File #1335: "Aircraft in Flight (1956).pdf"

Aircraft in Flight (1956).pdf

PDF Text



Copyright 1956 by Civil Air Patrol, Incorporated

Harold E. Mehrens
William E. Rowland
Art Director

Price FIFTY CENTS per copy




Aircraft in Flight .....................................................................


Wind and Wing .....................................................................


Forces of Flight .....................................................................


Throttle, Stick, and Rudder ..........................................


Maneuvers, Gusts, and Load Factors .....................


High Speed Flight ...............................................................


Structure and Structural Units .................................... 3 9

Hydraulic and Electric Systems ....................................


Aircraft Instruments ............................................................ 49

Station Numbering ...............................................................


Summary .................................................................................... 5 8

The influences of aviation upon the modern
world are so great that they still are not fully
comprehended by those that they affect.
Although aviation's effects are of importance
to all of us, they are of greatest importance
to our youth. It is the young men and women
of America upon whom we must soon call for
solutions to the problems which an air-age has
The Civil Air Patrol recognizes this fact.
Its cadet program centers around an aviation
education curriculum. It is in the interest of
the success of its cadet program that this book
and the others of the Aviation Education series
have been prepared.
I f o u r n a t i o n i s t o f u l fi l l i t s d e s t i n y, t h e
young people of America must learn to employ
toward this objective many aviation understandings and many aviation skills. The Civil
Air Patrol's Aviation Education series will help
lay the foundation upon which these unders t a n d i n g s a n d s k i l l s m a y b e b u i l t . M o r e o v e r,
t h e b a s i c i n f o r m a t i o n o ff e r e d b y t h i s s e r i e s
can help give proper direction to the aviation
understandings and attitudes of the general,
adult public.

Major General, USAF
National Commander
Civil Air Patrol

Aircraft in Flight is one of a series of six pocket-sized
books prepared for use in the aviation education
program of the Civil Air Patrol. It is to be used with an
instructional 35 mm. color, sound filmstrip which
illustrates 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 flight, chiefly of heavierthan-air craft.
The forces affecting an aircraft in flight are defined;
simple problems of design encountered by
aeronautical engineers are considered; the effects of
high speed flight upon design are reported. Finally, the
relationship of the principles of hydraulics and
electricity to the operation of modern aircraft are
summarized. Its treatment of the several areas with
which it is concerned is sufficiently general to be of
basic importance to all aviation career objectives. Yet,
its content is detailed enough to challenge the interest
of students and adults alike.
Although the first use of this booklet 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 flight.
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
education program. Those working with adults may
also find this material helpful if the instructional or
informational goal is general education as it relates to

Special acknowledgments for the technical advice
given are due the following persons: Frank W. Hansley,
C o l o n e l , U S A F, D e p u t y C h i e f o f S t a ff , O p e r a t i o n s a n d
Training; Walter W. Thompson, Lieutenant Colonel, USAF,
D i r e c t o r o f Tr a i n i n g ; S e y m o r e E . L a t h a m , M a j o r, U S A F,
Chief, Cadet Training; Evarice C. Mire, Major, USAF, Chief,
Senior Training; and E. L. Jacobs, Captain, USAF, Execut i v e t o D e p u t y C h i e f o f S t a ff , O p e r a t i o n s a n d Tr a i n i n g .
Members of the Civil Air Patrol National Educational
Advisory Committee, whose names appear elsewhere in
t h i s b o o k l e t , a n d t h e C i v i l A i r P a t r o l R e g i o n a l Av i a t i o n
Educationists whose names follow offered helpful suggestions: Victor E. Moore; John M. Ogle; Charles W. Webb;
John E. Sims; Everett E. Collin; Arthur I. Martin; and John
V. Sorenson.
Mervin K. Strickler, Jr.
Director of Aviation Education
Headquarters, Civil Air Patrol


No doubt you have watched airplanes fly "Cubs," "Helicopters,"
" J e t s , " o r " F l y i n g B o x C a r s . " Yo u m a r v e l e d a t t h e w i s d o m t h a t d e s i g n e d
them and at the skill which built them. Perhaps you have seen the
"Thunderbirds" or the "Blue Angels" perform and were impressed by the
s k i l l o f t h e s e d a r i n g p i l o t s . Ve r y l i k e l y y o u w o u l d l i k e t o l e a r n w h y t h e
airplane flies and how to fly it. If you look ahead to a career in aviation,
you need eventually to learn these and other things about aircraft.
You have the same potential abilities as the men whose imagination
conceived the airplane, whose energies created it, and whose skills operate
it. These men are ahead of you only in terms of achievement. You are in
a position not only to match their accomplishments but also to out-distance
these--for you can put your imagination and energy to work adding to
their discoveries. But first you must learn what already has been done in
aviation. You must develop general understandings about aviation so that
you have a basis for developing aviation skills and solving the problems
of an aviation-age.
T h e r e a r e a i r c r a f t w h i c h a r e l i g h t e r, a n d a i r c r a f t w h i c h a r e h e a v i e r
t h a n a i r. L i g h t e r - t h a n - a i r c r a f t i n c l u d e b a l l o o n s , b l i m p s , a n d d i r i g i b l e s .
The balloon was actually the first aircraft. At the close of the 19th
Century, balloon ascensions were featured events at every special celebration. After its gas bag was filled with hot air and smoke produced by a
bonfire right on-the-spot, the balloon rose into the air.
If it did not catch fire, it carried its aerialist aloft a few hundred
feet. After this gentleman cut his parachute loose, the balloon turned
over lazily, emitting a trail of smoke; the aerialist then competed with the
balloon for the spectators' attention, performing, as he floated toward
the ground, on a trapeze attached to his parachute.

In later years, hydrogen (a gas lighter-than-air) served much better
f o r l i f t i n g b a l l o o n s a n d d i r i g i b l e s t h a n d i d h e a t e d a i r. H e a t e d a i r l o s t i t s
lifting power when it cooled; hydrogen exercised lifting power at normal
temperatures. However, hydrogen easily catches fire. Eventually, helium,
a more expensive but non-burnable gas, replaced hydrogen as the gas
used to inflate the suspension bags of lighter-than-air craft.
Heavier-than-air craft include gliders, airplanes, and helicopters. All
of these fly because some part of them is shaped to obtain a proper
reaction from the air. The name of this special part is the wing or airfoil.
Helicopter rotors and airplane propellers are airfoils that have special
uses. They are built strong enough to withstand the stresses resulting from
these uses.



Another of these principles is the Bernoulli law of pressure-differential.
One way to state this principle is to say that as the velocity of a fluid
increases, its pressure decreases; or as its velocity decreases its
pressure increases. A simple application of this principle is illustrated
by the venturi tube of the carburetor used by some types of aircraft and
automobile engines. The venturi tube is a tube, wide at each end but
narrow at the throat between these openings. Moving air will speed up
when passing through the narrow part of a venturi tube; hence, the
pressure at this point becomes reduced. Fuel will consequently feed
through a nozzle located at the venturi throat, since air pressure at the
nozzle opening will be lower than that in the fuel chamber.

The moving wing deflects the air. The air in turn deflects the wing.

Heavier-than-air craft fly because, through the application of power
to the resistance of the air, airfoils are made to lift and support a given
weight in flight. This is true whether the airfoil is the wing of the airplane
or the rotor of the helicopter.
To help you understand the nature of the reaction between the moving wing and the air through which it flies, it is necessary to discuss the
scientific principles whose applications make this reaction possible.
One of these principles is known as Newton's Law of Action and
Reaction. You have observed it operating in many situations. The recoil
of a gun; the back-lash of a hose as the water leaves its nozzle; gravel
thrown by an automobile tire, each illustrates its application. Newton's
observations led him to conclude that an object acted upon tended to
resist this action with a force equal to the force applied to it. His conclusions may be stated as follows: "For every action there must be an
equal and opposite reaction." A moving airplane wing acts upon the air.
Consequently, behaving in accordance with Newton's Law, the air must
act upon the airplane wing.

Air has weight; It is compressible; It exerts pressure.

Air can be compressed or expanded.
The upper surface of an airplane wing is shaped somewhat like the
inner surface of one-half a venturi tube split lengthwise. Consequently,
the air moving over the wing moves more rapidly than that moving under
it. As a result the air pressure above the wing is less than that below it.
In order to fully understand what happens when an airplane flies,
you must learn something of the air through which it flies. Air is the least
dense form of matter, as we are commonly acquainted with it. The ground

As the air velocity increases, its pressure decreases.
upon which we walk (a solid) is the most dense form. The water through
which you swim (a liquid) is a third form. Arranged in the order of their
increasing density, the forms of matter are gaseous, liquid, and solid.
Density is defined as mass per unit volume. But regardless of its
degree of density, air is a substance. It has weight; it is compressible; at
sea level it exerts a pressure of about 15 pounds per square inch, and a
cubic yard of it weighs about two pounds. However, with the increase of

50000 ft 3.44 P.I.

Normal Atmospheric 1"
at Sea Levei

30000 ft. 8.88 P.h
20000 ft.

13.75 P.h

1 0 0 0 0 f t . 2 0 . 5 8 P. h

5000 ft.

The pressure of the atmosphere can be measured.

altitude above sea-level, both its pressure and weight decrease. Since
gases and liquids are fluid substances, air is a fluid. As such, it is
capable of flowing like water. Like water, it takes the form of any vessel
that contains it.
THE ONE DISCOVERED BY NEWTON. The moving wing deflects the
air, and the air in turn deflects the wing. The moving wing also causes
pressure differences to occur. Thus, the wing does something to the air,
and the air does something to the wing. Consequently the airplane flies.
During this process, the moving airplane creates the relative-wind (air
movement in relation to an aircraft), and the relative-wind contributes to
the force called lift. The direction of the relative wind is always parallel to
the line of flight. One way to state the relationship between the moving
wing and the air through which it moves is to say that they are involved
in a process of interaction.





An airplane in flight is acted upon by four forces called lift, weight,
thrust, and drag. Lift in technical terms is defined as the force perpend i c u l a r t o t h e r e l a t i v e w i n d . We i g h t o r g r a v i t y i s t h e f o r c e w h i c h a c t s
downward, vertically. Thrust is the force that drives the airplane forward.
It may be derived from a sailplane launching device, from propellers
driven by reciprocating engines, or from the action of a jet or rocket
engine. Drag is the force which opposes the forward motion of the airplane. Drag always acts parallel with and in the direction of the relative
In the discussion of the action of wing and wind upon each other, we
were actually talking about forces. These were observed emerging as
the result of a condition brought about by the wing moving through the
a i r. O n e o f N e w t o n ' s l a w s o f m o t i o n s t a t e s t h a t o n c e a b o d y i s s e t i n
motion it tends to move along a direct line until acted upon by another
force. It is concluded, consequently, that when an airplane is flying straight


Lift also depends on
angle of attack.

and level, the forces acting upon it are in balance. Actually, there is a
complexity of such forces. '*Lift" is a term employed to denote the effect
of all those forces generated as the wing attacks the relative wind which
lifts the airplane within the air mass. The point at which these
aerodynamic forces are concentrated is called the center of pressure.
related to wing dimension, to airspeed, to angle of attack, and to air
density. Within certain limits, to increase any one of these increases lift.
Hence, the ratio is said to be direct. However, in some instances the
relationship is not quite so simple as the statements above would make it
Wing area is the product of the wing span and its average chord.
Span in relation to an airplane's wings is defined as their maximum tip-totip distance. The wing chord is the straight-line distance between the
leading and trailing edge of an airplane wing. Sometimes it is defined as
distance between imaginary perpendiculars erected to the leading and
trailing edges of the wing. Within practical limitations the larger the wing,
if other factors remain the same, the greater lift the wing will exert.
Air speed is the rate at which the airplane travels through the air. Of
course should there be no surface wind or, with reference to the earth's
surface, no movement of the air mass through which the airplane flies,
the air speed and ground speed will be the same. However, it is in direct
relationship to the airplane's movement not over the ground but through
the air that lift increases. Within practical limitations the faster the
airplane moves through the air, the greater will be the lift which results.
As you have already learned, it is not only the shape of the airfoil (wing)
but also the angle at which it attacks the air which creates lift. The angle
of attack causes deflection of the air which, in turn, causes an equal and
opposite reaction of the wing or airfoil. Up to a point, the greater the
angle of attack, the greater will be the lift which results.
Angle of attack must not be confused with angle of incidence. Angle of
attack is the angle formed by the airfoil chord line and the direction of
relative wind. It varies during flight, since it is controlled by the pilot. It

is one of the factors which determines the airplane's rate of speed
through the air. The angle of incidence is the angle at which the wing
root is fixed to the airplane's fuselage. Or more properly stated, it is the
angle formed by the chord line of the airfoil and the longitudinal axis of
the airplane. (See page 22.)
Density has been defined as mass per unit volume. Air density varies
directly as pressure varies and indirectly as temperature varies. That
air density varies with pressure means that if pressure is imposed upon
a body of air, its density will increase. Such pressure will decrease its
Since air at sea level is pressed down by all the air above it, density at
sea level is greater than density at higher altitudes. You can think of
many implications of this fact. For example, when conditions at sea
level are compared with those at higher altitudes, shorter take-off runs
are possible; hence, runways at sea level are shorter. Also, at sea
level, rate of climb may be higher and angle of attack may be greater.
That air density varies indirectly with temperature means that as air is
heated, it becomes less dense. And, as its temperature increases, the
volume of a given mass of air increases. Also, as its temperature
increases, a given mass of air can contain a comparatively greater
amount of water vapor.
There is relationship between density of the air and the amount of
water vapor it contains. The reason for this is that water vapor is less
dense than the other gaseous substances that make up the air about
Water vapor weighs about five-eighths as much as an equal volume of
dry air. Humidity is the term used to denote the presence of water
vapor in the air. Relative humidity is the ratio, expressed in per cent, of
the water vapor present in the air to the amount of water vapor which,
under equal temperature and pressure conditions, it can contain.

Air flows over and under the wing.

Air tending to spill over the wing tips causes vortices behind the wing.

AIRPLANE AND THE CARGO IT CARRIES. The center of gravity is the
point at which the total weight of the airplane is concentrated. If a
fulcrum were placed at this point, the aircraft would balance. The center

OF AN AIRPLANE. As lift acts in opposition to weight, so thrust acts in
opposition to drag. Consequently, when the drag forces acting on an
airplane are great, the power required to develop thrust is great.
Therefore, in the interest of economy of aircraft operation, aeronautical
engineers are concerned with the problems of reducing drag. Drag, like
the other forces acting' on an aircraft in flight, is made up of a number
of components. Among components of the total drag are included
induced drag and parasite drag.
Induced drag is an unavoidable result of lift. As the airplane speeds
forward, air from the high pressure area below the wings, as it tends to
move into the low pressure area above the wings, causes trailing
vortices to form. That is, this tendency imparts a whirling motion to the
air at both the trailing edges and the tips of the wings. At the center of
the vortices thus formed are low pressure areas. Obviously, such low
pressure areas behind the wing will tend to retard the forward
movement of the airplane.
Induced drag is a function of factors over which pilots and designers
have some control. It has a direct relation to the angle of attack. If the
angle of attack is increased, induced drag will increase. It has an
indirect relation to aspect ratio. If the aspect ratio is increased, the
induced drag will decrease. Aspect ratio is defined as the quotient
resulting when the span, the maximum distance from airplane wing tip
to wing tip, is divided by the average chord. A rectangular airfoil with a
span of 36 feet and a chord of 6 feet will have an aspect ratio of 36/6,
or 6.

of gravity (CG or c.g.) of the aircraft is normally located at approximately
one-third the distance of the mean chord aft the leading edge of the
wing. The relationship between the center of gravity and the center of
pressure should be such that no serious instability of the airplane will result.
It always acts in the direction of the line of flight. Its source may be the
propellers driven by reciprocating engines or it may be the reaction produced by a jet engine. Regardless of how this force is produced, it is of
great importance. Its effect enables interaction of wing and wind. It
bears a direct relation to engine speed and to the airplane's acceleration.

Air from under the wing tends to spill into law pressure area over the wing.


The facts stated above are important to aircraft designers. For
example, in order to develop required lift, slow cargo aircraft which
must fly at a high angle of attack have comparatively long, narrow
wings (high aspect ratio). In order to develop required lift, fast fighter
aircraft need fly at lower angles of attack. Hence, their wings may be
comparatively wider and shorter without causing excess, induced
drag. The designer must take these matters into account when
planning the construction of an airplane wing.
THOSE CAUSING INDUCED DRAG. Skin friction drag and eddy drag
(or form drag) are names applied to two of these components. The
drag caused by the friction between the outer surface of the aircraft
and the air through which it moves is called skin friction drag.
Whatever interferes with the streamline flow of air about the aircraft
causes eddy drag (sometimes called form drag). To understand the
causes of parasite drag you must learn about the boundary layer.
Air can be defined as a viscous fluid, and like other such fluids,
gaseous or liquid, it tends to cling to an object passing through it.
Have you ever observed, when one cuts griddle cakes drenched with
syrup, how the syrup sticks to the knife? And, that which clings to the
knife pulls more syrup along with it? Likewise, if you were to move
your hand in a single direction through a basin of water, you would
observe that your hand imparted motion to the water. In this instance
the water against the surface of your hand will move as rapidly as
your hand and in the same direction, However, as the distance of the
water from your hand becomes greater, the motion of the water
becomes less, until at a certain distance the water is not affected.

Turbulent Flow

Now let us consider the resistance due to form drag. As you move your
hand through water, you will observe that to change your hand's
position will change the behavior of the water which flows around it. If
you hold your hand rigid and move it edgewise through the water, the
disturbance will be less than if you move the flat of your hand through
the water. In this latter instance you will observe that more eddies
occur in the water and -that the water offers more resistance to the
forward motion of your hand. The behavior of air when objects speed
through it is quite similar to that of water when objects move through it.

Streamlined Flow

greater the drag, the greater is the horsepower needed to develop
required thrust. And, the greater the horsepower required, the greater
is the amount of fuel consumed. You observe that there are sound
practical reasons for airplane designers to attempt to reduce drag.
reduced if the airplane is kept clean and well polished and surface
irregularities such as those caused by protruding rivet heads are
removed. Form drag is comparatively easy to reduce. To reduce it the
designer has only to create designs which offer least disturbance to the
airflow about the airplane.
it has been found that the best way to prevent formation of eddies in
the air through which an object moves is to streamline the object--that
is, to give it a shape approaching that of a tear drop. Consequently, to
reduce eddy drag, airplane parts exposed to the airstream are
streamlined. When an airplane part cannot be streamlined, it is
inclosed in a streamlined cover. Th!s auxiliary structure reduces the
drag of the part to which it is fitted. Such a cover or structure is called a
You have observed that among the four composite forces that affect
aircraft in flight there is one, the force of gravity, over which, since it is
a constant force, neither designers nor pilots exercise control. The
others are controlled to varying degree by those who design and build
aircraft or by those who fly them.

Best streamlining approaches a teardrop shape.


H O T E TC , N U D R



In addition to its forward motion, an aircraft in flight may move about three
axes, the vertical, the lateral, and the longitudinal. These axes remain
flxed, not in terms of the earth's surface, but in terms of the aircraft.
Each is actually defined by the planes of rotation through which the aircraft
moves as a pilot operates the controls.
AND ROLL. Yaw is movement around the vertical axis; pitch is that around
the lateral axis; and roll is that around the longitudinal axis. If there is a
tendency for the aircraft in straight and level flight to execute any of these
movements voluntarily, it is termed unstable. An aircraft in flight which has
desirable stability will not only respond readily to the controls but also
tends to maintain a straight and level flight attitude. A stable aircraft can
almost fly by itself. An unstable aircraft needs continuous control by the
pilot in order to keep it at a constant altitude and on a constant heading.
The designer and the airframe mechanic actually build stability into the
airplane. One means of preventing voluntary yaw is to use a surface called
a vertical fin. A vertical stabilizer or vertical fin is the fixed portion of the tail
assembly to which the aircraft rudder is attached.

The most important method used by the designer to obtain stability of
movement around the longitudinal axis relates to wing dihedral. If you
could draw a line from the tip of one airplane wing to the tip of the other,
you would observe that such line did not coincide with the wing surfaces.
Now, should you draw lines from the tip of each wing so that they did
coincide with the wing surfaces and extend these lines until they meet,
you would observe that the three lines drawn form a triangle. In order to
find the degree of dihedral, you should measure the angle formed by the
first line drawn and either of the other lines.
A tendency of an aircraft to roll is caused by propeller rotation.
This tendency is called torque and is in the direction opposite propeller
rotation. To compensate for torque effect, the angle of incidence is
decreased toward the tip of the right-hand wing and increased toward
the tip of the left-hand wing . (See page 13.)
STICK, AND RUDDER. Learning to fly is in part learning to coordinate
these controls so that the desired maneuver is successfully performed.
The throttle is used by the pilot to supply fuel to the engine. To increase
the fuel supply produces additional horsepower. When this is done the
airplane tends to climb (unless the pilot applies forward-pressure to the
elevator controls).
To decrease the power (fuel intake) of the engine causes the aircraft to
enter a glide (unless the pilot applies back-pressure to the stick, or
The stick (or wheel) operates both elevators and ailerons, hence are the
pilot's controls over pitch and roll. The rudder pedals operate the rudder
and are the pilot's controls over yaw and the horizontal direction of flight.
Control surfaces are connected to cockpit controls by cables, push-pull
rods, and linking devices, such as turnbuckles (See page 43).
In practice the pilot may make use of one or all controls at the same
time. To apply forward pressure to the wheel of an aircraft in-flight tends
to place the aircraft in a diving attitude; to apply back pressure, tends to
place it in a climbing attitude. To turn the wheel to the right (to apply right
aileron) tends to lower the right wing; to turn the wheel to the left (to
apply left aileron) tends to lower the left wing. To exert pressure on the
right rudder pedal causes the rudder to move toward the right and the
nose of the aircraft in flight to move clockwise around the vertical axis of
the aircraft; to apply pressure on the left rudder reverses the process.
In order to perform certain maneuvers properly, the student-pilot must
learn through practice to coordinate the operation of cockpit controls.

For example, should he want to turn the aircraft to the right, he must
use right rudder and right aileron together, placing the aircraft in the
turn. Just the right degree of pressure on each control is necessary, or
the aircraft will either slip or skid. Once in the turn, he eases pressure
on rudder and aileron, applying sufficient back pressure to complete
the degree of turn desired. In order to resume straight and level flight,
he eases elevator pressure and applies proper, coordinated pressure
upon left rudder and aileron.
Ailerons, elevators, and rudder are equipped with trim tabs, also
controlled by the pilot. These enable him to keep the aircraft in straight
and level flight when its stability has been affected by cargo, fuel load,
or other cause.
Simply stated the aircraft controls enable the pilot to take the aircraft
off~ the ground, execute necessary climbs and turns, set it upon its
course in straight and level flight, keep it on course until his destination
is reached, enter the traffic pattern, and make a landing. Control
surfaces function in accordance with the principle of action and
reaction. Just as the rudder of a boat reacts with the water through
which it is moving to change the heading of the boat, so do rudder,
ailerons, and elevators react with the air to change the attitude of the
Handling of the aircraft on the ground brings other controls into play,
such as brakes, but it also employs some of the controls used in flight.
And certainly, landings or take-offs from a busy airport requires many
pilot skills in addition to those needed to operate the airplane's

Aircraft design determines stability.


M A N E U V E R S ,


Aircraft are built to withstand prescribed loads. The aeronautical engineer
who designs an aircraft is faced with problems similar to those
confronting the engineer who designs any other structure which has to
bear both its own weight (dead weight) and the weight of other loads (live
weight) placed upon it. The task of the aircraft designer is more complex,
however, than that other builders, because the dynamic forces affecting
the airplane in flight are more variable than forces affecting other
structures, and these act from many separate and combined angles. By
way of illustrating the comparative complexity of forces acting on an
airplane let us consider the nature of forces acting upon a bridge.

Designers consider forces affecting aircraft in flight.




The live load stresses placed upon a bridge are produced by traffic
passing over the bridge. The weights of the live loads vary continuously,
it is true, but the angles from which these act are not subject to much
change. On the contrary as the airplane assumes different attitudes and
flies through turbulent air, the stresses placed on its airfoils vary both
in degree and direction.
The term used to indicate the load placed upon an airplane under
certain conditions is load factor. The load factor is the ratio of a load to
the design weight of an aircraft. Because the weight of an object is
measured in terms of gravity, the load factor may be thought of as a ratio

Load factors placed upon an aircraft recovering from a dive.


between the normal weight of an airplane, its cargo, and passengers
and the pull of gravity under different conditions.
The acceleration produced by gravity is 32.17 feet per second per

second (32.17 ft/sec/sec). The letter "g" used in a formula refers to
t h e a c c e l e r a t i o n p r o d u c e d b y g r a v i t y. S o m e t i m e s a l o a d f a c t o r i s s y m bolized by the letter "g". This is because a maneuver which abruptly
checks acceleration will subject the aircraft to increased stresses. For


example, during a flight maneuver an airplane which weighs 3500
pounds may have a load of 10,500 pounds placed upon it. In this case
the load factor it sustains might be expressed as 3g.
STRESSES IN VARYING DEGREE. When design engineers specify the
kind of material the builder should use, they take this fact into
Tension tends to pull materials apart. Compression tends to push
materials together. Bending tends to distort a member by curving. Shear
tends to divide a member. Torsion tends to distort a member by twisting.
The load which may be imposed with safety upon an airplane depends
upon the degree of stress that it is designed to withstand. The maximum
safe loads for airplanes weighing less than 4,000 pounds in four principal
airplane classifications are as follows:

Aerobatic ..............................................................................
6.0 times gross weight
Utility (mild aerobatics including spins) .........
4.4 times gross weight
Normal (no aerobatics or spins) ...........................
3.8 times gross weight
Non-spinnable ..................................................................
3.5 times gross weight ;.
As airplanes increase in weight a corresponding decrease must be made in the
load factor to which they may be subjected. Consequently, airplanes of over
4,000 pounds have a corresponding load limit reduction. However, a safety
factor of 50% of the LIMIT-LOAD is always built into an airplane.
Actually the pull of gravity upon an airplane in a turn when the wings are
banked 60° is twice as great as that upon the airplane in level flight;

in a 60° bank twice its normal load is placed upon the aircraft structure.
Speed is also important in relation to limit load. This is not that speed by
itself imposes a greater load, but that maneuvers at high speeds do impose
greater stress than do maneuvers at lower speeds. An abrupt pull-out
from a dive can place a load upon the aircraft wing sufficient to cause
structural failure. Flying through gusty air or turbulence caused by storms
places extra stress upon an airplane. Flying speed should be reduced in
this latter instance to 90% of normal cruising speed.
During maneuvers no pilot (other than those testing experimental aircraft) will ever intentionally exceed the maximum speed limit established
for the airplane he is flying. However, should he accidentally approach the
never-exceed speed restriction marked on his air speed indicator, he will
handle the controls very carefully and use back pressure very gently to
"pull-out" of the dive. Incidentally, never-exceed-speeds can be attained
only when the airplane is diving.
The never-exceed-speed is marked on the air-speed indicator by a
red radial line. The range at which flaps may be operated is shown by a
white arc. The range from normal stalling speed to normal operating speed
is shown by a green arc. The cautionary range from normal operating
speed to never-exceed speed is shown by a yellow arc. Maneuvering speed
i s n o t g e n e r a l l y d e s i g n a t e d b y a i r - s p e e d i n d i c a t o r m a r k i n g s . H o w e v e r,
normal stalling speed is so indicated. Since maneuvering speed is 70%
greater than normal stalling speed, the pilot can readily compute the safe
speed for his airplane's operation during maneuvers or upon encountering
gusts. For example, a light aircraft whose normal stalling speed is 55 miles
per hour (mph) has a maneuvering speed of 83.5 mph.
THE PILOT'S USE OF THE CONTROLS. An airplane will not enter either of
these maneuvers if its trim and control systems are functioning properly.
Once it has been put into such a maneuver, if the altitude is sufficient, the
well-designed airplane will recover of its own accord. It is only when an
aircraft is handled unskillfully that accidental stall and spins occur.
An airplane stalls because it loses the lift necessary to sustain its
attitude. Lifting power is lost when the angle of attack is too great. For
most airplanes the maximum effective angle of attack is 20°. Any increase
in this angle causes the boundary layer to separate from the upper surface

of the wing. This creates eddies over the wing and a corresponding loss
of lift.
Stalls are associated with slow air speed. There is a relationship bet w e e n a i r s p e e d a n d s t a l l s . We s p e a k o f n o r m a l s t a l l i n g s p e e d , d e fi n i n g
this as the appropriate forward speed of the aircraft upon contact with
t h e g r o u n d w h e n l a n d i n g . H o w e v e r, a n a i r p l a n e c a n s t a l l a t a n y s p e e d
from its minimum to-its maximum, and the higher the speed the more
violent the stall. The controlling factor in a stall is the angle of attack; not
the air speed.
To recover from a stall the pilot needs only to release back pressure
on stick, or wheel. This permits the nose of the airplane to drop quickly,
the speed of the airplane to build up, and lift to increase. Once flying
speed has been reached and lift regained, gentle back pressure will return
the airplane to level flight. If too abrupt back pressure is applied after
stall or spin recovery, the angle of attack may be increased enough to cause
a second stall.
A spin results when, after it stalls, an airplane begins to travel downward along a spiral path. Rudder pressure applied after the stall will
cause the airplane to begin to rotate in the direction the rudder pedal
operated. When recovering from a spin the pilot proceeds as follows:
1. He holds complete back pressure on the stick until the application of
rudder opposite the direction of the spin checks the rotation of the airplane.
2. He returns the stick to a neutral position until flying speed is reached
and lift regained. 3. He then applies gentle back pressure on the stick
until the airplane is again in level flight
Stalls and spins are dangerous only when they occur unexpectedly at
a l t i t u d e s t o o l o w t o p e r m i t r e c o v e r y. U n i n t e n t i o n a l s t a l l s h a p p e n m o s t
often during turns. Since the lift required in a turn is greater than that
required for straight and level flight, an aircraft in a turn stalls at a comparatively higher airspeed. It is well to remember that the steeper the
bank, the greater the airspeed required to maintain adequate lift and
constant altitude
Abnormal use of the rudder during turns cause skids and slips. If the
Lower rudder of the airplane is held during a turn, its nose swings toward
the inside of the turn, and it skids sideways, just as a car sometimes skids
on a wet and slippery pavement. If during a skid, back pressure is applied
to the elevator control to aid in the turn or to maintain altitude, the angle

Stall and Spin

of attack might be increased enough to cause a stall or spin. To
execute a turn, the pilot uses the rudder control, coordinated with the
aileron and elevator controls, to place the airplane in the turn. He then
releases pressure upon rudder and aileron controls. The skillful pilot
never rides the controls and uses only the degree of control pressure
necessary under the circumstances.
To hold the upper rudder of the airplane during a turn causes its nose
to swing toward the outside of the turn and causes a slip with a
consequent loss of lift. Slips are sometimes executed deliberately to
lose excess altitude during a landing approach. Under such
circumstance, as long as the airplane's nose is kept down, there is no
danger of stalling or spinning.



Even before jet aircraft flew at supersonic speeds, the speeds of
airplanes with reciprocating engines began to approach the speed of
sound. Supersonic speeds are designated by a Mach number instead of
by miles per hour or knots. The Mach number of an aircraft is its air
speed in feet per second divided by the speed of sound. An airplane that
can fly at the sped of sound is flying at a Mach number of 1; one that can
fly twice the speed of sound is flying at a Mach number of 2.
Sound, as you know, is actually a sensation produced through the
organs of hearing. However, this sensation is caused by a disturbance
(called a wave) in the equilibrium of a substance. The speed of sound
waves in air increases as air density or air temperature increases. Under
standard conditions the speed of sound at sea level is 760 mph; at an
altitude of 35,000 feet, 660 mph.
When the speed of any projectile, a rifle bullet for example, reaches
Mach 1, a shock wave is formed at, or in front of, the nose of the
A shock wave may be simply defined as a piling up of the air in front of
an object moving faster than the speed of sound--it also may be
regarded as an accumulation of sound waves. Sometimes two shock
waves will form in front of an object moving at a supersonic speed. An
airplane flying at transonic speeds may create three shock waves; one in
front of the aircraft's nose, another in front of its wings, and a third in
front of its tail section.
Sometimes shock waves become attached to the upper surface of the
wing of an airplane flying at subsonic speeds. This is because the air
flow across the curved upper surface of the wing is moving more rapidly
than the aircraft and does reach the speed of sound. (See p. 9.) When
the speed of this relative airflow is not too great, the shock wave forms
near the trailing edge of the wings, and high speed difficulties may not
occur. However, as the speed of the relative airflow increases, the shock
wave moves toward the leading edge of the wing, and in this position it
causes a separation of the boundary layer (See p. 18), which makes the
airplane hard to control and causes violent shaking of the wing.
The so-called sonic barrier is reached when the speed of an airplane
reaches the speed of sound. Yet, aircraft, penetrating the transonic zone,
now fly at supersonic speeds. If the leading edges of the wings of



Shock waves are carried through the transonic zone.

the airplanes which fly at supersonic speeds are blunt, the shock wave
forms in front of the wings; if these are sharp or thin, the shock wave
attaches to them. Less drag results when the shock wave is attached-to
the leading edge of the wings. However, in either instance when aircraft
speed is above Mach 1, no boundary layer separation takes place, no
shaking of the airplane occurs, and no control difficulties are experienced.
A shock wave generated by an aircraft in supersonic flight tends to follow
the original line of flight after the airplane changes its course.
Consequently, a shock wave generated by an airplane diving at a supe

sonic speed will sometimes strike the surface of the earth with considerable impact. The noise of this impact is called the supersonic boom.
Airplane designers have learned that to decrease the thickness of
an aircraft wing, reduces the speed of the relative airflow over the wing.
They have also discovered that if they increase the degree of "sweepback," an airfoil moving at a high subsonic speed reacts as would an
airfoil with less sweep-back moving less rapidly. Hence, in order to delay
the formation of shock waves on the wings of both high subsonic speed
and supersonic speed aircraft, their wings are swept back, are comparatively thin, and have comparatively sharp leading edges. Airplane designers have also learned that drag builds up when shock waves are carried
through the transonic zone. Discovering that the shock wave drag of the
total airplane is greater than the sum of such drag on its parts, they have
formulated the Area Rule. Applying this rule, in order to enable air to
be displaced less violently, they now design aircraft with nose sections
tapered and lengthened and fuselages pinched at the waists.


have observed that some military aircraft are quite different from civilian
a i r p l a n e s . H o w e v e r, a m i l i t a r y a i r p l a n e u s e d t o t r a n s p o r t p e r s o n n e l i s
quite similar in appearance to one used by an air transport company to
transport passengers. Among military aircraft you will observe that fighters
do not look like bombers, nor do liaison aircraft look like fighters. Among
civilian aircraft, the single engine utility airplane appears to be a craft
c o m p l e t e l y d i ff e r e n t f r o m a t w o - e n g i n e c a r g o c a r r i e r,
The differences between two airplanes are the differences between
specific identical units of each. All airplanes are very much alike in that
each is made up of power plant, fuselage, wings, stabilizers, flight control
members, and landing gears. Each is alike in that pilot controls are throttle,
stick (or a substitute for it), and rudder pedals regardless of airplane
classification. Each requires for its operation the same general pilot skills
and understandings. In general airplanes are alike; specifically airplane
t y p e s d i ff e r i n d e t a i l o n e f r o m a n o t h e r.
The specific differences among airplane types makes possible aircraft
c l a s s i fi c a t i o n . C o n s e q u e n t l y, a i r p l a n e s m a y b e c l a s s i fi e d o n t h e b a s i s o f
p u r p o s e , e n g i n e , w i n g , a n d l a n d i n g g e a r. A n a i r c r a f t m a y b e p o w e r e d
by jet, reciprocating, or turbo-prop engine. It may have one engine, two
engines, four engines, or more. These may be radial or in-line engines.
The propeller may be behind the engine, as in the pusher-type aircraft.
O r, i t m a y b e i n f r o n t o f t h e e n g i n e , a s i n t h e m o r e c o m m o n t r a c t o r - t y p e
An airplane's wings may be characterized, by their position with
reference to the fuselage, as low-wing, mid-wing~ and high-wing. Wing
shape is also a factor in airplane classification. Consequently, descriptive
terms such as clipped wing, swept-back wing, and delta wing are employed.
.Landing gears are termed conventional when the traditional type
l a n d i n g g e a r i s u s e d . Tw o m a i n w h e e l s a n d a t a i l w h e e l c o m p r i s e t h i s
landing gear type. The tricycle landing gear uses two main wheels and
a nose wheel. On heavy aircraft each of these may be of dual construction. Some of the more recently constructed heavy bombers have two
sets of main wheels, one near each end of the fuselage, and smaller
wheels, one attached to each wing. This type is called a bicycle landing


AND LANDING GEARS. When these parts are assembled they are
termed the aircraft structure, or the airframe.
The fuselage is the principal structural unit. It houses crew, passengers,
cargo, instruments, and other essential equipment. On single engine
aircraft the power plant is attached to it. There are two basic types of
fuselage construction, the truss and the monocoque. In the construction
of some aircraft a combination of truss and monocoque design is used.
In truss type construction strength and rigidity are obtained by joining
tubing, steel or aluminum, to produce a series of triangular shapes,
called trusses. In monocoque construction rings, formers, and bulkheads
of varying sizes give shape and strength to the stressed-skin fuselage.
In practice a semi-monocoque construction is most often used. In this
type of-construction lengthwise members called Iongerons and stringers
hold the bulkheads, rings, rind formers in position. These members also
provide rigidity to the fuselage, a more adequate foundation for the skin,
and a means by which it can be attached. The Iongerons are
comparatively heavy and run the full length of the fuselage. The strlngers
are lighter and serve as "fill-ins." The skin is aluminum alloy, generally
covered on both sides with pure aluminum. After it has been heat treated
it is about as strong as a light steel. It is fastened to bulkheads, rings,
stringers, and Iongerons by means of rivets.
The partition between the rear of the engine and its nacelle must be
fireproof. This partition is called a firewall and is made of a highly hea





resistant, stainless steel. Nacelles house aircraft engines or
others objects larger than the boundaries of the airfoil section.
Their purpose is the fairing of such an object to reduce drag.
Wings are of two main types, semi-cantilever and cantilever.
The semi-cantilever wing is braced both externally by means of
wing struts and internally. The cantilever wings require no
external bracing. In such wings the stress is carried by the wing
spars, ribs, and stringers. Generally in this type wing, the skin
or metal wing covering is constructed to carry wing stresses.
Aircraft with wings so stressed are called stressed skin types.
The principal structural parts of the wing are spars, ribs, and
These are reinforced by trusses, I-beams, tubing, or other
appropriate devices. Cap strips provide a base to which the
skin is attached and carry a portion of the "bending-load." The
web forms the depth portion of the spar. Stiffeners give strength
to the spar structure. Treated aluminum alloy is most commonly
used as wing covering. The thickness of the covering depends
upon the load carried or the stresses imposed upon the wing
area where it is used.
The wing ribs actually determine the shape of the wing. Since a
wing rib is an outline of a wing cross section, it can be used to
help understand such terms as chord line, camber, and mean
line. The chord line is a straight line, which extends from the
leading to the trailing edge of the wing section. The camber is
the rise of the curve from this line to the outline of the airfoil. Or
to state this more simply, the distance from the chord to the
upper surface of the airfoil is upper camber; that from the chord
to the lower surface is lower camber. The mean line is a curved
line equidistant at each point from both upper and lower airfoil
One type of wing construction.

Sometimes wings are constructed with open spaces near their leading
edge. Such an opening is called a slot. Slots delay the separation of
boundary layer from the airfoil while its angle of attack is being increased.
Hence, they help reduce the possibility of unintentional stalls.
WIND TUNNELS. However, models constructed to scale, although
miniature replicas of airplanes, do not show the same aerodynamic
characteristics that these airplanes display. A scientist named Reynolds
discovered this. He also discovered that the airflow around an unconfined
object changed when the size of the object changed, when the velocity of
the flow changed, or when the air density changed. As a result of these
discoveries, he concluded that if he were to increase, during a wind tunnel
experiment, either the speed of the airflow or the density of the air, or
both, a scale model would react as the full-sized airplane reacted under
normal airspeeds and under standard atmospheric conditions.
Experiments proved that his conclusions were correct. When he knew its
dimension, its airspeed, and the density of the moving air, he was able to
compute a value for any aircraft. He called this value the Reynolds
Number (RN). The RN changed not only as the size of the aircraft or airfoil
changed, but also as the speed, density, and viscosity of the air changed.
Yet different aircraft with the same Reynolds Number displayed identical
aerodynamic characteristics regardless of difference in their dimensions.
Moreover, when the Reynolds Number of a full sized aircraft, its
dimensions, its air speed, and the density and viscosity of the atmosphere
are known, the formula for the Reynolds Number can be used to find
proper airflow speeds and/or air density necessary when during wind
tunnel tests models rather than full-size airfoils are used.
The main group of controls consists of ailerons, elevators, and rudders.
The auxiliary group consists of trim tabs, balance tabs, servo tabs, and
landing flaps. The ailerons are attached by means of hinges to the trailing
edge of the wing sections of both right and left wings. The elevators are
similarly attached to the trailing edge of the horizontal stabilizer; the
"rudder, to the trailing edge of the vertical stabilizer. The horizontal
stabilizer with the elevators and the vertical stabilizer with the rudder
comprise the tail assembly, generally called the empennage. Ailerons,
elevators, and the rudder are controlled by the pilot from the cockpit.
The tabs listed among the auxiliary controls help the pilot trim and balance
the airplane in flight. They also help him operate the main

One type of control system.

controls by reducing the amount of pressure he would otherwise need
to apply in order to actuate one of them. Landing flaps increase wing
camber and consequently lift and drag, enabling the reduction of
landing speeds and the distance required for landing runs. Flaps are
either attached to the trailing edges of the wing or recessed into
Landing gears must provide both a rolling medium between aircraft
and ground and a shock absorbing system. A landing gear must be
stressed to absorb great shocks. Two types of shock struts are in
common use on light aircraft, the spring strut and the air:oil strut. The
spring strut is made of spring steel and bends like the leaf-type
automobile spring. The air-oil strut is composed of piston and cylinder.
The cylinder is filled with air and fluid. The piston operates in the fluid.
Holes in the piston permit the passing of fluid from one side of it to the
other. In this manner abrupt shocks are prevented.
Landing gears are generally the wheel type, although pontoons are
used on aircraft operating from water surfaces. Wheel type gears are
either fixed or retractable. Retractable gears reduce drag. They may
retract into the fuselage or into nacelles. They may retract backward,
forward, or sideways, and they are operated manually, electrically, or


Piston B

As an airplane passenger you have no doubt noted changes in the sound
of the propellers. These sound changes take place as the airplane
engines are "warmed-up" just before "take-off," shortly after "take-off,"
and after the airplane reaches the desired cruising altitude and speed.
During the "warm-up" period, the propeller pitch is changed to find out
whether or not the pitch-changing mechanism is working properly. During
"take-off” the propellers are turning at maximum rpm (revolutions per
minute) with the blades set at a low angle of attack (Iow-pitch). When the
airplane becomes airborne, its engine rpm changes from maximum to
climbing and the propeller pitch changes from low toward high pitch.
After the airplane reaches its cruising altitude and changes from climbing
to level flight, its engine rpm changes from climbing to cruising and the
propeller changes from intermediate to high pitch. It is these changes in
propeller pitch which cause the changes in sound made by the propellers.
Some airplanes have reversible propellers. After one of these lands, the
pitch of the propeller is reversed and, although the engine rpm is
increased, the forward speed of the airplane is abruptly reduced. This is
because the reversed propeller acts as a brake. Incidentally, airplanes
with reversible propellers have safety devices which keep propellers from
being reversed while the aircraft is in flight.
The scientific principles whose applications have developed the devices
which control airplane propellers or start airplane engines are the same
as those upon which are based many of the commonplace appliances
which you use every day. When you apply the brakes to stop your car,
you make use of the principles of hydraulics which, on some airplanes,
enables changes of propeller pitch in flight, changes in the position of
flaps, and operation of the airplane's brakes. When you step on the
starter-switch of your car you make use of the principles of electricity
which underlie the construction and operation of your radio and television,
and of such household conveniences as electric lights, heaters, freezers,
mixers, and the like. In order to fly a modern airplane, a pilot makes use
of many auxiliary airplane-accessories which employ the principles of
hydraulics and electricity. During your airplane journey, while you relaxed
in comfort, the pilot, co-pilot, and engineer were busy with many tasks.
The successful completion of each of these depended not Only upon the
skills with which the pilot operates the aircraft's controls, but also upon the
proper functioning and skillful use of the airplane's hydraulic and electrical

20 Sq. In. Area


Cylinder A

aircraft hydraulic system employs fluid to bring about the movement and
force needed to operate brakes, to lower landing gears, and to extend and
lower flaps. Also, the mechanism which controls the pitch of the propeller
may be hydraulically operated. A physicist named Pascal discovered the
principle of hydraulics. It may be stated as follows: “A pressure exerted
anywhere on a confined fluid is transmitted undiminished to every portion
of the interior of the vessel containing the fluid. This pressure acts at right
angles with an equal force on equal areas.”
The application of the hydraulic principle makes it possible to increase a
force originally exerted. Assume that attached to the container of the
hydraulic fluid were two pistons and their cylinders. Assume that one of
these has an area of 1 square inch; the other an area of 10 square inches.
If 5 pounds of pressure is placed on the smaller piston, 10 x 5 pounds of
pressure will be created by the larger. This is true since the pressure
applied on the I square inch surface of the small piston will be transmitted
undiminished to each of the 10 square inches of the surface of the larger
piston. Also, pressure applied to one piston in an hydraulic system is
transmitted undiminished to all pistons throughout the system.
Pascal's principle is the principle upon which the braking system of your
car is based. The pressure of your foot on the brake pedal is multiplied by
the hydraulic mechanism so that by the time it reaches the brake bands it
is sufficient to control the car's weight. It is a similar system that enables
the pilot of a large aircraft to retract landing gears weighing hundreds of
pounds, operate other component parts when great power is
required. One significant difference between the hydraulic system of your
car and that of the aircraft is the manner in which pressure is built up in the
system. Hydraulic pumps generate and maintain pressure in the aircraft's
hydraulic system. The pilot, instead of generating it as

you do when you step on the brake pedal of your car, regulates it,
applying only that needed to do the work required.
The air-route that an airplane travels is marked by signals broadcast
from a range-station. Pilots can communicate, while in flight, with
stations on the ground or aircraft in the air. The use of electricity makes
such communication possible. The electricity which is generated in
flight is also used for many other purposes. Generators charge storage
batteries; magnetos provide current which spark plugs convert to
sparks that in turn ignite the fuel mixture which keeps engines
operating. Solenoids use electric currents from batteries to supplement
the pilot's muscles, making it possible by cockpit control to operate
large switches, valves, and mechanical devices. Electric motors further
increase the power at the disposal of the pilot. They help him start the
engines, and when these are not included in the hydraulic system, they
help him operate flaps, and change the pitch of the propeller.
In fact they can be adapted to serve wherever power is required.
Whenever a conductor of electricity is moved through a magnetic field,
voltage is induced in such conductor (wire, cable, etc.). Voltage is the
force that "pushes" an electrical current through the conductor.
Generators and magnetos, a special kind of generator, make use of
this principle to produce electricity which is used to help the pilot
navigate, control his aircraft, inspect the weather ahead of him, and
analyze his engines operation, discovering the location of trouble spots
before these can do series damage.
All this means that when aircraft are designed and their parts built and
assembled, provision must be made for magnetos, generators, storage
batteries, and motors and for the proper location of these. It is also
necessary, when assembling an aircraft, to install the electric cables,
linkage, and servo-mechanism which makes it possible for the different
electric motors to do their work.



A centrifugal
The operation of modern aircraft as they perform their civilian or military
tasks is possible because they are equipped with instruments. It is said
that the instrument board is the nerve center of an airplane. The number
of instruments housed in this panel range from a few, such as those
found on small airplanes--a magnetic compass, airspeed indicator, oilpressure gauge, tachometer, and altimeter--to well over 100 instruments
and other devices employed by large multi-engine aircraft.
Instruments are classified either in terms of their use or with respect to
the principle underlying their construction. When instruments are
classified according to use, there are four major groups---engine
instruments, aircraft instruments, flight instruments, and navigation
instruments. Engine instruments keep the pilots and flight engineer
aware of engine rpm, engine temperature, oil pressure, fuel on board,
fuel flow, manifold pressure, and carburetor pressure. Aircraft
instruments reveal to the pilots and flight engineers, air temperature,
position of landing gears and flaps, hydraulic pressure, and the like.
Flight instruments inform the pilot of his altitude, air speed, and the
attitude of the airplane. Navigation instruments help the pilot find his way
from point of departure to destination. These include clock, compass,
directional gyro, drift meters, sextant, radio, radar, and radio direction

With respect to the theory basic to the operation of the instruments
found on an airplane's instrument board, there are three groups:
mechanical, pressure, and electrical instruments.
measurement may be obtained by mechanical linkage. One kind of
tachometer uses mechanical linkage to measure the number of times
per minute (rpm) that the engine crankshaft rotates. One aircraft
instrument, the bank indicator, employs a gyroscope and rate of
change farces. The gyro-horizon, which indicates fore and aft attitude
of an airplane, is activated by these forces. The accelerometer, an
instrument used to indicate the degree of stress which might be placed
on an aircraft in flight, is an instrument activated by the force of gravity.
The drift meter, certain types of fuel level gages, and the navigator's
clock are all considered mechanical instruments.
One type employs the bourdon tube. The bourdon tube is made of
spring-tempered brass, bronze, or beryllium copper. It is curved, so that
it tends to straighten when pressure is imposed upon the fluid which it
contains and to assume its original shape when the pressure is
released. Among instruments which use a bourdon tube are
temperature gages, fuel-pressure gages, oil-pressure gages, and
hydraulic-pressure gages. Sometimes a single unit incorporates the
temperature, oil, and fuel-pressure gages of the engine.
The second type of pressure instrument employs a diaphragm. The
altimeter and the airspeed indicator are instruments of this type. The
diaphragm employed by an altimeter is sealed after all the air has been
removed from it. The diaphragm of the airspeed indicator is attached to
the pitot tube. The instrument case of the airspeed indicator is attached
to the static side of the pitot-static system. The difference between the
pressure of air upon the opening of the pitot tube and that contained in
the instrument case is indicated on the clial of the instrument as knots
or miles per hour.
An altimeter measures the air pressure at an airplane's position above
the ground. It is actually an aneroid (or diaphragm type) barometer
which reads in feet rather than in units of barometric pressure. Since
we know that as altitude increases, pressure decreases, we need only
to measure pressure to discover our approximate altitude above sea
level. We also

The Bourdon Tube
k n o w t h a t a t m o s p h e r i c p r e s s u r e i s a ff e c t e d b y t e m p e r a t u r e a n d o t h e r
w e a t h e r c o n d i t i o n s . C o n s e q u e n t l y, i t i s n e c e s s a r y t o c o r r e c t i n d i c a t e d
readings by taking temperature and pressure variations into account when
exact altimeter or airspeed indicator readings are needed. The rate of
climb indicator, the manifold pressure gage, and vaccum gages all use the
diaphragm as their pressure element.
Electrical instruments may be used instead of some of the mechanical
and pressure instruments. This is the case when the distance between
source and cockpit is comparatively great and, as a consequence, no other
method can transmit the measurements so readily. Electrical instruments
i n c l u d e t h e e l e c t r i c t a c h o m e t e r ; c y l i n d e r t e m p e r a t u r e i n d i c a t o r s ; o i l , a i r,
and coolant temperature indicators; fuel-air mixture indicator; pressure
warning units; and even the magnetic compass.
that the earth itself acts as a great magnet. A magnetized bar of iron
suspended so that it may turn freely in any direction will always align
itself so that one end points to the earth's north magnetic pole and the

The altimeter uses a diaphragm.

other end points to its south magnetic pole. Designers and builders of
magnetic compasses discovered these facts long ago and have since
that time employed them to devise instruments by which men orient
themselves in their surroundings.
Two wires made of different metals, when connected at one end, will,
when heated, generate an electric current, provided the other ends of the
wires remain at the normal temperature. This fact is recognized in the
construction of cylinder temperature indicators. The combination of
dissimilar wires in called a thermocouple. The very small voltage
generated by the thermocouple is measured by an indicating instrument
which employs the principle of the galvanometer. The galvanometer
makes use of a magnetized needle, suspended within a coil of wire, and
a flexible spring.
When a current is passed through the wire, a magnetic field is produced.
The needle attempting to align-itself with this field activates the indicator
and shows the strength of the current passing through the coil of wire,
hence, of the current generated. The ordinary thermocouple gives only
30 millivolts (30/1000 of a volt) at a temperature of 1000°F. At 500°F it
produces enough voltage to deflect the indicator, but at temperatures
below 300°F it does not provide sufficient voltage to assure accurate
readings; consequently, it is not an appropriate measure for temperature
variations below 300°F. For this reason oil, air, and coolant temperature
indicators employ the principle of the Wheatstone Bridge.
The Wheatstone Bridge makes use of the fact that the resistance of a
metal to the flaw of an electrical current changes directly as its
temperature changes. The Wheatstone Bridge converts this change-inresistance to change-in-current in order to obtain pointer deflection
across the scale of the temperature indicator.

the Wheatstone Bridge







the stations on an aircraft part, a zero reference point is selected, and
other reference points are marked off from this measured in inches and
fractions thereof, For the fuselage, the zero reference point may be at
the nose of the aircraft, at an imaginary point in front of the nose, or at
some other forward point, such as the firewall.
Among the components of an aircraft whose locations are indicated by
station number are fuel tanks and baggage compartments. To assure
the aircraft proper balance, those who assemble it are guided by
station markings. In order to load the aircraft so that proper relationship
is kept between the center of pressure and the center of gravity, the
flight engineer also must calculate the effect of the contents of fuel tank
and baggage compartments upon the lateral and longitudinal stability
of an airplane.
ASSUMED TO BE CONCENTRATED. It is usually defined in terms of
the mean aerodynamic chord.2 A line drawn through the CG and
erected perpendicular to the longitudinal axis of an airplane wiJl bisect
the wing at approximately one-third of the chord length aft its leading
edge. Airplane manufacturers specify fore and aft center of gravity
(CG) limits to guide those responsible for aircraft loading.
The center of pressure is the point at which the aerodynamic forces
acting upon the aircraft are concentrated. This concentration of
pressure forces contributing to lift, like the concentration of gravity
forces, is assumed to be upon the chord. The center of pressure (cp)
moves forward as the angle of attack increases, and moves backward
as the angle of attack decreases. For most airfoil sections (wings) the
most forward cp position is about three-tenths of the chord length aft of
the leading edge of the wing, and the most rearward cp position is fourtenths of the chord length aft of the leading edge. It should be noted
that the center of pressure and the center of gravity always must be
sufficiently close together Io assure longitudinal balance.

1 Average.
2 Chord is defined as the distance between imaginary perpendiculars erected
at the leading and trailing edges of a wing.



In order to determine whether or not the proper relationship exists
between CG and cp, the flight engineer makes use of the theory of
weight and balance. If you were to hold a weight of some kind at arm's
length from your body, the downward force of the weight expressed in
inch-pounds would be equal to the length of your arm multiplied by the
weight in pounds of the object held. For example, if you held a tenpound weight 20 inches from your body, the downward force at your
hand would be 200 inch-pounds. From this condition we can derive the
formula M=AW, where M is a product known as a moment; A is the
length of the arm in inches; and W the weight of an object in pounds.
Suppose that two weights are suspended from a bar (assumed to be
weightless) and that we want to find the center of gravity of the bar and
its weights. One weight of 10 pounds is located two inches from the left
hand end of the bar as we stand facing it; the other weight of 5 pounds
is located 32 inches from the same reference point.I In order to solve
for CG, we find it convenient to use the following table.

W (in pounds) X A (in inches) = M (inch pounds) Object
20 ,

Consequently A=12. The distance of the CG from the reference point is
found to be 12 inches.
Do you see how easy it is to locate the center of gravity of an assembly of
weights attached to a bar? It is just as easy to locate the CG of an
airplane; or to find a new center of gravity when changes are made in the
weights that the airplane carries; or to determine readjustment of cargo so
that CG limits are not exceeded. All you have to do is establish a center of
reference and proceed much as we did in the above illustration.
Assume that we want to discover whether or not the CG of a loaded
airplane exceeds the prescribed forward or aft CG limits, which for this
particular craft are 27.38 inches and 31 inches from the reference point.
Also given are the following data.

W A M Airplane
18113.92 Fuel
25 812.5 Oil
--105 Baggage
1710 Pilot
6290 Passenger
37 6290 Totals

Remember: M/W=A; A= 33276.42/1092




= 30.47 inches


Since WXA=M, M/W=A. If M=180 and W=15, M/W=180/15 or 12.

For convenience in computing the CG of an aircraft the reference point is
generally, but not always, located so that moments will all be positive.

Therefore, the number 30.47 inches is the length of the arm which, when
multiplied by the total weight (1092 pounds), will give the total moments
(33276.42 inch-pounds). Hence, 30.47 inches is the distance of the CG
from the reference point. In this case the CG is within the specified fore
limit of 27.38 inches and aft limit 31 inches from the reference point.

Heavier-than-air craft fly because their construction is based upon
principles which have been established as scientifically sound. The moving
aircraft wing reacts with the air through which it moves. As a result lift
forces are created. Other forces act upon the aircraft-in-flight. Among these
are thrust, drag, and gravity. Since forces are balanced when an aircraft
reaches cruising speed in straight and level flight, thrust is equal to drag;
consequently, the greater the drag, the greater the required thrust.
In the interest of economy of aircraft operation aeronautical engineers are
faced with the problem of reducing drag. They must also design aircraft
which are inherently stable, yet which react readily when pilots operate
aircraft controls. Aeronautical engineers need to know of the stresses
which are placed upon an airplane in flight. Materials which will withstand
the stresses of all normal flight maneuvers must be specified for use in
aircraft construction. Pilots must prevent placing stress upon aircraft in
flight which the aircraft are not built to withstand.
High speed flight has introduced new engineering and operational
problems. Research has overcome many of these problem's. Research
and testing of new aircraft design make use of such devices as the wind
Research determines the type of construction best suited for each type of
The application of scientific principles, such as those of electricity and
hydraulics, have made possible great advances in aviation and its related
fields. Aviation safety requires understanding of, and observance of, these
principles. The pilot who flies the aircraft, the mechanic who maintains it,
and the engineer who supervises the loading of it, must all, in the interest
of its successful operation, be guided by fundamental understandings of its
nature and of the aerodynamic principles which make its flight possible.


Emmett A. Botts
Director, Betts Readinq Clinic Willis C.
Specialist for Aviation Education
Division of State and Local School
Systems Office of Education
Leslie A. Bryan
Director Institute of Aviation University
of Illinois
John H. Furbay
Air World Education
Trans-World Airlines, Inc.
George N. Gardner
Educational Director
Pan American World Airways System
John L. Godwin
Associate Professor
Department of Trade and
Transportation University of Southern
California Philip S. Hopkins
Department Hard
Department of Aviation
Norwich University
D. Clay McDowell
Director, Institute of Tropical
Meteorology University of Puerto Rico
Merlyn McLaughlin, LI. Col., USAF
2662d Air Reserve Center
Ray O. Mertes
Director, School and College Service
United Air Lines
Jordan L. Larson

Kenneth E. Newland, Chairman
Occupations Division
Stephens College
W. Earl Sams, Consultant
Aviation Education
California State Department of Education
Harry C. Schmld
State Director
Vocational Division
Department of Education
State of Minnesota
Frank E. Sorenson
ProFessor of Secondary Education
Teachers College
The University of Nebraska
Roland H. Spaulding
Professor in Education in Charge
of Aeronautical Education
School of Education
New York University
Parker Van Zandt
International Staff, NATO
USRO/Defense 1
Paul A. Wilkinson
Professor, School of Aeronautics
University of Denver
Harry G. Zaritsky
Audio-Visual Division
Naval Medical School
National Naval Medical Center

Superintendent of Schools
Mount Vernon, New York