Sukhoi's Black Bird


"The aviation world had waited several years for the event which finally took place on September 25, 1997, at the Flight Research Institute airfield at Zhukovskii near Moscow. On that day, a radically new fighter from the Sukhoi OKB performed its maiden flight. Sporting an unusual overall black paint scheme, but dubbed the S-37 Berkut (Golden Eagle), the aircraft was taken into the air for the first time by test pilot Igor Votintsev.

Flight Telsts

The Sukhoi team had wanted to fly the aircraft the day before, on September 24, but during a routine systems check it appeared that one of the four duplex fly-by-wire systems had failed. Test Pilot Votintsev declared himself satisfied with the flight and stated that the aircraft was highly manoeuvrable. A third flight took place on October 8, 1997, and Russian radio briefly reported on that day that test flying of a new fifth-generation fighter aircraft, the Sukhoi S-32 (sic), had commenced.

On October 18, 1997, which was a Saturday and a non-working day for the Flight Research fnstitute airfield, a demonstration of the new aircraft had been arranged for several leading figures within the Russian military-industrial complex as well as for some military officers, (although the Air Force CinC did not attend the presentation). Before the end of the month several more test flights had also taken place.

Almost nothing had been previously known about Sukhoi's work on the S-37, with the exception of the fact that this new fighter had a forward swept wing. In the project definition and construction phase the aircraft was known by the designation S-32 in order to mislead Western intelligence services, (S-32 is the internal OKB designation for the Su-17 fighter-bomber). After completion of the prototype, it was decided to give the aircraft a new designation. It remains a puzzle, however, as to why the new aircraft was given the same number as another current Sukhoi project, the Su-37 development of the Flanker series! (The S in the Berkut's designation simply stands for Samolyot or aircraft.)

Fifth Generation

Research into the overall layout and general appearance of a fifth-generation fighter was started, both in the former USSR and the USA, in the mid-1970s, when fourth-generation aircraft such as the Su-27 and MiG-29 were only just coming onto the scene. The new aircraft were intended to have considerably greater combat potential than their predecessors. Leading aviation and scientific research centres in Russia were brought in to work on the design, which was conducted within the I-90 programme (Istrebityel 90 / Fighter of the 90s). Tfte main ideas defining the new fighter were formulated in concert with the Air Force, namely the concept of multi-functionality, Le great effectiveness when attacking land, sea or air targets, the provision of all-round situational awareness and achievement of 'super cruise' capability. Additionally, it was deemed important to significantly reduce radar and infrared signatures.

A fifth-generation aircraft must be capable of achieving all-aspect attacks on airborne targets in close combat dog-fight mode as well as be capable of conducting 'multi-channel' missile launches for long-range combat (i.e. the ability to track and engage many targets simultaneously). Automation of the control of on-board information and ECM systems was also planned into the design, along with increased combat autonomy as a result of a tactical situation display in the single-seat cockpit. The use of data link systems for information exchange with external, off-board sources was also envisaged.

The aerodynamics and on-board control systems of the planned fighter had to provide the capability for instantaneously changing the angular orientation of the aircraft (i.e. without any perceptible control circuit delays). It was also expected to be forgiving of fairly rough handling over a wide range of flight conditions and be equipped with an automatic flight control system adapted to the requirements of its tactical role.

Forward Swept Wing

One of the most demanding requirements of the Russian fighter was the need for 'super manoeuvrability', the ability to maintain stability and controllability at angles of attack greater than 90deg. Thus, one of the solutions adopted to achieve the required manoeuvrability characteristics was to use a forward swept wing (FSW). Such a wing, which offers distinct advantages when compared with a conventionaIly swept surface, first saw military application in the 1940s. The first jet aircraft with an FSW was the German Junkers Ju 287 bomber and after the victory over Germany in WWII two experimental FSW bombers ended up in the former USSR as war 'booty'.

Russian combat aircraft designers only returned to the theme of FSW concepts in the mid-70s when work started in the former USSR and the USA on the study of what are now generally known as fifth-generation fighter designs. It is well known that a forward swept wing provides significantly increased manoeuvrability at low airspeeds and high angles of attack, less aerodynamic drag compared with a swept back wing, which in turn leads to an increase in range and improvements in take-off and landing characteristics. However, such a wing requires new manufacturing solutions, since the usual methods of construction do not provide the requisite level of strength and rigidity. The only materials that are suitable as structural elements are layered composites which guarantee accepted standards of aerodynamic elasticity without any weight penalty. A forward swept wing also possesses the quality of 'high divergence', that is, with increased loading, the leading edge deflects upwards, increasing the angle of attack and wing loading, especially at the root.

The lift created by a normal wing is, as a rule, limited by the breakdown of airflow at the tips. Consequently, lift is reduced, since not all of the wing area is being used. On a forward swept wing there are no tip vortices, lift is greater and, moreover, the ailerons remain effective even during flow breakaway from other parts of the wing. An aerodynamic layout which embodies an FSW guarantees a better blending of wing and fuselage and also optimises the pressure distribution over the entire wing and foreplanes. According to the estimates of US specialists, the use of an FSW on an aircraft like the F-16 should lead to an increase in turning ability by 14% and radius of action by 34% while take-off and landing distances would be reduced by 35%. In the 1980s, the US firm Grumman Aerospace actively conducted research with an experimental FSW aircraft, the X-29. However, up to the present time, it has never been made completely clear why the Americans did not continue with this work.

Work on formulating the design of a Russian highly-manoeuvrable forward swept wing aircraft was carried out by the country's largest aerospace research centres - at TsAGI (The Central Aero-hydrodynamics Institute) near Moscow and at SiBNIA (The Siberian Scientific Research Institute for Aviation) in Novosibirsk. In particular, a model based on the MiG-23 has been tested in the wind tunnel at TsAGI, and at SiBNIA a layout based on the Su-27 with a similar type of wing has also been studied. The scientific data obtained from this research has enabled the Sukhoi OKB to take the decision to go-ahead with the unusually complex task of creating the world's first supersonic combat aircraft with a forward swept wing.

High Maneuverability

In the Russian press, it has often been reported that the S-37 is a fifth-generation fighter which should enter service with the Russian Air Force in the next century. However, these comments have raised profound doubts among Russian designers in some of the competing firms. For example, the General Designer of the Yakovlev OKB, Alexander Dondukov, has openly criticised this project. The chief visible characteristic of the S-37 is its forward swept wing, which gives it a somewhat fantastic appearance. The use of FSW on the S-37 suggests that the main objective of the designers was focused on the achievement of extremely high manoeuvrability. Stealth characteristics, which typify most fifthgeneration designs, although present in some elements of the structure of the Berkut, have been relegated to second order significance by the Sukhoi team.

The overall layout of the Berkut is that of a so-called 'integral tri-plane', with controllable foreplanes, in addition to normal tail surfaces. The wing consists of 90% composite materials and the nose radome has a slightly flattened section. Evidently these features are the determining factors in the aircraft's superaerodynamics. The nose of the aircraft is shorter than the equivalent area on the Su27/35/37 family which improves the forward and downward view for the pilot. The housing for the electro-optical sensor is located on the right hand side of the windshield as on the Su35. The ejector seat (namely the K-36DM) is inclined rearwards by 30o to provide increased tolerance to g-forces on the pilot during vigorous manoeuvring. Doubtless, on a series production aircraft, an improved version of the K-36 seat could be installed which would permit egress at low altitude with the aircraft inverted. Indeed, Professor Gai Severin and his team at Zvezda have made a noteworthy contribution to the design of the next generation of advanced Russian combat aircraft by proposing the use of their new 'adaptive' ejector seat. This would allow the pilot to sustain higher g-force levels while conducting manoeuvring air combat than is currently possible on existing fighters. It is understood that the 'adaptive' seat varies its angle of inclination as a function of g-loading although details of its installation on a specific aircraft are currently unavailable.

Large semi-circular fixed air intakes are located under the short but broad span leading edge root extensions (LERXs). The widely separated and slightly outwardly canted fins are similar in shape and dimensions to those of the Su-27 family, whilst in the Berkut's tail section there are two large radomes, housing a rearward-looking radar and an ECM installation respectively. The wing has a forward sweep angle of -20deg and is equipped with leading edge slats, flaps and ailerons and the nose and main undercarriage legs are practically identical to those of the naval Su-33.

The S-37 is powered by two modified Soloviev D-30F6 engines (the same type as installed on the MiG-31 heavy interceptor) with increased thrust in afterburner, namely about 44,0001b (20,000kg) sea level static thrust. In the longer term it is anticipated that the AL41F with thrust vectoring nozzles will be installed and these engines also have a thrust rating of around 44,0001b (20,000kg). These engines would provide the S-37 with an unsurpassed thrust-to-weight ratio. Auxiliary air intakes are mounted on top of the fuselage to improve engine airflow in manoeuvring flight or during take-off and landing. The Berkut has internal weapons bays for the carriage of new, compact medium-range missiles, possibly the R-77 (AA-12 Adder) and also the R-73Eh (AA11 Archer) (or the new R-74) short-range dogfight missiles. In addition to missile armament, the Berkut is equipped with a builtin cannon, namely the GSh-301, the barrel of which is installed in the upper part of the left LERX.

Successful commencement of flight testing of the Berkut does not mean, however, that it has a cloudless future. Russia's Air Force is unable at present to afford such programmes and this was stated openly in a report from the Air Force press service published in the military newspaper, Red Star, where it was affirmed that the S-37 is not a priority programme, and the Russian Air Force does not consider it to be financially justifiable in the present economic climate. In turn, the General Designer of the Sukhoi OKB, Mikhail Simonov, told press officials at the Dubai Air Show that the S-37 i an experimental aircraft, intended for the development and refinement of new technica concepts to be embodied in the design and construction of new aircraft.

In the spring of 1998 flight testing of the Sukhoi S-37 Berkut re-commenced at Zhukovskii. According to some reports, the aircraft had completed 12 test flights by the middle of October last year. During the course of the test programme the aircraft has been subject to some minor modification work, mainly concerning the area of the rudders. Testing of the S-37 had evidently spurred on leading personnel in the Mikoyan Engineering Centre (formerly the MiG Aviation Scientific Production Complex) who, for several years now, have had to repeatedly postpone commencement of test flying of their 1-44 fighter (Russia's Fifth- Generation Fighter, March, p. 35-37).

In October last year MiG had planned several times to make a sensational presentation of the aircraft, but this was not authorised by the Ministry of Defence. So, for the time being, the S-37 is already some way ahead of its competitor in its flight test programme. However, it should be noted that the Mikoyan project is financed by the militar)6 in contrast to Sukhoi's of the Berkut programme and that the Russian Air Force is still interested in the earliest possible commencement of flight testing of the 1-44. As far as the S-37 is concerned, it is evident that given sufficient funding a second example will be built, probably embodying modifications resulting from observations made during flight testing of the prototype, on which it will also be possible to accomplish the Berkut's main programme objectives.

Piotr Butowski Adds

A farther element of intrigue concerning the S-37 has been provided by Piotr Butowski who has sent AFM details of a model of the aircraft photographed in 1996. The model was originally shown in the background of a photograph showing Sukhoi General Designer, Mikhail Simonov in discussion with the then Commanderin-Chief of the Russian Air Force, Piotr Deynekin. At that time, in early 1996, the aircraft carried the side number 32, which in the convention of the day was also the company's designation for this aircraft, as already mentioned in the main article. When certain Western publications started to give details ofpreparations for the maiden flight of an S-32 prototype in 1997, Sukhoi personnel simply changed the aircraft's designation to S-37 and denied the existence of a so-called S-321 Recently, Piotr Butowski had an opportunity to study the model at close quarters and was able to determine that the most significant difference between it and the flying prototype lies in the engine and nozzle installation. On what is believed to be the definitive version of the S-37 design, the engine nozzle is a single two-dimensional thrust-vectoring slot. Two sets of large auxiliary inlets draw cold ambient air in from above and below the fuselage to further reduce the already dissipated exhaust gases to keep the infrared signature as low as possible.

Other important differences noted are the inwardly canted fins and rudders and the large wing-tip pods, resembling those of the Sorbtsiya co-operative jamming suite on the Flanker series. There are also two possible electronic equipment housings between the nozzle and the tailplanes. In addition, the model revealed the probable location of two internal weapons bays. The first, in the central part of the fuselage between the main undercarriage, measures 15ft x 7.2ft (4.6m x 2.2m), whilst the second measures 12.3ft x 2.6ft (3.75m x 0.8m) and lies in front of the larger bay and extends up to the nosewheel housing.

Unlike the disputed internal weapons bays on the MiG 1.44 prototype, a production variant of the S-37 would need to carry its weapons internally in order to preserve the integrity of the forward swept wing."

(source: AFM, September 1999, p.64)