SUKHOI S-37 BERKUT UNVEILED
By Karl Schwarz
Big, black, threatening - Sukhois S-37 Berkut (royal eagle) has the ideal attributes of an enemy. Had he appeared at the height of the Cold War, western military experts would have painted dangerous threat scenarios onto the wall. Today the situation is seen more relaxed. German fighter experts at least do not see a "breakthrough into a new dimension" with this aircraft.
For one the aerodynamic design concept is not new. As early as in the mid 80s the US flew a similar experimental fighter with the Grumman X-29A. It had forward swept wings, canards and pitch control surfaces at the tail. The same arguments that led to the X-29 will have guided the Suchoi team in their considerations:
A forward wept wing, gives lift/drag-ratios. The reason for this is that it creates a considerable part of its lift through excess pressure underneath the wing, which in turn means that the shock wave on the top is weaker.
A larger part of the lift is created at the inner wing portion, which results in lower bending moments. Theoretically this leads to a lighter structure, or you are able to increase the aspect ratio, which has a positive effect on the induced during during manoeuvres with a lot of lift. Apart from this the range is greater.
The break off of the air stream starts in the inner area of the wing. Thus the ailerons stay effective up to high angles of attack. An early stall at the root would be very disadvantageous. This is why the wing is extremely tapered towards the back here. The influence of the canards, which are placed closely to the leading edge, plays an important role as well.
The wing is mounted far back on the fuselage, wich gives greater freedom in shaping.
For a long time a grave structural problem outweighed all these advantages. If a forward swept wing bends with increased lift, it twists at the same time and causes the angle of attack to become bigger and the lift increasing even further.
If the plane was built in metal, this would need some strengthening and lead to a lot of additional weight. Not so with composite materials Here it is possible position the fibres skilfully to counteract these forces. They are arranged at an angle to the main axle of acting forces. One side effect of the aeroelastic tailoring is the better resistance against flutter.
All in all a three surface configuration with forward swept wings gives a very good performance especially during trans-sonic flight around Mach 1. Experience shows that all close air combat between fighters takes place at this speed.
According to the Suchoi designers, short distance clashes cannot be avoided even in times of stealth fighters and long distance air-to-air guided weapons. From this viewpoint the S-37 is a logical step to surpass the superb characteristics of the Su-27/Su-30/Su-35/Su-37 family. However, this view is not shared by western experts.
It is obvious that a design like the Berkut cannot be created over night. Although there is no concrete information, it can be assumed that the TsAGI (Russian Aerospace Research Institute) will have undertaken detailed studies about forward swept wings after the American X-29A appeared in the 80s. In 1993 scientific treatises were published about this topic, which pointed out that Sukhoi was working on a new fighter with this design.
Claims like these were hardly acknowledged in the west, although at this time two S-37s were already being constructed. However, work progressed only slowly after the collapse of the Soviet Union. Since there was a lack of government funds, Suchoi was forced to invest their own money in this project from export business. One was only able to complete one airframe, which had to be used for static tests as well.
In spring 1996 a photo appeared which showed the Sukhoi General Designer Michail Simonov and the then Air Force Commander Pjotr Deinekin with two models: the Su-35 and an aircraft with forward swept wings. The later had the number 32, which means the design bureau's internal type designation must have been S-32 (for strelowidnoje = swept wing, as opposed to delta wing).
At the beginning of 1997 there were more rumours that the maiden flight of the S-32 was to take place soon. Presumably to prove that there was no S-32, the Suchoi Deign Office, which was being very secretive again, changed the name to S-37.
It is assumed that the Berkut, which had meanwhile been transported to Zhukovski, started taxi tests in July 1997. It was, however, not possible to have it airborne for the MAKS '97 airhsow in August. Finally on 25 September Igor Votinzev took off on the fighter's maiden flight. The Soviet press celebrated the aircraft as the world's most modern fighter, which flew only weeks after the Lockheed Martin F-22.
This patriotic enthusiasm was not shared in wide circles of the Air Force. When the aircraft was presented in October 1997, only low-ranking officers turned up. Furthermore in the Krasnaja Zvesda (Red Star), the newspaper of the military, the Berkut was described to be an experimental aircraft, which still had to come a long way before it could develop into a fighter, ready for action. During the air show in Dubai in November 1997, General Designer Michail Simonov also expressed himself very carefully on the status of the programme.
It is, however, obvious that the S-37 is by far more than an X-aircraft. It is far too big and too expensive, being even larger than the Su-35, with a commensurate weight as well. It is obvious that in order to save money, tried and tested components from the "Flanker" production were used. The cockpit section, undercarriage and tail units were used in the new fighter.
It is said that the fuselage has been made rather conventionally, although the biggest possible pieces of skin were used. One assumes that there is a weapons bay like in the F-22 Raptor with room for four to six air-to-air guided weapons. This causes the radar reflection area to be reduced and radar-absorbing material has probably been used also for this reason.
The air intake canals lead in an s-shape to the engines, which also helps with Stealth characteristics. The S-37 is currently powered by two Aviadvigatel D-30F6-turbofans, which derive from the MiG-31. They were the only aggregates, which were available with the necessary output. One assumes, however, that AL-41Fs from Saturn (Ljulka) will be used in future. They had originally been developed for the MiG 1.42 (MFI) and have a thrust of 175kN (393,300lbs) with afterburner. They feature variable bypass ratios as well as thrust vector nozzles in order to increase the Berkut's agility beyond the stall.
Weapons systems have up to now not ben installed. The S-37 will certainly have enough room for new Russian radar with active antennas. Two booms offer space in the tail for an aerial, which is pointed backwards. A canon would be installed above the right hand side air intake and a probe for air fuelling will be available, once the fighter goes into mass production.
It should not be a problem to take over almost all weapons from the "Flanker" family, when it comes to arming the aircraft. Additionally there will be new developments like the air-to-air-guided missile R-77PD (RVV-AE-PD) with ramjet propulsion or the big K-37 with extreme range.
However, Suchoi is not yet ready for this. After the first eight flights in autumn 1997 a few modifications had to be made, before further test flights were carried out in April 1998. According to Suchoi General Director Michail Pogosjan, the S-37 has made 50 flight up to date, which have proved that "the concept works well".
The initial resentment of the Russian Air Force has made way to a more positive attitude, which means that the government has been making funds available for the S-37 during this year. However, it remains a fact that for years to come Russia can ill-afford a fighter of this size and price tag. It will be difficult to answer the question whether the Berkut will be more than a very impressing experimental jet before the year 2005 or so.
From page 58 of FLUG REVUE 11/99
In knowledge we trust!