Rolls-Royce Model 250
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Image:Allison (MTU) 250 C20B.jpg 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.
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.
