Moscow--Remember the Cold War, when disclosure of a new Soviet aircraft would be met in the West with warnings of the threat it posed? Contrast that with the recent revelation of Russia's Sukhoi S-37 Berkut (Golden Eagle) --seen more with curiosity than caution by aircraft experts. But make no mistake, the aerodynamics, materials, and components in this prototype are pushing Russian technology such that an after-the-turn-of-the-century competitor to the U.S. Lockheed Martin F-22 Raptor might well be on the world arms market.
The Berkut first flew late last September. Sukhoi says the design has maneuverability and stealth to rival the F-22 and can be refined into a production airplane. Berkut is clearly based on its operational Su-27 family roughly equivalent in function to the US F-15 Eagle. The most striking feature is its forward swept wing (FSW). This "inverted arrow," as the Russians term it, smoothes the nose-to-tail cross-sectional area distribution, cutting transonic (Mach 1) drag. Airflow along the wing is toward the fuselage, rather than the tip, eliminating tip stalling, thus the roll-controlling ailerons on the wing trailing edge are more effective in high angle-of-attack fighter maneuvering. Russian aircraft expert Bill Sweetman of International Defence Review also points out the center of pressure is closer to the fuselage, reducingthe wing bending moment. For the same of bending, a longer wing can be used, again lowering drag.
Only a composite wing can be used, and the Russians say the wing on S-37 is 90% composite. This tailors the wing. It twists leading-edge down as it bends upward under lift. If it simply bent upward, as with a metal wing, the angle of attack closer to the tip becomes greater, raising lift and bending the wing further. Structural failure can take place.The FSW was demonstrated on the Grumman X-29 in the mid-eighties and, notes Sweetman, on a Sukhoi demonstrator in 1982.
Queen of the slipstream. Also obvious is the "tandem triplane:" the FSW; a forward maneuver-trigger canard; and a small-area horizontal tail. Lift from the canard and tail lift can balance the airplane in pitch, maximizing overall aircraft lift. The fly-by-wire electronic flight controls "dither" the canard for stability and displace it for maneuvers. The canard also affects airflow over the wings and tail.
Future developments are said to include digitally-controlled hydraulically-powered vectorable nozzles to aid maneuvers and trim the aircraft. Current vectored thrust Su-27 derivatives have demonstrated eye-catching low-speed aerobatics such as summersaults and "cobra-strike" moves, but these may be of limited use in combat because at low speed an airplane is a sitting duck. S-37 maneuvering limits are likely around that for current fighters, 9 g. Anything greater "needs a superhuman to fly it," says aerospace consultant Jay Miller.
Stealth features center on application of radar absorbing materials to the airframe, curved inlet ducts to prevent radar from "seeing" the flat engine faces, and internal or faired-over conformal carriage of weapons to reduce radar reflection. Miller, who has visited Sukhoi facilities says, Russian workmanship and manufacturing tolerances are not fine enough to produce the tight radar-stifling"fit and finish," found on high-stealth US aircraft.
Russian reports say the S-37 cockpit is equipped with LCD multi-function displays. "They are ten years behind the West in displays and electronic controls," notes Miller, "and I would expect the guts of their avionics are based on Western technology," using microchip technology but without integrated digital data-bus architecture. Noting the Russian hallmark of high-thrust engines, he adds, "What they can't do with finesse, they do with power, making up for weight. It pays off: propulsion growth is linear but systems shrink exponentially. resulting in great thrust-to-weight."
So when is it likely that flocks of Golden Eagles will be seen in the skies? Not soon, according to Sweetman and Miller. Neither sees an operational airplane for up to a decade, and then only if some export customer bankrolls development. The threat to the West, says Miller, may be in setting up the S-37 as a "straw man" in order for Western air forces to obtain scarce funding for weapons to counter it.
Berkut by the (provisional) numbers
Length 74 ft
Wingspan 54 ft
Height 21 ft
Speed Mach 2
Current max. thrust 2 x 34,200 lb
Empty weight 52,900 lb
Max. takeoff weight 75,000 lb
Service ceiling 60,000 ft