Lesson 11: Aircraft
Hydraulic Systems
Aircraft Hydraulic Systems
• The Average modern aircraft utilizes hydraulic
systems to operate several systems.
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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.
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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
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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
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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)