Design and development
At the 1987 Paris Air Show, the Soviet Union displayed scale models of several aircraft in development,[3] including a 150-seat aircraft powered by two pusher propfans mounted on the rear fuselage.[4] This aircraft was unnamed at the time, but the Soviets disclosed that the Yakovlev Design Bureau was developing the aircraft.[4] Later in 1987, the Soviet civil aviation minister noted that Yakovlev was building a twin-propfan airliner based on its Yak-42 model.[5] In 1989, Yakovlev planned to test the D-236 propfan engine from the Ivchenko-Progress engine design bureau (also known as Progress, Lotarev, Muravchenko, ZMKB, and Zaporozhye) in flight on a Yak-42 testbed aircraft by the end of the year.[6]
Yakovlev revealed details in early 1990 about Yakovlev's propfan design, which was given the name of Yak-46 and planned to enter service in 1997.[7] The Yak-46 and the Yak-42M, a 4-metre stretched derivative (13-foot; 4,000-millimetre; 160-inch) of the Yak-42 that would enter service in 1994, would have fly-by-wire (FBW) controls, an electronic flight instrument system (EFIS), a supercritical airfoil wing of added aspect ratio, span, and sweep, seating capacity of 150 passengers or more, and new engines with thrust reversal capability. However, the Yak-46 would have two unshrouded propfan engines mounted on the aft fuselage, instead of the Yak-42 and Yak-42M's three aft-mounted turbofan engines (that includes one attached to the vertical stabilizer). Yakovlev also proposed an unnamed interim derivative situated between the Yak-42M and the Yak-46, which would be powered by two underwing engines based on the contra-rotating, integrated, shrouded propfan (CRISP) engine concept. Since this derivative required greater airframe changes, Yakovlev was less sure of its eventual production.
The Yak-46 would hold 150-162 seats in a six-abreast, single-aisle configuration, fly as far as 1,540 nautical miles (2,850 km; 1,770 mi), cruise at a speed of 445 to 460 knots (800 to 850 km/h), and be powered by two Lotarev D-27 propfan engines. The 3.8-metre diameter (12 ft; 3,800 mm; 150 in) propfan engines would have contra-rotating propellers with eight blades in front and six blades in back, have a thrust specific fuel consumption of 0.44 lb/(lbf⋅h) (12 g/(kN⋅s)), and deliver 9,700 kilowatts (13,000 hp), resulting in a thrust of 11,200 kgf (24,700 lbf; 110 kN). The fuel consumption per available seat kilometer of the Yak-46 was 13.8 grams per kilometre (0.78 ounces per mile) per seat. This value compared to 14.5 g/km (0.82 oz/mi) for the Yak-46's underwing engine alternative and 21 g/km (1.2 oz/mi) for the Yak-42M, which was already 35-40% more efficient than the Yak-42. At the time, airline interest but no orders were reported for the Yak-46, but Yakovlev was negotiating to sell 200 Yak-42M planes to Aeroflot,[8] which was then the world's largest airline.[9]
By October 1990, the two versions of the Yak-46 were in competition with the 102-126 seat Tupolev Tu-334, which like the higher-capacity Yak-46 had an interim turbofan version and a final propfan version, to replace hundreds of aging Tu-134 airliners at Aeroflot. To prepare for Yak-46 development, Yakovlev created a joint venture with Ivchenko Progress and the Soviet Ministry of Civil Aviation. Aeroflot would help fund a prototype, which would be built starting early in 1991 if the Yak-46 were selected. Annual production would eventually be as high as 100 airliners and would run through 2005.[10]
On March 15, 1991, Yakovlev finally began flight tests of one D-236 propfan engine on a Yak-42E-LL aircraft testbed, making it the first propfan flight test program led by an individual Soviet aircraft design bureau.[11] The testbed appeared on static display at the 1991 Paris Air Show in June.[12] Like the D-27 engine, the D-236 was a contra-rotating system with an eight-bladed front propeller and a six-bladed back propeller. It had a fan diameter of 4.2 m (14 ft; 4,200 mm; 170 in), a power rating of 8,195 kW (10,990 hp),[13] and a thrust rating of 10,500–11,000 kgf (23,000–24,000 lbf; 103–108 kN). The testbed engine was limited to a smaller thrust, though, because the D-236 was more powerful than the Lotarev D-36 engine that it replaced.[14]
Meanwhile, conflicting reports appeared about the Yak-46 power plant: one article stated that the D-236 would be the eventual engine,[15] but another article said Yakovlev was deciding between a gearless unducted fan, which would yield a fuel consumption of 12 g/km (0.68 oz/mi), and a less-efficient, but now considered more realistic, ducted fan with an ultra-high bypass ratio between 20 and 27.[11] In August, a report stated that the initial version of the Yak-46, now named the Yak-46-1, would have two Progress underwing power plants resembling the International Aero Engines (IAE) SuperFan engine, while the follow-up version, called the Yak-46-2, would again be aft fuselage-mounted D-27 engines.[16] A Soviet aviation publication named the initial engine as the Progress D-627, a quiet, super-high bypass ratio turbofan.[17] Derived from the D-27, the D-627 had ducted, contra-rotating fans and used a differential gearbox.[18] The D-627 had a takeoff thrust of 11,000 kgf (24,000 lbf; 110 kN); its thrust specific fuel consumption (TSFC) at cruise would not exceed 0.5 lb/(lbf⋅h) (14 g/(kN⋅s)) at Mach 0.8 and 11,000 m (36,000 ft) altitude, equivalent to a speed of 461 kn (854 km/h; 531 mph); and the Yak-46's per-passenger fuel consumption with the D-627 would be 15.5 g/km (0.88 oz/mi). The subsequent version of the Yak-46 would have the same characteristics and performance outlined for the D-27 earlier, but its cruise TSFC after losses would be 0.47 lb/(lbf⋅h) (13 g/(kN⋅s)) at 460 kn (850 km/h; 530 mph) speed. In addition, the Yak-46 would have a planned service life of 60,000 hours, based on an annual average flying time of 3,000 hours.[17] Yakovlev was constructing a wooden mockup of the Yak-46 as of November 1991.[19] By August 1992, it was planning for co-production of the aircraft in South Africa.[20]
As of March 1993, Yakovlev had received one order to develop the Yak-46, which temporarily saved the firm from bankruptcy.[21] In mid-1994, the Progress engine design bureau was looking to create a turbojet derivative of its D-27 to use on the Yak-46.[22] Later that year, it was reported that the Yak-46 would use two Progress D-727 high-bypass turbofan engines, without mention of a propfan aircraft version.[23] In 1996, though, another airliner census still described a Yak-46-1 version with D-727 underwing engines followed by a Yak-46-2 with D-27 aft-mounted engines in a T-tail empennage configuration. Because of engine development issues, neither of the planes would fly before the year 2000.[24]
