Allison Model 250

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Model 250 / T63
Allison (MTU) 250 C20B.jpg
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[edit]

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 were unsure whether to have a fixed or rotary wing aircraft, so Allison were 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. Thus was born the Model 250. 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 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 (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 turn through 90 degrees to exit the engine in a radial direction through twin exhaust ducts, which form a V-shape in 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.

Earlier versions have 7 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.

Variants[edit]

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-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[3]
250-C47M
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.

Applications[edit]

Fixed-wing
Rotary-wing
Other applications

Specifications (T63-A-5)[edit]

Data from Jane's All the World's Aircraft 1962-63.[4]

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

Model Weight (kg) Air flow rate (kg/s) Pressure
ratio
Take-off power (WPS) Continuous power (WPS) Height (mm) Width (mm) Length (mm)
C20B/F/J 73 1.230 6.2:1 420 370 571 483 986
C20R 78 1.733 7.9:1 450 380 589 528 986
B17D 90 1.624 7.2:1 420 370
B17F 93 1.733 7.9:1 450 380
C30M 113 2.54 8.6:1 650 540 638 555 1041
C30S 114 2.54 8.6:1 650 501 638 555 1041
C30P 114 2.54 8.6:1 650 501 638 555 1041
C40B 127 2.77 9.2:1 715 613 638 555 1040
C47B/M 124 2.77 9.2:1 650 600 638 555 1040
C47E[5] 133 9.2:1 700 630 630 1091

See also[edit]

References[edit]

  1. ^ http://www.enginehistory.org/Convention/2009/Presentations/Model250Hx.pdf
  2. ^ http://www.atlasaviation.com/powerplant/aircraftapp.htm
  3. ^ http://www.rolls-royce.com/news/press_releases/2013/04032013_military_helicopters.jsp
  4. ^ Taylor, John W.R. FRHistS. ARAeS (1962). Jane's All the World's Aircraft 1962-63. London: Sampson, Low, Marston & Co Ltd. 
  5. ^ "M250-C47E Turboshaft Specification" (PDF). Rolls-Royce. Retrieved 14 June 2016. 

External links[edit]