Grnd-54 Theory of Flight 7 Hydraulic Systems

Hydraulic Theory

Hydraulic systems use an incompressible fluid to transmit force from one location to another within the fluid. A large mechanical advantage can be obtained.

Pascal’s Law

Pressure applied to a confined liquid at any point is transmitted without loss in every direction in that it acts at right angles to each portion of the entire area of that container.

Mechanical Advantage

The ratio between the effort (input force) required to move a load (output force). In other words, it is the multiplying of force.

Makeup of Basic Hydraulic System

Hydraulic Fluid

Pressure Generator (pump)

Plumbing (reservoir, lines, and valves)

Hydraulic motor or actuator

Desirable qualities of hydraulic fluid

High flash point

Adequate viscosity

Lubricant properties

Thermal capacity/conductivity

Hydraulic fluid warning

May cause skin irritation, avoid prolonged or repeated contact with skin and ingestion and contact with eyes.

Types of hydraulic pumps

Manual

Engine driven

Electric

Pneumatic

Hydraulic

Ram Air Turbine

Hydraulic plumbing components

Reservoir

Filters

Valves (shut-off, control, pressure relief)

Accumulators

Hydraulic Motors

Converts hydraulic pressure and flow into torque or rotation. Most often used to drive jackscrews which power flaps, stabilizer trim, and some vertically extending landing gear applications.

Hydraulic Actuator

Provide a reversible force in a single direction. Landing gear extension and retraction, cargo door operation and movement of flight control surfaces.

Aircraft Hydraulic Systems operating pressure

May vary from 200 - 5000 psi, typically 3000 - 5000 psi.

Advantages of hydraulic sources for operating aircraft systems

Light weight

Ease of installation

Simplification of inspection

Minimum maintenance requirements

Almost 100% efficient

Hydraulic Aircraft Components

Wheel brakes, NWS

Landing gear retraction/extension

Flaps and slats, spoilers and speed brakes

Flight control surfaces

Propeller pitch control, thrust reversers

Cargo doors, loading ramps/bomb bay doors

Hoist/winches

System redundancy

Multiple pressure sources (pumps), multiple hydraulic systems (actuators)

Hydraulic System Threats 1

Hydraulic System Threats 2