Electricity and Electronics for Aerospace Vehicles (Northrop Institute of Technology series)

604 Pages·1972·43.4 MB·English

by  Northrop Institute of Technology

PREFACE
This textbook provides a thorough coverage of basic This book is one of a series of texts prepared by the
electrical and electronic theory at a level which is staff of Northrop Institute of Technology on the con-
easily understood by the student who does not have a struction, inspection, operation, maintenance, over-
knowledge of advanced mathematics. Following the haul, and repair of aircraft, space vehicles, and power-
chapters explaining fundamental theory, the applica- plants. The purpose of this text is to provide informa-
tions to electrical and electronic systems are de- tion to students, technicians, inspectors, maintenance
scribed. Although a detailed study of advanced elec- engineers, shop foremen, and others who may wish to
tronic systems is beyond the scope of the text, the last become familiar with the electrical and electronic
several chapters are devoted to descriptions of many (avionic) systems installed in aircraft and space
of these systems as installed in modern aircraft and vehicles.
space vehicles. These systems are usually described In the earlier sections of the text, specific informa-
as avionic systems, inasmuch as they represent avia- tion is given concerning typical aircraft electrical
tion electronics. With the background knowledge ob- equipment, power systems, and basic electronic cir-
tained in earlier chapters, a student is able to under- cuits. A thorough study of these portions will give the
stand the electronic systems in modern airliners and technician a solid foundation on which to build for
space vehicles. more advanced work in electric and electronic tech-
The title "Electricity and Electronics for Aerospace nology.
Vehicles" has been carefully selected to be descriptive For the person who is not an electrical or elec-
of the material contained in the text. The word "aero- tronics specialist but who is assigned to work on
space" has been generally accepted as an inclusive equipment in which electrical and electronic systems
term to describe any vehicle that flies, either in the are installed, the information contained in this text
atmosphere (aero) or outside the atmosphere will provide an increased appreciation of the systems
(space). Because the text includes material appli- installed in aerospace vehicles.
cable to all aerospace vehicles, the term "aerospace" Each topic in the Northrop series has been ex-
is used in place of the word "aircraft." plained in a logical sequence so that the student may
advance step by step and build a good foundation for Collins Radio Company
increased learning. The student's understanding of Continental Air Lines, Los Angeles, California
the explanations and descriptions given in the text is Delco-Remy Division, General Motors Corporation
Electronic Instrument Company, Long Island City, New
greatly enhanced by the use of numerous pictures,
York
charts, and drawings.
Exide Industrial Division, Electric Storage Battery Com-
The subjects in the Northrop series are so orga-
pany
nized that instructors in public and private technical
Federal Aviation Administration
schools, training departments of aerospace rnanufac-
General Electric Company
turing companies, vocational schools, high schools, Granger Associates
and shop departments of colleges are provided with International Rectifier Company
a wealth of classroom material. The series may be Jack and Heintz, Inc., Cleveland, Ohio
used, also, by those who seek self-development. Jet Propulsion Laboratories, Pasadena, California
Kollsman Instrument Corporation, Glendale, California
Lear, Inc., Santa Monica, California
Motorola, Incorporated
Narco Avionics
National Aeronautics and Space Administration
ACKNOWLEDGEMENTS National Carbon Company, Division of Union Carbide
and Carbon Corporation
The authors acknowledge with thanks the generous Piper Aircraft Corporation, Lock Haven, Pennsylvania
contributions of technical information and illustra- Radio Corporation of America
tions by the following organizations: Sky Stores, Hawthorne, California
Sperry Gyroscope Company, Division of Sperry Rand
AiResearch Manufacturing Company, Division of the Corporation
Garrett Corporation, Los AngeIes, California Sperry Phoenix Company, Division of Sperry Rand
American Airlines Corporation
AMP Incorporated, Harrisburg, Pennsylvania Sundstrand Aviation, Division of Sundstrand Machine
Bendix Corporation, Eclipse Pioneer Division Tool Company, Rockford, Illinois
Bendix Corporation, Electric Power Division United Airlines
Boeing Company Western Air Lines
Burgess Battery Division of the Clevite Corporation Westinghouse Electric Corporation
Cannon Electric Company, Los Angeles, California Weston Instruments, Division of the Daystrom Corpora-
Cessna Aircraft Company, Wichita, Kansas tion, Newark, New Jersey
FUNDAMENTALS OF ELECTRICITY
This present period in history may well be called last few decades, the causes of electrical phenomena
the age of electronics because electricity and elec- have been accurately determined, and we are now
tronics have become so vital in every facet of modern able to employ electricity to perform a multitude of
technology. This is particularly true in the aviation tasks.
and aerospace fields because all modern aircraft and Today electricity is so common that we take it for
spacecraft are very largely dependent upon elec- granted. Without it there would be no modern auto-
tronics and electricity for communications and mobiles, refrigerators, electric irons, electric lights,
control. Electronics is merely a special application of streetcars, airplanes, missiles, spacecraft, radios, x-
electricity wherein precise manipulation of electrons ray, telephones, or television. Life, in the modern
is employed to control electrical power for a vast sense, could not continue, and we would soon revert
number of functions. to the "horse and buggy" era.
The airframe- and powerplant-maintenance tech- One function of electricity in an airplane is to
nician is not usually required to have an extensive ignite the fuel-air charge in the engine. Electricity
knowledge of electronic phenomena ; however, he for this purpose is supplied by magnetos coupled
should understand the basic principles of electricity to the engine. In the case of gas-turbine engines such
and electronics and be able to perform a variety of as turbojets or turboprops, electrical ignition is
service operations involved in the installat ion of needed only at the time of starting the engines. In
electrical and electronic equipment on an airplane. addition to providing engine ignition, electricity
The repair, overhaul, and testing of electronic supplies light, heat, and power. For example, it
equipment is usually performed by avionic specialists operates position lights, identification lights, landing
who have had extensive training in this type of work. lights, cabin lights, instrument lights, heaters, re-
Previous to the last century, little was known con- tractable landing gear, wing flaps, engine cowl flaps,
cerning the nature of electricity. Its manifestation radio, instruments, and navigation equipment.
in the form of lightning was considered by many Modern jet airliners contain many miles of electric
to be a demonstration of divine displeasure. In the wiring and hundreds of electrical and electronic
components; hence it is obvious that any person en- into which any compound can be divided and still
gaged in the servicing, operation, maintenance, or retain its identity is called a molecule.
design of such aircraft must have a thorough under- If a molecule of a substance is divided, it will be
standing of electrical principles. This applies to found to consist of particles called atoms. An atom
pilots, aircraft and powerpiant technicians, instru- is the smallest possible particle of an element, and
ment technicians, flight engineers, design engineers, until recently it was considered impossible to divide
maintenance engineers, and many others interested or destroy an atom.
in the technical aspects of aircraft operation and There are more than 100 recognized elements,
maintenance. several of which have been artificially created from
Furthermore, electricity is as essential to the firing various radioactive elements. An element is a sub-
and operation of rockets, missiles, and spacecraft stance that cannot be separated into different sub-
as it is to the operation of aircraft. With such stances except by nuclear disintegration. Common
devices, electricity (electronics) is needed for ground elements are iron, oxygen, aluminum, hydrogen,
control, operation of servomechanisms for various copper, lead, gold, silver, and so on. The smallest
in-flight control functions, computers, tracking, division of any of these elements will still have the
automatic navigation systems, homing on a target, properties of that element.
communications, etc. A compound is a chemical combination of two
or more different elements, and the smallest possible
THE ELECTRON THEORY particle of a compound is a molecule. For example,
a molecule of water (H,O) consists of two atoms of
Many persons who are unfamiliar with electricity
hydrogen and one atom of oxygen. A diagram
believe that an understanding of the subject is
representing a water molecule is shown in Fig. 1.1.
extremely difficult to attain and that only a few
individuals of superior intelligence can hope to
learn much about it. This is not true. A few hours
In recent years, many discoveries have been made
of study will enable almost anyone with sufficient
which greatly facilitate the study of electricity and
interest to understand the basic principles. These
provide new concepts concerning the nature of
principles are Ohm's law, magnetism, electro-
matter. One of the most important of these dis-
magnetic induction and inductance, capacitance, and
coveries has dealt with the structure of the atom.
the nature of direct and alternating currents. These
It has been found that an atom consists of in-
fundamentals are not difficult to master, and almost
finitesimal particles of energy known as electrons,
all electrical applications and phenomena may be
protons, and neutrons. All matter consists of one
explained in terms of these principles.
or more of these basic components. The simplest
MOLECULEASN D ATOMS atom is that of hydrogen, which has one electron
Matter is defined as anything which occupies space; and one proton as represented in the diagram of
hence everything which we can see and feel con- Fig. 1.2~.T he structure of an oxygen atom is
stitutes matter. It is now universally accepted that indicated in Fig. 1.26. This atom has eight protons,
matter is composed of molecules, which, in turn, are eight neutrons, and eight electrons. The protons
composed of atoms. If a quantity of a common and neutrons form the nucleus of the atom ; electrons
substance, such as water, is divided in half, and the revolve around the nucleus in orbits varying in shape
half is then divided, and the resulting quarter divided, from an ellipse to a circle and may be compared to
and so on, a point will be reached where any further the planets as they move around the sun. A positive
division will change the nature of the water and charge is carried by each proton, no charge is carried
turn it into something else. The smallest particle by the neutrons, and a negative charge is carried by
each electron. The charges carried by the electron an atom has more than two electrons, it must have
and the proton are equal but opposite in nature; more than one shell, since the first shell will accom-
thus an atom which has an equal number of protons modate only two electrons. This is shown in Fig.
and electrons is electrically neutral. The charge car- 1.2b. The number of shells in an atom depends
ried by the electrons is balanced by the charge carried upon the total number of electrons surrounding the
by the protons. nucleus.
Through research on the weight of atomic The atomic structure of a substance is of interest
particles, scientists have found that a proton weighs to the electrician because it determines how well
approximately 1,845 times as much as an electron the substance can conduct an electric current.
and that a neutron has the same weight as a proton. Certain elements, chiefly metals, are known as
It is obvious, then, that the weight of an atom is conductors because an electric current will flow
determined by the number of protons and neutrons through them easily. The atoms of these elements
contained in the nucleus. give up electrons or receive electrons in the outer
It has been explained that an atom carries two orbits with little difficulty. The electrons that move
opposite charges: a positive charge in the nucleus, from one atom to another are called free electrons.
and a negative charge in each electron. When the The movement of free electrons from one atom to
charge of the nucleus is equal to the combined another is indicated by the diagram in Fig. 1.3, and
charges of the electrons, the atom is neutral; but it will be noted that they pass from the outer shell
if the atom has a shortage of electrons, it will be of one atom to the outer shell of the next. The only
positively charged. Conversely, if the atom has an electrons shown in the diagram are those in the
excess of electrons, it will be negatively charged. outer orbits.
A positively charged atom is called a positive ion, An element is a conductor, nonconductor (in-
and a negatively charged atom is called a negative sulator), or semiconductor, depending upon, the
ion. Charged molecules are also called ions. number of electrons in the outer orbit of the atom.
If an atom has less than four electrons in the outer
ATOMICST RUCTURE AND FREE ELECTRONS orbit, it is a conductor. If it has more than four
The path of an electron around the nucleus of an atoms in the outer orbit, it is an insulator. A semi-
atom describes an imaginary sphere or shell. Hydro- conductor material such as germanium or silicon
gen and helium atoms have only one shell, but the has four electrons in the outer orbit of its atoms.
more complex atoms have numerous shells. When These materials have a very high resistance to current
Figure 1.1 Diagram of a water molecule. Figure 1.2 Srructure ofatoms. Figure 1.3 Assumed movement of
free elecfrons.
HYDROGEN ATOM OXYGEN ATOM
(a) (b)
4 flow when in the pure state ; however, when measured hair causes the electrons to transfer. The hair now
amounts of other elements are added, the material becomes positively charged because it loses electrons,
can be made to carry current. The nature and use of and the comb becomes negatively charged because
semiconductors is discussed in a later chapter. it gains electrons.
To cause electrons to move through a conductor, When the hair is thus charged, it will tend to
a force is required, and this force is supplied in part stand up, and the single strands will repel one another
by the electrons themselves. When two electrons because each has a similar charge. If the comb is
are near each other and are not acted upon by a then brought near the hair, the hair will be attracted
positive charge, they repel each other with a rela- by the comb because the hair and the comb have
tively tremendous force. It is said that if two electrons unlike charges. The attraction is the result of the
could be magnified to the size of peas and were electrons on the comb being attracted by the positive
placed 100 ft apart, they would repel each other charge of the hair.
with tons of force. It is this force which is utilized Static charging by friction between two or more
to cause electrons to move through a conductor. dissimilar materials is called triboelectric charging.
Electrons cluster around a nucleus because of the This type of charging is an important factor in the
neutralizing positive force exerted by the protons design and installation of electric and electronic
in the nucleus and also because of an unexplained equipment in aircraft or space vehicles.
phenomenon called the nuclear binding force. If A charged body, such as a comb or plastic rod,
the binding force were suddenly removed, there may be used to charge other bodies. For example,
would be an explosion like that of the atomic bomb. if two pith balls are suspended near each other on
The force of the atomic-bomb explosion is the fine threads, as in Fig. 1.4a, and each ball is then
result of an almost infinite number of atoms dis- touched with a charged plastic rod, a part of the
integrating simultaneously. charge is conveyed to the balls. Since the balls will
The movement of electrons through a conductor now have a similar charge, they will repel each other
is due, not to the disintegration of atoms, but to as in Fig. 1.46. If the rod is rubbed with a piece
the repelling force which the electrons exert upon of fur, it will become negatively charged and the
one another. When an extra electron enters the outer fur positively charged. By touching one of the balls
orbit of an atom, the repelling force immediately with the rod and the other with the fur, the balls are
causes another electron to move out of the orbit given opposite charges. They will then attract each
of that atom and into the orbit of another. If the other as shown in Fig. 1.4~.
material is a conductor, the electrons move easily The behavior of a charged body indicates that
from one atom to another. it is surrounded by an invisible field of force. This
We are all familiar with the results of passing a field is assumed to consist of lines of force extending
hard rubber or plastic comb through the hair. When
the hair is dry, a faint crackling sound may be heard
and the hair will stand up and attempt to follow
Figure 2.4 Reaction of like and unlike charges.
the comb. As the comb moves through the hair,
some of the electrons in the hair are dislodged and
picked up by the comb. The reason for the transfer
is probably that the outer orbits of the atoms of the
material in the comb are not filled; they therefore
attract electrons from the hair. When the hair is
agitated by the comb, the unbalanced condition REPULSION
existing between the atoms of the comb and of the
in all directions and terminating at a point where and the movement is from negative to positive as 5
where there is an equal and opposite charge. A field explained in this section. The student should fix
of this type is called an electrostatic field. When two this principle firmly in his mind so that he will not
oppositely charged bodies are in close proximity, the be confused when he encounters an application of
electrostatic field is relatively strong. If the two the old "conventional" current-flow theory.
bodies are joined by a conductor, the electrons from It is expected that eventually all writers and
the negatively charged body flow along the con- teachers will teach the principle as it actually is;
ductor to the positively charged body, and the however, it often takes many years to correct a
charges are neutralized. When the charges are false idea, and the student is warned to exercise
neutral, there is no electrostatic field. care as he continues to study electricity. He must be
particularly careful when he applies rules dealing
DIRECTIOONF CURRENT FLOW with current flow and its effects.
It has been shown that an electric current is the
result of the movement of electrons through a con- STATIC ELECTRICITY
ductor. Since a negatively charged body has an
excess of electrons and a positively charged body a
deficiency of electrons, it is obvious that the electron The study of the behavior of static electricity is
flow will be from the negatively charged body to called electrostatics. The word static means stationary
the positively charged body when the two are con- or at rest, and electric charges which are at rest are
nected by a conductor. It is therefore clear that called static electricity. In the previous section it
electricity flows from negative to positive. was shown that static electric charges may be
Until recently, however, it was assumed that produced by rubbing various dissimilar substances
electric current flowed from positive to negative. together and triboelectric charging takes place. The
This was because the polarities of electric charges nature of the charge produced is determined by the
were arbitrarily assigned names without the true types of substances. The following list of substances
nature of electric current being known. The study is called the electric series, and the list is so arranged
of radio and other electronic devices has made it that each substance is positive in relation to any
necessary to consider the true direction of current which follow it, when the two are in contact:
flow, but for all ordinary electrical applications, the
I. Fur 6. Cotton 11. Metals
direction of flow may be considered to be in either
2. Flannel 7. Silk 12. Sealing wax
direction so long as the theory is used consistently.
3. Ivory 8. Leather 13. Resins
Even though there are still some texts which adhere
4. Crystals 9. The body 14. Gutta percha
to the old conventional theory that current flows
5. Glass 10. Wood 15. Guncotton
from positive to negative, it is the purpose of this
text to consider all current flow as moving from If, for example, a glass rod is rubbed with fur, the
negative to positive. Electrical rules and diagrams rod becomes negatively charged ; but if it is rubbed
are arranged to conform to this principle in order with silk, it becomes positively charged.
to prevent confusion and to give the student a true When a nonconductor is charged by rubbing it
concept of electrical phenomena. with a dissimilar material, the charge remains at the
The student will sometimes read or hear the state- points where the friction occurs because the electrons
ment "electron flow is from negative to positive, cannot move through the material; however, when
and current flow is from positive to negative." a conductor is charged, it must be insulated from
This statement is a fallacy because current flow other conductors or the charge will be lost.
consists of electrons moving through a conductor, An electric charge may be produced in a conduc-
6 tor by induction if the conductor is properly insu- centers at speeds which are sometimes in excess of
lated. Imagine that the insulated metal sphere shown 100 mph. The turbulence caused by these updrafts
in Fig. 1.5 is charged negatively and brought near is largely responsible for the development of the
one end of a metal rod which is also insulated from electric charges which cause lightning.
other conductors. The electrons constituting the Although serious damage to an aircraft as the
negative charge in the sphere repel the electrons result of lightning is rare, studies have been made
in the rod and drive them to the opposite end of the to establish safe procedures when lightning may be
rod. The rod then has a positive charge in the end encountered. Such studies have indicated that a
nearest the charged sphere and a negative charge positive charge develops in the forward portion of
in the opposite end. This may be shown by suspend- the cloud, where the updrafts are more pronounced.
ing pith balls in pairs from the middle and ends of Thus it seems that the rising air currents are re-
the rod by means of conducting threads. At the moving electrons from that portion of the cloud.
ends of the rod, the pith balls separate as the The negative charge develops in the rear portion of
charged sphere is brought near one end; but the the cloud and is separated from the positive charge
balls near the center do not separate because the by a neutral area. When the difference between the
center is neutral. As the charged sphere is moved charges becomes great enough, a flash of lightning
away from the rod, the balls fall to their original occurs and the cloud becomes neutral for a time in
positions, thus indicating that the charges in the that particular area.
rod have become neutralized. The use of weather radar in modern airliners has
helped pilots to avoid flying through thunderstorms
where the danger of lightning would be greatest.
The familiar flash of lightning is nothing but an Danger areas show up clearly on the radar scopes
enormous spark caused by the discharge of static at a sufficient distance for the pilot to have adequate
electricity from a highly charged cloud. Clouds time to fly around them.
become charged because of friction between their
many minute particles of water, air, and dust.
Lightning is most commonly found in cumulus and As mentioned previously, the effects of static elec-
cumulonimbus clouds. These latter are the towering, tricity are of considerable importance in the design,
billowy clouds frequently seen in the summer; they operation, and maintenance of aircraft. This is
are caused by warm moist air moving up into particularly true because modern airplanes are
colder areas where condensation takes place. Such equipped with radio and other electronic equipment.
clouds have air currents moving up through their The pop and crackle of static is familiar to everyone
who has listened to a radio receiver when static
conditions are prevalent. An airplane in flight
picks up static charges because of contact with rain,
Figure 1.5 Charging by induction. snow, clouds, dust, and other particles in the air.
The charges thus produced in the aircraft structure
result in precipitation static (p static). The charges
flow about the metal structure of the airplane as
they tend to equalize, and if any part of the airplane
is partially insulated from another part, the static
electricity causes minute sparks as it jumps across
the insulated joints. Every spark causes p-static
noise in the radio communication equipment and
also causes disturbances in other electronic systems. Corona discharge occurs as short pulses at very 7
For this reason, the parts of an airplane are bonded high frequencies, thus producing energy fields which
so that electric charges may move throughout the couple with radio antenna fields to cause severe
airplane structure without causing sparks. Bonding interference. The solution to the problem is to cause
the parts of an airplane simply means establishing a the charge on the airplane to be partially dissipated
good electrical contact between them. Movable in a controlled manner so that the energy level of
parts, such as ailerons, flaps, and rudders, are con- the discharge will be reduced and the effects of the
nected to the main structure of the airplane with discharge will cause a minimum of interference. In
flexible woven-metal leads called bonding braid. the past, static-discharge wicks were used to reduce
The shielding of electronic devices and wiring is the charge on the airplane. Such an installation is
also necessary to help eliminate the effects of p static shown in Fig. 1.6.
on electrical equipment in the airplane. Shields Because of the high speeds of modern jet aircraft
consist of metal coverings which intercept un- and the fact that they are powered by jet engines
desirable waves and prevent them from affecting which tend to increase static charges, it became
sensitive electronic systems. necessary to develop static-discharge devices more
An airplane in flight often accumulates very high effective than the wicks formerly used. A new type
electric charges, not only from precipitation, but of discharger has proved most successful. It is called
also from the high-velocity jet-engine exhaust as it a Null Field Discharger and is manufactured by
flows through the tailpipe. When the airplane charge Granger Associates. These dischargers are mounted
becomes sufficiently high, electrons will be dis- at the trailing edges of outer ailerons, vertical
charged into the surrounding air from sharp or stabilizers, and other points where high discharges
pointed sections of the airplane. The level at which tend to occur. They produce a discharge field which
this begins is called the corona threshold. Corona has minimum coupling with radio antennas. Typical
discharge is often visible at night, emanating from installations are shown in Fig. 1.7.
wing tips, tail sections, and other sharply pointed Static charges must be taken into consideration
sections of an airplane. The visible discharge is when an airplane is being refueled. Gasoline or jet
often called "St. Elmo's fire." fuel flowing through the hose into the airplane will
Figure 1,6 Static-discharge wicks. Figure 1.7 Installation of Null Field Dischargers. (Granger
Associates)