The Allison Model 250, now known as the Rolls-Royce M250, (US military designations T63 and T703) is a highly successful

turboshaft A turboshaft engine is a form of gas turbine that is optimized to produce shaftpower rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust ...

engine family, originally developed by the

Allison Engine Company The Allison Engine Company was an American aircraft engine manufacturer. Shortly after the death of James Allison in 1929 the company was purchased by the Fisher brothers. Fisher sold the company to General Motors, which owned it for most of it ...

in the early 1960s. The Model 250 has been produced by Rolls-Royce 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), to replace the Cessna O-1A Bird Dog. At this stage the US Army was 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 The Bell 47 is a single-rotor single-engine light helicopter manufactured by Bell Helicopter. It was based on the third Bell 30 prototype, which was the company's first helicopter designed by Arthur M. Young. The 47 became the first he ...

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. The

Hughes OH-6 The Hughes OH-6 Cayuse is a single-engine light helicopter that was designed and produced by the American aerospace company Hughes Helicopters. Its formal name is derived from the Cayuse people while its "Loach" nickname comes from the acronym f ...

design, powered by the T63, was selected for the US Army LOH in May 1965. The Model 250 powers a large number of helicopters, small aircraft and even a motorcycle ( MTT Turbine Superbike). 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.

Design

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 ported rearwards via two transfer pipes, which go around the outside of the turbine system, before the air is turned through 180 degrees at 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 through 90 degrees to exit the engine in a radial direction through twin exhaust ducts, which form a V-shape seen in the front 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. There are four modules: *compressor module, at the front of the engine *gearbox module (including accessory drives) *turbine module (including V-shaped exhaust ports) *combustion module (including twin compressed air transfer ducts) at the rear Earlier versions have seven 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 . One of the latest versions of the Model 250 is the -C40, which has only a centrifugal compressor producing a pressure ratio of 9.2:1, at an airflow of 6.1 lb/s (2.8 kg/s), and develops, at the shaft, .

Variants

;250-B15 ;250-B15A ;250-B15C ;250-B15G ;250-B17 ;250-B17B ;250-B17C ;250-B17D ;250-B17Fg ;250-B17F/1 ;250-B17F/2 ;250-C10D ;250-C18 : ;250-C18A : ;250-C20 ;250-C20B ;250-C20F ;250-C20J : ;250-C20R ;250-C20R/1 ;250-C20R/2 ;250-C20R/4 ;250-C20S ;250-C20W ;250-C22B ;250-C28 ;250-C28B ;250-C28C ;250-C30 ;250-C30G ;250-C30G/2 ;250-C30M ;250-C30P ;250-C30R ;250-C30R/3 ;250-C30R/3M ;250-C30S ;250-C30U ;250-C34 ;250-C40B ;250-C47B ;250-C47E ;250-C47M ;250-E3: Experimental engine containing a

regenerative heat exchanger A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. To accomplish this the hot fl ...

. First regenerative engine to fully power a VTOL aircraft in flight. Ran on a Hughes YOH-6A Light Observation Helicopter in 1967. engine delivering . ;T63-A-5 ;T63-A-5A ;T63-A-700 : ;T63-A-720: ;T703-AD-700 ;Soloy Turbine-Pac:Typically 2x 250-C20S driving a single propeller via a combining gearbox, able to operate individually. ;Mitsubishi CT63: Licence production for

Kawasaki-Hughes 500 The MD Helicopters MD 500 series is an American family of light utility civilian and military helicopters. The MD 500 was developed from the Hughes 500, a civilian version of the US Army's OH-6A Cayuse/Loach. The series currently includes the M ...

/ OH-6A helicopters.

Applications

Fixed-wing

Rotary-wing

Other applications

Loral GZ-22 The Loral GZ-22 (also known as the Goodyear GZ-22) was a class of non-rigid airship, or blimp first flown in 1989 and operated by Goodyear as its flagship promotional aircraft, with civil registration ''N4A'' and christened ''Spirit of Akron'' ...

, non-rigid airship * MTT Turbine Superbike, motorcycle

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

References

External links

Rolls-Royce M250 Official page

Video of running model
{{USAF gas turbine engines 1960s turboprop engines Model 250 1960s turboshaft engines M250