Chapter 5 - Hydraulic Systems (18-07-18) Flashcards
What are the features of a single acting actuator?
Oil acts on one side only
a spring returns the actuator to its orginal position.
the spring makes the actuator fail safe.
What are the features of a double acting balanced actuator?
The are actuator rods on both sides of the piston
Oil acts on both sides of the piston and so moves it either way when desired.
What are the features of the double acting unbalanced actuator?
The piston only as a rod on 1 side of the piston
This means the oil provides a larger force in one direction than the other
(this is good for things like the landing gear)
the oil can act on either side of the piston when desired
How does a non return valve work?
A spring pushes a ball into a socket which blocks flow
When ‘forward’ flow occurs the spring is compressed and the ball is displaced from the socket, allowing fluid to flow.
What are the 3 types of selector valves?
Closed centre
open centre
linear (spool) valve
What are the features of a closed centre selector valve?
Pressure is fed to a closed section and the centre is rotated to the desired system link.
The pressure from the closed section is then allowed to flow to the desired system until the centre is rotated away from the link and it closes again.
What are the features of an open centre selector valve?
The fluid is able to flow through the valve when not in use.
The centre then rotates and a new junction aligns with the flow and the desired system
it then rotates back to local the system.
What are the features of a liner (spool) selector valve?
The piston is positioned in a way that blocks both the input pressure and actuator lines in the neutral position.
The piston can be moved to allow the flow to be directed through 1 of 2 directions to the actuator, allowing for the actuator to be used in either direction.
The piston can then be moved back to local the actuator.
When selector valves are closed what are the actuators said to be?
Hydrostatically locked
How does a servo control valve work?
One out of 2 solenoids is electronically displaced which allows oil to take the easiest route to the return link.
This causes the pressure to be higher on one side of the spool and this displaces it
This allows more fluid to flow to one side of the actuator than the other
What is an issue with an open centre (in series) hydraulic system?
When multiple systems are being used the overall output is lessened
What does a closed centre hydraulic system require?
An off-loading valve to allow the pump to unload due to maintaining a constant supply of hydraulic pressure.
When the hydraulic system of the landing gear fails, what can be done?
The dump valve breaks the hydraulic lock and allows the landing gear to lower under its own weight
How much oil needs to be stored in the hydraulic reservoir?
Enough to have all systems at full extension with extra for potential leaks.
How and why is the hydraulic reservoir usually pressurised?
With bleed air from engines and to raise boiling point.
How can you check the amount of oil in the hydraulic reservoir?
With the sight glass or pressure gauge on the unit.
What is the purpose of baffles and fins in the hydraulic resevoir?
The baffles limit oil movement through g changes
The fins limit side to side and swirling movement of the oil
What is the purpose of the stack pipe in the hydraulic resevoir?
If there is a leak in the main system the oil will drain down to the top of the stack pipe and then another outlet will keep emergency systems going
What is the purpose of the de-aerator plate in the hydraulic resevoir?
Removes air bubbles from the fluid.
How do accumulators work?
As the system is pressurised the accumulators charge with pressure and can be used as an emergency source of hydraulic pressure.
What are the purposes of accumulators?
An emergency source of hydraulic pressure
to remove pump pulses
What is the pressure of the nitrogen in an accumulator when the oil pressure is 0 and when fully charged?
1000psi and 3000psi
How can hydraulic pumps be driven?
By the engine accessory gearbox
electrically
by hand
by a Ram Air Turbine (RAT)
by a hydraulic motor or Power Transfer Unit (PTU)
What kind of hydraulic pumps are used in light aircraft?
Spur and rotary
What kind of hydraulic pumps are used in large A/C?
Piston pumps
What kind of hydraulic pumps are used as emergency back ups?
Electrical
What kind of hydraulic pumps are used for maintainance purposes?
Hand pumps
What are the features of a constant delivery pump?
Usually spur gear system
pump emits a constant volume
normally requires an automatic cut out valve (ACOV)
