Hydraulic Systems Flashcards
Hydrostatic Pressure
Pressure generated purely and simply by the height of the fluid.
The pressure exerted is dependant only on the height of fluid.
Pascal’s Law
Pressure will increase equally throughout, at 90° to the sides of the container, irrespective of area, diameter or shape.
What is the effect of the hydraulic force by increasing the surface area?
A larger force will be felt
Relationship between force, pressure and area
Force is equal to pressure x area
FAP Triangle
Passive hydraulic system
No pump
Pressure is only applied when this force applied to a piston
What is the role of a hydraulic pump?
The pump’s purpose is to deliver a flow of fluid to obtain a correct pressure
What are symptoms of air in a passive hydraulic system?
Spongy
If air in the passive hydraulic system is detected what is the correct action to take?
Air must be bled normally
What is the normal hydraulic operating region?
3000psi
Advantage of a high-pressure hydraulic system?
Saves weight, eliminating the need for actuators
Chevron seals in a hydraulic system
Effective in one direction, ideal for high-pressure usage.
The viscosity of a hydraulic fluid should be:
Low to minimise power consumption and resistance to flow.
Filtration in a modern hydraulic system is usually ensured by filters where?
Filters in both the pressure and return lines.
A separate pressure regulator is used in a hydraulic system:
In conjunction with a constant delivery type pump.
What type of piston pumps are used in a hydraulic system?
Axial piston pumps - powerful, but can be off loaded to reduce power consumption.
Formula for Hydraulic Pressure
F = A x P
A = F ÷ P therefore.
Synthetic hydraulic fluid
Skydrol
Phosphate ester-based synthetic oil, irritable to skin and eyes.
Fire resistant less prone to cavitation and has a high boiling point.
What colour identifies synthetic hydraulic fluid
Green or purple
What colour identifies mineral oil
Red
What is the purpose of backing rings in a basic hydraulic system
Prevents extrustion
If the incorrect fluid was used in a hydraulic system;
Damage to the seals could result
Basic System layout of the hydraulic system
Reservoir, pump, actuator/jack and selector valve
Actuator/Jack
Convert pressure energy into linear motion.
Provides the output.
Consist of piston, seals and ram.
Present on undercarriages and flap system.
Selector valve
Allows the supply and return either side of service
For example, extension and retraction of undercarriage.
Extension- small force.
Retraction- lots of force
Single Acting Actuator
Is powered in one direction only by hydraulic power.
The return movement being under another force, for example a spring.
What is the purpose of the arrow on non-return valve?
Prevents the direction of flow
Function of the non-return valve
As soon as pressure drops the valve shuts.
Prevents return.
Selectors on the hydraulic system
Purpose is to direct hydraulic flow, dependent on the pilot selection.
Hydraulic Lock
When fluid is trapped between the piston of a jack, and a non-return valve. Hydraulic lock is formed.
Fluid cannot flow through the system, and the piston is locked.
Open-centre hydraulic system
Found on light aircraft.
- Flaps
- Undercarriage
Can only operate one service at a time
Types of hydraulic circuits
Passive:
No hydraulic pump
Brakes on the light aircraft
Brahma press
Active:
Mechanically driven pump
Hydraulic Power Pack
Self-contained independent system
Main disadvantage on light aircraft: can operate one service at a time.
Where is the temperature sensor located in a hydraulic system?
In the reservoir
Hydraulic accumulators
Gives impetus by energy stored in nitrogen gas.
Store of hydraulic fluid under pressure
Dumps out pressure fluctuations
What is the purpose of the separator in the accumulator
Separate liquids from gas
Constant Delivery/ Constant Volume hydraulic pump
Produces a constant delivery
Needs to be equipped with an ACOV
Automatic cut out valve (ACOV)
Senses power downstream to keep within a reasonable range.
System leaks will cause cut in/ cut out
Effect of low hydraulic pressure on accumulators
Fluctuation occurs on an increased level
Hammering of valves
What’s the purpose of a Case Drain Filter?
Monitor conditions of the parts
Thermal relief valve
PRESSURE SENSING, extreme temperatures
Restrictors in a hydraulic system
Located in up line
Restricts flow in the flap system during up flap selection.
Restricts flow in landing your system and creates a restriction gear down selection.
Implications of a single pump failure on a hydraulic system
Pressure is constant
Delivery and flow is halved.
“Operating times extended”
Shuttle valves
Enables an alternate supply to operate an actuator
Switch hydraulic unit to most appropriate supply
Pressure maintaining valves
Priority valve ensure pressure is provided to basic systems in case of failure
Pressure reducing valves
Reduces from 3000 to 1800 for brakes.
Hydraulic fuse
Prevent total system loss in an event of a hydraulic leak
Power transfer unit (PTU)
Transfers power not fluid
Consists of: pump, shaft, motor
Mechanical Overload Protection (RELIEF)
Fitted in flat circuits and spoiler circuits
Prevent damage during high-speed
Flaps move to intermediate position
As we slowdown they will deploy normally
With what type of air is the pressurisation managed in the reservoir?
Bleed Air
Purpose of the filter in the hydraulic system
Prevent ingress of debris and dirt
Should a hydraulic pump seize during operation:
The quill drive will shear to offload and protect the gearbox.
A stack pipe is fitted in a hydraulic reservoir to provide:
Emergency supply
Check Valve
Allows hydraulic fluid to flow in one direction only.
Operates similarly to a diode.
