Russian S-37 Accelerates Fighter Technology



Russian S-37 Accelerates
Fighter Technology

The prototype is Russia's most advanced in 20 years, and Sukhoi would like to show it at Farnborough

Russia's Sukhoi S-37 forward-swept-wing fighter has completed about 100 flights and is progressing through its supersonic test regime at the Zhukovsky Flight Test Center near the Russian capital.

Russia's Sukhoi S-37 forward-swept-wing fighter is the most advanced military aircraft developed by the former Soviet Union in 20 years.
The company would like to bring the large twin-engine fighter to the Farnborough air show in July, Sukhoi General Director Mikhail A. Pogosyan told Aviation Week & Space Technology. But whether the Russian government will allow unique aircraft to fly to England is still an open question.

Sukhoi has not been able to obtain Russian Defense Ministry permission to even display the S-37 publicly at Russian venues other than Zhukovsky, although the aircraft made two brief overflights of Moscow after it first flew in 1997.

S-37 lands at Zhukovsky Flight Test Center near Moscow. The 74-ft.-long fighter has a wingspan of nearly 55 ft.
The prototype is the most advanced Russian military aircraft since the Su-27 operational fighter first flew more than 20 years ago. "The S-37 remains highly classified, and the Ministry of Defense is not in a hurry to make information available about this aircraft," Pogosyan said.

But the Sukhoi director has close ties to new Russian President Vladimir Putin, and this could aid the S-37 program, which earlier had been shunned by some senior officers in the Russian Defense Ministry. "Putin has taken a special interest in our company," Pogosyan said.

Contrails stream off wing tips and canards as S-37 pulls out of dive.
The S-37 is Russia's primary testbed for new composite materials and multiple-string fly-by-wire computer and software technology that could form the foundation for a Russian fifth-generation heavy fighter, although the Russian economy is so bad that it will be years before such a program emerges.

A new Sukhoi heavy fighter design will not necessarily involve a forward-swept wing. But the composite materials development and fabrication techniques and flight control technologies embodied in the S-37 will benefit Russian fighter designs well into the next two decades.

S-37 performs hard-bank turn in afterburner. Twin-engine fighter has 68,400 lb. of takeoff thrust. Russian prototype is twice the size of the earlier U.S. X-29.
The more conventional MiG-1.42 flight test aircraft flying at Zhukovsky also will be a key element in any Russian fifth-generation fighter decision made during the next several years. But the MiG-1.42 has flown considerably less than the S-37 and features fewer cutting-edge technologies than the Sukhoi forward-swept-wing aircraft.

A Russian fighter to succeed the Su-27 would be developed to counter the U.S. Air Force/Lockheed Martin F-22, while both Sukhoi and the MiG Russian Aircraft Corp. also continue to refine new lightweight fighter candidates for possible development to counter the U.S. Joint Strike Fighter.

Pogosyan said the S-37 has successfully validated the preflight design models developed by Sukhoi and the Central Aerohydrodynamic Institute (TsAGI), although the aircraft needed its vertical tails enlarged after initial tests showed this would improve controllability.

"We are continuing to press forward on S-37 flight test very heartily," he added.

The program funding is split between the Russian Defense Ministry and Sukhoi, although initially it was largely funded by Sukhoi. The S-37 carries a Sukhoi "S" designation because it is considered an experimental prototype, compared with the design bureau's "Su" designation for operational fighters.

Russian President Vladimir Putin (left) meets with Mikhail Pogosyan, Sukhoi general director (right), and former general designer Mikhail Simonov (center). Putin has flown in the Su-27 and Su-25.
The forward-swept-wing aircraft is performing to its design models for both slow and supersonic flight. "The aerodynamics during supersonic flight have been proven. It has also satisfied one of our main goals to correlate ground test structural data with structural loads information obtained during flight," Pogosyan said. The use of advanced composites in the wing permitted aeroelastic tailoring to allow it to bend, but without the twisting that would create intolerable structural loads.

"We are proud to say that all the advantages of this configuration have been successfully proven during flight test," he said. These include algorithms associated with flying the forward-swept wing at high angles of attack, a key element for defining the efficiency of the design.

Air flowing over the S-37's thin supercritical airfoil travels inward toward the wing root instead of outward toward the tips as in conventional wings. As this inward flow occurs across the airfoil, it prevents the wing tips and ailerons from stalling at high angles of attack.

As the forward sweep maintains uniform airflow over the ailerons, this--coupled with computer control of the movable canards--enables the pilot to maintain excellent control response at angles of attack of 45 deg. or more.

The S-37 is designed with "relaxed static stability," which greatly reduces drag compared with conventional aircraft designs where aircraft fly with drag-producing downforce on the horizontal tail to balance out wing lift. On the S-37, the canards, in addition to providing primary pitch control, share the lifting load with the wing, resulting in relaxed static stability and therefore less overall aircraft drag. Strake flaps on either side of the vertical tails assist the canards with pitch control.

The aircraft is inherently unstable and can fly only because of its fly-by-wire control system. Both Pogosyan and the Russian Defense Ministry declined to provide any detail on the S-37's fly-by-wire control system or the number of redundant computer strings in the aircraft.

The two Grumman X-29 forward-swept-wing test aircraft that flew 442 research flights during 1984-92 had three digital flight control computers, each with an analog backup.

Pogosyan, who headed the S-37 design team for 10 years, said many of the forward-swept-wing structural elements and flight control aspects of the S-37 are substantially different from the X-29s, which were less than half the size of the new Russian fighter.

The S-37 dwarfs the X-29. The wingspan of the S-37 is 54.7 ft., compared with only 27.2 ft. for the X-29. The S-37 is 74 ft. long, versus 48 ft. for the U.S. aircraft, and the normal takeoff weight of the S-37 is about 56,600 lb., compared with only 17,600 lb. for the X-29.

With full fuel and internal weapon stores, the S-37 can have a maximum gross takeoff weight of up to 75,000 lb. This is about 10,000 lb. more than an F-15C with a maximum fuel and missile load.

While the X-29 was powered by a single 16,000-lb.-thrust General Electric F404 engine, the S-37 generates 68,400 lb. of takeoff thrust from two Russian Aviadvigatel D-30F6 turbofans like those in the MiG-31M. The S-37 also has two large rectangular supplemental air intakes with flapper doors atop its fuselage to supply extra air to the engines during takeoff and landing.

The aircraft's basic air-to-ground and air-to-air maneuvering capabilities and radar signature are being explored during the flight tests. The S-37 also has an infrared sensor pod mounted on its nose and an internal weapons bay. But Pogosyan said weapons test details about the aircraft are classified.

In addition to S-37 flight tests and the ongoing sale and upgrade of Su-27 variants, Pogosyan said his other major near-term goal is to totally reform the management and field service capabilities of Sukhoi. Pogosyan wears two hats in this regard: He is the director of both the Sukhoi Design Bureau and director of the Sukhoi Aviation Industrial Complex, the broader business side of the organization.

Pogosyan believes Putin will aid the reform at Sukhoi and other Russian companies. "Now there is deep interest by the government in the Russian military industrial complex," he said. And Putin is also especially familiar with Sukhoi. Last fall, he flew in a Su-25 ground attack aircraft; then just days before the Russian presidential election in March, Putin flew into Chechnya in the rear cockpit of a Su-27 (AW&ST Mar. 27, p. 28).

The Sukhoi director said one of Putin's main objectives is to exert more government oversight of hard currency flowing into Russia for arms sales and funneling some of that funding into the establishment of aerospace holding companies.

Pogosyan said this should help to focus Russian military aerospace priorities and better coordinate the development efforts that are selected.

Alexey Komarov contributed to this story from Moscow.

Copyright May 29, 2000 The McGraw-Hill Companies

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