The S-37 prototype aircraft is designed to test the control
systems and flight dynamics of agile aircraft and their new aerodynamic
capabilities as well as to accumulate precious design solution experience, put
up-to-date materials and production techniques to use and introduce radically
Among the set of requirements to the prospective fighter the super-maneuverability, i.e. ability to maintain stability and controllability at virtually any angle of attack (alpha), is far from being unimportant. It was specified in the requirements set forth by the US Air Force to its prospective fighter - ATF (prototype of the present-day F-22A). However, having faced the difficult issue of meeting the requirements in full, the Americans had to sacrifice some of its maneuverability in favor of a reduction in its radar cross-section (RCS).
The actual air combat experience of third, fourth and fourth-plus generation fighters shows that dogfighting remains virtually unavoidable. In air combat fought by large groups of fighters of generally equal capabilities, neither of the opposing parties had enough time to produce decisive results at medium range. Even if an overwhelming superiority is available, there are no guarantees that advanced medium- and long-range air-to-air guided missiles will be efficient enough. For instance, there have been precedents of the 100-per cent inefficiency of American long- and medium-range weapon systems (AIM-54 Phoenix, AIM-120 AMRAAM and AIM-7M Sparrow) in F-14 and F/A-18 clashes with Iraqi warplanes.
Besides, further sophistication of electronic warfare (EW) capabilities and reduction in aircraft radar cross-section makes the decrease in long- and medium-range missiles' relative efficiency unavoidable.
However, even in combat at long range waged by fignters with similar speeds and comparable weapons, it is the one of the opponents to enjoy superiority who, while engaging consecutive targets in different sectors of fire, will manage to turn his fighter towards a target by the time of launch to maximize the combat capabilities of its weaponry.
In such a situation maximum turn rates are of special importance, as are the speed brackets for launching weapons. The aircraft's aerodynamics and on-board systems should enable the fighter to alter its angle of attack and flight path in a timely manner. Therefore, the stringent requirements to the fifth-generation Russian fighter maneuverability persist.
The S-37 experimental aircraft program took the baton from the upgrade program of the Su-27 that enjoys the top maneuver-ability among the fighters of every nation in the world. In search of ways to further enhance its fighters' maneuverability, the Sukhoi Design Bureau went for a forward-swept wing aircraft that was supposed to boast extremely high agility at subsonic modes while matching the maneuverability of its best rivals in supersonic modes.
Compared to the sweptback wing, the advantage of the forward-swept wing of the same area are as follows:
The forward-swept wing's lift is not subject to the limits
imposed by the wingtip stall, with the ailerons remaining effective up to high
angles of attack, while the aircraft retains its controllability even if the
flow separates from the rest of the wing surface. Substantial portion of the
forward-swept wing's lift is realized on the inner section of the wingspan,
which results in lesser bending force being generated on the wing surface. This
allows an increase in the wing aspect ratio, thus contributing to the inductive
impedance during high lift ratio maneuvering. High wing aspect ratio provides
However, the employment of the froward-swept wing in developing advanced agile aircraft causes a multitude of special problems. Such a wing is subject to twisting and requires new design and technological solutions. Carbon fiberglass composites enabled developers to design aeroelastic structures through enhancing the necessary wing rigidity. The S-37's wing panels are nearly 90 percent composite. In addition, a design solution to this problem has been found for the aircraft, too.
The aircraft's aerodynamic configuration has been thoroughly wind-tunnelled at the TsAGI. To gauge the S-37's stability and controllability, a free-fall dynamic model was used. It ensured an evaluation of the actual behavior of the fighter in critical flight modes, determination of stall and spin characteristics and development of relevant recovery techniques. Controlled by the onboard complex, the flying model reached very high alpha, stuck to the heading and avoided going into spin. It maintained this position three to four times longer than the Su-27.
For the first time in developing such a complex aircraft, the Sukhoi Design Bureau has introduced a unique technique of nondestructive testbed structural testing. The technique enabled the developer to start build-ing the flying prototype at once. A large num-ber of innovative technical solutions were employed, which allowed the same airframe to be used for both static and flight trials. In developing the S-37, innovative techniques have been devised ensuring accurate extrapo-lation of the design's ultimate structural loadings based on the testbed-shown operational loadings and structural stresses, i.e. without damaging the aircraft in the tests. This resulted in a significant reduction in the program costs and terms.
The flight trials proved that the fighter's structural development was done in the right manner and the results produced are in full compliance with the estimates.
The flight trials confirmed the estimated advantages the forward-swept wing aircraft enjoy. With the S-37 going supersonic, the Sukhoi Design Bureau completed the major test phase. The results produced have surpassed the designers' preliminary estimates. The aircraft retains its stability and controllability at very high angles of attack in subsonic modes, with its maneuverability being far superior to that of the Su-27. According to test pilot Igor Votintsev, while travelling at supersonic speed the S-37 shows no specific peculiarities and is controlled just as other supersonic aircraft, with its stability and ease of control remaining as good as in subsonic modes.
The set of technical solutions in developing, building and testing the S-37 prototype reflects Sukhoi's concept of the new-generation fighter gradual development. Some solutions employed in the S-37 program were first embodied in other Sukhoi aircraft. Building the prototype enabled the company to focus its efforts on most critical issues.
The results of developing numerous drastic innovations will be put to good use in designing aircraft of other types - both military and civilian ones. Thus, the Berkut is a means enabling the nation to maintain its access to high technologies in the field of aircraft manufacture as well as helping the national aircraft industry in making a revolutionary breakthrough.
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