File #1337: "Power for Aircraft (1956).pdf"

Power for Aircraft (1956).pdf

PDF Text


Copyright 1956 by Civil Air Patrol, Incorporated

Harold E. Mehrens
William E. Rowland
Art Director


EARLY in the Twentieth Century
an engine was built and used to
power an aircraft. Within two
decades after this event the
aircraft powerplant had been
proved dependable. In 1929 I
had the opportunity to help
dramatize this dependability.
During the quarter century that
has passed since that occasion,
sources of tremendous power,
undreamed of then, have been
discovered. Today we face the
stark reality that this terrible
power makes available to
scientists and engineers on
both sides of the globe a force
capable of annihilating the
human race. We must keep our
slim margin of technical
superiority until we find the way
to use the power we now
command to provide mankind
with peace and prosperity.



Aircraft Power Plants .........................................................

Internal Combustion Engines ..........................................





How the Engine Gets Its Fuel .......................................... 1 9

Power Plant Electric Systems .......................................... 2 7

The Use and Loss of Heat ................................................ 3 3

Lubrication and Cooling ................................................... 4 1


Thrust and the Propeller ................................................... 47


Instruments and Controls ...................................................


Summary .................................................................................... 57

Among the major events of this century
only one other ranks with the invention of the
powered aircraft. This event is the development of the engine which can release the
power potentials of uranium atoms. The technological advances following in the train of
these two events have created sociological
conditions which demand immediate attention.
The young men and women of America
must help the world adjust to the impacts of
its inventions and discoveries. The Civil Air
Patrol recognizes the task that this world adjustment places upon the youth of America.
Its cadet program is designed to assist organized formal education in properly guiding
young people toward the completion of this
The Civil Air Patrol aviation education
series will help lay a foundation of basic
understanding upon which can be built skills
and attitudes essential to this task--that is to
the solution of the problems spawned by technical acceleration.
Major General, USAF
National Commander
Civil Air Patrol

Power for Aircraft is one of a series of six pocketsized books prepared for use in the aviation education
program of the Civil Air Patrol. It is to be used with an
i n s t r u c t i o n a l 3 5 m m . c o l o r, s o u n d fi l m s t r i p w h i c h i l l u s trates the concepts which it introduces.
The purpose of this book is to describe in terms of
secondary-school student understandings the scientific
principles basic to aircraft power plant operation. Descriptions and explanations are given of the methods by
w h i c h t h e p o t e n t i a l o f a f u e l i s c o n v e r t e d i n t o e n e r g y,
and straight line motion into propeller thrust. The operation of starting systems, fuel induction systems, and fuel
ignition systems are explained. Cooling systems and
lubricating systems are discussed. Simple explanations
of thermal efficiency and mechanical efficiency are offered. The operation of jet-propelled engines is described.
Power plant developments currently under way are briefly
c o n s i d e r e d . F i n a l l y, t h e s y s t e m o f p o w e r p l a n t c o n t r o l s
the pilot uses is described. The book's treatment of the
several areas with which it is concerned is sufficiently
general to be of basic importance to all aviation career
o b j e c t i v e s . Ye t , i t s c o n t e n t i s d e t a i l e d e n o u g h t o c h a l lenge the interest of students and adults alike.
Although its first use will be with Civil Air Patrol cadets,
it will be found of considerable value in science classes
and any other class that stresses the role of scientific
concepts related to engines and power plant systems.
The books and filmstrips of this series are not limited
to use with Civil Air Patrol cadets only. They will be found
of value to students and teachers in any aviation program. Those working with adults may also find this material helpful if the instructional or information goal is
general education as it relates to aviation.

The history of aviation records achievements and development in
the design and construction not only of airframes, but also of
power plants. It is the aircraft power plant that makes sustained
flight possible. Without a dependable power plant the airplane
could not have been put to practical use.
A good many years before the Wright Brothers made the first
powered flight, men had flown man-carrying gliders. Otto
Lilienthal, a German, Octave Chanute, a Frenchman, and John
Montgomery, an American, were among these men. Before 1903,
the Wright Brothers themselves had built and flown soaring
planes. It may be that the greatest accomplishment of the Wright
Brothers was to design and build, with the help of Charles Taylor,
the power plant for the airplane which they flew at Kitty Hawk.
Early aircraft power plants were not very reliable. However, by
1919 one had been built which would operate long enough to
enable Alcock and Brown to fly non-stop from Newfoundland to
In 1927 Lindbergh flew non-stop from New York to Paris, Spaatz
and Eaker kept the Question Mark aloft, for over six days, finally
demonstrating that during the twenty-three years following the
advent of powered flight, the aircraft power plant had become
dependable. A new transportation system could now emerge; the
creation of a new military arm was destined; a new way of life was
to open.

* Major Carl Spaatz and Captain Ira C. Eaker. Later both of these officers became
generals of the United States Air Force. General Spaatz became its first Chief of
Staff. From 1948 to 1956 he was Chairman of the National Board of the Civil Air




Starter; Carburetor, Magneto, Propeller--Links in the chain of power.

Power Plant Systems
You have heard people call the airplane power plant an engine or
perhaps a motor. As a matter of fact the engine is only one of the links
in the chain of power. Sometimes the electric motor is another such
The engine proper changes heat energy jnto mechanical energy.
In order to do this it must be assisted by a carburetion system which
supplies fuel; by an ignition system which kindles fuel; by a lubrication
system which reduces friction of its moving parts. To initiate the
operation of a modern aircraft engine requires a starting system. To
maintain effective operation of the engine requires a control system

and enqine instruments. Moreover, the power generated by a
reciprocating engine is productive only when harnessed to an
airplane propeller or helicopter rotor by a transmission system.
The aircraft engine is the most important of the power plant
essenrials, but other power plant equipment also requires study
and understanding. Furthermore, beyond the engine must be a
source of potential energy--a liquid fuel, such as gasoline; an
unstable compound, such as gun-powder; or an element, such
as uranium-238, capable of atomic fission.

So that you can better understand how heat energy is converted into
mechanical energy and how such energy is put to work, it is necessary
to learn about some simple machines. Some of these were discovered
long ago. Men who lived in the dawn of civilization used them to increase
the force they could apply with their muscles. Other men in later ages
refined the early machines, using the principles of these machines to
develop more complicated devices. Today we use the same basic
principles to increase and to apply the force an engine generates. You
will observe devices based upon them in use within hydraulic and
electrical systems. In the aircraft power plant, they are found in rather
complicated combinations. Sometimes in their adaptations, they are hard
to recognize..There are six of these devices.
They are the lever and fulcrum, the wheel and the axle, the pulley, the
wedge, the inclined plane, and the screw.

Wheel and Axle
The wheel and axle may be exemplified by the steering wheel of
an automobile and the shaft to which it is attached. Any twisting
force applied to the outside of the steering wheel produces a greater
twisting force on the shaft. On occasion, a single spoke of a wheel
is used to obtain a similar result. The wheel then becomes a crank,
b u t t h e p r i n c i p l e r e m a i n s t h e s a m e . C o n s e q u e n t l y, a c r a n k s h a f t i s
an adaptation of the wheel and axle.
The Pulley
Yo u u s e a n a d a p t a t i o n o f t h e p u l l e y w h e n e v e r y o u a d j u s t t h e
drapes over the windows of a room. Other adaptations of this
principle are employed whenever men sail ships, or operate the
controls of airplanes.

A gain in force means a loss in distance.
The Lever and Fulcrum

The child's teeter-totter on the playground is an example of the
lever and its fulcrum (pivot-point) with which all are familiar. Many
tools, in everyday use, illustrate this principle. The pinch-bar with
which the railway worker can move a freight car is an example of
a lever with a built-in fulcrum. A nutcracker, a pair of pliers, a
wheelbarrow—all are adaptations of the lever for specific
purposes. A gear may be regarded as a spinning lever.

Inclined Plane, Wedge, and Screw
The inclined plane, the wedge and the screw are closely related
devices. When a truck driver wants to load a barrel of oil, he lays
a plank from the edge of the truck-platform to the ground. He then
rolls the barrel up the inclined plane thus formed.
The wedge is a movable inclined plane. In the instance in which
the barrel was moved up the inclined plane, force was applied to

the barrel. When the wedge is used, the initial force is applied to
the wedge which in turn transmits this, magnified, to the object to
be moved.
The screw may be regarded as an inclined plane wrapped around
a shaft. Wood screws, cork screws, bolts and other familiar devices
employ the principle of the screw. The aircraft propeller also makes
use of this principle. In fact the English call an aircraft propeller an
You will recognize characteristics of the six simple machines in the
mechanisms of power-plant systems. As a matter of fact, to gain
mechanical advantage, these six simple machines each employs the
same, single, fundamental principle. The object of each machine
is to accomplish work. In such instances, work is defined as force
times distance. When one of these machines is used to increase
a force, that force cannot move as for as the original smaller force
moves. The force applied by the man who used a pinch bar to
start the railway car rolling may have moved through a distance of
three feet; the force transmitted to the car in this instance may have
moved through no more than a distance of one inch. Under such
circumstance a pressure of 100-pounds at one end of a five-foot bar
w o u l d b e t r a n s m i t t e d a s a p r e s s u r e o f 6 , 0 0 0 - p o u n d s a t t h e o t h e r.
Yo u w i l l n o t e t h a t t h e g a i n i n f o r c e m e a n t a l o s s i n d i s t a n c e . ( S e e
illustration, page 5.)

Another characteristic of energy is that although it cannot be
created, nor destroyed, it can be transformed. Heat energy can
be changed into mechanical energy; mechanical energy and
chemical energy into electrical energy; electrical energy, in turn,
into chemical, heat, or mechanical energy.


V A L V E - -




The Engine and Its Job

The engine releases stored energy.

Potential and Kinetic Energy
The first task of any engine is to convert potential energy into
kinetic energy. Energy is the capacity for performing work.
Potential energy is stored energy---energy ready to act. Kinetic
energy is energy in action. The one can be illustrated by a boulder
poised on the edge of a canyon; the other by the moving boulder
crashing down a canyon wall after its balance has been disturbed.
Energy cannot be created nor destroyed, but potential energy can
be converted into kinetic energy. A mixture of a fuel, such as
gasoline and air, has potential energy. When this mixture is ignited,
the combustion generates gases that have kinetic energy and
perform work.

The Combustion Chamber
The first task of any engine is to release the potential heat energy
of a fuel. In order to transform the potential of a fuel into heat;
a n e n g i n e e m p l o y s a c o m b u s t i o n c h a m b e r. F u e l i s i n t r o d u c e d i n t o
this chamber, is ignited, and burned there. An external combustion
engine, such as a steam engine, burns its fuel outside the engine
to change water into steam under pressure. This steam is used to
operate the engine's movable parts--piston or turbine. The engine
with which we are most greatly concerned is called the internal combustion engine. Airplane engines, whether reciprocating, jet, or
turbo-prop are internal combustion engines and burn their fuel within
an integral part of themselves.

Heat and Mechanical Energy
The second task of any engine is to transform heat energy into
mechanical energy. As far as the internal combustion engine is
concerned, some relationships discovered by two scientists who
lived many years ago help us to understand how this is done.
Robert Boyle, who lived about the middle of the sixteen-hundreds
(1627-1691) discovered that the volume of a gas kept at a uniform
temperature varied inversely as the pressure varied. That is, when
we double the pressure of a gas, we reduce its volume by one-half,
or if we treble the pressure, we reduce its volume to one-third of the
original volume. Jacques Charles, who lived a century after Boyle,
discovered that the pressure of a confined gas is directly
proportional to its temperature. (lncidentally, Charles in 1783 was
the first man ever to make a balloon ascension.)
Internal Combustion
The internal combustion engine puts to practical use the two
principles discovered by Boyle and Charles. The mixture of fuel and
air inducted into an engine cylinder, or combustion chamber, is
therein compressed, ignited, and burned. It gives off heat which
raises the temperature of the gas in the cylinder or chamber. As the
temperature of the gas increases, it expands and its pressure
increases. The higher its temperature, the more pressure it exerts.
This pressure acts on the head of the piston in a reciprocating
engine, on the vanes of a turbine in a turbine engine, and on the
chamber walls of the jet engine.
Internal combustion engines are classified in a number of ways.
Reciprocating engines, turbine engines, and ram engines depend
for their operation upon oxygen contained in the atmosphere.
Rocket engines carry their own oxygen, hence are independent of
the atmosphere. There are over-lappings of engine classifications.
For example, a turbo-prop and a turbo-jet are both turbine engines.
Yet a turbo-prop is a combination, external-internal reaction engine
in that some of the power it generates is transmitted to its propeller
and some of it is converted into direct thrust.
Ram engines and rocket engines are internal reaction engines, yet
ram engines are atmospheric-dependent while rocket engines are


It appears likely that reciprocating engines will be used as the
principal element of light airplane and helicopter power plants for
many years to come. However, the turbo-jet engine, which in some
respects is less complicated than a reciprocating engine, appears to
be rapidly replacing the latter as an aircraft engine, particularly in
military aircraft. In any event, a general knowledge of reciprocating
engines, as background information, will help you better understand
current research and development in aircraft propulsion.
The most simple reciprocating engine consists of a cylinder, a
piston, a connecting rod, and a crankshaft. The ends (journals) of
the crankshaft are mounted in oiled bearings. At one end of the
crankshaft there may be a fly wheel. If there is no fly wheel, some
other counter balancing device will be used. There are valves or
ports in the cylinder which provide both an entrance for the air-fuel
mixture and an exhaust for the burned products of this mixture.
There is also provision for the ignition of the air-fuel mixture inside
the cylinder.
Cylinder and Piston
The cylinder is a hollow tube closed at one end (the cylinder head).
The piston is a second, shorter, hollow tube closed at one end (the
p i s t o n h e a d ) w h i c h s l i d e s f r e e l y w i t h i n t h e c y l i n d e r. I t s w a l l s a r e
grooved to accommodate rings which fit closely against the cylinder
walls and help seal the open end of the cylinder against escaping

From straight-line motion to rotary motion.

Connecting Rod and Crankshaft
The connecting rod is a straight rod which helps convert the
straight-line motion of the piston to the rotary motion of the crankshaft. It is attached at one end to the piston by means of a wrist pin
and a bearing. It is attached at the other end to the offset portion
(the crank) of the crankshaft by means of a bearing. The crankshaft
is actually an adaptation of one of the simple machines--the wheel
and axle. The moving piston, through its connecting rod, applies
force to the crank in much the same may that the muscles of a
bicycle rider, through his legs, apply force to the bicycle pedal.
The Flywheel
If you have ever ridden a bicycle you will remember that, if you
did not release the pressure on one pedal at about the time you began
t o e x e r t p r e s s u r e o n t h e o t h e r, t h e p e d a l c r a n k w o u l d s t o p a t t h e
l o w e s t p o i n t o f i t s c i r c u l a r p a t h . To p r e v e n t a n e n g i n e c r a n k s h a f t
stopping in this fashion, it is equipped with a flywheel or counterweights. Once the wheel is set in motion and the pressure on the top
of the piston is spent, the inertia of the flywheel or counterweights
will carry the crank past its lowermost position and make possible
the continuing operation of the engine.

Power Stroke
During the fourth event, the air-fuel mixture is ignited and burns.
As this mixture burns, its temperature rises; it expands and drives the
piston downward. This event is called the power stroke. Throughout
the stroke, both valves remain closed, as they did throughout the
compression stroke.






Five events happen during the four-stroke cycle.

The Four-Stroke-5-Event-Cycle-Engine
A cycle is a series of events that repeats itself. An engine cycle
i s t h e s e r i e s o f e v e n t s p e r f o r m e d b y a n e n g i n e d e l i v e r i n g p o w e r.
When the term stroke is used in relation to the engines, it means
the distance a piston travels from top dead center (TDC) to bottom
d e a d c e n t e r ( B D C ) . To p d e a d c e n t e r i s t h e p o s i t i o n o f t h e p i s t o n
just before it begins its downward stroke; bottom dead center is the
position of the piston at the completion of this stroke. Although some
engines complete a cycle of events in two-strokes, reciprocating aircraft-engines employ four strokes to complete a five-event series.

Intake Stroke
The first event of the engine cycle is the intake stroke. During
this stroke, the fuel mixture is admitted through a valve opened
by a projecton (a cam) on a shaft (cam shaft) operated by gears
driven by the engine crankshaft. At the completion of the intake
stroke, the cam shaft has revolved sufficiently to allow a spring to
close the valve.
Compression and Ignition
The second event is the compression stroke. The rotation of cranks h a f t f o r c e s t h e p i s t o n t o w a r d t h e t o p o f t h e c y l i n d e r. D u r i n g t h i s
stroke, both intake and exhaust valves are closed. As the piston
approaches the top of the stroke (TDC) the fuel-air mixture is ignited
by an electric spark. This is the third event--the ignition event.

Exhaust Stroke
The fifth event is the result of the exhaust stroke. The exhaust
valve is opened by action of the cam shaft, or other cam operating
mechanism. The moving piston then forces through the exhaust valve
opening the burned and burning gases left in the cylinder at the end
of the power stroke. This leaves the cylinder ready to begin the
cycle all over again. As long as the engine remains in good mechanical condition and unless fuel supply or ignition switch is turned off,
each event of the cycle will take place in proper sequence.

Instead of valves, the two-stroke-cycle engine makes use of
openings in the cylinder called ports. These are opened or
closed as a result of piston movement. The piston head is
constructed so that the exhaust port is opened before the intake
port is opened and is closed before the intake valve is closed.
During the first part of the compression stroke, both intake and
exhaust events occur. The compression event takes place during
the second part of this upward stroke. An arrangement of fuel
intake and crank case makes it possible for the compression
stroke also to draw an air-fuel mixture into the crank case.
Following the ignition event, the downward stroke—the power
event—occurs. During this stroke, action of the piston also builds
up pressure in the crank case which helps to force the fuel-air
mixture through the intake part. Although the two-stroke-cycle
engine is sometimes used in aviation to operate related and
auxiliary equipment, in the main it has proven of no practical
value as an aircraft power unit.

M o s t d i e s e l - e n g i n e s e m p l o y t h e f o u r - s t r o k e c y c l e . H o w e v e r, n o
fuel is mixed with air drawn into the cylinder during the intake
e.vent. The fuel is injected info-the cylinder under very I~gh pi~es~
sure as the piston approaches TDC on the compression event. You
will remember that Boyle's law states that the pressure of a gas is
inversely proportional to its volume, if its temperature is held constant.
However, under ordinary circumstances, it is not possible to compress
a g a s w i t h o u t r a i s i n g i t s t e m p e r a t u r e . C o n s e q u e n t l y, a t t h e e n d o f
the diesel-engine compression stroke, the compressed air in the
cylinder is hot enough to ignite the injected and atomized fuel.

Different engine designs for different engine uses.

Therefore, th,~e no.spark plugs. The diesel compresses the air within its cylinders to approximately one-half the
volume of the compressed fuel-air mixture of the conventional fourstroke cycle engine, pressures and high tem~qture of the
d i e s e l - e n g i n e r e q u i r e t h a t i t b e o f h e a v y c o n s t r u ~ : ~ C o n s ~ n t l y,
"7~ ~ , ' , . . . . - . . ~ . ~ , . . . . . . . . . . . . . . . . . . . . . . . . . ." ~
. .
since an aircraft en~n_e must J~e of comparahvely light we,ght, little
use is made of the diesel as a power~lant unit for~

S i n c e a i r c r a f t w h i c h a r e b u i l t f o r d i ff e r e n t p u r p o s e s d i ff e r f r o m
one another in design, so the engines of each type of aircrafi are
l i k e l y t o d i ff e r i n d e s i g n . F o r e x a m p l e , t h e c y l i n d e r s o f a n a i r c r a f t
engine may vary in number and in plan of arrangement. T,.be~.m~to_
common ~m~hods of arranging cylinders of aircraft engines ar~,_j~line, V-type, opposed, single-row radial, and multi-row radial.
.... .
. -,
A n a i r c r a f t e n g i n e m a y. b e h q u l d - c o o l e d o r a i r - c o o l e d . I n - h n. e
a n d V- t y p e e n g i n e s a r e l i q u i d - c o o l e d . R a d i a l - t y p e e n g i n e s a r e a , r , . . . . . . . . . . . . . . . . . . . . ~."°" ~ ~'~, '~C",
cooled,. Excessive engine heat is undesirable. If the eng,ne cyhnder
temperature becomes too great, pre-heating and pre-ignition of the
fuel-air mixture will take place. Such premature combustion will
cause detonation, or knocking, which in turn affects the eng!ne_e~adv eer~-~xc~e~'~h~i'h'e~'temperatures may weaken heat-treated, airp l a n e p a r t s o r d e s t r o y t h e e ff e c t i v e n e s s o f l u b r i c a t i n g o i l . C o n s e quently, the cooling system is an important part of the aircraft power


The cylinders of a radial engine branch out from a center in the
same way that the spokes of a wheel spread out from an axle.
Radial engines always have an odd number of cylinders. A singlerow, radial engines may have five, seven, or nine cylinders. Multirow radial engines have two or more banks of seven or nine
cylinders each.
From the outer surface of cylinders and cylinder heads, thin metal
fins project. Air moving over and around these fins carries away
excessive heat. A system of flaps {cowl flaps) hinged to the engine
cowling (metal cover) is used to control this air flow.and
consequently, to regulate engine temperature. Although the
principles of operation are the same for all reciprocating engines
used on modern aircraft, different engines may use different
operating devices. For example, rather than a crankshaft with an
offset for each connecting rod, a single-row, radial engine uses a
crankshaft employing one crank with balancing counterweights. A
master rod is attached to the crankshaft, and all other connecting
rods are attached to the master rod. Also the radial engine instead
of a cam shaft must use a cam ring.
Those who design and build reciprocating aircraft engines are
influenced in their work by several important, general, aircraftengine requirements. The weight of the engine must be kept as low
as possible so that a maximum useful load may be carried by the
aircraft. It should be built as inexpensively as possible without
sacrificing efficiency of performance. It should have a long

life, a low fuel consumption at cruising speed, and ability to run
smoothly and perform adequately at all its speeds under all
variations of atmospheric conditions.

the last few years. Such research has discovered heat resisting
alloys, new fuels, new lubricants, and more efficient designs for
jet and rocket engines.


Principles of Jet Propulsion
Jet propulsion is based upon Newton's second and third laws of

The principal advantage of jet-propulsion of aircraft is speed.
Another advantage is its comparative simplicity. Air drawn into the
front of the jet engine is compressed, fuel is added, and the fuel-air
mixture set on fire. The ignited gases expand and discharge from
the rear of the engine. The reaction to this discharge moves the
airplane forward. The propeller of the reciprocating engine creates
pressure differences by pushing the air backward; the expanding
gases of the jet create pressure differences which move the
aircraft forward as it blows the heated air mixture backward.

Jet and Rocket Development
The idea of the reaction engine is almost as old as recorded
history. It is said that as early as 100 A.D., Hero of Alexander
invented a device known as the Aeropile, which used the
principle of jet propulsion. It consisted of a hollow sphere with
nozzles on its surface mounted on an axle between two supports.
When steam under pressure was introduced into the sphere and
allowed to escape through the nozzles, the reaction caused the
sphere to spin between its supports.
Centuries later in 1849 a man named Charles Golightly was
granted a patent on an aircraft to be powered by a jet engine. It
never got out of the design stage. The first jet propelled aircraft
that actually flew was developed by a man named Campine, an
Italian. Air Commodore Frank Whittle, of the R.A.F. (Royal Air
Force), designed the first turbo-jet powerplant. An aircraft
powered with this engine flew successfully in 1941. However, it
was not until the Germans powered the buzz bombs which they
used against England in World War II with a pulse jet engine that
any practical use was made of jet propulsion.
Recent progress in jet and rocket propulsion has been the result
of research and development efforts made by many people

motion. The recoil of a gun or the reaction of a deflating toy balloon
demonstrate in principle these laws of motion. One way to state
Newton's second law of motion is to say that the force acting upon
a body is equal to the mass times the acceleration of the body.
One way to state Newton's third law is to say that for every action
there is an equal and opposite reaction.
Another way to explain the operation of the jet is to say that its
forward motion and that of the aircraft to which it is attached is a
reaction to the force necessary to blow the burning air-fuel mixture
out of the engine. This force is equal to the mass of the air blown
out times its change of speed in relation to the jet engine.
A common application of jet propulsion is the rotating water
sprinkler. Water under pressure enters the hollow shaft of the
sprinkler which serves as an axis for the rotating sprinkler head.
The water flows to the nozzles and, because of pressure, is ejected
from each nozzle at high velocity. As a consequence, thrust-forces
develop at each nozzle and cause the sprinkler head to rotate.
The turbo-jet employs a compressor which is somewhat like the
turbo-supercharger used on some reciprocating aircraft engines. Its
purpose is to compress the air which it acts upon much as a piston
of a reciprocating engine compresses the air introduced into its
cylinders, reducing its volume and increasing its density and
Guide vanes of the turbo-jet direct the course of the air into a
compressor. From the compressor the air is discharged into the
diffuser channels of the compressor casing. These channels slow
down its velocity, increase its static pressure, and distribute the air
evenly through adapters into the turbo-jet's combustion chambers.
The combustion chamber is equipped with removable liners. The air
enters these through a series of holes, mixes with the fuel
continuously injected into the combustion chamber, and is ignited.
The resulting combustion causes heat which raises the temperature
of the gases. The hot expanding gases escape with great force
from the

A turbo-jet powerplant.

combustion chamber liners through the curved blades of the
nozzle diaphragm and strike the curved blades of the turbine
wheel, causing it to rotate.
The turbine wheel is attached to a shaft which drives the impeller
wheel of the compressor. The operation of the turbine wheel
takes much of the energy of the expanding gases. The remaining
energy is transformed into forward thrust. One type of turbo-jet
engine has an afterburner where extra fuel is injected into the
escaping gases and ignited, giving added thrust to the engine.

Aircraft have been successfully powered by rocket engines.
However, research must yet solve many engineering problems
before such engines are generally used either in military or civil
aviation. The use in aircraft operation of the tremendous power
released by atomic fission is also contemplated. Research
projects are now underway which very likely will produce an
"atomic powered" aircraft. Again many practical problems confront
research engineers and manufacturers who are working on these
projects. An interesting experiment is also underway which would
employ the jet principle to obtain lift as well as thrust, making the
airplane wing unnecessary. An aircraft powered with this kind of jet
engine would be a flying fuselage.
Whether or not engine projects now in the experimental stages
prove practical, we can be certain that in the course of time
research and development will produce new types of aircraft
powerplants based upon newly discovered principles and


upon auxiliary equipment for proper operation; consequently, even
the mast simple carburetion system includes fuel tank, fuel line, strainers, fuel pump, and manifold. Gasoline from the fuel tank enters the
carburetor through the fuel line. After the carburetor vaporizes the
gasoline and mixes it with air, the manifold directs proper amounts
o f t h e m i x t u r e i n t o e a c h c y l i n d e r. M o r e c o m p l e x c a r b u r e t i o n s y s tems include superchargers which keep the manifold air pressure
proper for engine-operation at high altitudes.

Yo u h a v e l e a r n e d t h a t t h e i n t e r n a l c o m b u s t i o n e n g i n e , w h e t h e r
reciprocating or jet, makes use of the fact that when a gas is heated
it expands. A fuel-air mixture enclosed in the cylinder of a reciproc a t i n g e n g i n e e x p a n d s i n o n e d i r e c t i o n o n l y, a g a i n s t t h e y i e l d i n g
piston. A fuel-air mixture entering the combustion chamber of a
jet engine also expands in one direction, but the force of this expansion imparts movement to the engine itself. Before fuel can
burn and release its potential heat energy, it must reach the engine
cylinder or combustion chamber. Either a carburetion system, a fuel
injection system, or some combination of these two introduces the
fuel required for the engine's operation.

task of the carburetor is to meter (measure) the fuel. In the float
type carburetor this measuring begins as the fuel enters the float
chamber. A needle valve is operated by a float in the float
chamber. When the level of the fuel in the float chamber rises,
the float rises. As it rises, the needle attached to a level (the
lever-fulcrum principle) lowers. When the proper fuel level is
reached, the lowering needle cuts off the fuel flow through the
The second task of the carburetor is to atomize the fuel (break it
into tiny particles and mix these with the air). To do this task the
carburetor uses a fuel nozzle, an air intake, a venturi, and a
throttle valve. (The illustration below shows these in their relation
to each other and to other parts of the carburetion system.)
The principal parts of a float type carburetor.


A carburetor is used with reciprocating type engines that do not
employ the fuel injection system. The carburetors first used by early
aircraft engines were of the same type as those used by automobile
engines. The carburetors used on modern reciprocating aircraft
engines are still similar in principle to early carburetors.
However, the modern aircraft engine carburetor is much more
complex than the automobile carburetor, since it must function
regardless of the attitude an aircraft assumes while in flight.
Aircraft carburetors may be classified as float type, diaphragm type,
and injection types. All types of carburetors like all engines depend





When the piston moves downward during the intake stroke, a
partial vacuum is created within the cylinder. The decreased
pressure occurring in the cylinder as a result of this action causes
several things to happen. To fill the partial vacuum created, the
fuel-air mixture from the manifold portion of the carburetion system
rushes into the cylinder. Air from outside the carburetor is drawn by
this lowered pressure into the air intake and through a constriction
in the carburetor barrel called the Venturi section. The air passing
through this narrow part of the carburetor barrel has to move faster
than the air passing through the other sections of the barrel.
Because of this, it has to use more of its energy for speed and as a
result has less for pressure. (You remember the relationship of the
velocity and pressure of gases.) Since the fuel nozzle enters the
carburetor barrel at the Venturi section, when the pressure in this
section lowers, the higher atmospheric pressure in the float
chamber to which the fuel nozzle is connected causes the fuel to
spray through the nozzle into the barrel of the carburetor. The
butterfly valve, as it is opened or closed by throttle action, controls
the amount of the fuel-air mixture entering the manifold for
distribution to the cylinders.
In addition to the carburetor parts whose functions have been
described above, the modern carburetor also requires an air bleed,
an idling system (idle air bleed and idling jet), an accelerating
pump, an economizer valve, and a mixture control. The air bleed
permits air to be drawn into the discharge nozzle along with the
fuel. By forming air bubbles in the fuel, it helps break the fuel into
small particles as it is discharged through the nozzle. The idling
system allows fuel to enter the carburetor barrel above the throttle
valve (butterfly valve) so that, even with the throttle closed, a fuel
supply sufficient for engine idling operation will be provided.
When the throttle is opened quickly and air and fuel are drawn into
the carburetor, the heavier fuel does not move as rapidly as the
lighter air. Consequently, the fuel-air mixture may become so lean
(less fuel, more air) that the engine will not operate. The
accelerating pump, under these conditions, forces extra fuel into
the Venturi section. The economizer valve works in cooperation
with the accelerating pump. At cruising speeds it remains closed.
When the throttle is opened wide, its needle is raised and more
fuel is permitted to flow

through the fuel nozzle. The mixture control keeps the amount of
fuel flowing through the discharge nozzle in proper relation to the
density of the air through which the aircraft flies.
You will remember that air at sea level is heavy enough to exert a
p r e s s u r e o f 1 4 . 6 9 p o u n d s p e r s q u a r e i n c h . H o w e v e r, t h i s p r e s s u r e
decreases as an aircraft increases its flight altitude, so that when an
aircraft flys at 15,000 feet above sea level, the pressure exerted upon
it is only half this amount. The mixture control makes it possible to
reduce the amount of fuel flow to correspond to the weight of the
air intake at high altitudes. (See illustrations, page 21.)



A diaphragm mechanism controls fuel flow.

When the float system carburetor is used, any unusual change in
the attitude of the aircraft will cause fuel to slosh around in the fuel
c h a m b e r, i n t e r f e r i n g w i t h p r o p e r f u e l m e t e r i n g . To p r e v e n t s u c h
malfunctioning, aircraft engines are often equipped with the diaphragm type carburetor. The Venturi on this type carburetor is made
of two parts shaped like half cylinders. These can be rotated to close
the Venturi opening; consequently, no throttle valve is necessary.




The diaphrams are made of metal disks set in larger, tough fabric
disks. In the diaphragm type carburetor, diaphragms are placed one
o n e a c h s i d e o f a c y l i n d r i c a l f u e l c h a m b e r. T h e y a r e j o i n e d t o gether, yet held a certain distance apart, by a metal spring. Levers
connected to them operate the fuel inlet valves. Fuel entering through
these valves fills the chamber and, pressing against the diaphragms,
forces them apart, thus closing the inlet valves. As the quantity of
the fuel in the chamber is used and decreases, the diaphragms are
pulled together by the action of the spring connecting them, thus
opening the inlet valves. (See the illustration, page 23.)

The Injection Carburetor
There are two great problems which the aircraft engine
carburetion system must solve. One is to assure a constant
flow of fuel; the other is to prevent ice formation in the
carburetor. For the ordinary carburetor is a miniature
refrigerator. It changes the liquid fuel into a gas--vaporizes it.
To vaporize a liquid requires heat. This heat is taken from the
surrounding air. The decrease in air temperature which results
may amount to 30° or 40° F. The lowered temperature causes
the condensation and perhaps the freezing of water vapors
which may be present in the air passing through the Venturi.
Fuel injection helps prevent carburetor icing.








A turbo-jet fuel system.
T h e r o t a t i n g Ve n t u r i m e n t i o n e d a b o v e h e l p s p r e v e n t i c e f o r m a tion. Another effective method is to use the injection system of
i n t r o d u c i n g t h e f u e l i n t o t h e c a r b u r e t o r. T h r e e p r i n c i p a l f e a t u r e s
set this carburetor apart from the ordinary carburetor: 1. Fuel under
pressure from a fuel pump enters the fuel section of a cleverly des i g n e d f u e l r e g u l a t o r. 2 . T h e f u e l r e g u l a t o r, b y u s e o f a s m a l l
booster Venturi and a rod-diaphragm assembly, controls the amount
of fuel injected into the carburetor. 3. Fuel enters the carburetor not
at the Venturi throat, but beyond both this constriction and the throttle
valve. (See the illustration, page 24.)

The fuel injection system discharges fuel in the form of a sprayjet into the intake manifold at the intake valve. Fuel is pumped
into fuel jets by a plunger-cylinder arrangement activated by a
gear and a cam operating off the main cam shaft. Obviously, the
operation of the system must be synchronized with the intake
event of the reciprocating engine.
The fuel injection system eliminates completely the problem of
ice formation, since fuel vaporization takes place at the intake
valve where temperatures are normally quite high. The system
also reduces fire hazards, is reported to give more power per unit
of fuel, and functions equally well regardless of the attitude of the
plane. Its principal disadvantage stems from the fact that when
the system is adapted to a many-cylindered engine it is very
complex. The jet engine uses a fuel-injection system, with these
differences: 1. Fuel is injected directly into each combustion
chamber. 2. The fuel flow is continuous.

A supercharger is a type of air-compressor. Its purpose is to pump
enough air into the carburetor or engine to assure efficient engine
operation. You will remember that as altitude increases, air
pressure decreases. This means that air at high altitude weighs
less than air at low altitude. As a matter of fact, while at sea level
13 cubic feet of air weighs about one pound, at 18,000 feet it
takes 26 cubic feet of air to weigh one pound. At 36,000 feet of air
it takes about 52 cubic feet of air to weigh one pound.
Before an engine operating at 18,000 or 36,000 feet can attain
sea-level operating efficiency, the supercharger must compress
the fuel-air mixture as it enters the manifold, the air as it enters
the carburetor intake, or both. All superchargers operate on the
same principle. Revolving blades press the air into the
carburetion system and consequently build up manifold pressure.
The turbo supercharger impeller (or blower) is operated by a
turbine driven by the exhaust gases from the engine. One type of
supercharger is driven by means of shafts geared to the engine
crankshaft. A method much like the transmission system of the
modern automobile is used to drive the supercharger-impeller of
the modern reciprocating engine. This method employs a fluid
rather than a gear assembly as the driving mechanism.
Like the impeller of a turbo-jet, the impeller of the supercharger is
driven by a turbine.



After the fuel-air mixture drawn into the engine combustion chamber is compressed, it must be ignited, if the sequence of the engine's
operating events takes place. That is to say, if the gases taken into
the cylinders or combustion chambers are to expand, they must be
set on fire. Of course, if a gas compressed is sufficiently reduced in
volume, its temperature will rise high enough to ignite the vaporized
fuel which it contains. The diesel engine uses this method of ignition.
However, in aircraft engines most commonly used, it is found best to
ignite the fuel-air mixture by means of an electric spark.

Starting the Engine
Before the operating cycle of an engine can get under way, some
force outside the engine must be applied to "turn" the engine over.
One way of starting the small engine of a light plane is by
"swinging the prop." This method, which is rapidly being
discontinued, requires that the propeller be used (as the crank of
the ear,automobile was used) to start the engine cycle operating.
As a matter of fact, detachable cranks are still used in starting
some types of aircraft engines. One method of starting aircraft
engines employs the crank directly; another method uses the crank
to assist in inertia starting. In direct starting, the crank turns a
clutch which engages the rear end of the crankshaft. In inertia
starting, the crank turns a flywheel, causing it to rotate at high
velocity. When the flywheel reaches o proper high speed, it
engages the crankshaft, also by means of a clutch arrangement.
The energy of the spinning flywheel then rotates the engine several
times, putting, it into operation. However, as the application of
electricity provides the most satisfactory ignition system, so does
the application of electricity provide the most satisfactory aircraft
engine starting systems.

Some Essential Facts About Electricity
To help you understand the operation of aircraft engine ignition
systems and starting systems, it is necessary to explain some
simple facts that will help you understand how electricity is
generated: 1. A magnetic field composed of magnetic lines of
force surrounds any magnet.
2. A current flowing through a conductor sets up a magnetic field
around that conductor.
3. Magnetic lines of force (or lines of flux) take the path of least
resistance through a conductor.
4. When a conductor of electricity (a wire) is moved through a
magnetic field, a current will flow through the conductor.
5. If the magnetic field is moved across the conductor, the effect
is the same as that which occurs when the conductor is moved
through the magnetic field.
The generating of electricity for any purpose makes use of these
facts. Electric generators produce electric currents either by
moving properly arranged conductors through a stationary
magnetic field or by moving a magnet so that the magnetic field
moves across the stationary conductors.

Igniting the Air-Fuel Mixture
Ignition systems are of two types: battery and magneto. Each
system has a source of electrical energy, an induction coil, an
interrupter, a condenser, a distributor, wiring, and control
switches. Both systems are shielded to reduce electrical
interference with radio communication. Modern aircraft engines
are equipped with dual magneto ignition systems which assure
both greater variety and greater efficiency of engine operation.
However, on aircraft using magnetos, the source of electricity for
purposes other than ignition is provided by electric generators
and batteries.
The Magneto
A magneto is a special kind of generator. As it operates, magnets
are rotated between "pole shoes" by means of an accessory shaft
geared to the engine crankshaft. The pole shoes are joined by a
core around which are wound the primary and secondary coils.

The primary and secondary coils of the magneto serve as the
induction (or spark) coil. An induction coil is really a transformer
which steps up the voltage (electric pressure) so that it is great
enough to cause a spark to jump between the spark plug
electrodes (wires).
When the magnets turn, the lines of flux of their magnetic fields
move along the path of least resistance, the pole shoes and the
soft iron bar which joins them. As a consequence, a voltage is
introduced in the primary coil--the comparatively few turns of
heavy copper wires wrapped around the bar between the pole
shoes. Interrupting the flow of electricity through the primary coil
induces a voltage in the secondary coil. (See Essential Facts
About Electricity, p. 29.) The magnitude of the voltage induced in
the secondary coil depends in great part upon the comparative
number of turns of wire that it has. If a primary coil which has 50
turns of wire produces 12 volts, its secondary coil which has 500
turns will produce 120 volts. Since the average magneto
produces 20,000 volts, the secondary coil of such a magneto will
be made up of thousands of turns of wire.
Remember that a changing current through one coil can induce a
current in a second coil only when wires of the secondary cut the
lines of flux in the magnetic field of the primary. Consequently, a
current will flow in the secondary circuit only when the current in
the primary circuit is building up from zero to its maximum or
collapsing (when the primary circuit is broken) from its maximum
to zero. This explains the purpose of the interrupter or circuit
breaker. Its action makes it possible for the secondary windings
to be cut the lines of flux of the primary coil as these build up or
collapse. (See illustration, page 31 .)
One type of magneto, the low tension magneto, does not "step
up" the voltage of the primary circuit. When this type of magneto
is employed, an induction coil (secondary coil) is used with each
spark plug to obtain the voltage needed for the "spark-jump."
The Condenser
The purpose of the condenser of an ignition system is to store up
o r a c c u m u l a t e e l e c t r i c i t y. C o n d u c t i n g b o d i e s w h i c h a r e s e p a r a t e d
by insulation are able to do this. Consequently, condensers may be
m a d e o f a l t e r n a t e l a y e r s o f a m e t a l l i c f o i l a n d w a x e d p a p e r, r o l l e d

The magneto is o special kind of electricity-generator.

into a cylindrical shape and inclosed in a moisture-proof container.
A condenser does not provide a path for the transmission of an
electrical current. Its chief purpose is to help make the collapse of
the primary circuit instantaneous and, as a consequence, increase
the voltage produced by the secondary coil. That is to say, it helps
build up voltage for the "spark-jump."

The Distributor
From the secondary windings the current passes to the distributor.
The distributor is a revolving contact point which passes over a
circle of other stationary contact points. There is one of these for
each engine cylinder. As the revolving contact point passes over a
stationary contact point, a spark jumps the gap between the two
electrodes of the appropriate spark plug. It is important that the
timing mechanism of an engine is adjusted so that this ignition
event occurs in each cylinder at the proper time. (See illustration,
above.) It is necessary to supplement the magneto with a booster
coil which furnishes sparks for starting the engine. Until the engine
rpm builds up, magnetos cannot furnish the necessary spark.

Jet Ignition
The ignition system of the jet engine is much less complicated than
that of the reciprocating engine. Spark plugs are used to ignite
the fuel-air mixture in two of the combustion chambers, as the engine
is started. Cross ignition tubes connecting these with other combustion chambers ignite the fuel-air mixture of the remaining combustion chambers. A battery is used as the source of voltage. An
induction coil steps up the voltage to produce the essential spark.

Electric Starting
Electric starting uses an electric motor whose source of
electricity is produced by a generator and stored in an electric
battery. As a matter of fact, an electric motor is a generator
operating in reverse.
Mechanical energy converted into electrical energy can be
recovered as electrical energy is converted back into
mechanical energy. The device that generates electricity can,
by using electricity, generate mechanical energy.
The mechanical energy of the electric starting motor can be
applied directly or indirectly. In this latter instance it turns the
flywheel of the inertia starter which engages the crankshaft.
However, direct electric starting is the most widely used of all
starting systems.
Solenoids (coils with moveable cores) are used
both as starting switches and as mechanical hands to mesh
the starter gears of motor and crankshaft. (See illustration,
The turbo-jet employs an electric starting system which
operates the turbine wheel and compressor until necessary
air-fuel density is built up in the compression chambers.




Yo u k n o w t h a t t h e i n t e r n a l c o m b u s t i o n e n g i n e i s a h e a t e n g i n e .
This is true whether the engine be a reciprocating, a turbine, or a
jet propulsion type. The principle of operation of the aircraft engine
is based upon the fact that heat energy can be changed into mechanic a l e n e r g y. A n E n g l i s h p h y s i c i s t n a m e d J o u l e d e m o n s t r a t e d m a n y
years ago that it was possible also to transform a certain amount
of work (mechanical energy) into the same amount of heat. The
controlled observations of Joule and other scientists led to the definition of the first law of thermodynamics (the principle of the conservation of energy). The law states that although energy may change
its form, no energy is ever destroyed.

So that calculations involving heat energy and mechanical energy can be
made, it is necessary to establish units of heat and units of work, and to
describe the relation of these one to another. The unit of heat is called
the British thermal unit It is defined as the amount of heat required raise
temperature of one pound of water one
degree Fahrenheit (F). The unit of work is the foot-pound. !t is defined as
the amount of mechanical energy required to move a one-pound weight
a distance of one foot. It has been learned that if all the heat energy in
one B.t.u. could be salvaged it would produce 778 ft./lbs, of work.

Thermal Efficiency
It has been learned that one pound of gasoline has a potential heat
energy content of 20,000 B.t.u. This means thai when we mix one
pound of fuel with fifteen pounds of air and set fire to the mixture,
20,000 B.t.u. are given off. An internal-combustion engine which
can transform all this heat into mechanical energy will be able to
produce from one pound of fuel, 778 x 20,000 or 15,560,000 ft./lbs,
of work. Such an engine would be 100% effecive. Engineers would
say that its thermal efficiency was 100%.
Thermal efficiency is the ratio (numerical expression of
relationship) of the useful work developed by an engine to the heat
energy produced. Unfortunately, no engine yet manufactured can
turn into useful work more than 35% of the heat energy produced
by the combustion of the fuel-air mixture. (See illustration, page
36.) You remember that in the reciprocating engine the heat made
available for conversion to work takes the form of pressure upon
the piston head. The high speed of the reciprocating engine's
operation forces the burned gases out of the exhaust port while
their temperatures are still very high (approximately 2000° F).
About half of the total heat produced during the engine operation is
expended through the exhaust process.
The temperatures (2000° F) resulting as the fuel-air mixture burns
are so high that metals used in engine construction cannot
withstand them. Far this reason the engine must employ a cooling
system. (See Chapter VI.) The cooling system carries away about
20% of the total heat produced during the engine operation.
Friction (the rubbing together of the engine's moving parts) raises
the temperature of the metals involved. This temperature rise is
made possible only by using the nearest source of heat. The
burning gases of the engine cylinders supply an easy source and
ample amount of heat. About 5% of the total heat produced by the
reciprocating engine is expended because of friction.

Mechanical efficiency is the ratio of the ft-Ibs, of work actually delivered to the
propel.ler to the ft.-Ibs, of work produced by the engine. Generally the ratio is
expressed in horsepower and is stated

5000 B.t.u.
or 25% Useful Work

20% Cooling
Where the heat goes.

as the ratio of the useful horsepower to the total horsepower
James Watt established
horsepower as the unit of measurement to describe the power of
his steam engine. He concluded that the largest weight that
could be raised in one minute by a draft horse was 33,000
pounds. Consequently, one horsepower came to be defined at
33,000 lbs./min. (33,000 foot pounds per minute).
in the formula used to calculate horsepower. This fact accounts
for the figure 33,000 in the formula used to calculate horsepower.
You must remember that to find the total horsepower of an
engine, the horsepower of one cylinder is multipled by the
number of the engine's cylinders. The result of this multiplication
is called the indicated horsepower. However, friction not only
causes the expenditure of heat energy, it also causes an
expenditure of mechanical energy. The amount of horsepower
needed to overcome friction

is called friction horsepower. In order to learn how much
horsepower is left to turn the propeller, friction horsepower is
subtracted from indicated horsepower. The remainder is called
brake horsepower.
The relationship between the brake horsepower and the indicated
horsepower, expressed in percent, is the mechanical efficiency of
the engine.
One way to discover the brake horsepower of an engine is to use a
device called a Prony brake. This device uses a flywheel which is
fastened to the shaft of the engine. The flywheel is then clamped
between two blocks. As the engine operates, the flywheel tends to
twist the blocks. This torque (the force of the twisting movement) is
measured on a scale by a pointer attached to the block assembly.
The scale is assumed to record the brake horsepower.

Horsepower Factors
In order to learn how much indicated horsepower an engine will
produce, it is necessary to know a number of things:
1. We need to know the pressure of the expanding gases in the
cylinder. The average value of this pressure expressed in
pounds per square inch, called the mean effective pressure
(m.e.p.), is the value used in the horsepower formula. In order to
find this value, an indicator card (pressure, volume diagram) of
the engine in question is used. This card shows the rise and drop
of pressure during the piston strokes. An approximation of the
m.e.p, value can be determined by taking an overage of pressure
values plotted on the indicator card.
2. We need to know the stroke of the piston (distance of piston
travel) since this is the distance through which the force on the
piston head is exerted.
3. We need to know the area of the piston head, since m.e.p.
is expressed in lbs./sq. in.
4. We need to know how many times the crankshaft turns each
minute (r.p.m.). In the four-stroke-cycle engine, this value must
be divided by two because the power stroke lasts only one half of
one revolution and the crankshaft makes two complete
revolutions for each complete four-stroke cycle.
5. We need to know the number of cylinders of our engine.

By using a formula we can learn from this information the indicated horsepower of our engine.

formula is PLANK/33,000 = IHP
P = m e p., L = the length of the stroke;
A = the area of the piston head, or cylinder bore;
N = the number of r.p.m. divided by 2;
K = the number of cylinders, 33,000 is the weight in Ibs. that can be
raised by 1 HP in 1 min. (See page 36 )

thrust will equal 1 horsepower; if it is 750 m.p.h., one pound of
t h r u s t w i l l e q u a l 2 h o r s e p o w e r. F o r e x a m p l e , a j e t e n g i n e o f 4 0 0 0
pounds thrust develops 4000 horsepower at 375 m.p.h, and 8000
horsepower at 750 m.p.h.
In theory, if a jet propelled airplane was flown in a perfect vacuum,
its forward velocity would equal the velocity of the jet its engine
p r o d u c e d . C o n s e q u e n t l y, u n d e r c e r t a i n c o n d i t i o n s , t h e l e s s d e n s e
the atmosphere through which it flies and the greater the speed at
which it flies, the better will be the jet engine's performance.
The jet engine has a comparatively high rate of fuel consumption.
A jet developing 4000 pound thrust may use as much as 5000 pounds
o f f u e l i n o n e h o u r. O b v i o u s l y, a l t h o u g h r e s e a r c h b r i n g s a b o u t
almost daily improvement in the performance of jet engines, their
thermal efficiency is low when compared with that of reciprocating

Let us assume that the m.e.p, of our engine is 110 lbs./sq. in.; that
its stroke is 3.5 inches; its bore diameter 3 inches; that its r.p.m.
is 2000; and that it has 9 cylinders. Substituting these values for
the symbols in the horsepower formula we have:

After completing the necessary multiplication and division we find
the IHP (indicated horsepower) of our engine to be 735.
The power developed by jet propulsion engines is expressed in
pounds of thrust rather than horsepower. Since the thrust delivered
by the reaction type engine is constant, its power is in proportion to
its speed. If the speed of a jet aircraft is 375 m.p.h., one pound of

Crude petroleum is the source of modern aviation fuels. Petroleum
belongs to a chemical family called hydrocarbons. Hydrocarbons are
formed by the chemical elements, hydrogen and carbon. After the
crude oil is pumped from wells it is piped to refineries where a process
called fractional distillation takes substances from it, such as gasoline,
kerosene, and lubricating oils. Fractional distillation is possible bec a u s e e a c h o f t h e s e d i ff e r e n t s u b s t a n c e s h a s a d i ff e r e n t b o i l i n g
p o i n t , a n d a l l v a p o r i z e a t d i ff e r e n t t e m p e r a t u r e s . C o n s e q u e n t l y,
when crude oil is raised to one temperature, gasoline is evaporated;
when raised to a higher temperature, kerosene is evaporated, etc.
After a substance is vaporized, it is then cooled to its condensation
point and is ready for use.
Fuel for the modern high-speed, high-compression, reciprocating
e n g i n e m u s t m e e t c e r t a i n s t a n d a r d s . Vo l a t i l i t y ( i t s a b i l i t y t o t u r n
readily into vapor) is one of these. Its antidetonation (antiknock)
v a l u e , o r o c t a n e r a t i n g , i s a n o t h e r.
Rapid burning of fuel vapors in the combustion chamber means
not only detonation, but also a sudden rise in pressure followed by

an immediate decrease in pressure. As a result, the average
pressure during the power stroke (mean effective pressure,
m.e.p.) is comparatively low. The octane rating is an index to the
slow-burning quality of a fuel. Slow burning aviation gasolines
have octane ratings from 90 to 120.



How friction is reduced.

When two surfaces move across one another there is a!ways a
resistance to this motion. The resistance is called friction. It causes
the temperatures of the surfaces involved to rise. This rise in heat,
due to the friction of the engine's moving parts not only causes a
loss of thermal efficiency, but also may weaken these parts.
Furthermore, friction also reduces the engine's mechanical
If friction can be reduced, the deterioration of the engine parts which
rub against each other can be prevented, friction horsepower of the
reciprocating engine will be made less, and mechanical efficiency
will be increased. There are three ways of reducing friction.
In practice these are generally used in combination.
1. Since it has been learned that friction is less when the moving
parts in contact are of different metals, it is customary to make plain
bearings of metals or metal alloys which are unlike their journals.
(Journals used in this sense means the shaft, spindle, or rod which
turns within the bearing.)
2. Since balls or rollers offer less resistance than a metal block when
moving across a surface, ball bearings and roller bearings are used
to reduce friction. (See illustration, above.) 3. Since oil placed
between moving surfaces reduces friction, engine lubrication
systems are used on all types of aircraft engines.

The use of oil between surfaces that rub together changes dry
friction to fluid friction. The molecules of fluids (liquids and gases)
are not held so closely together as those of solids; hence, they
slide more easily. Oil molecules are also long and flat, and when
placed between moving parts, they tend to form a layer only one
molecule deep. This is known as a monomalecular layer.
Through experimenting with different kinds of oils (animal,
vegetable, and mineral) it has ben learned that mineral oil
(petroleum) is best for aircraft engine lubrication.
Petroleum is a member of the great hydrocarbon family. There
are different kinds of petroleum. Petroleums from the eastern oil
fields are designated as paraffin-base crudes, those from
California and the Gulf Coast oil fields as asphalt base crudes.
Many different kinds of lubricants and lubricating oils are made
from crude oils. The purpose for which they are made has a lot to
do with their characteristics. For example, the oil you use in your
automobile engine would not serve for your aircraft engine.
Aircraft engine pistons are of aluminum alloy construction; they
operate at higher temperatures than do automobile engines;
these pistons are twice as large as automobile pistons; they
expand more; there are larger clearances between them and
their cylinder walls.
Lubricant Tests
To make sure that an oil is right for a certain type of engine, it is put
through a series of tests. The test measures viscosity, flash point, pour
point, carbon residue, and sulfur content.
Viscosity is used as the measure of ability of an oil to flow at certain
temperatures. (For an explanation of viscosity see Aircraft in Flight, p.
18.) A heavy-bodied oil has a high viscosity. A thin bodied oil has a low
viscosity. The higher the viscosity of an oil, the greater the weight it
can support without breaking. The lower the viscosity of an oil, the less
drag it adds to moving engine parts.
Viscosity is affected by temperature. The S.A.E. (Society of Automotive
Engineers) rating for viscosity is found by heating the oil to

a temperature of 100 C. or 212 F., then finding how long it takes
for 60cc. of it to flow from a container through a hole of a
specified size.
The flash point test discovers the temperature at which an oil
gives off inflammable vapor which will catch fire. The pour point
test finds the lowest temperature at which an oil will flow when it
is cold. The carbon residue test shows the amount of carbon
which is precipitated from oil operating at high temperatures.

The filter removes from the circulating oil any dirt, sludge, or
metal particles accumulated by the oil as it passed through the
engine. The radiator serves to regulate the oil temperature by
dissipating excess heat taken from the engine. On some small
engines the engine crank case serves as both sump and oil tank.
Such engines are sometimes called wet-sump engines; other
engines that have engine oil tanks separate from the engine are
sometimes called dry-sump engines.


The oil system
The oil system consists of an oil storage tank, an oil pump,
How oil gets between moving engine parts.
oil lines, a sump and scavenger pump, an oil filter, and a
Because of the pressure put upon the moving parts of the
aircraft engine, oil cannot flow between them unless forced
to do so. The oil pump is used to feed the oil through the
oil lines to the engine parts. After the oil has been forced
through the engine, it collects: in a sump from which the
scavenger pump (which is actually another oil pump)
forces the oil through a filter and radiator and back into the
storage tank. (See illustration, above.)

Heat has been defined as the motion of the molecules of a
substance. Heat causes the expansion of engine parts; excessive
heat can cause the warping of engine parts; and heat in sufficient
quantities can cause solid metals to melt (become liquid). It is for
these reasons that means must be provided to dissipate the
excess heat absorbed by the parts of internal combustion
engines. As a result of compression and ignition, cylinder
temperatures of an uncooled aircraft engine could reach 4500 F.
Such temperatures would do damage to the engine.
Oil carries off a great amount of excess heat and dissipates this
through the oil temperature regulator radiator. Strange as it may
seem, a rich fuel mixture helps keep cylinders cool. (Remember
what happens in the carburetor when fuel vaporizes.)
Two systems of engine cooling are in general use. One of these
uses a liquid and a radiator. This works much as the cooling
system of your car does. The coolant, generally a liquid such as
ethylene glycol (popularly known as Prestone or by some other
trade name), is circulated by means of a pump through a jacket
which encloses the cylinders and a radiator. Air passing among
the radiator tubes and fins actually carries away the heat.
The other cooling system uses cowlings to scoop up the air and
baffle plates to direct air flow around the cylinder heads and
barrels. The cylinders and the intake and exhaust ports are
covered with fin-like metal projections. These projections serve to
present a greater surface area to the inflowing air and,
consequently, to speed

Liquid cooled and air cooled engines.

up the transfer of heat from the hot metal to the cold air. After
passing around the cylinders, the hot air passes out of the rear of
the engine cowling through cowl flaps. These can be opened or
closed and are used to control the amount of air flowing over the
engine during flight, thus regulating engine temperatures.
Liquid-cooled engines are usually in-line engines of one or another
type, although some radial engines have been liquid cooled
Aircooled engines are always radial or opposed cylinder type
(See illustration, above.) Jet engines are air cooled. Air enters the
airplane through air inlet louvers and flows rearward between a
stainless steel shroud and the hot surfaces of the power plant. In
addition to cooling the engine, this device insulates the engine,
exhaust cone, and tail pipe, and protects the fuselage structure
from the high, exhaust-gas temperatures.
High-speed flight made possible by jet propulsion causes friction
between aircraft and air, resulting in high temperatures which can
be controlled only by a refrigerating system such as commonly
used to preserve foods. (See Aviation and You, page 34.) However,
there is no relationship between the situation causing heat in this
instance and that causing excessive powerplant temperatures.
Jet engines also need cooling systems.





To develop power and to convert this into thrust are the major
purposes of the aircraft power plant. The let engine virtually succeeds
in combining these two tasks into one. In the jet, the potential chemical
energy of the fuel-air mixture is converted into thrust directly.
The reciprocating engine converts the potential energy of the fuelair
mixture into mechanical energy, which is applied through a
transmission system to propeller operation. This difference between jet
and reciprocating engines accounts for the methods of reporting power
output of the two types. We speak of jet performance in terms of
pounds of thrust; and of reciprocating engine performance in terms of

The Propeller
The blade of a propeller is a type of airfoil, just as the aircraft
wing is an airfoil. (See Aircraft in Flight, page 10. Like the rotor of
a helicopter, the propeller, in a sense, is a rotating wing. The
helicopter, of course, uses its rotating airfoil to produce both
vertical motion (lift) and forward motion (thrust). By changing the
pitch of the helicopter rotor as it reaches a certain position during
its turn, a difference of forces is obtained. The resultant of these
forces controls the direction of the helicopter's movement.
The helicopter rotor (a rotating airfoil) can produce both lift and
thrust. The propeller also has a double function: 1. It produces
thrust which overcomes drag and pulls the airplane forward. 2.
The forward movement of the airplane sets up the reaction
between wing and wind and causes lift, which overcomes the
force of gravity. The principle of propeller action is based on the
fact that a decrease in pressure in any direction will cause the air
to produce a force in that direction.
If a propeller blade is cut in cross section at any point, this cross
section view will look very much like an airfoil section of an
airplane wing. If the propeller blade is cut into several sections it
will be noted that while each section retains the appearance of a
wing section, the camber and chord of each section differ one
from another. As a matter of fact, the aircraft propeller is not as
simple in

action as the aircraft wing. Whereas the wing has only one
motion--forward, the propeller has two motions--forward and
Since these two motions take place at the same time, the path of
the motion is shaped like a corkscrew). Also, each segment of the
blade takes a path different from each other segment. Regardless
of the length of a propeller blade, the tip section will travel, as it
rotates, a distance twice as great as a section midway between
propeller hub and tip.
Since the comparative rotary speed is different for every section of
the propeller, the thrust produced by a propeller whose sections all
have the same shape will decrease in value throughout propeller
sections from tip to hub. Consequently, to equalize the thrust
(forward lift) producing qualities of each propeller section, the blade
elements toward the propeller tip decrease in chord length, upper
camber, and angle of attack. (See illustration, below.) Since the
amount and direction of thrust (forward lift) changes with each
propeller element, propellers must be designed to produce a
resultant force in the proper direction and amount.
There are different ways to classify propellers. For example, some
are made of metal and some of wood, and are classified
accordingly. Another classification is based upon whether or not
the pitch of the propeller blade is fixed or variable.

Propeller camber and chord.

Both types of constant-speed propellers use a governor as the
core of the automatic pitch control. As propeller speeds increase, the
flyweights of the governor turn faster; as propeller speeds decrease,
t h e fl y w e i g h t s t u r n m o r e s l o w l y. T h e fl y w e i g h t s , r e a c t i n g t o t h e
centrifugal forces resulting, activate a control rod.
Blade pitch--the propeller's angle of attack.
The Fixed-Pitch Propeller
Blade pitch is defined as the angle made by the chord of the
blade element and the plane in which the propeller rotates. When
this angle is large, the blade takes a deeper bite into the air and is
said to be in high pitch; when it is less, the blade takes a lesser
bite and is said to be in low pitch. During take-off and climb , greater
loads are placed upon the engine than during cruising flight; hence
low propeller pitch is desirable for these operations. At cruising
speed, high pitch is desirable. The fixed-pitch propeller is a compromise. It is neither the best possible for take-off nor for cruising.

The Variable-Pitch Propeller
There are two types of variable-pitch propellers; the adjustable
and controllable. The adjustable is little better than the fixed-pitch
propeller except, before take-off, the pitch may be changed to
meet flight conditions anticipated.
The controllable-pitch propeller can be adjusted by the pilot in
flight. Propellers used on modern aircraft have been improved so
that they are not only controllable in flight, but are also automatic.
They are called constant-speed propellers since they vary their
pitch with varying power requirements of engine loads, keeping
engine and propeller operating at a constant r.p.m.
Constant-speed propellers are of two principal types depending
upon the system controlling their operation. One type uses a
hydraulic system, the other, an electrical system. In both of these
systems the propeller blades are attached to their shafts by
means of bevel gears. Changes in pitch are effected by rotating
these gears.

The Hydraulic Propeller
In the hydraulic propeller, the control rod opens or closes a pilot
valve and also operates an oil pump. By means of its pilot valve
and pump, the governor regulates and varies the oil pressure on
either side of a piston in the dome section of the propeller assembly.
( S e e i l l u s t r a t i o n , b e l o w. ) B y c h a n g i n g t h e p r e s s u r e o n t h e p i s t o n ,
it is moved forward or backward, rotating a cam. The moving cam
turns the propeller blades one direction when the piston moves
forward, the opposite direction when the piston moves backward.

The Electric Propeller
In the electric propeller, the governor control-rod opens or closes
an electric contact switch. Closing the switch completes a circuit
with a reversible electric motor in the propeller hub. The pitchcontrol motor drives reduction gears, which in turn drive the bevel
gears which change blade pitch.
Constant speed propellers are full feathering, which means that
the blade pitch can be increased to 90. Sometimes in case of an
engine failure, in order to decrease propeller drag and windmilling
A governor is the core of automatic pitch control.




with their accompanying ill-effects, it is necessary to vary propeller
pitch so that the sharp propeller edge is presented to the wind.
Reversible pitch propellers are often used on heavy aircraft. Upon
landing the pilot can change the blades of such propellers so that
they create a backward thrust. Reversing propeller pitch helps
shorten the length of the landing run. The reversed propellers help
brake the airplane to a stop.

The Contraprop Propeller
Some propellers are dual rotating. They are called contraprop
propellers, since each propeller assembly really consists of two
propellers, one behind the other, rotating in opposite directions.
This contra-rotation is accomplished by a special type of reduction
gear system. Contraprops have wide blade sections near the hubs
which throw air into the engine to help in engine cooling.
Since some propellers have a rather large diameter, at high engine
r.p.m., the tips of these would travel very rapidly. For example, if
the diameter of a propeller is 16 feet, in one revolution the blade tip
will travel 16 ft. x or 50.24 ft. At 2000 r.p.m, the blade tip will travel
1674 feet per second, which is faster than the speed of sound at
sea level (1100 ft./sec.). Propellers lose their effectiveness under
such circumstances, consequently, a system of reduction gears
must be used. Types of reduction gears differ, but their purpose is
the same, to keep propeller tips operating below the speed of


As aircraft engines have developed and become more complex,
the number of instruments they use has increased. The purpose of
the engine instruments is to keep the pilot informed of the operating
conditions of his engine. The engines of comparatively low horsepower and low compression ratio used in the early days of aviation
required only tachometer, oil temperature gauge, and water temperature gauge. The modern engine requires these and, in addition,
gauges which show pressure of oil, fuel, and manifold. It also requires indicators which show the temperature of oil, air, the carburetor, and cylinder heads.
P r e s s u r e a n d Te m p e r a t u r e G a u g e s
Pressure gauges operate either on the Bourdon-tube principle or
on the aneroid principle. The temperature indicators may employ
the principle of the Wheatstone bridge, the thermocouple, or the
vapor-pressure type thermometer and Bourdon tube. The principles
of the Bourdon tube, aneroid, thermocouple, and Wheatstone bridge
have been explained. (See Aircraft in Flight, Chapter Vl.)
The vapor-pressure type temperature indicator uses a bulb containing a highly volatile liquid such as methyl chloride, a capillary
tube, and a Bourdon tube. The capillary joins the Bourdon tube,
which is connected to the indicator needle by a linkage system, and
the bulb, which is located at the point where the temperature is to
be measured. As the temperature of the liquid in the bulb changes,
t h e v a p o r p r e s s u r e t h r o u g h o u t t h e s y s t e m c h a n g e s , a ff e c t i n g t h e
Bourdon tube and indicator needle.

Remote Indicating Systems
On large multi-engine aircraft where considerable distances
separate the point where a measurement is to be made, and the
instrument panel where the reading is taken, self-synchronizing,
remote-indicating systems are in common use. These instruments
are used to reveal fuel, oil, and manifold pressures and
temperatures, fuel flow and levels, and engine r.p.m. They may also
be used to indicate landing gear, flaps, and tail wheel positions.


How information reaches the instrument panel.

The self-synchronizing system is an adaptation of the synchronous
motor principle. The synchronous motor assembly consists of two
s e p a r a t e m o t o r s w h i c h o p e r a t e i n e x a c t t i m i n g o n e w i t h t h e o t h e r.
T h e s e l f - s y n c h r o n i z i n g s y s t e m d i ff e r s f r o m t h e s y n c h r o n o u s m o t o r
a s s e m b l y i n t h a t t h e r o t o r s n e i t h e r s p i n n o r p r o d u c e p o w e r. T h e i r
design is such that the rotor of the indicator moves only the distance
r e q u i r e d t o m a t c h a m o v e m e n t o f t h e r o t o r o f t h e t r a n s m i t t e r. ( S e e
illustration, above.)
Engine Control Systems
Engine control systems may be classified in accordance either with
their construction or with their method of operation. In terms of
construction there are two systems, the push-pull tube control and the
wire-and-cable control. In terms of operating method, there are
three classifications: the manually operated, the semi-automatic, and
the automatic.
As engines have become more complex, semi-automatic and automatic controls have been adopted. However, some method of manual
control is almost always kept as a safeguard in the event of failure
of the automatic mechanisms. The principles underlying the operation of automatic controls are hydraulic, electric, or electronic. Electronic mechanisms are electric mechanisms which employ vacuum


Instrument panel and cockpit controls.
Reciprocatng engine controls include controls for throttle,
fuel-air mixture, cowling flaps or radiator shutter,
supercharger, supercharger intercooler shutters, oil coolers,
and perhaps carburetor heat. One of the most important
controls is the engine throttle; it helps the pilot govern the
power of the engine and the speed of the airplane. Another
important control is the fuel-air mixture control.
Engine controls often combine a control lever, which
operates through a 90° arch (a quadrant, one fourth of a
circle), and the base upon which these levers are mounted.
The control pedestal is a frame secured to the floor of the
cockpit which supports the control quadrants and control
Controls are identified, as by a letter T for throttle, or a letter
M for fuel-air mixture. Control lever positions are marked on
the quadrant. When the pilot wants to increase engine
power, he moves the throttle control lever toward open
position; should he want to reduce engine power, he moves
this control toward closed position. On some aircraft the
mixture control has four positions, idle cut-out, automatic
lean, automatic rich, and emergency rich. A manually
operated mixture control has any number of positions. It is
by means of the mixture control that a proper relationship
(by weight) of fuel to air is kept in the fuel-air mixture.
Jet engine controls are much less complicated than
reciprocating engine controls. In this as in other respects
the jet engine has the advantage of simplicity.

Power for flight is produced by engines all of which convert potential
chemical energy into heat energy. Reciprocating engines are internal
combustion engines that use machine-principles to convert heat
energy into mechanical energy. Jet engines are reaction engines
that convert the pressure forces of expanding gases directly into
The engine is the most important of the power-plant systems.
However before the engine can produce power and before this can
be converted into thrust, other systems must be brought into play.
These include the fuel, ignition, lubrication, control, and propeller
The reciprocating engines used by aircraft are four-stroke-cycle
engines. This type of engine delivers power only one out of every
four piston strokes, or two crankshaft revolutions. Turbo-jet and
turbo-prop engines employ turbine wheel and compressor rather
than piston to compress the fuel-air mixture. The purpose of the
aircraft engine is to change, through the process of combustion, the
potential energy of a fuel into heat and the heat energy into work.
Energy means the capacity to do work. There are several different
kinds, such as chemical, heat, electrical, and mechanical. One kind
of energy may be changed into another.
The law of conservation of energy states, in effect, that although
energy can be changed from one form to another, it cannot be
destroyed. However, no engine can convert all the heat energy it
produces into mechanical energy. Some of the heat escapes
through the exhaust, some through essential cooling, but none of it
is destroyed.
Thermal efficiency is the ratio of heat an engine converts into useful
work to the heat potential of the fuel used. Mechanical efficiency is
the ratio of the brake horsepower an engine develops to its indicated
The carburetor vaporizes the fuel used by reciprocating type engines
and meters this to the cylinders. Some carburetors make use of the
Venturi principle, others use an injection system to introduce fuel into
the carburetor barrel. A jet engine uses a fuel injection system which
feeds fuel directly into each of its combustion chambers.


The ignition system is an important part of the aircraft power plant.
Generally, reciprocating engines use a magneto to provide the
spark which sets fire to the fuel-air mixture drawn into its cylinders.
The jet engine also uses a spark to ignite its fuel when starting.
However, since jet engine combustion is continuous, jet engine
ignition does not present engineers the difficult problems
encountered by those who design ignition systems for
reciprocating engines.
When two surfaces move over one another friction results. Since
friction causes heat loss, the weakening of power plant parts, and
other bad effects, it is necessary to reduce it as much as possible.
To solve the problem of friction reduction, engines use special
types of bearings and employ lubricating systems.
The propeller is the device which converts the power produced by
the reciprocating aircraft engine into thrust. Propeller blades are
actually airfoils that rotate. The pitch of the propeller blade can be
compared with the angle of attack of the airplane wing. The
propellers of large modern aircraft are controllable-pitch, constant
speed propellers. Their pitch changes automatically in flight,
adjusting to the power-load placed upon the aircraft engine.
Reduction gears are used with some propellers so that the tip
speeds of the propeller blades do not exceed the speed of sound.
Men designing aircraft engines have met and solved many
problems. Each day they meet and solve new problems in aircraft
propulsion. The solutions of these problems require scientific
understandings and technical skills. Each day adds to these
understandings and skills. As a result, great improvements are
being made in aircraft power plants. In military aviation, with some
exceptions, jet aircraft are replacing aircraft powered by
reciprocating engines. Even helicopters make use of jet power. As
a matter of fact turbine engines (both turbo-jet and turbo-prop) are
very much in the limelight.
Research appears already to have developed turbine engines
which are economically feasible from the commercial air-transport
viewpoint. Turbo-props are now in use; turbo-jets have been
ordered by airline companies. In the course of time, rocket engines
and atomic engines will likely be used as sources of power for

Emmett A. Betts
Director, Betts Reading Clinic Willis C.
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Director, Institute of Aviation
University of Illinois
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Air World Education
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Associate Professor
University of California
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Director, School and College Service United
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Director of Aviation Education

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