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CHAPTER 2

ALLISON 250C–20J TURBOSHAFT ENGINE

2.1.

Identify the type of gas turbine engine use in the TH – 57.

Allison 250C-20J series turboshaft engine.  This engine is an internal combustion gas turbine engine featuring a “free” power turbine consisting of a combination axial-centrifugal compressor; a single “can” type combustor; a turbine assembly which incorporates a two-stage power turbine and exhaust collector; and an accessory gear box which incorporates a gas producer gear train and a power turbine gear grain.

2.1.1.

Stated the power output of the 250C–20J engine.

Allison’s 250C-20J engine is a 420 SHP engine derated by Bell Helicopter to 317 SHP due to power train limitations.

2.2.

Identify the four sub-assemblies of the engine.

1.

Compressor Section

2.

Accessory Gearbox Section

3.

Turbine section

4.

Combustion Section

2.2.1.

Identify the four sub-assemblies of the compressor.

1.

Front Support

2.

Compressor rotor wheels and blades

3.

Case Assemblies

4.

Diffuser Scroll

2.2.1.1.

State the three functions served by the front support of the compressor and identify how each function is achieved.

1.

Hollow struts serve a part of the anti-ice system- 500oF air from the diffuser scroll is channeled into the struts via the anti-valve.  This heated air prevents the inlet vanes from cooling to the point of freezing.

2.

Struts serve as inlet guide vanes, ensuring ambient air strikes the first stage compressor at the proper angle.

3.

Support the forward end of the compressor shaft via bearings and struts.

2.2.2.

Described the compressor  used in the 250C–20J engine by identifying the components, the types of compression utilized, the number of stages of compression used, and where the components are mounted.

Compressor: A 6 stage axial compressor (rotor followed by stator) followed by a  single stage centrifugal compressor provides a compression ratio of 6.5 to 1 and a temperature of 500oF.  (The compressor bleed air valve is located at the 5th stage of axial compression)

Diffuser Scroll: Mounted on output of centrifugal compressor; used to direct air from compressor to the Air Transfer Tubes.

Houses 4 bleed air taps:

1.

Anti-Ice

2.

Cabin Heating

4

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