Fluids, Pipelines & Seals - Hydraulics - Airframes & Aircraft Systems #5

Summary

This lesson delves into the critical properties of aircraft hydraulic fluids, emphasizing the need for incompressibility, lubrication, appropriate viscosity, and resistance to extreme temperatures. It highlights the importance of proper pipeline design and effective seals to prevent fluid leakage. The script contrasts two common fluids: Skydrol, with a high flash point and less prone to cavitation, and Dex-Oron 91-48, suitable for high-pressure, low-temperature operations. It warns of the toxicity of these fluids and stresses the necessity of using specified fluids to avoid system damage.

Takeaways

• 💧 Hydraulic fluids should be incompressible up to 276 bar (4000 psi) for instantaneous operation.
• ⚙️ Good lubricating properties are essential for both metal and rubber components.
• 💧 The viscosity of hydraulic fluid should be low for minimal friction but high enough to prevent leaks.
• 🔥 A high boiling point is needed to prevent cavitation, and a low freezing point is important for outdoor reservoirs.
• 🔥 Hydraulic fluids should be non-flammable with a flashpoint above 100°C for safety.
• 🔬 Chemically inert and stable to ensure long-term performance without degradation.
• 💧 Resistance to evaporation and free from sludging and foaming is crucial for consistent performance.
• 💰 Hydraulic fluids should be reasonably priced and readily available for practical use.
• ⚠️ Two common types of hydraulic fluids are Skydrol and Dexron, each with specific properties and applications.
• 🛠️ Proper maintenance, including using the correct fluid type and avoiding skin contact, is vital for system efficiency and safety.

Q & A

Why is it important for hydraulic fluid to be incompressible?

Hydraulic fluid must be relatively incompressible to ensure instantaneous operation of the system, especially up to 276 bar or 4000 psi. This property ensures that the system responds quickly and efficiently to pressure changes.

What is the significance of a hydraulic fluid's viscosity?

Viscosity is a fluid's resistance to flow. Hydraulic fluid needs to have a viscosity low enough to minimize friction but high enough to prevent leaks, ensuring smooth operation and sealing efficiency.

Why should hydraulic fluid have a high boiling point and a low freezing point?

A high boiling point helps prevent cavitation, while a low freezing point ensures the fluid remains functional even in cold environments, such as when the reservoir is outside the heated fuselage.

What are the safety considerations when handling hydraulic fluids?

Hydraulic fluids are toxic and can damage skin and paintwork. They can also be irritants to the eyes. It's crucial to protect eyes and avoid skin contact, washing any contaminated skin immediately with soap and water.

What are the two common types of hydraulic fluids mentioned in the script, and what are their characteristics?

The two common types are DEF STAN 91-48, a super-clean mineral hydraulic fluid, and Skydrol, a phosphate ester base synthetic fluid. DEF STAN 91-48 is red and used with synthetic neoprene rubber seals, while Skydrol is purple and used with synthetic butyl rubber seals.

Why is it crucial to use the specified hydraulic fluid in a system?

Using the incorrect fluid can lead to seal breakdown, internal and external fluid leakage, pump overheating, filter blockage, and corrosion of parts, compromising the system's integrity and performance.

How does the color of hydraulic fluid assist in its identification and leak detection?

The color of hydraulic fluid aids in identification and helps in spotting leaks by visual inspection. However, it should not be the sole method of identification as the color may change over time or with use.

What factors influence the efficiency of a hydraulic system?

The efficiency of a hydraulic system is influenced by the resistance to motion encountered by the fluid, including static resistance from friction in cylinders and dynamic resistance from fluid rubbing against pipe walls.

What is the role of seals in a hydraulic system?

Seals are crucial in preventing fluid leakage. Static seals are used between non-moving surfaces, while dynamic seals are fitted between sliding surfaces and require lubrication to remain effective.

How can grit damage seals in a hydraulic system?

Grit can damage seals by causing abrasive wear, which degrades the sealing quality and may lead to component failure. Wiper rings are often installed on actuators to prevent grit from contaminating the seals.