Allison 250-C20B Engine

Question 1

How many stages to the compressor section?

Answer 1

7 total
6 axial
1 centrifugal

Question 2

Describe the engines gas path

Answer 2

• Inlet
• 1st stage compressor
• Out centrifugal stage
• Into 2 external air transfer tubes
• Combustion chamber
• 2 stage compressor turbine (N1)
• 2 stage power turbine (N2)
• Spent gases directed out exhaust ducts & outlets

Question 3

What are the 4 major engine sections?

Answer 3

1. Compressor section
2. Combustion section
3. Turbine section
4. Power & Accessory gearbox section

Question 4

Total weight of engine?

Answer 4

158 lbs
72 kgs

Question 5

Brief overview of the compressor section.

Answer 5

Consists of inlet housing, combination axial (6) centrifugal (1) compressor & diffusers at the rear

Question 6

Brief overview of the combustion section.

Answer 6

Consists of 2 external air transfer tubes which carry air from diffuser to a single basket type reverse flow annual combustor.
Contains one duplex fuel nozzle and a single ignitor plug.

Question 7

Brief overview of the gas producer turbine.

Answer 7

Sometimes referred to as compressor turbine or N1, is a two stage turbine with its purpose being to drive the compressor and accessory gearbox directly from the combustor discharge.

Question 8

Brief overview of the power turbine.

Answer 8

The power turbine or N2, is a two stage turbine with the function being to drive the power output gear through a pinion gear coupling. The power output gear drives the helicopter transmission.

Question 9

What sort of turbine is the power turbine?

Answer 9

A Free Turbine, allowing it to rotate at different speeds to the Gas Producer Turbine.

Question 10

What are the 3 advantages of the free power turbine system?

Answer 10

1. Added flexibility that allow different speeds between N1 and N2 rotors/turbine.
2. Improved overall performance as each turbine can be operating at the point of maximum efficiency.
3. Ease of starting as the starter only rotates the N1 rotor during start cycle.

Question 11

What are the 2 separate gear trains located in the power and accessory gearbox?

Answer 11

Power turbine gear train
&
Gas producer gear train

Question 12

What is incorporated in the power turbine gear train?

Answer 12

• Torquemeter assembly
• Power turbine tachometer generator
• Power turbine governor

Question 13

What is incorporated in the gas producer gear train?

Answer 13

• Oil pump
• Fuel pump
• Gas producer fuel control
• Gas producer tachometer generator
• Starter generator

Question 14

How are the main bearings lubricated?

Answer 14

Through a circulating dry sump type, with an external oil tank and oil cooler.
Oil jet lubrication is provided to all major bearings while the rest is lubricated via oil mist.