Hydraulic Systems Flashcards
Why are there two separate hydraulic systems
Redundancy. Can still provide hydraulic power to the most essential components
How are pumps driven
Engine driven pump are driven by a gearbox.
Gas turbines are driven by accessory gearbox.
What are actuators and what are the 3 types
Convert hydraulic pressure into movement.
Single-acting, Double-acting balanced, Double-acting unbalanced
What are single-acting actuators used for?
Move only in one direction. Spring applies return force. Normally used as a locking device eg landing gear.
What are Double-acting Balanced actuators?
Hydraulic pressure moves pistons in both directions. Pressure gives equal force. Used for nose steering wheel and flight control systems.
What are double acting unbalanced actuators?
Equal hydraulic pressure generates different forces on each side of piston area due to different surface area on piston face. Actuator moves left. Allows for different forces to be produced for example on extending/retracting. landing gear
What are non-return valves.
Allows fluid to flow only in one direction. Fluid pressure unseats ball under spring pressure to allow flow. When system pressure falls below spring pressure ball reseats preventing reverse flow.
What are selector valves
Direct hydraulic fluid to various parts of system. Can be operated mechanically or electrically. Can be used to hydraulically lock position of actuator.
What are the types of hydraulic circuit
Passive hydraulic system-No pump, power provided by Pilot
Active hydraulic system- Uses one or more pumps
What is an open-centre hydraulic system
2000psi. Used on small-midsized aircraft. Operates landing gear or flaps but not at same time. When one is used other is locked.
What is an on-demand system
Pressure only generated when a service is required. System pressure remains low until service selection made. Off load condition off-loads pump reducing engine power required to drive it and also reducing wear on pump
What is a closed centre Hydraulic system
Maintains a constant supply of pressure using a variable pump. Services can be operated simultaneously.. Off-loading valve allows pump to unload when not in use.
What are light aircraft systems
Often only one active hydraulic circuit used to extend/retract Landing gear. Uses self-contained hydraulic power pack operated by reversible DC electric motor.
What are the main points of a light aircrafts hydraulic system
Landing gear held in retracted position by hydraulic lock.
Pump comes online when required to restore system pressure.
Thermal relief Valve protects against over pressure.
When landing gear lowered mechanically locked and electric supply to pump switched off.
What is the hydraulic reservoir in large aircraft system
Stores H.fluid. Allows thermal expansion, holds excess fluid to compensate for leaks, accepts fluid returning from low pressure side.
Lower exit inside to ensure emergency supply if major fluid loss
What do fins, baffles and de-aerator tray do
Reduce fluid swirl due to aircraft manoeuvring.
Removes bubbles from returning fluid.
Why are hydraulic reservoirs pressurised
Prevents fluid from boiling at altitude. Pressurised by bleed air from engine. Relief valve protects from over pressurisation.
What are accumulators
Comprises piston moving freely inside sealed cylinder. On one side pre-charged inert gas (nitrogen) other hydraulic fluid.
What is the purpose of an accumulator?
Store fluid under pressure.
Provide limited supply of pressurised fluid in an emergency.
Dampen pressure fluctuations in system.
Allow for thermal expansion and small internal leaks.
What are the two types of engine driven pumps
Engine driven constant delivery (fixed volume) Constant pressure (variable volume)
How do constant delivery pumps work
Constant fluid supply. Flow rate dependent on speed of rotation. Spurs rotate at constant speed the pump emits a constant volume of fluid.
This pump requires a automatic cut out valve.
What is an ACOV
When system pressure increases above pre-set level poppet valve opens creating an idling circuit which dumps excess pressure. Valve open= ACOV “kicked out”
System leaks will cause ACOV to operate for greater periods.
What is a constant pressure (variable volume) pumps
Commonly used on large modern aircraft. Operate 3000psi+ Demand pump which delivers hydraulic pressure on demand.
How does constant pressure pumps work
Driven by gearbox connected to engine. Series of pistons in sleeves with each piston end in a sliding shoe which presses against swash plate. If swash plate at slight angle stroke of piston is minimal resulting in an idling state in which pump produces no output.
How does a hydraulic motor work
Uses hydraulic pressure to generate rotary motion. Useful for operating screw jacks that operate flaps and slat mechanisms.
