11.11 Hydraulic Power Flashcards
How efficient are hydraulic systems?
Nearly 100%
What two types of hydraulic pressure are there?
Hydrostatic and Hydrodynamic
What is hydrostatic pressure?
Pressure produced by static or nearly static fluids.
What is hydrodynamic pressure?
Pressure produced by fluids with no internal friction.
Which type of pressure is used in aircraft?
Hydrostatic
What is an open centre hydraulic system?
An open-centre system is one having fluid flow, but no pressure in the system when the actuating mechanisms are idle.
Unlike the closed centre system what are the selector valves of the open centre system always?
Connected in series with one another.
If a constant delivery pump is used in a closed system circuit then what must the pressure be?
Regulated
What is the advantage of an open loop system over a closed?
The need for constant pressurisation is eliminated.
Why are closed centre systems more commonly used?
Due to their instant operation.
What is a hydraulic power pack?
A small self contained unit which reduces weight and system maintenance.
If a fluid is hygroscopic what does it have the tendency to do?
Absorb water.
Which is more hygroscopic, mineral or synthetic?
Synthetic
What is used to measure viscosity?
Viscometer or a Viscosimeter
What does a Saybolt Viscometer measure?
The time required for 60mm of fluid to pass through an orifice at specified temps between 21-99 degrees c.
What are the units given from a Saybolt Viscometer?
Saybolt universal seconds or saybolt flurol seconds
What is chemical stability?
Ability to resist oxidisation and deterioration over time.
What is a localised hot spot?
The point where liquid under pressure is forced through a small orifice making the part warm up.
What are the three principal categories of hydraulic fluids?
Minerals
Polyalphaolefins
Phosphate esters
What is the oldest hydraulic fluid?
Mineral MIL-H-5606
Can MIL-H-5605 be mixed with 6083?
Yes, 6083 is just the rust inhibiting version.
What is a Phosphate Ester hydraulic fluid known as?
Skydrol
What types of skydrol are used today?
Types IV and V
What classes of Skydrol IV are there?
Class I- Low density
Class II- Standard density
Why are type V skydrol fluids being developed?
The industries requirement for more thermally stable fluids.
What are two general contaminants found in hyd fluid?
Abrasives
Non-Abrasives
What are the two main service activities for hydraulic systems?
Reservoir filling
Fluid sampling
When should hydraulic systems be sampled?
Once a year or 3000 flight hours.
What two classifications of reservoir are there?
Pressurised and Non-pressurised
Where is the return pipe located on a resevoir?
Bottom
Contrary to their name, non-pressurised reservoirs do actually have some pressure, why is this?
To ensure positive supply of fluid to the inlet ports of the pumps.
For what reasons are reservoirs pressurised?
Provide positive flow and also prevents foaming, evaporation, and cavitation of the hydraulic fluid.
What does the reservoir pressurisation module do?
Supplies aircraft bleed air to the reservoir.
When a reservoir is filled correctly the top sight glass and bottom sight glass show what?
Top-red
Bottom-black
What is the purpose of an accumulator?
To accumulate energy and smooth pulsations
What type of accumulator operation is commonly used today?
Hydro-Pneumatic piston type.
What classifications of pump are there?
Positive displacement and Non-positive
What are centrifugal and impeller pumps examples of?
Non positive displacement
What is a gerotor pump?
Similar to a rotary engine, it is a positive displacement fixed volume pump.
What type of mounting bases do piston pumps have?
Flanged mounting bases for mounting the pumps on the accessory drive cases of aircraft engines.
What protects a cylinder pump from damage if it seizes?
There is a shear shaft between the splines.
What type of pump is the EDP?
VDP
What are air driven pumps driven by?
Typically the RAT
When are ADP’s used?
When the EDP is not consistent enough.
What types of hand pump are there?
Single action
Double action
Rotary
How is emergency and back up pressurisation acheived?
RAT, PTU and ACMP.
What is an ACMP?
Alternating current motor pump
What type of pump is the RAT?
VDP
Why do systems have filters?
Because 85% of failures are due to clogs.
What is a wire mesh filter for?
Made of steel, this type of filter is usually used for surface cleaning.
What filter material gives the best filtration?
Micro glass
What micron element does a micro glass filter prevent from passing through?
Greater than 10 microns
What is the advantage of cellulose paper filters?
They can be recycled
What is the service life of a pressure filter, return filter and case drain filter?
Pressure 3000 hours
Return 1500 hours
Case Drain 600 hours
What are the most common types of pressure relief valve?
Ball
Sleeve
poppet
What are skydrol seals made from?
Butyl rubber or ethylene-propylene elastomers.
What three main classes of seal are used?
Packing
Gasket
Wipers
Hydraulic seals used internally on a sliding or moving assembly are normally called?
Packing
Hydraulic seals used between non-moving fittings and bosses are normally called?
Gaskets
What is the part number for v ring seals?
AN6225
Where are U ring seals used?
Brake assemblies
In what pressures are u ring seals used?
Less than 1000psi
What is the operating temps of o rings?
-54 to 135 degrees c
What does a red stripe indicate on an o ring?
Fuel compatible
What does a blue stripe indicate on an o ring?
Hydraulic compatible.
What are static seals used for?
Steady or pulsating pressure.
What are dynamic (buffer) seals used?
Stroking or rotational movements
What are some common gasket materials?
Asbestos
Copper
Cork
Rubber
What are asbestos gaskets good for?
Heat resistance
What does an open centre selector valve allow?
Allows a continuous flow of system hydraulic fluid through the valve, even when the selector is not able to actuate a unit.
What is the purpose of a shuttle valve?
To isolate the normal system from an alternate or emergency system.
What are hydraulic fuses used on?
Brake system
Leading edge flap and slat extend and retract lines
Nose landing gear up and down lines
Thrust reverser pressure and return lines
What does a hydraulic fuse react too?
Sudden increase in flow.
What does a hydraulic actuating cylinder do?
Transforms energy in the form of fluid pressure into a mechanical force, or action, to perform work.
What two types of actuating cylinder are there?
Single
Double
What angles can rotary actuators acheive?
Up to and beyond 720 degrees
What can pressure sensors be divided into?
Pressure switches
Pressure transducers
What is a pressure switch?
Active electromechanical device which measures the pressure in a system.
What is a pressure transducer?
Samples and records the pressure in the system.
What is a pressure transducers input and output voltage?
Input under 10v
Output can be hundreds of thousands volts
What does HMDGS stand for?
Hydraulic motor driven generator,
What does an HMDGS do?
Provides electrical power of 400Hz in the event of a primary gen failure.
Why has power by wire come into existence to replace FBW?
The disadvantage of FBW is that if a single actuator fails then the entire hydraulic system has to be stopped.
What is an Electro Hydrostatic Actuator?
A self-contained, modular, and compact actuator system, used in the power by wire technology.
