Sukhoi T-50 Shows Flight-Control Innovations

By Bill Sweetman
Source: Aviation Week & Space Technology
August 19, 2013
Credit: Sukhoi

A highlight of the MAKS air show, which opens at Zhukovsky Airport near Moscow next week, is likely to be the demonstration of the Sukhoi T-50 PAK FA (Perspektivny Aviatsionny Kompleks Frontovoy Aviatsii—Future Tactical Air System) fighter.

The T-50 appeared at MAKS two years ago, but is now flying with updated control laws that expand its flight envelope. (The program had flown fewer than 100 test sorties between its January 2010 maiden flight and its MAKS debut.) Recent videos show the aircraft performing what appear to be sustained-altitude flat rotation maneuvers and high-angle-of-attack turns similar to those demonstrated at the Paris air show by the Su-35S. Four T-50 prototypes have now flown and a fifth is expected to fly by the end of the year. The first state acceptance trials are due to start in 2014, United Aircraft Corporation President Mikhail Pogosyan said earlier this year, and production should start in 2015.

Russian President Vladimir Putin has said that production aircraft will enter service in 2016. However, since the aircraft has yet to fly with its definitive engine, this most likely indicates that the Russian air force is reverting to Soviet-era practice by equipping an operational test unit with interim-standard aircraft while development of the objective system is completed.

Many details of the fighter's equipment and armament remain classified or unpublished. However, in recent months the Sukhoi design bureau has obtained several patents relating to the T-50, including the rationale behind the stealth fighter's configuration.

One Sukhoi patent opens by outlining a reference design similar to the Lockheed Martin F-22, but notes perceived shortcomings and areas where the Russian designers, starting a decade later after work on the Su-27 and its descendants, tried to do better. The F-22's thrust-vector control (TVC) system cannot provide roll or yaw control because the engines are too close together. The engine installation leaves no place for weapon bays in the same plane as the engines—they have to be installed around and below the inlet ducts. The serpentine inlet ducts add length and weight. Post-stall recovery is problematic if TVC fails, and the fixed fins and rudders are large.

The T-50 is a blended wing-body design, resembling the Su-27 in one key respect: the core of the structure is the “centroplane,” a long-chord, deep-section inner wing to which the rest of the airframe components—the forward fuselage and widely separated engine nacelles, wings and tail surfaces—are attached. Compared to the Su-27, however, the centroplane is deeper between the engines, to accommodate weapon bays.

The flight control system has 14 effectors—12 moving flight control surfaces and the engine nozzles. The wing leading-edge flaps are used symmetrically to maintain lift at high angles of attack and adjust the wing profile to the Mach number. The ailerons are used only at low speed and takeoff and landing, when the flaperons are used to increase lift. At higher speeds, roll control comes from the flaperons and horizontal tails.

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