Sukhoi Su-47 (formerly known as the S-37 Berkut)

The Sukhoi’s candidate for the Russian air force requirement for a Mnogo-funktsional’ny Frontovoy Istrebitel’ (MFI – multifunctional frontal fighter) is less known than its rival Mikoyan. Vladimir Ilyushin, Sukhoi’s veteran test pilot, revealed in mid 1997 that the aircraft was “close to completion”, adding that it will be a “worldwide sensation” when it is unveiled. The scarce information on Simonov’s new fighter indicate that it had already underwent high-speed taxi tests by the end of the summer and made its maiden flight at Zhukovsky at September 25th, 1997, in hands of Sukhoi’s test pilot Igor Votintsev.

Specification and Dimensions of theSukhoi Su-47 (formerly known as the S-37 Berkut)

What’s in the Name?

The S-32 (do not mix with Su-32!) is an internal Sukhoi OKB designation which is rationalized in terms of commonly used yet controversial indexing originated with Sukhoi Su-7 and Su-9 prototypes. These were designated S-1 and T-1 respectively, with “S” being a first letter of swept wing in Russian “Strelovidnoe krylo” and “T” from the Russian for delta wing “Treugol’noe krylo”. Clearly, the “S” in S-32 implies that new Sukhoi has a swept wing but the index conflicts with another S-32 taken by Sukhoi Su-17 prototypes few decades ago. There is a great deal of hints that S-32 is a phony designation and presently a different designation is used in conjunction with new Sukhoi fighter — S-37 (do not mix with Su-37!). The S-37 index was formerly allocated to a single-engined lightweight multirole combat aircraft broadly similar to FrenchRafale which was cancelled in 1994. In any event, both S-32 and S-37 are internal bureau designation, and could become Su- anything. Reported name of S-37 is Berkut (Ber-koot) which means golden eagle in Russian. The NATO reporting name for this aircraft is “Firkin”.

From Tailless Canard to Tandem Triplane

The fifth-generation Sukhoi fighter features forward-swept wings (FSW), canards and twin outward-canted vertical stabilisers and incorporates low-observable and thrust-vectoring technologies. Comparing blown up photograph of the scale model and early speculative sketches and artist renderings, which appeared in western aviation press, it is clear that S-37’s forward-swept wing is closely coupled to canards. Nonetheless, the aircraft retains the horizontal stabilisers, evolving from pure canard to a tandem triplane layout. The first S-37 photographs show that the stabilizer are highly swept (ca 70 deg) and their leading edges appear to extend aft from the wing roots of FSW. This is very different fromX-29A strakes which were extensions of the wing roots themselfs ending with a trailing edge flaps.

The tandem triplane configuration was test flown in 1985 on T-10-24 which served as one of the naval Flanker prototypes. The addition of the canards, referred PGO in Russian (Perednee Gorizontal’noe Operenie – Forward Horizontal Stabilizers), solved the control problems encountered at high angle-of-attack (AOA) flight regimes when the tailerons lost their efficiency in the wake of the wing. The PGO’s seemed to be a favourite choice for forth-and-a-half generation of Sukhoi’s Su-34, -32FN, -35 and Su-37 and fifth-generation S-37 and S-54.

Although the genesis of the S-37 remains unclear, some sources suggested that original layout was much closer to X-29A tailless scheme and that aerodynamic of early S-37 directly benefited from Central Aerohydrodynamics Institute (TsAGI) wind-tunnel tests withX-29A scale models. It is possible that S-32 may be a technology demonstrator, built to examine FSW aerodynamics and composite wing structures or may have started its life as one, but realities of 90’s urged Simonov to take this project one step further in attempt to present the S-32 as a genuine contender for a fifth-generation fighter seeked by the Russian air force. This might explain the fact that the S-32 seems to be much too heavy for a research testbed, being a considerably larger aircraft than Northrop F-5sized X-29A.

In fact, the S-37 is in the class ofSu-27 as seen from comparison of its scale model to the advanced Su-27 Flanker model presented at the same meeting. Moscow Aerospace (MAKS 97) provided additional data on S-37 and confirmed that dimensions, performance and tandem triplane layout of the S-37 are similar to that of Su-37. First public photographs of the S-37 suggested that the front part of the fuselage including the “hooded cobra” LERX could have come from the original S-37 canard-delta. If true, this could possibly clear up the origins of the S-37 index.

Forward Swept Wing

The early Soviet designs to feature moderately forward swept wing were Belyaev’s DB-LK and Babochka aircraft and Mikoyan Gurevitch PBSh-2 (MiG-6) biplane. Captured at the end of WWII, German FSW Junkers Ju-287 was test flown by German and Russian crews. A six engined EF-131 was build and underwent extensive structural and flight testing until 1947, when theme was closed. At about the same time Pavel Tsybin build several testbeds LL (Letauchaya Laboratoriya) -1, -2 and -3 with straight, swept back and forward swept wings respectively (40 degrees). The LL-1 and LL-3 rocket powered gliders performed number of powered flights and provided TsAGI with much needed FSW data. In one of the flights LL-3 reachedMach 0.97 in dive.

Sukhoi Fifth-generation Fighter Philosophy

The FSW is a better performer at high angles of attack in post-stall manoeuvring much needed in close-in dogfight. The fact that Simonov had chosen FSW for his fifth-generation fighter once again confirms Sukhoi’s commitment to the super agility as a crucial requirement for the next generation air-superiority fighter. This approach, so much different from western concepts ofstealth, supercruise and BVR engagements, was taken to the limits in Su-37. The FSW S-32 fitted with TVC expected to outperform its stalemate in close-in dogfight involving post-stall flight regimes. Having the edge in manoeuvring, the S-32 is clearly catching up instealth with US and European new-generation fighters. However even with its internal weapon bay and RAM coating, the new Sukhoi is a very different concept than F-22. The heavy accent on RAM rather than radar absorbing structures (RAS) is obvious. The reason for such attitude is not clear, although a combination of the technology limitations and operational doctrine is most likely candidate. The major components of radar stealth — RAM coatings and surface quality — are subject to the production and maintenance tolerance as it was shown by USAF F-117 and B-2 operational experience. Untightened screws, scratches or unfastened access panels were known to greatly deteriorate the RCS of the aircraft, reducing the engineering efforts put into aircraft design. It remains to be seen how Sukhoi will overcome the looser production standards of the Russian aircraft plans.

The Afghanistan experience where Sukhoi’s encountered a thread of the shoulder launched infrared homing surface-to-air missiles such as Redeye, Stinger and SA-7, forced Sukhoi team to work on the reduction of the infrared signature of the Su-25. The results materialized in the Su-25T development — Su-25TM (Su-39 in Sukhoi’s nomenclature). The installation of the intake cones hiding the turbine blades and efficient mixing of the exhaust with cold air reduced the IR signature of the Frogfoot from front and rear aspects. This fourfold reduction at expense of 2% lower SFC is indeed an impressive achievement. Further experiments with low visibility involved the advanced Flanker development prototypes, aircraft of 700 (Su-35,-37) and 600 (Su-30) series. These fighters wear eye catchy new camouflage schemes designed to reduce the visual signature of the aircraft on the ground and in the sky. One of the most interesting examples of Sukhoi experiments was a scheme applied to 701, designed to deceive space based optical systems. Some effort was directed in reduction of the radar cross sections of advanced Flankers as well. The Su-34,-32FN have optimised radar random shape, lack variable geometry intakes and were reported to have partial RAM coating. Recently Sukhoi stated that basic export models of Su-30MK can be treated with RAM to fulfil customer requirement for a lower RCS aircraft. Clearly benefiting from previous research, the S-37 prototype relies heavily on the Sukhoi’s state of the art low observable technology. The forward swept wing, a conformal underfuselage weapon station(s), use of RAM and the inward-canted tailfins, suggest a further reduction of the aircraft radar signature down from similarly sized Flanker’s 3-5 sq m. The extend of the reduction of the IR signature of the S-37 exhausts will depend on the choice of the trust vectoring nozzle. TheF-22 type flat 2D nozzle can give a better results while 2D nozzle might contradict to Simonov’s superagility ideas favouring 3D exhaust. The Saturn-Lulka was reported to work on reduction of the IR signature of the axi-symmetric trust vector controlled (TVC) Al-37FU power plant on non-afterburning regimes.

Powerplant

The scarce availability of trust vectoring Saturn-Lulka Al-41F engineered for the Mikoyan’s article 1.42 forced Sukhoi to seek a replacement for the originally planned powerplant. According to MAPO MIG sources, the limited number of Al-41F are involved in Mikoyan’s Article 1.42 tests and not available to Sukhoi’s competitor. Reluctance of MAPO MIG made a trust-vectoring control (TVC) Al-37FU (sometime referred as Al-31FU where FU stands for Forsazh, Upravlaemoye soplo – afterburning, articulated nozzle) powerplant used in Sukhoi’s Su-37 a natural choice for fifth-generation fighter, but would have been premature for the first S-32 airframe. Additionally, the availability of the Al-37FU could be a problem since all prototypes are involved in flight tests on the Su-37 and in the bench endurance tests. At the time of the Su-37 first flight only three Al-37FU were built.

The ultimate S-32 powerplant — Al-37FU — operates in automatic and manual modes. In manual mode the nozzle deflection angle is set by the pilot, and in automatic mode the axi-symmetric nozzles are controlled by the MNPK Avionika full-authority, digital fly-by-wire flight control system (FCS). The movable in pitch axis nozzle deflects ±15 degree at 30 deg/s by a pair of hydraulic jacks. The production Al-37FU will use jet fuel instead of hydraulic liquid to drive the nozzles. Surprisingly, as a temporal solution, instead of similar and widely available Su-27 Flanker’s Al-31F powerplants, the S-32 prototype received a pair of Perm Aviadvigatel D-30F6 engines used on MiG-31 Foxhound interceptors. Designed by the 1980, this full authority digital engine control (FADEC) engine comprises six interchangeable modules and a core module. Although powerplant accumulated several thousand flight hours and experienced no operational drawbacks, it has estimated 300 hrs life between overhauls (Russian engine maintenance is very different from western philosophy and term “overhauls” has a different meaning). There were no reports on TVC versions of D-30F6.

The photographs of S-37 Berkut, show two details: the starboard tail sting is slightly longer than the port one and the two auxiliary intakes on the top of the fuselage. There are three reasonable explanations to the sting asymmetry: a) it houses a breaking or a spin recovery shute b) it is due to the asymmetric engine installation typical for prototypes. The port engine will be used to test a 3D TVC nozzle which will require adequate space for the yaw vectoring c) Sukhoi used two 2D nozzles oriented perpendicular to each other to control pitch and yaw separately. Combined action give a pseudo 3D effect. This last explanation is least likely since Lulka reported to have 3D TVC nozzle “in the pocket” at the time of Farnborough 96. Auxiliary intakes could be used during take offs for increased air flow to the engines. These could have been repositioned from the underside of the aircraft due to the reduces radar cross section considerations or/and lack of the space taken by internal missile bay(s).

Avionics

In early September, defence-ministry acquisition chief Col Gen Anatoly Sitnov noted: “What is the use of developing the Sukhoi fifth-generation fighter, if the aircraft’s cockpit dates back to a second- or third-generation design?” While Sitnov statement clearly implying the state of the art of the S-32, one can hardly expect that a first test airframe will incorporate all innovations planned for the series production. Similarly, the sole Su-37 demonstrator flies with a counterweight instead of the advanced radar hence the aircraft is intended to explore among other things the trust vectoring modes of the new powerplant. However, the Su-37 fighter will have the top notch avionics suit which is tested on other 700 series airframes — Sukhoi Su-35s. It is expected that the sophistication of S-32 cockpit and avionics suit should at least match that of forth-and-a-half generation Su-35 and Su-37 aircraft. The cockpit of the S-32 does most certainly feature the color liquid crystal MFDs and wide angle HUD. The test proven in Su-37 demonstrator inclined pilot seat, a fixed pressure sensitive throttle and side-stick controller will also find its way to the cockpit of new fighter expected to impose even greater G-loads on pilot than superagile Su-37.

The type of the radar intended for S-32 is not known. The size of the random seems to be somewhat smaller than that of Su-27 family, possibly implying the smaller diameter antenna. Since the S-32 lacks the Flanker’s sting, the placement of the rearward facing radar will be challenging at best.

Armament

Thearmament of the S-32 will most likely never get close to the air-to-air arsenal of Mikoyan’s article 1.42, enjoying super long range K-37. However the ram jet version of AA-12 Adder, R-77PD (RVV-AE-PD), seems to be the most appropriate long stick for the new fighter. The missile’s collapsible lattice stabilizers give R-77 family the compactness well suited for the internal weapon bay(s) of thestealth S-32. However, the aerodynamically superior lattice stabilizers have reportedly a much greater RCS than conventional surfaces, thus potentially revealing the position of the aircraft at the moment of the missile launch. The exact number of weapon bays is not known, although the total number of the hardpoints will be fourteen. The use of the internal/external weapon loads will depend on the mission.

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