Sukhoi T-50 Shows Flight-Control Innovations

By Bill Sweetman
Source: Aviation Week & Space Technology

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, F-35 and Eurofighter Typhoon), so they also feature a radial blocker similar in principal to that used on the Boeing F/A-18E/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 the Lockheed Martin F-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.

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