T-50 design rationale unveiled
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,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Many details of the fighter's equipment and armament remain classified or unpublished.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, 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.
The all-moving vertical tails sit on short fixed pylons that contain the actuators, and air intakes for engine compartment cooling and heat exchangers. One purpose of the pylons is to make room for a longer bearing arm for the vertical tail pivot, between the top of the pylon and the lower surface of the blended wing. This reduces loads and allows the bearings and structure to be lighter. At supersonic speeds, the T-50 is directionally unstable and uses active control via the vertical tails. That is why the all-moving surfaces can be much smaller than the F-22's fixed fins and movable rudders. The vertical tails replace the airbrake, moving symmetrically to increase drag with minimal pitch moment.
The large and unique moving leading edges on the centroplane help optimize the lift generated by that section in cruising flight, but their most important function is to recover the aircraft in the event of a TVC failure at post-stall angles of attack. They do this by deflecting sharply downward, reducing the plan-projected area of the wing-body section in front of the center of gravity.
The engines are widely separated, to make room for weapon bays and provide roll and yaw vector control. The engine centerlines are splayed outward to reduce effects of asymmetric thrust with one engine inoperative, placing the thrust vector of the good engine closer to the center of mass of the aircraft.
As on the TVC-equipped versions of the Su-27/30/35 family, the individual engine nozzles vector only in one plane, but the vector axes are rotated outward. Consequently, symmetrical movement of the nozzles creates a pitch force (each nozzle creates an equal and opposite yaw moment) and asymmetrical movement creates both roll and yaw moments. If yaw only is required (for example, in the Su-35's “bell” maneuver, a high-alpha deceleration followed by a 180-deg. change of direction) the roll moment can be counteracted by flaperons and ailerons.
The T-50's inlets are a compromise design. They are serpentine but the curvature is insufficient to obscure the entire engine face (as on the F-22,and ), so they also feature a radial blocker similar in principal to that used on the /F Super Hornet. Unlike the F-22 inlets, however, they feature a variable throat section and spill doors on the inboard, outboard and lower surfaces of the ducts. The result is a complex multiple-shock pattern at supersonic speed, which the Russians consider essential for efficient operation at Mach 2. The inlets also feature clamshell-like mesh screens and diverter slots to keep foreign objects out of the engine, as used on the Su-27 family.
The main challenge in the structural design was to provide space for tandem weapon bays running the entire length of the center section. This ruled out the structural concept used on theF-35 and F-22, which have multiple full-depth bulkheads carrying the wing loads, because this forces all the weapon bays to be ahead of the wing. The centerline structure on the T-50 has to be quite shallow, so that designing it to resist peak wing bending loads will be a very difficult challenge. The solution on the T-50 is to design the “centroplane” section as a stiff, integrated structure with two sets of full-depth longitudinal booms, located at the outer edges of the nacelles and at the wing-to-centroplane junction. These are connected by multiple (the patent drawing shows eight) spanwise spars that also carry the wing attachment fittings. The result is a structure that spreads the bending loads over the centroplane and reduces the peak loads at the centerline.
It is believed that the target maximum speed of the T-50 is around Mach 2. The goal was originally Mach 2.35, but this was reduced to Mach 2.1 and then to the current figure, compared to Mach 2.25 for the Su-35S. The main reason for the difference is that the T-50 uses more composite materials in its primary structure than the Su-35S, which makes heavy use of titanium.
The T-50 aircraft flying today are equipped with the izdeliye (Type) 117 engine, described by its designer in a 2011 interview as being more advanced than the 117S used on the Su-35S. The 117S appears to be an evolution of the AL-31 engine series with some technology from the 117. The 117 is claimed to have a thrust/weight ratio of 10:1.
However, Saturn Managing Director Ilya Fyodorov confirmed at a press conference last month that the company is designing a follow-on engine (referred to by the 117 designer as izdeliye 30) for the T-50, which is expected to offer higher performance than the 117 from 2020 onward.
More details of the fighter's weapons may be revealed at MAKS, but it appears that the T-50 is designed to carry variants of in-service missiles initially. Tactical Missiles Corporation General Director Boris Obnosov identified several T-50 weapons in an interview early in 2012, including the existing Kh-35UE anti-ship missile, Kh-38ME air-to-surface weapon and the RVV-MD, an improved version of the R-73E short-range air-to-air missile with an enlarged seeker field of view and a claimed 30% range increase. A significant development is the new Kh-58UShKE, a long-range (up to 245 km), Mach 4-capable anti-radar missile originally produced for the MiG-25BM Foxbat-E, fitted with folding wings for internal carriage.
However, Obnosov identified these specifically as being weapons at service entry, which he projected in 2014. There is still no definitive information about the T-50's internal weapons capability, but it seems likely that there are four separate weapon bays. Two bays outboard of the inlets each accommodate a single RVV-MD. Tandem bays between the engines each hold two missiles, but it is likely that the forward bay is deeper to house weapons such as the Kh-58UShKE, with the aft bay dedicated to air-to-air missiles in the R-77 family.
Tap the icon in the digital edition of AW&ST to see the T-50 performing sustained-altitude, flat-rotation maneuvers and high-angle-of-attack turns, or go to AviationWeek.com/video