Sukhoi T-4 (Part 1)

The Sukhoi T-4 at the ramp.

The Sukhoi T-4 is arguably called the “world’s first fly-by-wire” aircraft. It is however the first aircraft in Russia, to be built using titanium (like Lockheed’s SR-71 Blackbird).

Design configuration for the T-4 was finalized in December 1964 after extensive research done at TsAGI (the Central Aerohydrodynamic Institute, the Soviet counterpart to NASA) over the previous 2 years. The T-4 was do be a Mach 3+ so-called “strategic weapons system” i.e. a bomber, somewhat similar to the North American XB-70 (to which the T-4 bears a more than passing resemblance).

Sukhoi T-4 3-view drawing.

The Sukhoi T-4 (sometimes called the “Sotka” meaning 100) is 146ft 2in long and has a wing span of 72ft 2in (smaller than the XB-70) and is 36ft 9in tall. She weighs in empty 120,370lb. Loaded at 252,205lb (normal) or a maximum weight of 299,824lb. The wing has an area of 3,183 sq ft2.

The design performance specifications are as follows: Maximum speed was to have been 1,988mph (Mach 3.01). At sea level 715 mph (Mach 0.94). The service ceiling was 78,740ft with a range of 3,780 miles in the clean configuration. Take off and landing runs were 3,281ft and 3,117 respectively.

The T-4s fuselage at construction.

The T-4 aircraft under construction without the nosecone.

Overall, the structure of the T-4 used VT-20, VT-21L and VT-22 titanium alloys. As mentioned, the T-4 was the first aircraft in the Soviet Union to be constructed of titanium. VIS-2 and VIS-5 stainless steels were also used in the structure of the T-4. VKS-210 structural steel was used for fuel system piping and VNS-2 steel was used for hydraulic system piping.

The T-4’s flight control system consisted of a main, electro-hydraulic, fly-by-wire control system and a back mechanical system. The FBW system operated in 3 modes :

-a damper mode operating jointly with the mechanical control system.

-the takeoff and landing mode.

-the enroute mode.

The quadruplex FBW system had full authority but would automatically fail-over to the mechanical system if any of the 2 FBW channels failed. Each channel of the mechanical system was equipped with an automatic cable tension control unit and a changeover mechanism. The FBW and mechanical control system both had common artificial feel units and trimming mechanisms. Duplicate electromechanical actuators operated the canards via electrical signals from the pilot’s controls. All flights were done with the FBW control system switched on from the moment of takeoff.

An in-flight view of the T-4s wing surface.

The wing featured 0 degrees of anhedral. The inboard leading edge was angled at 75 degrees 44’. The outboard leading edge was angled at 60 degrees 17’. The wing’s thinkness/chord ratio was 2.7%. Both leading edges were fixed. Each wing also had 4 evelons (a combination of elevators and ailerons). There were also flapped canard foreplanes and a 2 part rudder.

A composite photo of the movable nose in the lowered and raised positions.

The fuselage diameter was 20 inches. The nose could be drooped (for better crew visibility during takeoff and landing) to 12 degrees 12’ at speeds up to 435 mph. The nose was driven by screwjack driven hydraulic motors located behind the pilot. The first T-4 (aircraft serialed “101”) had a periscope for the pilot when the nose was fully raised. The cabin as fully pressurized and located behind the cabin was a refrigerated fuselage section containing the avionics. Behind that were 3 fuel tanks that carried a a total of 57 tonnes of fuel (a specially developed fuel called RG-1, a naphthyl fuel similar to JP-7)In the aft section of the fuselage, there is provision for braking parachute.

T-4 landing run after the first flight trailing the braking parachutes.

The fuel system was automated to maintain the T-4s center-of-gravity throughout the flight envelope. Production versions had provision for a ventral-mounted drop tank and air refueling recepticle.

A view of the 4 RD-31 engine nozzles as installed on the T-4.

A view of the T-4’s landing gear, engine bay and air inlet.

Under the wing of the T-4 was a huge box containing the air inlet system and the RD-36 turbojet engines.  The T-4 was powered by 4 Kolesov RD-36-41 turbojets each rated at 35,273lb in afterburner. The RD-36 has a specific fuel consumption of 1.94lbs/h at military power and 4.19lbs/h. The FBW system controlled the engines, the 3-section variable nozzles and the variable geometry air inlets. The T-4 had an automatic engine control system. For the most part it was used during decent and final approach phases to maintain speed. The system was doubly redundant and had built-in test equipment.

The T-4 main gear.

Each main landing gear had 4 twin tired wheels that retracted forward, rotating 90 degrees to lie on it’s side just outboard of the air inlet box. The nose gear has a levered suspension on 2 tires with wheel brakes and steering. The nose gear retracts backwards into a a bay between the engine ducts.

The T-4 has 4 autonomous hydraulic systems pressured at 3,980lb/in2. The aircraft also had a 400Hz 3 phase electrical system rated at 220/115V powered by 4 oil-cooled alternators each rated at 60kVa.

Sukhoi T-4 cutaway drawing.

In part 2 I’ll cover the T-4 flight-test program.