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Chapter IX

Hydraulic and Pneumatic Pow er Systems

he word hydraulics is based on the Greek word forater, and originally meant the study of physical

havior of water at rest and in m otion. Today, theeaning has been expan ded to include the physicalhavior of all liquids including hydraulic fluids.

A. Aircraft Hydraulic Systemsydraul ic systems are not new to aviat ion. Some

r ly aircraft used hydraulic brake systems. A s

rcraft became more sophisticated, newer systemsith greater complexity were developed.

Although some aircraft manufacturers make

eater use of hydraulic systems than others, the

ydraul ic system of the average m odern aircraf t

rforms many functions. Among the units com-

only operated by hydraul ic systems are landing

ear, wing flaps, speed and w heel brakes, and flightontrol surfaces.

Hydraul ic systems have m any advantages as a

ower source for operating various aircraft units.

hey combine the advantages of light weight, ease ofstallation, simplification of inspection, and mini-um maintenance requirements. Hydraulic opera-ons are almost 100% efficient, with only a negligibless due to fluid friction.

Aircraft hydraulic systems belong to that branchf physics concerned with fluid power/mechanics.hey do their wo rk by moving fluid, and the fluid

ey use is incompressible. Pneumatic systems workmuch the same way, obeying many of the same

ws, but the fluid they u se (air) is compressible.

To bet ter understand how a hydraulic system

ccomplishes its task, a brief review of the physicsnvolved is necessary.

1. Pascal s Law

his is the bas ic law w e use when we th ink of

ansmit t ing power by a hydraulic system. Therench m athematician Blaise Pascal observed thatny increase in the pressure on a confined liquid wasansmitted equally and undiminished to all parts of

he container, and acts at right angles to the enclos-ng walls of the con tainer. This means simply that ife hav e an enclosed vessel full of liquid, and we

pply a force to a p iston in the vessel to raise the

ressure, this increase in pressure w ill be the samenywhere in the system. Each of the gauges attached

to the container shown in figure 9-1 will have thesame reading.

2. The Hydrostatic ParadoxThe pressure produced by a column of liquid is directly

proport ional to i ts densi ty and the height of the

column, and in no way depends upon the shape of thecontainer or the amount of liquid the container holds.For example, 1 cu. in. of water weighs 0.036 lb. A tubethat is 231 tall with a cross section of 1 sq. in. willhold 1 gal. of wa ter (1 gal. = 231 c u. in.). If the tube is

Figure 9-1. Pressure exerted on a fluid in an enclosedcontainer is transmitted equally andundiminished to all parts of the containerand acts at right angles to the enclosingwalls.

Figure 9-2. The pressure exerted by a colum n of liquidis determined by the height of the columnand is independent of its volume.

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FORCE RE x PRESSURE

RE FORCE R SSUR

RESSURE

ORCE RE

A B C)

standing straight u p, the 1 gal. of water will exert apressure of 8.32 PSI at the bottom of the tube.

If the tube were 231 high and had an area of 100sq. in. , i t would ho ld 100 g al . of water, but thepressure at the bottom w ould still be 8.32 PSI. Theforce exerted by the column of water is, of course,equal to the pressure act ing on each square inch

times the number of square inches, or 832 lbs.I t makes no difference as to the shape or s ize of

the vessel that contains the liquid; it is the height ofthe column that is the critical factor. In figure 9-3,

the pressure (P) read by the gauges will be the samein all four instances, since the height (H) is the same.Naturally, all of the vessels must be filled with thesame liquid.

3. Relationship Between Pressure, Force,and Area

Pressure is a measure of the amoun t of force thatacts on a unit of area. In most American hydraulic

systems, pressure is measured in pounds per squa reinch PSI).

The relationship between force, pressure, and

area may be expressed by the formula:

Force = Pressure x Area

This may be visualized by looking at figure 9-4.

The bottom half represents the product of the area

in square inches and the pressure in PSI. This givesus the amount of force in pounds, which is repre-

sented by the top h alf of the circle.

In order to find pressure, divide the force by the

area:

Pressure =ForceArea

In order to find the area, divide the force by the

pressure:

Area = ForcePressure

4. Relationship Between Area, Distance,and Volume

Another relationship in hydraulics is between the

area of the piston, the distance it moves, and the

volume of the fluid displaced. We can visualize thisrelationship in figure 9-5. One half of the circle

represents the volume in cubic inches, and the otherhalf of the circle the area in square inches and the

distance the piston moves in inches. Distance is alsoknown as stroke.

The relationship between volume, area, and dis-

tance may be expressed by the formula:

Volume = Area x Distance

To find the area divide the volume by the distance:

Area =Volume

Distance

To find the distance divide the volume by the area:

Distance =Volume

Area

Figure 9-3. Neither the shape nor the volume of acontainer affects the pressure.

Figure 9-4. Relationship between area pressure and force.

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AAREA DISTANCE

VVOLUME

VOLUME = AREA x DISTANCE AREA = VOLUME / DISTANCE DISTANCE = VOLUME / AREA(B) (C)

1n W = 20

AREA20 SO. INCH

llllllll

AREA. 1 SO. INCH

D 1 INCH

1 = 1/20 INCH

Figure 9-5. Relationship between volume area and distance.

5 M echanical Advantage in a HydraulicSystem

A hydraulic system has two major advantages overother types of mech anical systems. One is the easewith which force can b e transmitted over large dis-tances and into and out of sealed compartments. Theother is the mechanical advantage ma de possible byvarying the size of pistons.

In f igure 9-6, we see the way m echanical ad-

vantage is achieved in a hydraulic system. If we havea piston whose area is 1 sq. in. pressing dow n witha force of 1 lb., it will produce a pressure of 1 PSI

and for every inch it moves, will displace 1 cu. in. of

fluid.If the cylinder containing this piston is connected

to one having a pis ton with an area of 20 sq. in .,

every square inch will be acted on by the same 1 PSIpressure, and a force of 20 lbs. will be produced. The1 cu. in. of fluid displaced w hen the sm all piston

Figure 9-6. The product of the force times the area ofthe large piston is equal to the product ofthe weight times the area of the smallpiston.

move s down 1 in. spreads out und er all 20 sq. in. ofthe large piston, and will move up o nly 1/2o .

This may be expressed as:A (small) x D (small) = A (large) x D (large)

1 x 1 = 20 x 1 2o

1 = 1

All hydraulic systems are essentially the same,

whatever their function. Regardless of application,each hydraulic system has a minimum number of

componen ts, and some type of hydraulic fluid.

B. Hydraulic FluidWhile we m ay not normally think of fluid as being acomp onent, the fluid used in aircraft hydraulic sys-tems is most important. This fluid must flow with aminimum of o pposition, and be incompressible. It

must have good lubricating properties to prevent

wear in the pum p and valves. It must inhibit cor-

rosion and not chem ically attack seals used in the

system. And it must not foam in operation, becauseair carried into the com ponents will give them a

spongy action.

Manufacturers of hydraulic devices specify thetype of fluid best suited for use w ith their equipment.W orking conditions, service, temperatures, pres-

sures, possibilities of corrosion, and other condi-tions must be considered Some of the

characteristics that must be considered when select-ing a satisfactory fluid for a particular system are

discussed in the following paragraphs.

1. ViscosityOne o f t he m os t imp or t an t p rope r t ie s o f any

hyd raul ic f lu id i s i t s v i scos i ty. Viscos i ty i s ameasure of internal resis tance to f low. A l iquid

such as gasoline flows easily has a low viscosity)

while a liquid such as tar flows slowly has a high

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HEATINGUNIT IQUID

BATH

THERMOMETER

CORKCONTAINER

6c c

OIL

D

RESERVOIR

viscos i ty ) . Viscos i ty increases as tem pera ture

decreases.

The viscosity of a liquid is measured with a vis-

cosimeter. There are severa l types, but the instrumentmost often used is the Saybolt universal viscosimeter(figure 9-7). This instrument measures the number ofseconds it takes for a fixed quantity of liquid (60 cc) to

flow through a small orifice of standard length anddiameter at a specific temperature . This time of flow ismeasured in seconds, and the viscosity reading ex-pressed as SSU (sec onds, Saybolt universal).

2. Chemical StabilityChemical stability is another property which is impor-tant in selecting a hydraulic fluid. It is the ability of the

liquid to resist oxidation and deterioration for longperiods. Mostl liquids tend to undergo unfavorablechemical changes during severe operating conditions.

This is the case when a system operates for a consid-erable period of time at high temperatures.

Excessive temperatures have an adverse effect onthe life of a liquid. The temperature of the liquid inthe reservoir of an operating hydraulic system doesnot always represent a true state of operating con-

ditions. Localized hot spots occur on b earings, gearteeth, or at the point where liquid under p ressure isforced through a small orifice. Continuous passageof a liquid through these points may produce local

temperatures high enough to carbonize or sludge the

liquid, yet the liquid in the reservoir may not indicatean excessively high temperature. Liquids with a highviscosity have a greater resistance to heat than light

Figure 9-7. Saybolt viscosimeter.

or low viscosity liquids which have been derived fromthe same source. Fortunately, there is a wide cho iceof liquids available for use within the viscosity rangerequired of hydraulic systems.

Liquids may break dow n if exposed to air, water,salt, or other impurities, especially if in constant

motion or subject to heat. Some metals, such as zinc,

lead, brass, and copper have an u ndesirable chemi-cal reaction on certain liquids.

These chem ical processes result in the formationof sludge, gums, carbon or other deposits which clogopenings, cause valves and pistons to stick or leak,and give poor lubrication to moving parts. As soonas small amo unts of s ludge or other deposi ts are

formed, their rate of formation gen erally increases.As they are formed, certain changes in the physicaland chem ical properties of the liquid take place. Theliquid usually becomes d arker in color, higher in

viscosity, and acids are formed.

Flash PointFlash point is the temperature at which a sub stancegives off va por in suff ic ien t quant i ty to ign i te

momentarily (flash) when a flame is applied. A highflash point is desirable for hydraulic fluids becauseit indicates a good resistance to comb ustion and a

low degree of evaporation at norm al temperatures.

Fire PointFire point is the tempe rature at which a substancegives off vapor in sufficient quantity to ignite and

continue to burn when exposed to a spark or flame.Like f lash point , a high f i re point is required of

desirable hydraulic fluids.

5 Types of Hydraulic FluidTo assure proper system operat ion and to avoid

damage to nonm etallic components of the hydraulicsystem, the correct fluid mu st be used.

When adding fluid to a system, use the type specified

in the aircraft manufacturer's maintenance manual oron the instruction plate affixed to the reservoir or unitbeing serviced. There are three types of hydraulic fluidscurrently being used in civil aircraft.

a. Vegetable-base FluidMIL-H-7644 f luid has been used in the past w hen

hydraulic system requirements were not as severe

as they are toda y. This fluid is essentially castor oiland alcohol. Althoug h it is similar to autom otive

brake fluid it is not interchange able. This fluid is

used primarily in older typ e aircraft. It is dyed blue

for identification. Natural rubber se als are used withvegetable base fluid. If this system is contaminatedwith petroleum base or phosphate ester base fluids,

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e seals will swell, break dow n and block the sys-m. The system ma y be flushed with alcohol. Thispe of fluid is flammable.

Mineral-base Fluid

IL-H-5606 is the most widely used hydraulic fluidgeneral aviation a ircraft today. It is basically a

erosene-type petroleum product , having good

bricating properties and additives to inhibit foam-g and prevent corrosion. It is quite stable chemi-l ly and has very l i t t le viscosi ty change with

mperature. MIL-H-5606 fluid is dyed red for iden-f icat ion, and system s using this f luid ma y beushed with naphtha, varsol, or Stoddard solvent.

eoprene seals and hoses may be used with MIL-H-606 fluid. This type of fluid is flammable.

Synthetic Fluid

on-petroleum ba se hydraulic fluids were intro-

uced in 1948 to provide a fire-resistant hydraulicuid for use in high performance piston engine andrbine powered aircraft.

The m ost comm only used f luid of this type is

MIL-H-8446 or, Skydrol a registered trade namef the Monsanto C hemical Co.). This fluid is coloredght purple, is slightly heavier than water, and haswide range of operating temperatures from around5F to over 225F for sustained operation. Cur-

ntly there are two grades of Skydrol in use, Skydrol00B4, and Skydrol LD. Skydrol LD has a lower

ensi ty and offers some advantage in jumbo jetansport aircraft where weight is a prime factor.

Skydrol is not without its problems however, as it quite susceptible to contamination by w ater from

he atmosphere an d must be k ept t ight ly sealed.

When servicing a system using Skydrol, be extreme-y careful to use only seals and hoses having the

roper par t number. Skydrol sys tems m ay beushed out w ith trichlorethylene.

Intermixing of Fluidsue to the difference in composition, vegetable base,etroleum base and p hosphate ester base fluids willot mix. Neither are the type of se als for any one fluidsable with or tolerant of any of the other fluids.

hould an aircraft hydraulic system be serviced withhe wrong type of fluid, immediately drain and flushhe system and maintain the seals according to the

manufacturer's specifications.

Compatibility with Aircraft Materialsi rcraf t hydraul ic systems designed for Skydrol

uids should be virtually trouble-free if properlyerviced. Skydrol does not appreciably affect com-

mon aircraft metals as long as the fluid is kept freef contamination.

Due to the phosphate es te r base of syn the t ichydraulic fluids, thermoplastic resins, including

vinyl comp ositions, nitrocellulose lacquers, oil basepain ts , l ino leum and asphal t m ay be sof tenedchem ically by these f luids. Skydrol wil l at tackpolyvinyl chloride, and must not be allowed to dripon to electr ical wir ing, as i t wil l break dow n the

insulation. However, this chem ical reaction usuallyrequires longer than just momentary exposure; andspills that are wiped up with soap and w ater do notharm mo st of these materials.

Skydrol is compatible with natural fibers and w itha num ber of synthe t ics , inc luding nylon andpolyester, which are used extensively in many

aircraft.

Petroleum o il hydraulic seals of neoprene orBuna-N are not compatible with Skydrol and mustbe replaced with seals of butyl rubber or ethylene-propylene elastomers for units that are intended foruse in sys tems u t i l iz ing pho sphate es te r basehydraulic fluid. These seals are rea dily available

from suppliers.

8 Health and HandlingSkydrol fluid does not present any particular healthhazard in its recommended use. Skyd rol has a verylow order of toxicity when taken orally or applied tothe skin in liquid form. It causes pain on contact

with eye t i s sue , bu t an imal s tud ies and hum an

experience indicate that i t causes no permanent

damage. First aid treatment for eye con tact includesflushing the eyes immediately with large volumes ofwater and the application of an anesthetic eye solu-t ion. If pain persists , the individual should b e

referred to a physician.

In mist or fog f orm, Skyd rol is quite irritating tonasa l o r r e sp i r a to ry pas sages and ge ne ra l lyproduces coughing and sneezing. Such irritation

does not persist following cessation of exposure.

Silicone ointments, rubber gloves, and careful

washing procedures should be utilized to avoid ex-cessive repeated contact with Skydrol in order to

avoid solvent effect on skin.

C. Basic Hydraulic SystemsA hydraulic system is much like an electrical system.It must have a source of power, a means of transmit-ting this power, and finally some type of d evice to

use the power.

1 Open Hydraulic SystemsThe most basic form of an open h ydraulic system isthat used by hydroelectric power plants. Large damsblock streams of wa ter to form lakes that s tore

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