principles of hydraulics

8/10/2019 Principles of Hydraulics

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Hydraulic Systems In Aircrafts

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INDEX

Introduction

Why hydraulic systems are used in aircrafts?

A simple hydraulic system

Hydraulic fluids for aircrafts

Hydraulic systems for aircraft

System with pressure regulator or unloading valve

The open-center system

Light-plane hydraulic system

ase Study

hydraulic system for !oeing "#"airplane$

Hydraulic system for landing gear

Hydraulic !ra%e operating system

onclusion

&eferences

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Chapter No: &

HY HYDRAULIC SY'E(S ARE USED IN

AIR)CRAF'%

The main aspect of designing an air-craft is to reduce the weight* And

hydraulic systems ma%e possi!le the transmission of pressure and energy at the !est

weight per horsepower ratio* Hydraulic system ensures safety) (uality and relia!ility

and reduces costs*

ollowing are some important properties of hydraulic system due to which it is

used in Air-craft*

Propertie! o* h"drau#ic !"!te$!:

The components in hydraulic systems are simple in construction and light

in weight*

Hydraulic systems can produce high pressure at less area*

.ower can o!tain at comparatively at low cost*

/ood operating speed*

High relia!ility

0nsure safety*

Ad+anta,e! o* h"drau#ic !"!te$! o+er other !"!te$! *or aircra*t u!e:

It is lighter in weight than alternate e1isting systems*

It is dead !eat) that is) there is an a!sence of sloppiness in its response to

demand placed on the system* It is relia!le2 either it wor%s or doesn3t*

It can easily maintain*

It is not a shoc% ha4ard2 it is not much of fire ha4ard*

It can develop practically unlimited force or tor(ue*

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Chapter No: -

A SI(PLE HYDRAULIC SYS'E(5asically) a hydraulic system re(uires a source of hydraulic power 6pump72

pipes or hoses to carry the hydraulic fluid from one point to another2 a valve

mechanism to control the flow and direction of hydraulic fluid2 A device for

converting the fluid power to movement 6actuating cylinder or hydraulic motor72

and reservoir to store the hydraulic fluid* A simple hydraulic system is shown in

diagram of figure 8*,*

This system is similar to the hydraulic !ra%e system* Hydraulic fluid is stored

in cylinder and is directed through a valve into the master cylinder as it is needed

when !ra%e pedal is depressed) the fluid is directed to the !ra%e cylinders2 these

cylinder push the shoes apart) thus causing them to !ear against the !ra%e drum and

provide !ra%e action) when pedal is released) springs attached to the !ra%e shoes

cause shoes to contract and push inward 9n !ra%e cylinders) thus causing some of

fluid to return to the master cylinder*

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Chapter No: .

HYDRAULIC FLUIDS FOR AIRCRAF'S

Purpo!e o* h"drau#ic *#uid:

Hydraulic fluid ma%es possi!le the transmission of pressure and energy* They

also act as a lu!ricating medium) there!y reducing the friction !etween moving part

and carrying away some of heat*

H"drau#ic *#uid t"pe!:

There are three principal type of fluids2 +e,eta/#e)/a!e *#uid!0 $inera#)/a!e

*#uid0 *ire)re!i!tant *#uid!

2e,eta/#e)/a!e *#uid!are usually mi1ture containing castor oil and alcohol

and are colored !lue) !lue-green) or almost clear* They are still used in !ra%e systems

!ut are not generally found in hydraulic-power systems*

(inera#)/a!e *#uid!consist of high (uality petroleum oil and are usually

colored red they are still used in many systems) especially where the fire ha4ard is

comparatively low* Small aircraft which has hydraulic-power systems for operating

flaps and landing gear will usually use mineral-!ase fluids conforming to :IL-'-

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Chapter No: 3

HYDRAULIC SYS'E(S FOR AIRCRAF'

Hydraulic systems have !een designed in many configurations for wide

variety of aircraft* Some very simple systems are used for light aircraft while

e1tremely comple1 systems have !een designed for operating modern =et aircraft) !oth

military commercial* 9n a light airplane) a power hydraulic system may !e used for

landing gear and flaps* 9n large transport aircraft the system is used for landing gear)

flaps) spoilers) control-surface !oost) retracta!le stair operation) !ra%es) leading edge

salts) and possi!ly other devices* In this section we shall e1amine a few sample

systems and few specific systems used on modern aircraft*

S"!te$ 4ith pre!!ure re,u#ator or un#oadin, +a#+e:

igure ;*, is a schematic drawing of a direct pressure system that has its

pressure regulated !y an unloading vale 6pressure regulator7 operating in con=unction

with an accumulator*

When the actuating e(uipment is not in use) pressure is relieved from the

continuously operating engine driven pump !y means of unloading valve after the

accumulator is charged to the correct level* As soon as a su!system is operated)

pressure will first come from the accumulator for operation2 when accumulator

pressure has dropped to a predetermined level) the unloading valve will cut in@ and

direct pump flow to the operating system* A study of diagram ma%e it clear that the

pump output flows directly through the unloading valve and !ac% to the reservoir

when no pressure is re(uired* When a su!system 6!ra%es) landing gear) or flaps7 is

operated) pump pressure will then flow through the su!system and !ac% to thereservoir through the su!system return line*

The main relief valve in the system is located !etween the unloading valve and

the return line* If the unloading valve should !ecome stuc% in the %ic%ed-in position)

the e1cess pressure would !e !ypassed through the relief valve to the reservoir*

.ressure gage would show higher than normal pressure) and the system would

pro!a!ly !e ma%ing noise !ecause of the operation of relief valve* The relief valve is

generally set at least ,'' psi a!ove the normal operating pressure*

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'he open)center !"!te$:

An open-center hydraulic system consists !asically of a fluid reservoir) chec%

valve) pump) relief valve) selector valves) actuating units) and tu!ing to connect the

units* As shown in figure ;*#* The system consists of a single circuit for fluid flow

with selector valve in series* The fluid flows continuously through the system under

low pressure e1cept when a su!system is operated* or e1ample) if the landing gear is

to !e raised) the gear selector valve is moved to the up position and the main flow of

fluid is rerouted to the gear selector valve* The return fluid from the actuator flows

!ac% through the selector valve passage and then continues on the ne1t selector valve

in the system* As soon as the actuator reaches the end of its operation) the valve

automatically %ic%s out@ and allows free flow of fluid through the system*

9ne of the advantages of open-center system is that it does not re(uire

e1pensive and complicated pressure regulators2 the power pump can !e simple gear

pump) although the fi1ed-displacement piston pump is li%ely to !e used*

A disadvantage of the open-center system is that the operation of only one

su!system at a time possi!le without interference from other systems* or e1ample if

flap su!system precedes the landing-gear su!system) and the two systems are

operated at the same time) the speed of gear operation will !e limited !y the amount

of fluid returning from the flap actuating cylinder* As soon as the flap operation is

complete and the flap selector valve %ic%s out) the landing-gear operation will proceed

at its normal rate*

Li,ht)p#ane h"drau#ic !"!te$:

A schematic diagram of a hydraulic system for a modern light

airplane is shown in figure ;*8* This system is installed in the piper apache .A-#8 air

plane to operate the wing flaps and landing gear*

rom the diagram it is seen that the engine-driven hydraulic pump ,' draws

hydraulic fluid from the reservoir ,; and pumps it through the pressure port of the

powerpa%@ assem!ly into the landing-gear selector pressure cham!er* When the two

selector valves are in the neutral position) the fluid travels from the landing gear

selector valve , through the flap selector valve # and !ac% to reservoir*

The powerpa% assem!ly is a modular unit which includes the reservoir) relief

valve) hand pump) landing-gear selector valve) wing flap selector valve) filters and

numerous small parts essential to the operation* When !oth selector valves are in


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the neutral position) the system acts as an open-center system in that the fluid flows

first through one selector valve and then through the other !efore returning to the

reservoir* uring this time the fluid flows freely at a reduced pressure* Since the fluid

supply line runs first through the landing-gear selector valve) the flaps can not !e

operated while the landing-gear su!system is in operation* 0ach selector valve has a

separate return line to allowed fluid from the actuating cylinder or cylinder to flow

!ac% to the reservoir*

The diagram of figure ;*> shows the fluid flow when the landing selector is

placed in up position* When the selector valve is placed in either operating position) it

is held in position !y a detent which consists of a !all) o ring) plunger) and spring* The

!all snaps into a groove in the spool which operates the poppet valves and prevent the

spool from moving linearly until the operation is complete* When the actuating

cylinder reaches the end of its movement) pressure !uilds up to appro1imately ,,;'

psi* This pressure acts against the plunger of the detent mechanism and relives the

pressure of the spring) thus allowing the !all to pop out of the groove* A spring then

causes the spool and selector lever to return to the neutral position*

igure ;*; shows the fluid flow in the system when the wing-flap selector

valve is placed in the down position* The action is generally the same as that descri!e

in the foregoing paragraph* It should !e noted that the fluid to the flap selector valve

has passed through the landing-gear selector valve first) and that the landing-gear

valve is in the neutral position*

The arrangement of the hydraulic system components in the piper .A-#8

Apache airplane is shown in the drawing of figure ;*


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Chapter No: 5

Ca!e Stud"

HYDRAULIC SYS'E( FOR 6OEIN7 8&8 AIRPLANE:

The hydraulic power system for the !oeing "#" airliner is typical of those

employed for modern =et transport type aircraft* These systems are of the high

pressure type) utilising pressure up to 8''' psi* The advantage of high pressure

systems is that they can deliver more power for given weight of fluid and system

component than the lower pressure system

The !oeing "#" incorporates three separate and independent hydraulic power

systems* These are designated hydraulic system A@) hydraulic system 5@) and

stand !y hydraulic system@* Hydraulic fluid is supplied from two pressuri4ed

reservoir and one unpressuri4ed reservoir) each system !eing supplied !y one of the

reservoirs* System A receives fluid under pressure from two engine-driven pumps

installed on engine Do*, and #* System 5 receives fluid under pressure from two

electric) motor-driven pumps installed in the left fairing ad=acent to the rear of the left-

wing root*

The stand!y system receives fluid pressure from one electric motor driven

pump installed in the left side wall of the aft stairwell* This system operates only on

demand and hydraulic power for the leading edge devices and the lower-rudder

operation*

igure


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removal without disrupting tu!e connections* As an e1ample of the function of a

modular unit) the one installed in a system A contains a pressure filter) two chec%

valves) two pressure warning switches) a pressure relief valve) and !ypass valve*

The hydraulic fluid used for the !oeing "#" systems is s%ydrol ;'') a fire-

resistant) phosphate ester-!ase) synthetic fluid* This purple fluid has an operating

temperature range of -


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that it will deliver fluid under pressure as needed for operation !ut will not !e

pumping fluid when there is no need for it*

A filter in the pressure line from each pump filters the fluid !efore it enters the

various su!systems* A pressure switch in the pressure line from each pump is

connected to a pump low-pressure warning light on the third crewman3s panel to

provide an indication of low hydraulic pressure* A piston type accumulator is

provided in the system to a!sor! sudden pressure surges* .ressure gauge in the control

ca!in and accumulator servicing station are connected to the gas side of the

accumulator to monitor hydraulic pressure when the system is pressuri4ed and

accumulator preload when system is not pressuri4ed* The accumulator preload

consists of nitrogen gas under pressure 6appro1imately #''' psi when the system is

depressurised7*

.ressure relief valve protects the system against damage in case a malfunction

permits the pressure to rise to an a!normally high level* A pump-case drain filter in

each pump return line is provided to detect incipient pump failure and to filter return-

line fluid !efore it enters the reservoir* A hydraulic-fluid heat e1changer in the pump

return line is provided to cool the hydraulic fluid !y transferring heat from the fluid to

a cooling airflow* A system return filter =ust ahead of the reservoir filters return fluid

from the su!systems supplied !y the system A* hydraulic-fluid overheat is sensed !y a

switch installed in the system return filter assem!ly and indicated !y a warning light

in the control ca!in*

A !ra%e interconnect valve is installed to supply hydraulic power to !ra%es

from system A whenever system 5 is inoperative and the !ra%e system is intact*

When the airplane is on ground) system A can !e depressuri4ed through the

manual !y pass valve* .lacing the !ypass valve handle in open position connects the

pressure and return lines) thus permitting pressuri4ed fluid to return to the reservoir*

or ground operation) system a can !e pressuri4ed to supply normal pressure without

engine operation !y attaching e1ternal hydraulic pressure source to the airplane at the

engine hydraulic self-sealing disconnect fittings* System a can also !e pressuri4ed

without engine operation !y attaching an e1ternal electrical ground power supply to

the airplane electrical system) opening the ground interconnect valve and opening

system 5 pumps* System 5 pumps are then feeding pressure into system A*

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H"drau#ic re!er+oir:

System A on the 5oeing "#" airplane is supplied with a fluid from a ;*>-gal

reservoir which is pressuri4ed to appro1imately >; psi !y !leed air from engine Do* ,

and #* .ressurisation of the reservoir assures a positive supply of fluid to the engine-

driven pumps* Air from the engine !leed is fed through chec% valves) a filter) and

pressure regulator* The regulator to admit air to reservoir if the pressure is !elow the

re(uired amount and it will relieve air from the reservoir if pressure rises a!ove the

correct level* It also acts as a vacuum relief valve when the reservoir pressure drops

!elow '*;'.si under am!ient pressure* In this case) air enter in the reservoir is filtered

!y the vent filter* A schematic drawing of the reservoir pressurisation is shown in

fig*


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The lower drawing shows the position of controlling elements when the

system is using fluid and the cam is in full flow position* Since system pressure has

dropped) the spring has pushed the high pressure compensator spool to the right) thus

opening passage so the fluid pressure against the stro%ing pressure is released* The

rate piston spring has moved the hanger to the right against the stro%ing piston and

changed the angle of cam* am can !e seen clearly in the drawing of the pump in

fig*


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At a fluid temperature of ,;;G 6 ;G7 the !ypass valve will !e fully closed) porting

all return fluid through the oil cooler*

Accu$u#ator:

The accumulator for system A is a cylindrical) free floating piston-type unit)

precharged with nitrogen gas* As e1plained previously) the purpose of unit is to store

fluid under pressure and to protect the system against sudden pressure surges* A fluid

line is connected to the fluid end of the accumulator and a nitrogen line is connected

to the opposite end* The accumulator is mounted vertically in airplane with the fluid

end at the top* 9n the nitrogen is connected a direct pressure gauge) a pressure

transmitter) and a nitrogen charging valve* The pressure transmitter is connected

electrically to the pressure gauge on the third crewman3s panel in the ca!in*

The preload gas pressure placed in the accumulator is appro1 #'''.si when

the am!ient temperature is


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The ground interconnect valve is provided so system A can !e pressurised for

ground operation and testing through system 5* system 5 is pressuri4ed !y

electrically driven pump2 hence it can operated without running the engines*

The !ra%e interconnect valve connect system A to the !ra%e system when

system 5 is inoperative and the !ra%e system is intact* This allows !ra%e operation

and !ra%e-accumulator charging using system a pressure* The valve is electrically

operated !y a switch on the third crewman3s panel*

The !ypass valve is installed in the modular unit to provide for

depressuri4ation of the system during ground maintenance operations* The valve

provides a flow of only ; gpm2 hence the system will operate when the engines are

running) even if the valve is open* &emem!er that the engine-driven pump furnish

##+# gpm each at full-flow position* uring flight) the !ypass valve is loc%wired in

the closed position*

S"!te$ 6 and !tand/" !"!te$:

The system 5 and stand!y systems were discussed earlier in this chapter2 since

their operations is similar to that e1plained for system A*

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Chapter No: 8

HYDRAULIC SYS'E( FOR LANDIN7 7EAR

(ain #andin, ,ear:

9ne type of retracta!le main landing gear is illustrated in fig*"*,* The

assem!ly consists principle of the shoc% strut2 the wheel2 the !ra%e assem!ly2 the

trunnion and side !race2 the tor(ue lin%) or scissors@) the actuating cylinder2 the

down and up loc%s2 and the !ungee system*

To retract the gear) the actuating cylinder is e1tended !y hydraulic pressure*

Since the actuating cylinder can provide greater force during the e1tension of the

cylinder that it can during retraction !ecause of the greater piston area e1posed to

fluid pressure) the e1tension movement of the actuating cylinder is used to retract the

gear* &etraction of the gear re(uires the greater force !ecause of gravity* 01tension of

the actuating cylinder causes the gear to rotate on the trunnion pin until the gear is

appro1imately in a hori4ontal position* When the gear reaches the full up position) a

pin on the strut engages the up latch and loc%s the gear in the up position*

When the gear is e1tended) the first moment of the actuating cylinder releases

the up loc%* This permits the gear to fall of its own weight) and the actuating cylinderacts to snu! the rate of fall* sually there is an orifice chec% valve in the up line of the

landing gear hydraulic system2 this restricts the fluid flow from the actuating cylinder

to the return line) thus slowing the rate of gear descent* As the gear approaches the

down position) the actuating cylinder moves it to the full down position* In the down

position) a !lade engages the down-loc% trac% and slide into the down-loc% latch as

shown in figure "*#* The down loc% prevents the gear from retracting after it has !een

lowered*

When the operation of retracta!le gear system includes the opening and

closing of the gear doors) an associated system controlled !y se(uence valves is often

use to operate the doors* The se(uence of operation is 6,7 opening of doors and 6#7

lowering of gear* uring retraction) the gear retracts and then the door closes* The

door can !e operated through the hydraulic system or !y mechanical lin%age in

connection with the movement of the lending gear mechanism* In some designs)

lending gear door are closed when the gear is e1tended or retracted* In such cases the

door must operated twice for either retraction or e1tension of gear*

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door-latch cylinders and !ungee cylinders to unloc%* As the gear moves down) the

door follow up mechanism !egins to drive the door se(uence valves toward the up

position* 5y the time the gear is down and loc%ed) the door se(uence valves have

reached the closed position and in!oard doors close* The nose-gear doors are actuated

mechanically through lin%age with the gear2 they are se(uenced to the open position

during gear travel and to the closed position at the end of each travel cycle*

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Chapter No:

H"drau#ic /ra;e operatin, !"!te$

The !ra%e systems for aircraft all serve the same purpose even though there

are a variety of designs* The ma=ority of !ra%e systems fall into one of four categories$

6,7 Shoe !rea%s) 6#7 e1pander-tu!e !ra%es) 687 single-dis% !ra%es and

6>7 multiple-dis% !ra%es*

The shoe-type !ra%es have !een used for many years on some of the older

aircraft and are still used e1tensively for automo!iles* Atypical shoe !ra%e is

illustrated in figure B*,* This !ra%e system consists of two master cylinders with a

single reservoir for storing the reverse hydraulic fluid* The master cylinder contains a

piston operated !y foot pressure to force hydraulic fluid through tu!ing the wheel

cylinders) which e1pands the !ra%e shoe and press them against the inner surface of

the !ra%e drums* The !ra%e shoes are lined tough) heat-resistant material to withstand

the wear and developed !y the friction of the shoe against the drum*

01pander-tu!e !ra%es consist of a flat) synthetic ru!!er tu!e mounted on a

drum-shaped mem!er attached to the inner of the a1le* 5ra%e !loc%s 6linings7) shaped

to the contour of the !ra%e drum) are mounted over the e1pander tu!e and held inplace !y leaf-springs which pass through slots in the ends of the lining !loc%s and

loc% in slot in the flanges on each side of the e1pander tu!e* The inside of the tu!e is

connected to the hydraulic fluid line !y means of suita!le fittings* When hydraulic

fluid pressure from the !ra%e master cylinder or !ra%e valve enters the e1pander tu!e)

the tu!e is e1panded and presses outward against the !ra%e !loc%s* The !loc%s) in

turn) press against the inner surface of the !ra%e drum on the wheel) thus tending to

stop the wheel rotation when the pressure of the fluid is released2 the leaf-springs pull

the !ra%e linings away from the !ra%e drum and released the !rea%ing action*

A single-dis% !ra%e consists of a steel dis% which is %eyed to the wheel and

the !ra%e linings arranged so they can apply pressure to !oth sides of the dis% when

hydraulic pressure is applied to the !ra%e pistons* ig B*# illustrates the principle of

the single dis% !ra%e* It will !e noted that stationary and movea!le !ra%e linings are

placed on opposite sides of the dis% so the dis% will !e gripped !etween the linings

when force is applied to the movea!le lining* The movea!le lining is actuated !y

,B


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means of cylinder and piston which receives hydraulic-fluid pressure from the !ra%e

master cylinder or !ra%e valve*

The multiple-dis% !ra%e is constructed somewhat li%e a dis% clutch*

5ra%e dis%s) alternately stationary and moving) are arrange to provide a ma1imum of

!rea%ing surface when the dis%s are compressed !y means of the annular piston or !y

means of num!er of individual pistons arrange in a circle such that their radial

distances from the a1le are e(ual* The mova!le dis%s 6rotor dis%s7 are %eyed to the

wheel) and the stationary dis%s 6stator dis%s7 are %eyed to the a1le* When the wheel is

turning) each mova!le dis% is turning !etween two stationary dis%s* When the !ra%e

piston or pistons receive the fluid pressure) all the dis%s are pressed together) thus

tending to stop the movement of the mova!le dis%s* Since the mova!le dis%s are

%eyed to the wheel at the outer edges) the wheel stop turning when the dis% stop*

6ra;e actuatin, !"!te$!:

5ra%e actuating system for aircraft can !e classified as mechanically operated)

hydraulically operated or pneumatically operated* All !ra%e actuating system provide

for applied either one or !oth !ra%es !y operating foot pedals*

In some airplanes) the hydraulic !ra%e system is a su!system of the main

hydraulic system* In other airplanes) there is an entirely independent !ra%e system*

:any of the large airplanes have a power-!ra%e system which is a su! system of the

main system* The smaller airplanes usually have an independent) master !ra%e

cylinder !ra%e system*

6a!ic /ra;e !"!te$ u!in, $a!ter c"#inder!:

In figure B*,) a comparatively simple !ra%e system utili4ing master cylinders

and shoe type !ra%es is illustrated* As shown) the system includes !ra%e pedals) a

fluid reservoir) two master cylinders) fluid lines) actuating cylinder at the wheels)

!ra%e shoes) and !leeder valves*

The master cylinder is energi4ing unit* There is a one for each main lending

gear wheel* The master cylinder is actually foot-operated) single action reciprocating

pump) the purpose of which is to !uild up hydraulic fluid pressure in the !ra%e

system*

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The reservoir is a storage tan% that supplies the fluid to compensate for small

lea%s in the connecting lines or cylinders* The reservoir may !e a part of the master

cylinder or it may !e a separate unit as shown in the drawing* It is a vented to the

atmosphere to provide for feeding the fluid to the master cylinders under the force of

gravity2 or air will enter the system and reduce its effectiveness*

The fluid lines may consist of fle1i!le or rigid tu!ing or com!ination of !oth*

sually fle1i!le tu!ing is employed with retracta!le gear system and !etween the

mova!le parts of the shoc% strut*

:echanical lin%ages are re(uired to transmit the energy of the food to the

master cylinder* Some airplanes have the master cylinder mounted on the rudder

pedals and other have the master cylinders mounted at a distance from the pedals* A

system of rods) livers) !ell cran%s) and ca!les is often employed to carry the

mechanical energy to the master cylinders* In figure B*,) the /ra;e peda#! are toe

!ra%es mounted on the rudder pedals* When the !ra%es pedals are pressed) the lin%age

causes the master cylinder piston to move into the cylinder and force fluid into the

!ra%e lines* When !ra%es of this type are pressed) it necessary for the pilot to !alance

the force on the pedal with e(ual force on the other pedals unless he wishes to turn the

airplane* The !ra%es and the rudder control are operated independently2 however)

since the !ra%e pedals are on the rudder pedals) the pilot should !e practiced in their

use*

The /ra;e actuatin, c"#inder! are units of the !ra%es assem!ly that causes

the !ra%es shoes to press against the inner surface of the !ra%e drum when pressure

from the master cylinders is transmitted to them* If e1pander-tu!e !ra%es or dis% type

!ra%es are employed) the effect is the same even thought the !ra%ing force is applied

in a different manner*

The par%ing !ra%e mechanism is a su! assem!ly of the usual hydraulic !ra%e

system* The control for the mechanism is in the pilot3s compartment and usually

consists of a pull handle or lever* When the !ra%e pedals are depressed and the

par%ing !ra%e lever is pulled !ac% !y hand) the !ra%es are loc%ed in the on position*

epressing the !ra%es pedals again releases the !ra%es* Farious types of master

cylinders can !e used) !ut the action of depressing the pedals the second time must

cause the master cylinder piston return to the off position* epending upon the type of

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master cylinder used) depressing the pedals will either !uild up enough pressure to

unseat the par%ing valve) or it will unload a ratchet type par%ing loc%*

7ood"ear $a!ter c"#inder!:

9ne type of master cylinder for light air craft is illustrated in fig*#>* This

master cylinder is a type designed and !uilt !y the /oodyear tyre and &u!!er

ompany*

A more recently designed /oodyear master cylinder is used on num!er of

modern) light aircraft* This cylinder is illustrated in fig*B*8* 9n the essna 8,'

airplane) these cylinders are mounted on the rudder pedals as shown in the fig* #) this type of master cylinder incorporates the fluid reservoir

6B7 on the top of the cylinder 6,,7 within the same !ody 6"7* A plastic filler plug 6,B7

is used to close the opening the opening in the cover 6>7) which is threaded into the

!ody* The filler plug is not vented !ecause sufficient ventilation is provided !y

clearance !etween the piston rod 687 and the piston rod opening through the cover

!oss 67)

loc%-o-seal@ 6,


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compensate for any e1cess fluid which may !e present in the system due to pumping

or from thermal e1pansion*

Po4er /ra;e contro# +a#+e!:

9n large aircraft with hydraulic power !ra%e actuating systems) the control of

fluid pressure to the !ra%e mechanism is accomplished !y means of !ra%e control

valves) or !ra%e metering valves* These valves are merely special type of flow control

valves designed to meet the re(uirements of !ra%e operation*

The !ra%e control valve or the uglas -C aeroplane is illustrated in figure

B*;*The upper drawing is the e1ternal view) and the lower drawing shows the internal

arrangement* As shown in the drawing) the !ra%e control valve has one pressure port)

one return port and two !ra%e ports*

It is seen from figure that during operation the !ra%e valve cran% is moved

towards the on position) and moves the spool inside the sleeve-and-shuttle assem!ly

to direct fluid from the pressure inlet to the outlet and hence to the !ra%e servo valve*

When the system is in the automatic position) the servo valves automatically control

the fluid pressure from the !ra%e control valves to the !ra%es in the response to

signals from the anti-s%id system* When the system is in the non-automatic) or

manual@) condition) the !ra%e control valves control pressure directly to the !ra%es*

uring the automatic operation) the pilot holds the !ra%e pedals in the full on position*

The power !ra%e control valve for the Boeing "#' air plane is illustrated in

figure B*


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pressure to the !ra%es* A passage through the valve rod permits the hydraulic fluid

under pressure to enter a compensating cham!er enclosing the inner end of the valve

rod* .ressure acting on the end of the rod creates a return force tending to close the

valve* This return force varies the intensity of !ra%ing force and providesfeel at the

pedals* The desired !ra%ing effort is o!tained !y depressing the pedals a greater or

lesser distance* a!le stretch and ad=ustment of pedal position permits the valve rod to

move !ac% until !oth pressure and return ports are closed* At this point the !ra%ing

effort remains constant* This condition is shown schematically in figure B*


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Chapter No:

principles of hydraulics

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