Allison Model 250

Model 250 / T63
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

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.

Design and development

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 HP turbine, which is connected, via the HP shaft, to the compression system, before expanding through the two stage power turbine; the exhaust gases then turn through 90 degrees to exit the engine in a radial direction; a stub shaft connects the power turbine to a compact reduction gearbox, located inboard, between the centrifugal compressor and the exhaust/power turbine system.

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

Some of the earlier versions have axial compressor stages mounted on the HP shaft to supercharge a relatively low pressure ratio centrifugal compressor. The -C20R is typical, with an overall pressure ratio of 8.0:1, at an airflow of 4.0 lb/s (1.8 kg/s), with a power output, at the shaft, of 450 hp (340 kW).

The Model 250 powers a large number of helicopters, small aircraft and even a motorcycle (MTT Turbine Superbike).^[1] As a result, nearly 30,000 Model 250 engines have been produced, of which approximately 16,000 remain in service, making the Model 250 one of the highest-selling engines made by Rolls-Royce.

Variants

Applications

Fixed-wing

• Aermacchi M-290 RediGO
• BAE Systems Mantis
• Beechcraft Bonanza {Prop-jet conversions}
• Britten-Norman BN-2T Turbine Islander
• Extra EA-500
• GAF Nomad
• Gippsland GA10
• Partenavia AP.68TP variants - Spartacus & Viator
• RFB Fantrainer
• SIAI-Marchetti SM.1019
• Soloy Cessna 206 turbine conversion
• Cessna P210 Silver Eagle O&N Aircraft Modifications - onaircraft.com

Rotary-wing

Other applications

• Loral GZ-22, non-rigid airship
• MTT Turbine Superbike, motorcycle

Specifications (T63-A-5)

Data from Jane's All the World's Aircraft 1962-63.^[3]

General characteristics

• Type: Turboshaft
• Length: 40.5 in (1,029 mm)
• Diameter: 22.5 in (572 mm)
• Dry weight: 136 lb (62 kg) dry

Components

• Compressor: 6-stage axial + 1-stage centrifugal compressors
• 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: 250 hp (190 kW) for take-off, sea level 59 °F (15 °C)
• Overall pressure ratio: 6.2:1
• Air mass flow: 3 lb (1 kg)/s
• Turbine inlet temperature: 1,805 °F (985 °C)
• Specific fuel consumption: 0.77 lb/hp·h (0.468 kg/kW·h) at 90% power cruise rating
• Power-to-weight ratio: 1.838 hp/lb (3.022 kW/kg)

See also

• Rolls-Royce RR300
• Rolls-Royce RR500

References

External links

Wikimedia Commons has media related to Allison T63.

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