century long monopoly on the design of Very Low Observable (VLO) or stealth aircraft.

The capabilities of the PAK-FA make a clear statement defining the Russian view of Within-Visual-Range (WVR) and Beyond-Visual-Range (BVR) air combat, which diverges fundamentally from contemporary Western thinking. The Russian paradigm is clearly centred on the idea that BVR and WVR combat are much alike, insofar as during the engagement endgame the fighter under attack is within tracking range of the weapon fire control system and where possible the weapon or fire control element should be defeated kinematically. The principal observed difference between WVR and BVR combat in the Russian model, is that the latter relies more heavily on long range sensors and their ability to defeat low observability measures, or active countermeasures.

Designed to compete against the F-22 in traditional Beyond Visual Range (BVR) and Within Visual Range (WVR) air combat, the PAK-FA shares all of the key 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. While the PAK-FA firmly qualifies as a fifth generation design, it has two further attributes absent in the extant F-22 design. The first is extreme agility, 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 a 25,000 lb internal fuel load. The internal and external weapon payload are likely to be somewhat larger, though comparable to those of the F-22A.

Russia intends to operate at least two hundred PAK-FAs, India two hundred and fifty of the Indian Fifth Generation Fighter Aircraft (FGFA) variant, with global PAK-FA exports likely to add at least 500 more tails to the production tally. The stated intent is to supply the PAK-FA as a replacement for existing T-10 Flanker series fighter aircraft.

Initial analysis of PAK-FA imagery and public disclosures by the Russian government and Sukhoi bureau indicate that a production PAK-FA will yield greater aerodynamic and kinematic performance to the current F-22A design, and similar low observables performance to the F-35A JSF2. 

While the basic shaping observed on this first prototype of the PAK-FA will deny it the critical all-aspect stealth performance of the F-22 in BVR air combat and deep penetration, its extreme manoeuvrability/controllability design features, which result in extreme agility, give it the potential to become the most lethal and survivable fighter ever built for air combat engagements3.

It is important to consider that the publicly displayed PAK-FA prototype does 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. A number of observers have attempted to draw conclusions about production PAK-FA VLO performance based on the absence of such treatments, the result of which have been a series of unrealistically optimistic commentaries.

PAK-FA Low Rate Initial Production is planned for 2013, and Full Rate Production for 2015, with initial deliveries of the Indian dual seat variant planned for 2017.

The evolution and development history of the PAK-FA, historically, has not been well documented in open sources, largely due to the high levels of secrecy surrounding this program since its inception. What is known from open sources largely amounts to a collation of various intentional and incidental Russian disclosures, and increasingly, disclosures by India, who have a 25% share in the development of the design.

Study of the aircraft's design features, and earlier Sukhoi demonstrators, indicate that much careful thought has been invested into this design and its progressive development over a period of two decades.

When the Soviets deployed the Su-27S Flanker B during the early 1980s, investment into a replacement was initiated. This resulted in the reasonably well known 1990s MiG I.44 MFI (Mnogo-Funktsionniy Istrebitel' or Multi-Role Fighter), which was a multirole fighter modelled on the aerodynamics of the three “Eurocanard” designs, but much larger and intended to be powered by the Al-41F supersonic cruise engine.

The MFI was built to supercruise, and to provide very high agility, but no investment was made into signature reduction, making it fundamentally uncompetitive against the early 1990s US Air Force Advanced Tactical Fighter (ATF) YF-22 and YF-23 demonstrators.

The lack of a future for an expensive high signature fighter, and the MiG organisations de facto bankruptcy due to the export market success of the larger Sukhoi Flanker, saw the MFI relegated to a demonstration program. The important product of the MFI program was the Al-41F supercruising engine, modelled on the United States' Pratt & Whitney F119 series, which powers the F-22A. The Al-41F is the basis of the high temperature core components used in the supercruise capable 117S series engine, which now powers the production Su-35S Flanker and PAK-FA prototypes.

During this period Sukhoi developed the unusual S.32/S.37 forward swept wing demonstrator, intended to combine supersonic performance with super-manoeuvrability. This design demonstrated the use of large LEX, over large quarter circular inlets. Like the MFI, this design was not stealthy and was used to prove basic technologies and design rules.

A more successful demonstrator built during this period was the Su-37 “Super Flanker”, derived from the earlier Su-27M/Su-35 Flanker E. The Su-37 was intended to extend the T-10 Flanker design to the limit, especially in avionic systems and manoeuvre performance. It introduced the first axi-symmetric 3D (three dimensional) Thrust Vector Control (TVC) nozzles, manually controlled, and later integrated with the Digital Flight Control System (DFCS); the first quadruplex DFCS in a Russian fighter; composite structural components; a modern glass cockpit and force sensitive sidestick controller; digital core avionics; the N-011M BARS hybrid Electronically Steered Array (ESA) radar; and, a compact ESA tail warning radar.

The combination of aerodynamic design refined through progressive evolutionary development, DFCS, twin 3D vectoring thrust supercruising engines interoperating in and on an advanced kinematic design airframe, extended the Flanker design squarely into the category of “extreme agility” - which can be defined as the harmonised and complementary balance of extreme manoeuvrability and extreme controllability.

The Su-37 Super Flanker demonstration effort extended the viability of the basic T-10 Flanker design by almost two decades, and yielded basic technology used in the design of the Su-30MKI/MKM Flanker H and, as seen in the latter part of 2008, the Su-35S, often labelled the “4++ Generation Flanker”. It also provided experience which was critical to the development of the replacement for the T-10 Flanker series.

The PAK-FA properly qualifies as a 21st century project, as formal tendering for the program was launched during the 2000 - 2001 period by the Russian MoD. Russian sources claim that Sukhoi, MiG and Yakovlev were invited to bid proposals. Initial thinking was to develop a fighter larger than the MiG-29 Fulcrum, but smaller than the Su-27 Flanker, with greater range/persistence to the Flanker, low observable capability, extreme agility, supersonic cruise capability, and near STOL short field capabilities. Sukhoi won the tender in 2002 with its T-50/I-21 proposal, with MiG and Yakovlev engaged as subcontractors in the development. Russian sources state that Sukhoi's ability to fund much of the development effort from company export revenue profits was a major factor in the decision.

The initial design of the PAK-FA was finished in 2004, amid public controversies about lower than intended maximum speed, and greater than intended empty weight. Full Russian MoD funding was not provided until 2005 - 2006.

The prototype flown on the 29th January, 2010, is intended to prove aircraft aerodynamics, structure, and compatibility of the VLO shaping with aerodynamic and structural constraints. It is claimed to be fitted with the 117S Al-31F engine variant, common to the Su-35S, as the intended new engine has yet to complete development and enter production. The latter is expected to be a variant or derivative of the Al-41F design. The existing prototype probably lacks a complete version of its final avionic suite, most likely employing large parts of the new and fully digital Su-35S avionic suite as a basis for evolving the design of the final avionics fit, as has been the case on the development of previous Flanker variants.

The first “public” flight lasted 47 minutes and was intended to test handling, engine behaviour, landing gear operation, and basic systems functions2.

The PAK-FA was designed with a stated requirement of being able to operate from short, unprepared runways in support of expeditionary operations. With the exception of an observable deployed arrestor hook, the PAK-FA design incorporates all of the key design characteristics that are required in aircraft built to operate from Russian Navy ski-jump equipped aircraft carriers. It is not known whether the extant structural design includes the necessary provisions for arrestor hook loads.

Equipped with 3D TVC and large LEX control surfaces, power approach speeds in the order of 100 knots, sink rates somewhat less than 20 ft/sec and quite flat aircraft approach attitudes can be expected from this design, as can commensurately low arrestor hook and related carrier landing loads. Such performance, when combined with the extensive field of view provided by the fighter/strike/attack canopy configuration, and a functional arrestor hook system, likely integrated into the rear ventral internal weapon bay, would make the PAK-FA an eminently suitable aircraft for maritime operations.

India was engaged early in the PAK-FA development effort, but Russian sources suggest that negotiations on the work share between HAL and Sukhoi/KnAAPO were protracted. Open sources suggest that India is responsible for 25% of the development of the PAK-FA, primarily in software and systems integration, areas where India has recent experience via the Su-30MKI program. India is to also contribute in composite materials, with claims the PAK-FA structure is, by total aircraft weight, rather than just the airframe structural weight, some 25% titanium alloys, and 20% composites. Indian sources suggest that both single and dual seat variants will be built for India.

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