Allison Model 250

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.[1] 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. 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 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:

i) compressor module, at the front of the engine

ii) gearbox module (including accessory drives)

iii) turbine module (including V-shaped exhaust ports)

iv) 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 420 hp (310 kW).

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, 715 hp (533 kW).

The Model 250 powers a large number of helicopters, small aircraft and even a motorcycle (MTT Turbine Superbike).[2] 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.

Fixed-wing

Rotary-wing

Other applications

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

Data from The Instrumentation Design And Control of a T63-A-700 Gas Turbine Engine [4]

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 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: 317 hp (236 kW) for Take-off, Sea Level 59 °F (15 °C) NH 51600rpm NL 35000rpm Nout 6000rpm
• 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)@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)

Related development

• Rolls-Royce RR300
• Rolls-Royce RR500

Wikimedia Commons has media related to Allison T63.

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