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Encyclopedia > Allison 250
MTU build Allison 250-C20B

The Rolls-Royce Model 250 is a highly successful turboshaft/turboprop engine family, originally started by a General Motors offshoot, the Allison Engine Company, in the early 60's. MTU Aero Engines, Germanys leading aircraft engine manufacturer, develops, manufactures and provides service support for aircraft engines, military and civil alike. ... This article is about the company. ... The Allison Engine Company was an aircraft engine manufacturer that was acquired by Rolls-Royce in 1995. ...

Allison adopted a rather unusual "trombone" style of 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 2-stage HP turbine, which is connected, via the HP shaft, to the compression system, before expanding through through the 2-stage power turbine; the exhaust gases then turn though 90 degrees to exit the engine in a radial direction; a stub shaft connects the power turbine to a compact reduction gearbox, located inboard, between the centrifugal compressor and the exhaust/power turbine system. The engine configuration is shown clearly in http://helicopterflight.net/engine1.jpg.

One of the latest versions of the Model 250 is the -C40, which has a hi-tech centrifugal compressor pulling a pressure ratio of 9.2:1, at an airflow 6.1lb/s, and developing 715shp.

Some of the earlier versions have axial compressor stages mounted on the HP shaft to supercharge a relatively low pressure ratio centrifugal compressor. The -C20R is typical, pulling an overall pressure ratio of 8.0:1, at an airflow of 4.0lb/s, with a power output of 450shp.

As the following site shows (http://www.atlasaviation.com/powerplant/aircraftapp.htm), the Model 250 propels a large number of helicopters and small aircraft. As a result, nearly 30,000 Model 250 engines have been produced. Of these, approximately 16,000 remain in service. The Model 250 is one of the highest-selling engines made by Rolls-Royce.

Oldler retired units have also had a second life as theatrical props as engines for Colonial Vipers that are being worked on in the background of the port Hangar bay on the Sci FI channel remake of Battlestar Galactica.

  Results from FactBites:
Allison 250-C18 T63-700 Gas Turbine Engine (500 words)
The Allison 250 is fitted to numerous aircraft including the ubiquitous Bell/Textron Jet Ranger- one of the worlds most successful Helicopters.
The 250 is not without it's flaws, the engine is very complicated in construction and is also relatively fragile.
The 250 is used as a Helicopter engine and a Turboprop engine, the differences are mainly in the gearbox output arrangements, the control system and the engine orientation.
Common maintenance events on the Allison 250 C20 @ AMTOnline.com (7455 words)
Evidence of oil spraying out of the bleed valve of an Allison 250 C20 engine should lead you to suspect an oil leak from the carbon seal for the No. 1 bearing at the front of the compressor.
To split a Allison 250 C20 gearbox, the gearbox cover or aft portion is lifted off of the housing or front half.
Allison recommends this clearance to be within 0.0005 to 0.0015 inch.
  More results at FactBites »



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