Hydraulics

Question 1

Hydraulics definition

Answer 1

Fluid dynamics used for the generation, control, and transmission of power by the use of pressurized liquids

•Relies upon the physical property of liquids, in that they are virtually incompressible

Question 2

Basic laws of fluid power

Answer 2

•Newton’s second law of Motion: Force
•Acceleration depends upon the net force acting upon the object.
•Energy is exerted and is the cause of motion.
•Pressure
•Force applied over a given area
•Pressure(PSI) = Force / Area
•Conservation of Energy
•Kinetic (hydraulic velocity)
•Potential (hydraulic pressure)
•Work
•Force applied for a given distance
•Work = Force x Distance
•Power
•Amount of work performed given a period of time
•Power = Work / Time

Question 3

Pascals law

Answer 3

Pressure in an enclosed system is transmitted equally and undiminished to ALL PARTS.
Pressure exerted by a column of liquid is dependent on height & density, but independent of its volume
•-> Pressure is expressed as a force per area (i.e. pounds per sq. inch)

Question 4

Hydraulic fluid properties

Answer 4

Different types of fluid based on:
•working conditions
•service required
•temps inside and outside the system
•pressure the liquid can withstand
•Viscosity
•Low = flows easily High = flows slowly
•Must have enough body to seal at pumps, valves and pistons but not be so thick that is resists flow
•Chemically Stable
•Must be able to resist oxidation and deterioration for long periods of time
•Liquids may break down if exposed to air, water salt, or metals
•Causing sludge and carbon build ups that clog the system

Question 5

Different types of hydraulic fluid

Answer 5

•Mineral Based Fluid
•MIL-H-5606
•Non-corrosive
•Flammable – use w/ low fire hazard
•Red in color
•Synthetic Based Fluid
•MIL-H-8446
•Skydrol
•Non-flammable
•Purple in color
•Corrosive
•Why not water?
•Corrosive, low boiling point, minimal lubricating properties, biological growth

Question 6

Reservoir

Answer 6

Stores fluid for hydraulic system operation
•Serves as overflow basin for excess fluids after expansion (temp change)
•Expansion chamber where fluid can be purged of air
▪Air is compressible!
•Strainers are used to keep buildup from clogging the system
•Baffles/Fins are installed inside to prevent sloshing leading to bubbles or foam
Non-Pressurized vs. Pressurized
•Non-Pressurized used in aircraft that are not designed for violent maneuvers or fly at low altitudes
•Pressurized used at high altitudes to compensate for loss of atmospheric pressure
▪Usually located in wheel wells or non-pressurized portions of an aircraft
•Used to prevent foaming
•Heat exchangers cool hydraulic fluid

Question 7

Pumps

Answer 7

All systems have one or more power driven pump that forces fluid through the system
•Can be:
•Engine driven
•Electric driven
•Air driven
•Hand driven

Question 8

Hand pump

Answer 8

•Hand Pump
•Single-action
▪Pump moves fluid only in one stroke of the piston
▪Like a car jack
•Double-action
▪Pump moves fluid in both strokes of the piston
▪More efficient

Question 9

Constant displacement

Answer 9

•Moves a specific amount of hydraulic fluid in a given amount of time.
•Deliver a fixed amount of fluid per revolution of the gear
•Fluid per minute depends on the revolutions per minute of the gear
•Pressure Regulator Valve must be used because the pump does not change

Question 10

Variable displacement

Answer 10

Fluid output is varied to meet the pressure demands of the system
•Pump output is changed automatically by a pump compensator

Question 11

Flow control valves

Answer 11

•Check Valves allows fluid to flow in one direction, but prevents it from flowing in the opposite direction

•As fluid flows it pushes the spring back which allows fluid to flow

•When fluid stops, the spring compresses back and seals up the fluid line

Question 12

Relief valve

Answer 12

Limits the amount of pressure exerted on the system
•w/o it we could have failed components or ruptured hydraulic lines
•Pressure is adjusted by tightening the spring
•Pressure regulator malfunctions
•Thermal expansion

Question 13

Hydraulic actuator

Answer 13

Convert potential energy (pressure) into kinetic energy (movement)
•Consists of:
▪Cylinder Housing
▪Piston Rod
▪One or more Pistons
▪Seals

•Two Types:
▪Single Action
▪Double Action

Question 14

Single action actuator

Answer 14

Fluid enters from one side of the cylinder
•Forces the piston in one direction
•As pressure becomes less than the spring, the piston is forced in the opposite direction and fluid is forced out

Question 15

Double action actuator

Answer 15

When selector valve is “on/extend” fluid is forced in one direction
•Piston moves and does “work”
•When selector valve is “off/retract” fluid is forced in the opposite direction

Question 16

Landing gear piper seminole

Answer 16

System Description

•Retractable tricycle landing gear
•Hydraulically actuated by an electrically powered reversible pump.
•Pump is controlled by a selector switch on the instrument panel.

•System Limitations
•The landing gear should not be retracted above a speed of 109kias, nor extended above a speed of 140kias

Question 17

Parts of gear system

Answer 17

Squat Switch – when on ground switch is not pressed (opens circuit). Prevents gear up when on the ground.

•Down Lock Switch – turns on 3 green lights when gear is fully down. Shuts off pump when gear are extended.

•Up-Limit Switch – turns off the “red” gear unsafe light when all gear are up

•Gear Indication Lights – green lights that illuminate when gear are down. Can become dim when Nav lights are also on.

•Pressure Switch – controls the hydraulic pump, when gear are up and pressure decreases past a certain PSI, switch turns the pump back on

Question 18

Pressure switch and control

Answer 18

•Pressure switch
•Off 1800 +/- 100 PSI
•On 1500 +/- 100 PSI
•Pressure control
•High (2400 +/- 200 PSI)
•Low (600 +/- 200 PSI)
•Thermal relief (3000 PSI)

Question 19

Retract gear

Answer 19

•Pressure 1500 - 1800 psi
•Hydraulic pressure holds gear up
•Pressure switch turns pump off

Question 20

Extend gear

Answer 20

Pressure 400 - 800 psi
•Down lock switches turn pump off

Question 21

Emergency manual gear extension

Answer 21

Relieves hydraulic pressure
•Gravity free fall
•Doesn’t require electrical power
•Utilize mirror to determine landing gear down/locked

Question 22

Landing gear warning systems

Answer 22

•WARNING GEAR UNSAFE
•Gear is in transit
•Gear is not full up or
•Gear is not down locked

•All Lights OUT
•Gear is fully retracted

•WARNING GEAR UNSAFE and Warning Horn
•Gear up and power is ≤ 14” MP
•Gear selector switch is UP while on the ground
•When flaps are extended to 25° or greater & landing gear are not down and locked.

Question 23

Seminole landing gear

Answer 23

Pressure Switch
•Turns the pump off when pressure reaches 1800 PSI during gear retraction
•If line pressure to the bottom of the actuators drops below 1500 PSI, the pressure switch closes activating the pump to increase pressure to~1800 PSI

•High Pressure Control
•Relief valve used during gear retraction if pressure in the lines exceeds 2,250 psi.
•E.G. the pressure switch failed and pump not turned off at 1800 psi, the high pressure control would allow fluid back into the reservoir instead of over pressurizing the lines

•.020 Diameter Bleed Hole
•Used by maintenance to purge air bubbles from the system.
•Air in the system would cause the pump to no

Question 24

Landing gear

Answer 24

Low Pressure Control
•Relief valve that is used during gear extension.
•If pressure in the lines were to exceed 650 psi during gear extension, the low control pressure would allow fluid back into the reservoir.
▪The likely cause of this would be a down limit switch malfunction.
▪If the down limit switch was not to work, the hydraulic pump would run infinitely. In this case, fluid would escape through the low pressure control.
•Thermal Relief
•A relief valve that is used when the gear is fully retracted.
•This relief valve allows fluid to return to the reservoir.
▪The likely cause of a pressure increase with the gear up would be thermal expansion due to an increase in outside air temperature.
•Emergency Gear Valve
•Pressure relief valve located in the lines to hold gear up in the retracted position
•Opening will divert fluid to the reservoir from the high pressure lines