Hydraulic System Case Study

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AIRCRAFT HYDRAULIC SYSTEM

1.1. Explain the reasons for using hydraulic power as a major source for nose wheel and landing gear

DEFINITION:

A hydraulic system uses a liquid under pressure to drive machinery or move mechanical components. Hydraulic system takes engine power and converts it to hydraulic power by using a hydraulic pump. This power can be supplied throughout the aircraft with the help of tubing that runs throughout the airplane. By using a turbine or an actuating cylinder, hydraulic power may be reconverted to mechanical power.

The hydraulic power is supplied by:
- Primary hydraulic systems during flight operation
- Auxiliary hydraulic supply
 On ground if primary hydraulic system is not available or for maintenance operation.
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However, these fluids are not fireproof and under certain states, they ignite. o The airframe manufacturers dubbed the new generation of hydraulic liquid as types depending on their performance. o Nowadays, types IV and V liquids are used. Two distinct classes of type IV liquids exist depending on their density:
I. Class I liquids are low density and provide weight savings advantages versus class II.
II. Class II liquids are standard density. o Type V liquids are being developed in regards to industry needs for a more thermally firm liquid at higher operating temperatures. o Compared to type IV liquids, type V liquids will be more resistant to hydrolytic and oxidation degradation at high temperature. (Reference:http://www.itrusky.ru/products/consumables_chemicals/aerospace_hydraulic_fluids/DOSEN-SKYDROL1.bmp)

PROPERTIES OF HYDRAULIC LIQUID:

 Viscosity
 Chemical stability
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Viscosity is nothing but an internal resistance to flow. o A fluid like gasoline that has a low viscosity flows easily, while a fluid like tar that has a high viscosity flows slowly. o As temperature decreases viscosity increases. o For a given hydraulic system, a tolerable fluid must have enough body to provide a good seal at valves, pistons and pumps, but it should not be too thick that it renders resistant to flow, leading to loss of power and higher operating temperatures. These factors results in additional wear of parts and heavy load. o The liquid that is so thin also give rise to rapid wear of moving parts or of parts that have heavy loads. o Viscometers or viscosimeters are the instruments used to measure the viscosity of a fluid. o The saybolt viscometer calculates the time needed, in seconds, for 60ml of the tested fluid at 100 °F to pass through a standard orifice. The calculated time is used to express the viscosity of the liquid, in Saybolt universal seconds or Saybolt furol seconds.

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