Allison Model 250
| Model 250 / T63 | |
|---|---|
_250_C20B.jpg) |
|
| MTU-built Allison 250-C20B | |
| Type | Turboshaft |
| National origin | United States |
| Manufacturer |
Allison Engine Company Rolls-Royce plc |
| Major applications |
Bell 206 MD Helicopters MD 500 MBB Bo 105 Sikorsky S-76 |
| Number built | >30,000 |
| Program cost | $6.4 million (1958) (equivalent to $53,100,000 in 2016) |
| Unit cost | -C20B : $323,400 (2012) -C30R/3 : $265,600 (2012) |
The Allison Model 250, now known as the Rolls-Royce M250, (US military designations T63 and T703) is a highly successful turboshaft engine family, originally developed by the Allison Engine Company in the early 1960s. The Model 250 has been produced by Rolls-Royce since it acquired Allison in 1995.
In 1958, the Detroit Diesel Allison division of General Motors was chosen by the US Army to develop a new light turbine engine to power a Light Observation Aircraft (LOA), to replace the Cessna O-1A Bird Dog. At this stage the US Army were unsure whether to have a fixed- or rotary-wing aircraft, so Allison was instructed to consider both applications. Design studies undertaken considered a wide range of possible mechanical configurations for the turboprop/turboshaft. These studies culminated in the testing of the first prototype engine, designated YT63-A-3, in April 1959. In 1960 the US Army settled for a rotary wing platform. The YT63-A-3 first flew in a variant of the Bell 47 helicopter in 1961. A modified version of the engine (YT63-A-5) with the exhaust pointing upwards (to avoid grass fires) soon followed. This version, rated at 250 hp, passed the Model Qualification Test in September 1962. Thus was born the Model 250. Incidentally, the Hughes OH-6 design was selected for the US Army LOH platform in May 1965,
Allison adopted a reverse-airflow engine configuration for the Model 250: although air enters the intake/compression system in the conventional fashion, the compressed air leaving the centrifugal compressor diffuser is ducted rearwards around the turbine system, before being turned through 180 degrees at entry to the combustor. The combustion products expand through the two-stage (single-stage on early engines) high-pressure turbine section, which is connected to the compressor via the HP shaft. The combustion products continue to expand through the two-stage power turbine which generates shaft horsepower for the aircraft. A coaxial stub shaft connects the power turbine to a compact reduction gearbox, located inboard, between the centrifugal compressor and the exhaust/power turbine system. The exhaust stream then turn through 90 degrees to exit the engine in a radial direction through twin exhaust ducts, which form a V-shape in elevation. An important design feature of the Model 250 engine is its modular construction which greatly simplifies maintenance and repair activity. Also the unique reverse-flow design provides for ease of hot section maintenance.
Earlier versions have 7 axial compressor stages mounted on the HP shaft to supercharge a relatively low-pressure-ratio centrifugal compressor. The -C20B is typical, with an overall pressure ratio of 7.2:1, at an airflow of 3.45 lb/s (1.8 kg/s), with a power output, at the shaft, of 420 hp (310 kW).
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