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
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) 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 and an airflow of , and a shaft power output 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), developing 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 Purpose-built for use with the Sikorsky S-76. Featured a single-stage centrifugal compressor instead of the multi-stage axial/centrifugal compressor of earlier models 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. First regenerative engine to fully power a vertical take-off and landing (VTOL) aircraft in flight. Ran on a Hughes YOH-6A Light Observation Helicopter in 1967. engine delivering . 250-KS4 Used aboard marine ships as a starter system for larger gas turbines. Redundant Independent Mechanical Start System (RIMSS). T63-A-5 T63-A-5A T63-A-700 T63-A-720 T703-AD-700 Soloy Turbine-PacTypically 2x 250-C20S driving a single propeller via a combining gearbox, able to operate individually. Mitsubishi CT63 Licenced production for Kawasaki-Hughes 500 / OH-6A helicopters.
Applications Fixed-wing
Rotary-wing
Other applications Gerhardt 66 – USAC IndyCar racing car Loral GZ-22 – non-rigid airship MTT Turbine Superbike – motorcycle Redundant Independent Mechanical Start System (RIMSS) (used on gas turbine ships)
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)
See also
References
External links
Video of running model