The Allison Model 250 (now known as the Rolls-Royce M250 and designated T63/T703 in US military service) is a highly 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), 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 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 helicopter in 1961. A modified version of the engine (YT63-A-5), with the exhaust pointing upward (to avoid grass fires), soon followed. This version, rated at 250hp, passed the Model Qualification Test in September 1962. The Hughes OH-6 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 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 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 90 degrees to exit the engine radially through twin exhaust ducts, which form a V-shape visible in the front elevation.

An important design feature of the Model 250 engine is its modular construction, which greatly simplifies maintenance and repair activities. Also, the unique reverse-flow design facilitates 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 and an airflow of 3.45lb/s (1.8kg/s), 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 6.1lb/s (2.8kg/s), and develops, at the shaft, .

Variants

1. 250-B15

2. 250-B15A

3. 250-B15C

4. 250-B15G

5. 250-B17

6. 250-B17B

7. 250-B17C

8. 250-B17D

9. 250-B17Fg

10. 250-B17F/1

11. 250-B17F/2

12. 250-C10D

13. 250-C18 :

14. 250-C18A :

15. 250-C20

16. 250-C20B

17. 250-C20F

18. 250-C20J :

19. 250-C20R

20. 250-C20R/1

21. 250-C20R/2

22. 250-C20R/4

23. 250-C20S

24. 250-C20W

25. 250-C22B

26. 250-C28

27. 250-C28B

28. 250-C28C

29. 250-C30: Purpose-built for use with the Sikorsky S-76. It featured a single-stage centrifugal compressor instead of the multi-stage axial/centrifugal compressor of earlier models

30. 250-C30G

31. 250-C30G/2

32. 250-C30M

33. 250-C30P

34. 250-C30R

35. 250-C30R/3

36. 250-C30R/3M

37. 250-C30S

38. 250-C30U

39. 250-C34

40. 250-C40B

41. 250-C47B

42. 250-C47E

43. 250-C47M

44. 250-E3: Experimental engine containing a regenerative heat exchanger. First regenerative engine to fully power a VTOL aircraft in flight. Ran on a Hughes YOH-6A Light Observation Helicopter in 1967. engine delivering .

45. 250-KS4: Used aboard marine ships as a starter system for larger gas turbines. RIMSS, Redundant Independent Mechanical Start System

46. T63-A-5

47. T63-A-5A

48. T63-A-700 :

49. T63-A-720:

50. T703-AD-700

51. Soloy Turbine-Pac:Typically 2x 250-C20S driving a single propeller via a combining gearbox, able to operate individually.

52. Mitsubishi CT63: Licence 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

• RIMSS, Redundant Independent Mechanical Start System (use 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.

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

See also

References

External links

• Video of running model