Lesson 11 Aircraft Hydraulic Systems Aircraft Hydraulic Systems

Lesson 11: Aircraft Hydraulic Systems

Aircraft Hydraulic Systems • The Average modern aircraft utilizes hydraulic systems to operate several systems. • • Landing gear Wing flaps Speed and wheel brakes Flight controls

Aircraft Hydraulic Systems • Pascal’s Law • Pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container and acts as right angles to the enclosing walls.

Aircraft Hydraulic Systems • Hydrostatic Paradox • The pressure exerted by a column of liquid is determined by the height of the column and is independent of its volume.

Aircraft Hydraulic Systems • Neither the shape nor the volume of a container affects the pressure. Only the height of the column does this.

Changes In Velocity and Pressure (Bernoulli) • When a fluid or gas is supplied at a constant flow rate through a duct, the sum of the pressure energy and velocity energy is constant. • If pressure increases, velocity decreases proportionally or, if pressure decreases, velocity increases proportionally. • Kinetic Energy = Velocity (Ram Pressure) • Potential Energy = Pressure (Static Pressure)

Changes In Velocity and Pressure (Bernoulli)

Aircraft Hydraulic Systems • Relationship Between Pressure, Force, And Area • Pressure is a measure of the amount of force that acts on a unit of area. • Pressure is measured in pounds per square inch (psi). • Force = Pressure x Area

Aircraft Hydraulic Systems • Relationship Between Pressure, Force, And Area

Aircraft Hydraulic Systems • Relationship Between Area, Distance, And Volume • The area of the piston, the distance it moves, and the volume of the fluid displaced. • Volume = Area x Distance

Aircraft Hydraulic Systems • Relationship Between Area, Distance, And Volume

Aircraft Hydraulic Systems • Mechanical Advantage In A Hydraulic System • Two major advantages – Ease with which force can be transmitted over large distances and into sealed compartments. – Large gain in mechanical advantage made possible by varying the size of pistons.

Aircraft Hydraulic Systems • Mechanical Advantage In A Hydraulic System

Aircraft Hydraulic Systems • Mechanical Advantage In A Hydraulic System • If a large amount of movement is need but only a small amount of force we can use a large piston to drive a smaller one. • The fluid moved by the large piston will enter the cylinder with the small piston and move it a distance equal to the volume of fluid divided by the area of the small piston.

Aircraft Hydraulic Systems • All hydraulic systems are essentially the same, whatever their function. • Regardless of application, each hydraulic system has a minimum number of components, and some type of hydraulic fluid.

Hydraulic Fluid • The fluid used in aircraft hydraulic systems is one of the system’s most important parts. • The fluid must flow with a minimum of opposition. • Must be incompressible • Good lubricating properties • Inhibit corrosion and not attack seals • Must not foam in operation

Hydraulic Fluid • Some characteristics that must be considered. • • Viscosity Chemical Stability Flash Point Fire Point

Hydraulic Fluid • Viscosity is the internal resistance to flow. • Gasoline flows easily (has a low viscosity) • Tar flows slowly (has a high viscosity) • A satisfactory liquid for a hydraulic system must have enough body t give a good seal at pumps, valves and pistons; but it must not be so thick that it offers excessive resistance to flow. • The average hydraulic liquid has a low viscosity.

Hydraulic Fluid • Chemical Stability is the ability of the liquid to resist oxidation and deterioration for long periods. • Excessive temperatures have a great effect on the life of a liquid. • Liquids may break down if exposed to air, water, salt, or other impurities.

Hydraulic Fluid • Flash Point is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily when a flame is applied. • High flash point is desirable for hydraulic fluids.

Hydraulic Fluid • Fire Point is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. • High fire point is required of desirable hydraulic fluids.

Types Of Hydraulic Fluid • Vegetable-base • Mineral-base • Synthetic Fluid

Types Of Hydraulic Fluid • Mineral-base Fluid • MIL-H-5606 is the most widely used hydraulic fluid in general aviation aircraft. • Kerosene-type petroleum product. • Dyed red for identification

Types Of Hydraulic Fluid • Synthetic Fluid • Non-petroleum base hydraulic fluid for use in high performance piston engine and turbine powered aircraft. • Most commonly used fluid of this type is Skydrol • Colored light purple.

Basic Hydraulic Systems • Open Hydraulic System

Basic Hydraulic Systems • Closed Hydraulic Systems

Basic Hydraulic Systems • More modern brake system

Basic Hydraulic Systems

Power-pack Hydraulic System

Basic Hydraulic Systems • System Components • • Reservoirs Pumps Selector Valves Check Valves Hydraulic Fuses Accumulators Actuators

System Components • Reservoirs • In an in-line reservoir, space is provided in the reservoir for fluid expansion and the escape of entrapped air. • Jet aircraft that operate at altitudes where there is not enough air pressure to assure a positive feed of fluid to the pump have hydraulic reservoirs pressurized.

System Components • Hydraulic reservoir pressurized by hydraulic system pressure.

System Components • Pumps • Powered Pumps – Constant displacement moves a specific volume of fluid each time its shaft turns. – Variable displacement does not move a constant amount of fluid each revolution, but only the amount the system will accept.

System Components • Constant Displacement Pumps • Gear Pump • Gerotor Pump • Vane Pump

System Components • Gear Pump • Medium volume of fluid under pressure.

System Components • Gerotor Pump

System Components • Vane Pump • Moves a large volume of fluid under low pressure

System Components • Valves • • Selector valve Sequence valve Priority valve Pressure Control valves

Plug-Type Selector Valve • Low pressure systems

Popper-Type Selector

Sequence Valves

Priority Valves

Hydraulic Fuses

System Components • Accumulators • All accumulators consist of a high strength container divided by some form of movable partition into toe sections, or compartments. • One compartments connected to the hydraulic pressure manifold, and the other is filled with compressed air or with nitrogen.

System Components • Accumulators

System Components • Actuators • Single-Acting Linear • Double-Acting Unbalanced Linear • Double-Acting Balanced Linear

System Components • Single-Acting Linear (Brakes)

System Components • Double-Acting Unbalanced Linear (Landing Gear)

System Components • Double-Acting Balanced Linear (Flight Controls)