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14.5.3. Hydraulic starters

Hydraulic starting systems fall into two categories:

- energy limited

- power limited

The energy-limited system uses a highly pressurized accumulator and a large, positive-displacement motor. Examples of other starting systems that are also ener­gy-limited are the electric motor, when supplied from a bat­tery, and the cartridge starter. The energy-limited system is designed to complete the start in as short a time as possible in order to minimize the amount of stored energy required. The accumulator system is best suited to small engines up to 100 kW.

A power-limited system uses an auxiliary power unit (sometimes a small gas turbine engine, which is itself started by an energy-limited system) to drive a pump that supplies the correct amount of flow and pres­sure to a variable-displacement hydraulic starter motor. The variable-displacement motor permits high torque to be applied without exceeding the power limits of the main engine at starter cutoff speed.

It is possible to adapt the hydraulic starter as a pump, but since the starter cutout speed is less than 50 percent of the normal engine operating speed, a two-ratio gearbox is nec­essary to provide proper speed for both pumping and start­ing. The hydraulic pump on the auxiliary power unit can also be used to supply power to the aircraft.

Hydraulic starter basic element is hydromotor (usually of plunger or gear type), which transmits a torque to the started engine accessory gear. After an engine start-up the hydraulic starter is used as the hydropump, which provides the airplane aggregates hydraulic-circuit system work.

The hydraulic energy can be brought to such starter from a hydraulic accumulator, hydropump of an auxiliary power unit, hydropump of the engine, which was activated earlier or from an aerodrome source.

Hand-crank starter

The hand-crank method of starting gas turbine engines is, of course, limited to very small units, on the order of 35 to 75 kW. As the name implies, starting is accomplished by turning a hand crank, which through a series of gears, turns the engine to the self-sustain­ing rotational speed. Hand-crank to engine-shaft speed ratios are on the order of 100:1.

Questions for self-check

1. Name the requirements to starting systems of a gas turbine engine.

2. Explain the block diagram of a gas turbine engine starting system.

3. Explain the gas turbine engine starting diagram.

4. How can indispensable maximum power of starting devices for the different types gas turbine engines be determined?

5. How can dependence of starting device torque versus gas turbine engine rotational speed be calculated and constructed?

6. How can dependence of rotor antitorque moment versus its rotational speed be calculated and constructed?

7. How can dependence of turbine rotor torque versus its rotational speed after main fuel ignition be constructed?

8. Give brief characteristics of electrical starting devices of different types.

9. Give brief characteristics of turbine starting devices of different types.

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