Allison Model 250

Model 250 / T63
MTU-built Allison 250-C20B
Type	Turboshaft/Turboprop
National origin	United States
Manufacturer	Allison Engine Company Rolls-Royce plc
Major applications	Bell 206 Britten-Norman BN-2T Turbine Islander MD Helicopters MD 500 MBB Bo 105 Sikorsky S-76 Agusta A109 A, B, C
Number built	>30,000
Developed into	Rolls-Royce RR500
Variants	Rolls-Royce RR300

The Allison Model 250 – now known as the Rolls-Royce M250 and designated the T63/T703 in US military service – is a successful turboshaft engine family, originally developed by the Allison Engine Company in the early 1960s. Rolls-Royce has produced the Model 250 since it acquired Allison in 1995.

Development

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); the goal was to replace the Cessna O-1A Bird Dog aircraft. At this stage, the US Army was unsure whether to use a fixed- or rotary-wing aircraft, so Allison was instructed to consider both applications. The design studies considered a range of possible mechanical configurations for the turboprop/turboshaft. These studies culminated in testing the prototype engine, designated YT63-A-3, in April 1959. In 1960, the US Army settled on 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 (the YT63-A-5) – with the exhaust pointing upward (to avoid grass fires) – soon followed. This version, rated at 250 hp, passed the Model Qualification Test in September 1962. The Hughes OH-6 design, powered by the T63, was selected for the US Army Light Observation Helicopter (LOH) in May 1965.

The Model 250 powers many helicopters, small aircraft, and a motorcycle (the MTT Turbine Superbike). As a result, nearly 30,000 Model 250 engines have been produced, of which approximately 16,000 remain in service; thus, the Model 250 is one of the best-selling engines made by Rolls-Royce.

Design

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Allison adopted a reverse-airflow engine configuration for the Model 250: although air enters the intake/compression system conventionally, the compressed air leaving the centrifugal compressor diffuser is ported rearwards via two transfer pipes – which go around the outside of the turbine system – before the air is turned 180 degrees upon entry to the combustor. The combustion products expand axially forward 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 turns 90 degrees to exit the engine radially through twin exhaust ducts, which form a V-shape visible in front elevation.

An important design feature of the Model 250 engine is its modular construction, which significantly simplifies maintenance and repair activities. The unique reverse-flow design also facilitates hot-section maintenance. The engine has four modules:

• a compressor module at the front of the engine,
• a gearbox module (including accessory drives),
• a turbine module (including V-shaped exhaust ports),
• and a combustion module (including twin compressed air transfer ducts) at the rear.

Earlier versions of the engine have seven axial compressor stages mounted on the HP shaft to supercharge a relatively low-pressure-ratio centrifugal compressor. The C20B version is typical, with an overall pressure ratio of 7.2:1, an airflow of 3.45 lb/s (1.56 kg/s), and a shaft power output of 420 hp (310 kW).

One of the latest versions of the Model 250 is the C40, which has only a centrifugal compressor, which produces a pressure ratio of 9.2:1, an airflow of 6.1 lb/s (2.8 kg/s), and develops 715 hp (533 kW) at the shaft.

Variants

Applications

Engines on display

A partially sectioned Allison 250-C20B is on public display at the City of Norwich Aviation Museum in Horsham St Faith, Norfolk, England.

Specifications Model 250-C18 (T63-A-700)

Data from The Instrumentation Design And Control of a T63-A-700 Gas Turbine Engine

General characteristics

• Type: turboshaft
• Length: 40.5 in (1,029 mm)
• Diameter: 22.5 in (572 mm)
• Dry weight: 138.5 lb (63 kg) dry

Components

• Compressor: 6-stage axial + 1-stage centrifugal
• Combustors: single-can combustion chamber
• Turbine: 2-stage axial gas generator power turbine + 2-stage axial free-power output turbine
• Fuel type: JP-4 aviation kerosene (alternatively JP-1 or JP-5)
• Oil system: pressure spray/splash, dry sump

Performance

• Maximum power output: 317 hp (236 kW) for take-off (Sea Level 59 °F (15 °C); 51,600 rpm N_H, 35,000 rpm N_L, 6,000 rpm N_out)
• Overall pressure ratio: 6.2:1
• Air mass flow: 3.3 lb/s (1.5 kg/s)
• Turbine inlet temperature: 1,380 °F (750 °C) at power turbine inlet
• Specific fuel consumption: 0.697 lb/hp·h (0.424 kg/kW·h)
• Power-to-weight ratio: 2.289 hp/lb (3.765 kW/kg)

See also

Related development

• Rolls-Royce RR300
• Rolls-Royce RR500

References

External links

Wikimedia Commons has media related to Rolls-Royce Model 250.

• Rolls-Royce M250 Official page
• Video of running model