Produced by James Simmons.

This file was produced from page images at Google Books.

Transcriber’s Note

This book was transcribed from a scan of the original found at Google
Books. I have rotated some images. Some of the more elaborate tables are
treated as images.

[Illustration: Curtiss Type JN4D Biplane Used by The United States Air
Mail Service]

                              *AEROPLANE*

                           *CONSTRUCTION AND*

                              *OPERATION*

                  Including Notes On Aeroplane Design

                      And Aerodynamic Calculation,

                            Materials, Etc.

         A Comprehensive Illustrated Manual of Instruction for

           Aeroplane Constructors, Aviators, Aero-Mechanics,

                 Flight Officers and Students. Adapted

                   Either for Schools or Home Study.

                                   BY

                            JOHN B. RATHBUN

                         AERONAUTICAL ENGINEER

      Consulting Aeronautical Engineer, Chicago Aero Works; Chief
                               Engineer,

         Automotive Engineering Company. Formerly Instructor in

        Aviation and Machine Design, Chicago Technical College.

                               *CHICAGO*

                      *STANTON and VAN VLIET CO.*

                              *PUBLISHERS*

                                  1919

                           Copyrighted, 1918

                      By STANTON AND VAN VLIET CO.

AEROPLANE CONSTRUCTION AND OPERATION

INTRODUCTION

Many aeronautical books of a purely descriptive nature have been written
for the average man, but as a rule they contain little of interest for
the more serious student of the subject. Other books of a highly
technical and mathematical class have also been published, but their
contents are all but unintelligible to anyone but a trained engineer. It
is the purpose of the author to compromise between these two extremes,
and give only that part of the theory and description that will be of
practical use for the builder and flyer. The scope of the subjects
covered in this volume has been suggested by the questions asked by
students and clients, and is the result of many years’ correspondence
with beginner aviators and amateur aeroplane builders.

I have endeavored to explain the principles of the aeroplane in simple,
concise language, starting with the most elementary ideas of flight and
finishing with the complete calculations for the surfaces, power,
weight, etc. When mathematical operations are necessary they are simple
in form, and are accompanied by practical problems worked out
numerically, so that a man with even the most elementary mathematical
knowledge will have no difficulty in applying the principle to his own
work. In cases where the calculations would necessarily be complicated,
I have substituted tables of dimensions for the mathematical operations,
these dimensions being taken from a number of representative machines.

While flying cannot be taught by books, and is only the result of actual
experience, the chapter devoted to the use of controls under different
flight conditions will be of great benefit to the prospective aviator.
The portion of the book devoted to operation will be of use in flying
schools and training camps since both training methods and control
manipulation are covered in detail. In addition I have presented
considerable data on the requirements of the modern aeronautical motor.

So many new firms are now entering the aeroplane industry that there is
an ever increasing demand for trained mechanics, designers and flyers,
and many technical men now working along other lines are taking a keen
interest in aeronautical engineering. If the contents of this book will
serve to inspire the technical reader to deeper interest and practical
research in the fascinating subject of aeronautics, the author will be
more than satisfied with the result of his labor. The aeroplane is
rapidly assuming a great commercial importance, and there is no doubt
but what it will develop into an industry rivaling that of the
automobile.

To keep fully abreast of the times in aeronautic development, one should
be a constant reader of the excellent aeronautical magazines. Too much
praise cannot be given to the aeronautical press in its effort to
maintain an interest in this subject, and as with all pioneering
movements, these magazines have met with many discouragements and
financial setbacks in the earlier days of flying. To the American
magazines, "Aerial Age" and "Flying" (New York), the author owes a debt
of gratitude for the use of several of the cuts appearing in this book.
The English magazines, "Flight," "Aeronautics" and the "Aeroplane," have
been similarly drawn on. "Aviation and Aeronautical Engineering" (New
York) has suggested the arrangement of several of the tables included
herein. All of these papers are of the greatest interest and importance
to the engineer, aviator and aero-mechanic.

JOHN B. RATHBUN.

AERONAUTICAL MAGAZINES

The following list of American and English aeronautic publications will
be of interest to those who wish to keep in touch with the latest
developments in aeronautics:

  - AVIATION AND AERONAUTICAL ENGINEERING (two issues per month). A
    technical magazine published by The Gardner-Moffat Co., Inc., 120 W.
    32d St., New York.
  - AERIAL AGE (weekly). Popular and technical. The Aerial Age Co.,
    Foster Bldg., Madison Ave. and 40th St., New York.
  - AIR SERVICE MAGAZINE (weekly). Military and popular subjects.
    Gardner-Moffat Co., Inc., 120 W. 32d St., New York.
  - FLYING (monthly). Popular and military subjects. Published by Flying
    Association, Inc., 280 Madison Ave., New York.
  - AIR TRAVEL (weekly). Popular subjects. Published by Air Travel, New
    York.

ENGLISH MAGAZINES.

  - FLIGHT AND THE AIRCRAFT ENGINEER (weekly). Technical and popular.
    Published by Flight and Aircraft Engineer, 36 Great Queen St.,
    Kingsway, W.C.2, London, England.
  - AERONAUTICS (weekly). Technical and industrial. Published by
    Aeronautics, 6-8 Bouverie St., London, E.C.4, or may be had from
    1790 Broadway, New York.
  - THE AEROPLANE (weekly). Technical and popular. Published by "The
    Aeroplane," 166 Piccadilly, London, W.1.

    AEROPLANE CONSTRUCTION AND OPERATION ..............................
      INTRODUCTION ....................................................
      AERONAUTICAL MAGAZINES ..........................................
      ENGLISH MAGAZINES. ..............................................
      CHAPTER I. PRINCIPLES OF THE AEROPLANE. .........................
      CHAPTER II. TYPES OF MILITARY AEROPLANES. .......................
      CHAPTER III. ELEMENTARY AERODYNAMICS ............................
      CHAPTER IV. EXPERIMENTAL LABORATORIES. ..........................
      CHAPTER V. AERODYNAMICS OF LIFTING SURFACES (AEROFOILS). ........
      CHAPTER VI. PRACTICAL WING SECTIONS. ............................
        CORRECTION FACTORS FOR WING FORM AND SIZE. ....................
      CHAPTER VII. BIPLANES AND TRIPLANES. ............................
      CHAPTER VIII. EFFECTS OF PLAN FORM. (TANDEM AEROPLANES.) ........
      CHAPTER IX. WING CONSTRUCTION. ..................................
      CHAPTER X. WING CONSTRUCTION DETAILS. ...........................
      CHAPTER XI FUSELAGE (BODY) CONSTRUCTION. ........................
      CHAPTER XII DETAILS OF FUSELAGE CONSTRUCTION ....................
        FUSELAGE WEIGHTS. .............................................
        SIZE OF LONGERONS .............................................
      CHAPTER XIII. CHASSIS CONSTRUCTION. .............................
      CHAPTER XIV. ESTIMATION OF WEIGHT. ..............................
        WEIGHTS OF AERONAUTICAL MOTORS. ...............................
      CHAPTER XV. BALANCE AND STABILITY. ..............................
      CHAPTER XVI. HEAD RESISTANCE CALCULATIONS. ......................
      CHAPTER XVII. POWER CALCULATIONS. ...............................
      CHAPTER XVIII. PROPELLERS. ......................................
      CHAPTER XIX. OPERATION AND TRAINING. ............................
      CHAPTER XX. AERONAUTICAL MOTORS. ................................
      CHAPTER XXI. GLOSSARY OF AERONAUTICAL WORDS. ....................
        A .............................................................
        B .............................................................
        C .............................................................
        D .............................................................
        E .............................................................
        F .............................................................
        G .............................................................
        H .............................................................
        I .............................................................
        J .............................................................
        K .............................................................
        L .............................................................
        M .............................................................
        N .............................................................
        O .............................................................
        P .............................................................
        P .............................................................
        S .............................................................
        T .............................................................
        U .............................................................
        V .............................................................
        W .............................................................

CHAPTER I. PRINCIPLES OF THE AEROPLANE.

*Mechanical Flight*. Although the elementary principles of mechanical
flight are not of recent origin, the practical development of the flying
machine is confined almost entirely to the present century. Gravity
propelled gliders and small models have been flown with success from a
comparatively early date, but the first actual sustained flight with a
power driven machine was performed by the Wright Brothers in 1903. There
was no single element on this first successful machine that had not been
proposed many years before by Langley, Chanute, Montgomery, Henson,
Mouillard, and others, but this first flight must be attributed
principally to the fact that the Wrights started carefully and
painstakingly to learn how to operate (By practicing with gliders)
before starting on the first power machine. If Langley had studied the
operation of his machine as carefully as he did its theory and design,
he would have been flying long before the Wrights as his original
machine was afterwards successfully flown by Curtiss.

When once actual flight was achieved, and the success of the Wright
Brothers became generally known, the development proceeded with leaps
and bounds. All the resources of science and engineering skill were at
once applied to the new device until our present scientific knowledge of
the aeroplane compares very favorably with the older engineering
sciences. In the few years that have elapsed since the first flight, the
aeroplane holds all records for speed, endurance, and radius of action.
A great deal of the success so rapidly acquired can be credited to the
automobile and motorcycle industries, since it was the development of
the light internal combustion motors used on these machines that paved
the way for the still lighter aeronautic motor. Again, the automobile
industry was responsible for the light and powerful materials of
construction, such as alloy steel, aluminum alloys, and also for the
highly important constructional details, such as ball bearings,
pneumatic tires, carburetors, magnetos, steel tubing, etc. The special
methods developed in automobile work have helped to make the aeroplane
an immediate commercial proposition.

[Illustration: Curtiss Type JN4-B Primary Trainer]

*Types of Flying Machines*. In general, flight apparatus may be divided
into two classes, (1) The Lighter Than Air Type, such as the balloon and
dirigible, and (2) The Heavier Than Air Machine, represented by the
aeroplane, helicopter and ornithopter. The lighter than air machine is
supported in flight by "buoyancy" in much the same manner that a piece
of wood floats in water. When a balloon or dirigible, because of its
large volume, displaces a volume of air equal to its own weight, the
device will float. When the weight of air displaced exceeds the weight
of the balloon or dirigible, it will continue to rise until it reaches
an altitude where the diminished air density again results in an
equality between the weight of the device and the air displaced. At this
point it rests, or is in equilibrium. The flotation of such a device is
entirely due to static forces and hence (1) is often called an
"aerostat."

The sustenation of a Heavier Than Air Machine is due to an entirely
different application of forces. Forces in motion (Dynamic Forces) are
essential to the support of a heavier than air machine, and it is the
resultant of these forces that performs the actual lifting operation,
this resultant corresponding to the buoyant force of the aerostat.
"Dynamic" flight is obtained by an apparatus in which an arrangement of
surfaces are moved in such a way as to cause an upward component of the
forces generated by the impact of the air on the surfaces. The surfaces
drive the air down and when the force necessary for the continuous
downward deflection of air becomes equal to the weight of the machine it
is sustained in flight. Dynamic flight therefore depends on the
continuous downward deflection of masses of air, and when this motion
ceases, sustentation also ceases.

An aeroplane is provided with a deflecting surface that is fixed rigidly
in regard to the body of the machine, and the motion necessary for its
support is provided by driving the machine forward, the forward motion
being produced by the horizontal pull of air screws or propellers. It is
at once evident that the forward horizontal motion of the aeroplane must
be maintained for its support, for the surfaces are fixed and there is
no other possible way of producing a relative motion between the wings
and the air.

To overcome the objection of forward motion, several other machines have
been proposed in which the surfaces are moved in relation to the body,
as well as the air, thus making it possible for the device to stand
stationary while the revolving or reciprocating surfaces still continue
in motion in regard to the air. One type of the moving surface machine,
the "Helicopter," is provided with revolving surfaces arranged in the
form of vertical air screws or propellers, the blades of the propellers
being inclined so that they drive down a continuous stream of air and
produce the continuous upward reaction that supports the machine. While
such machines have succeeded in raising themselves off the ground they
are not yet practical flying devices. The "ornithopter" or "orthopter"
is a flapping wing machine that maintains flight after the manner of the
bird (Ornis). Like the helicopter, the ornithopter has not yet proved
successful.

[Illustration: Fig. 1. Comparison Between the Kite and Aeroplane; Fig.
2, Showing the Lift and Drag Forces Produced by the Air Stream. The
Propeller (P), Acts in a Manner Similar to the Kite String (S) in
Producing Relative Motion Between the Air and the Lifting Surfaces.]

*Principles of the Aeroplane*. In its elementary principles, the
aeroplane can be compared with the kite, as both are supported by the
impact of a horizontal stream of air. In Diagram 1, the kite surface is
indicated by X-X with the relative air stream W-W-W-W moving from left
to the right as indicated by the arrow heads. On striking the surface,
the air stream is deflected vertically, and in a downward direction, as
shown by the streams lines R-R-R-R. The reaction of the air deflection
produces the lift shown vertically and upwards by the arrow L. The kite
surface is held against the impact of the air stream by the string S so
that there is relative motion between the air and the kite, and so that
the surface will not be carried along with the air current toward the
right. If the kite were allowed to drift with the wind there could be no
relative motion between the surface and the air stream, hence the kite
would fall as soon as it attained the velocity of the wind. The
horizontal force exerted by the wind tending to carry the kite toward
the right is indicated by the arrow D and is known as the "drag" or
"drift" force. There are thus three forces, the lift (L), the drag (D),
and the resultant of the two forces indicated by the string (S). The
forces of lift and drag are nearly at right angles to one another. The
kite tail T is simply a stabilizing device whose purpose is to maintain
a constant angle between the surface and the wind and it performs an
almost negligible amount of lift.

A few more words in regard to the "relative velocity" between the
surface and wind. In the figure, the kite is assumed as being
stationary, while the wind moves from left to right. With a thirty mile
per hour wind, the relative air velocity in regard to the surface would
be 30 M. P. H. If the air particles are now considered stationary, and
if the kite is towed toward the left (opposite to figure) at 30 miles
per hour, the relative velocity between the surface and air would still
be 30 M. P. H. In other words, the kite may be stationary, or may move
in regard to the earth, but its lift is unaffected as long as the
relative motion between the surface and air remains constant. The motion
between an aeroplane and the earth depends upon the difference of the
aeroplane and wind velocities. For example, a aeroplane with a relative
speed of 60 miles per hour, flying against a headwind of 30 miles per
hour, moves 60-30 = 30 miles per hour in regard to the earth. The same
aeroplane flying with the above wind would have a velocity of 60+30=90
miles per hour past a fixed point on the earth’s surface, yet in both
cases, the relative velocity of the surface in regard to the air would
be the same.

Fig. 2 is a diagram of an aeroplane that shows the connection between
the kite and aeroplane principles. In this figure, the wing surface of
the aeroplane, X-X corresponds to the kite surface X-X. The relative air
W-WW-W striking the wing from the left is deflected down along the
arrows R-R-R-R and results in an equivalent lift force L, and a drag
force D as in the case of the kite. The resultant force required to
maintain the relative velocity between the air and wings is indicated by
D¹, opposite and equal to the drag force D. The resultant required for
overcoming the drag is provided by the screw propeller P instead of the
string S shown in Fig. 1. The propeller thrust (D¹) is parallel to the
air stream instead of being inclined as in the case of the string, but
the total effect is the same since both are "Resultants of the lift and
drag." To sustain the aeroplane, the lift (L) must be equal and opposite
to the weight shown by M. The fact that M and L are opposite and equal
makes it only necessary for the propeller to overcome the horizontal
drag, and hence the thrust can be made parallel to the air flow—or
nearly so. The aeroplane is provided with a small tail surface (T) that
corresponds to the kite tail (T). It maintains the lifting surfaces X-X
at a given angle with the air stream. The tail may, or may not aid in
supporting the machine, but in modern machines it is common to employ a
tail surface that is non-lifting under ordinary conditions of normal
flight. The body (B) contains the pilot, motive power, fuel, and such
useful load as it may be necessary to carry.

[Illustration: Fig. 3. Caudron Monoplane. Side Elevation.]

Fig. 3 shows a Caudron monoplane in side elevation. This view
illustrates the application of the principles shown by Fig. 2, except
for the vertical rudder at the rear. The latter is used for steering in
a horizontal direction. Fig. 4 shows the construction even more clearly
since it is a perspective view. The machine is a Morane "Parasol"
monoplane with the wing placed over the body. This location of the main
lifting surface is for the purpose of improving the view of the pilot
and in no way affects the principles just described. The wires shown
above the wing are bracing stays. The tail is hinged near the rear so
that the angle of the rear portion can be changed (Elevator flaps), and
permits the angle of the main wings to be altered in regard to the air
stream, thus causing the machine to ascend or descend. The tail also
damps out oscillations or vibrations due to irregularities in the air
current. The wheels and running gear (Chassis) allow the machine to be
run over the ground until the relative air speed is sufficient to
support the machine in flight.

[Illustration: Fig. 4. Morane Umbrella Type Monoplane. Courtesy of
"Flight."]

*Multiplanes*. In order to support a heavy load, and at the same time
have a small compact machine, it is necessary to have more than one
"layer" of wing surface. It is evident that the wing length or "span"
can be made much less than that of the monoplane surface shown, if the
total area could be divided into two or more parts. A machine having its
main lifting surface divided into two or more separate sections is known
as a "multiplane," this term becoming "Biplane," "Triplane," or
"Quadraplane," depending on whether there are two, three or four
independent lifting surfaces. There is almost a limitless variety of
arrangements possible, but the most common arrangement by far is that of
the biplane, in which there are two superposed surfaces as shown by Fig.
5. In this type, the two lifting surfaces are placed over one another
with a considerable "gap" or space between. The body is placed between
the wings and the tail surfaces and chassis remain the same as in the
monoplane. This is known as a "Tractor" biplane since the propeller is
in front and pulls the machine along while Fig.6 shows a "Pusher" type
biplane in which the propeller is mounted behind the wings and therefore
pushes the machine.

[Illustration: Fig. 4-A. Deperdussin Monoplane with Monocoque Body.
Gordon-Bennett Racer.]

*Biplanes*. Besides the advantages of size, the biplane has a number of
other good features. The deep spacing of the upper and lower surfaces
permits of a powerful and light system of trussing being placed in the
gap, and therefore the biplane can be made stronger (weight for weight)
than the monoplane in which no such trussing can be economically
applied. The vertical "struts" of the bracing can be clearly seen in the
figure. The efficiency of this interplane trussing greatly increases the
possible size and capacity of the aeroplane. With monoplanes it is
seldom possible to exceed a wing span of 36 feet without running into
almost unsurmountable structural difficulties. The weight of the large
monoplane also increases is leaps and bounds when this critical span is
once exceeded. To maintain an equal degree of strength the monoplane
requires very careful attention in regard to the design and
construction, and is correspondingly more expensive and difficult to
build than the biplane, although the latter has by far the greater
number of parts. By suitable arrangements in the location of the biplane
surfaces a very fair degree of stability can be obtained, an advantage
which is impossible with the monoplane.

[Illustration: Fig. 5. S. P. A. D. Tractor Biplane Speed Scout.]

A distinct disadvantage of the two superposed surfaces of the biplane is
due to the fact that there is "interference" between the upper and lower
wings, and that the lift for equal areas is less than in the case of the
monoplane. With a given form of wing, 100 square feet of monoplane
surface will lift considerably more than the same area applied in
biplane form. The amount of the "drag" for the support of a given load
is increased, and with it the amount of power required. The greater the
separation or "gap" between the wings, the greater will be the lift, but
when the gap is unduly increased to obtain a great lift the length of
the interplane bracing is increased to such an extent that the
resistance of the bracing will more than overcome the advantages due to
the large gap. There is a fixed limit to the gap beyond which it is not
practical to go. The bracing has a very material effect on the air
resistance, no matter how small the gap.

[Illustration: Fig. 6. Pusher Type Biplane in Which the Propeller Is
Placed Behind the Wings.]

*Triplanes*. Of late the triplane has been rapidly increasing in use,
and in certain respects has many advantages over either the monoplane or
biplane. This type has three superposed surfaces which still further
diminishes the size for a given area. The interference between the
surfaces is even greater than with the biplane, and hence the lift is
less for a given area and aspect ratio. This latter defect is partly, or
wholly overcome by the possibility of using long narrow wings, and
because of the reduced span there is a corresponding reduction in the
bracing resistance. It should be noted at this point that the efficiency
of a lifting surface is greatly increased when the ratio of the length
to the width is increased, that is, a long, narrow wing will be more
efficient than a short, wide shape. The relation of the length to the
width is called "aspect ratio," and will be described in more detail in
a following chapter.

[Illustration: Fig. 6-A. Farman Type Pusher Biplane.... Note the
Propeller At the Rear of Body, and the Position of the Pilot and
Passenger.]

[Illustration: Fig. 6-B. The Mann Two-Propeller Pusher Biplane. The
Propellers Are Mounted on Either Side of the Body, and Are Driven by a
Single Motor Through a Chain Transmission. This Drive Is Similar to the
Early Wright Machines.]

Fig. 7 is a sketch of a Sopwith Triplane Scout and shows clearly the
three superposed wings. The small amount of interplane bracing, and the
great aspect ratio, makes this type very suitable for high speed. The
body, tail and chassis arrangements are practically the same as those of
a biplane. The Curtiss Triplane Scout is the pioneer of this type of
machine, although experimental work on the triplane had been performed
in England by A. V. Roe many years ago. The Roe triplane was lightly
powered and for its time was successful in a way, but the Curtiss is the
first to enter into active competition against the biplane scout. Owing
to the small span required for a given area, and the possibilities of
very light and simple bracing, the triplane is an ideal type for heavy
duty machines of the "bombing" species. Enormous triplanes have been
made that are capable of a useful load running up into the tons, the
large Curtiss and Caproni’s being notable examples. As the triplane is
much higher than the biplane of equal area, the interplane bracing is
deeper and more effective without causing proportionately higher
resistance.

[Illustration: Fig. 7. Sopwith Triplane Speed Scout.]

*Quadraplane*. The use of four superposed surfaces has not been
extended, there probably being only one or two of these machines that
can be said to be successful. The small "quad" built by Matthew B.
Sellers is probably the best known. The power required to maintain this
machine in flight was surprisingly small, the machine getting off the
ground with a 4 horsepower motor, although an 8 horsepower was
afterwards installed to maintain continuous flight. The empty weight was
110 pounds with the 8 horsepower motor. The span of the wings was 18’ 0"
and the width or "chord" 3’ 0", giving a total area of about 200 square
feet.

[Illustration: Fig. 7-A. Curtiss Triplane Speed Scout. Courtesy "Aerial
Age."]

*Tandem Aeroplanes*. A tandem aeroplane may be described as being one in
which the surfaces are arranged "fore and aft." The Langley "Aerodrome"
was of the tandem monoplane type and consisted of two sets of monoplane
wings arranged in tandem. This pioneer machine is shown in Fig. 8, and
is the first power driven model to achieve a continuous flight of any
length. Instead of two monoplane surfaces, two biplane units or triplane
units can be arranged fore and aft in the same manner.

While there have been a number of tandem machines built, they have not
come into extensive use. Successful flight was obtained with a full size
Langley Aerodrome, and this machine flew with a fair degree of
stability. The failure of other tandem machines to make good was due, in
the writer’s opinion, to poor construction and design rather than to a
failure of the tandem principle. The Montgomery glider, famed for its
stability, was a tandem type but the machine was never successfully
built as a powered machine.

The wings must be separated by a sufficient distance so that the rear
set will not be greatly influenced by the downward trend of the air
caused by the leading wings. As the rear surfaces always work on
disturbed air they should be changed in angle, increased in area, or be
equipped with a different wing curvature if they are to carry an equal
proportion of the load. Usually, however, the areas of the front and
rear wings are equal, and the difference in lift is made by changes in
the wing form or angle at which they are set. In some cases the wings
are approximately the same, the difference in lift being compensated for
by moving the load further forward, thus throwing more of the weight on
the front wings.

[Illustration: Fig. 8. Langley’s "Aerodrome," An Early Type of Tandem
Monoplane.]

*The Aeroplane in Flight*. Up to the present we have only considered
horizontal flight at a continuous speed. In actual flight the altitude
is frequently varied and the speed is changed to meet different
conditions. Again, the load is not an absolutely constant quantity owing
to variations in the weight of passengers, and variations in the weight
of fuel, the weight of the latter diminishing directly with the length
of the time of flight. To meet these variations, the lift of the wings
must be altered to suit the loading and speed—generally by altering the
angle of the wings made with the line of flight.

Fig.9 shows an aeroplane in horizontal flight and lightly loaded, the
machine traveling along the horizontal flight path F-F. With the light
load, the angle made by the wings with the flight path is shown by (i),
the tail and body remaining horizontal, or parallel to the flight path.
With an increased load it is necessary to increase the angle of the
wings with the flight line, since within certain limits the lift
increases with an increase in the angle of incidence (i). Fig. 10 shows
the adjustment for a heavier load (W₂), the angle of incidence being
increased to (i’), and the body is turned down through a corresponding
angle. The increased angle is obtained by turning the elevator flaps (T)
up, thus causing a downward force (t) on the tail. The force (t) acts
through the body as a lever arm, and turns the machine into its new
position. It will be noted that when the angle of incidence is great
that the rear of the body drags down and causes a heavy resistance. This
position of a dragging tail is known to the French as flying "Cabré."
With high angles cabré flight is dangerous, for should the propeller
thrust cease for an instant the machine would be likely to "tail dive"
before the pilot could regain control. This sort of flight is also
wasteful of power. Cabré flight is unnecessary in a variable incidence
machine, the wing being turned to the required angle independently of
the body, so that the body follows the flight line in a horizontal
position, no matter what the angle of incidence may be. In this type of
machine the wings are pivoted to the body, and are operated by some form
of manual control.

[Illustration: Figs. 9-10-11-12. Showing the Use of Elevators in
Changing Angle of Incidence. - Machine Shown in Four Principal Attitudes
of Flight. As the Body and Wings Are in a Single Unit, the Body Must Be
Turned for Each Different Wing Angle.]

In Fig. 11, the large angle (i’) is still maintained, but the load is
reduced to the value given in Fig. 9. With an equal load, an increased
angle of incidence causes the machine to climb, as along the new flight
line f-f. With the load (W) equal to that in Fig. 9, the angle of
incidence will still be (i) but this will be along a new flight line if
the large angle (i’) is maintained with the horizontal as shown by Fig.
11. With the wings making an angle of (i’) with the horizontal, and
angle of incidence (i) with the flight line, it is evident from Fig. 11
that the new flight line f-f must make an angle (c) with the original
horizontal flight line F-F. This shows how an increased angle with a
constant load causes climbing, providing, of course, that the speed and
power are maintained. With a given wing and load there is a definite
angle of incidence if the speed is kept constant. Should a load be
dropped, such as a bomb, with the wing angle kept constant, the new path
of travel will be changed from F-F to f-f.

Fig. 12 shows the condition when the rear end of the body is elevated by
depressing the elevator flap T. This occasions an upward tail force that
turns the wings down through the total angle (i’). With the former
loading and speed, the angle of incidence is still (i) degrees with the
new flight path f-f, the new flight path being at an angle (c) with the
horizontal F-F. The body is turned through angle (i’), but the angle (i)
with the flight path f-f is still constant with equal loads and speeds.

To cause an aeroplane to climb, or to carry a heavier load, the elevator
"flap" is pointed up. To descend, or care for a lighter load, the
elevator is turned down. In normal horizontal flight the machine should
be balanced so that the tail is horizontal and thus creates no drag.
When the elevator *must* be used to keep the tail up in horizontal
flight, the machine is said to be "tail heavy."

*Longitudinal Stability*. In Figs. 9-10-11-12 the machine was assumed to
be flying in still air, the attitudes of the machine being simply due to
changes in the loading or to a change in altitude. The actual case is
more complicated than this, for the reason that the machine is never
operating in still air but encounters sudden gusts, whorls, and other
erratic variations in the density and velocity of the air. Each
variation in the surrounding air causes a change in the lift of the
wings, or in the effect of the tail surfaces, and hence tends to upset
the machine. If such wind gusts would always strike the wings, body, and
tail simultaneously, there would be no trouble, but, unfortunately, the
air gust strikes one portion of the machine and an appreciable length of
time elapses before it travels far enough to strike another. Though this
may seem to be a small fraction of time, it is in reality of sufficient
duration to have a material effect on the poise of the aeroplane.
Vertical gusts due to the wind passing over buildings, hills, cliffs,
etc., not only tend to upset the machine, but also tend to change the
altitude since the machine rises with an up gust and sinks with a down
trend in the Stream.

Assume a machine as in Fig. 9 to be traveling steadily along a
horizontal path in still air. A sudden horizontal gust now strikes the
machine from the front, thus causing a sudden lift in the main wings. As
this gust strikes the wings before the tail, the tail will stand at the
old altitude while the wings are lifted, thus giving the position shown
by Fig. 10. After passing over the wings it lifts the tail, this effect
probably not being sufficient to restore the wing and the tail to their
old relative attitude since the gust generally loses velocity after
passing the wings. A head gust of this type often strikes the front
wings diagonally so that it never reaches the tail at all. To remedy
this upsetting action of the gust, the pilot must move his rear elevator
so that the elevator is in the position shown by Fig. 12, that is, the
flap must be turned down so as to raise the tail.

A gust striking from behind may, or may not affect the elevator flaps,
this depending on their position at the time that the gust strikes. If
the flaps are turned up, the rear end will be raised by the gust and the
machine will head dive: if turned down, the gust will depress the tail,
raise the head and tend to "stall" the machine. If the tail is of the
lifting type, the rear entering gust will reduce the relative velocity,
and the lift, and cause the tail to drop. On passing over the tail and
striking the wings, the rear gust will reduce the velocity and cause a
loss in lift. This will either cause the machine to head dive or drop
vertically through a certain distance until it again assumes its normal
velocity.

All of these variations cause a continually fore and aft upsetting
movement that must be continually corrected by raising and lowering the
elevator flaps, and in very gusty weather this is a very tedious and
wearing job. It requires the continual attention of the pilot unless the
action is performed automatically by some mechanical device, such as the
Sperry Gyroscopic, or else by some arrangement of the surfaces that give
"inherent" stability. Control by means of the elevator flaps (which
raise and lower the body in a fore and aft direction, as shown) is known
as "longitudinal control," and when the machine is so built that
correction for the longitudinal attitude is obtained "inherently," the
machine is said to be "longitudinally stable." Modern machines can be
made very nearly longitudinally stable, and are comparatively unaffected
by any than the heaviest gusts.

*Lateral Stability*. The gusts also affect the side to side, or
"lateral" balance by causing a difference in lift on either end of the
wings. Should the gust strike one tip before the other, or should it
strike one tip harder than the other, the tendency will be to turn the
machine over sidewise. This is a more difficult problem to solve than
the longitudinal moment, although perfect inherent stability has been
attained in one or two machines without the use of additional automatic
control mechanism. Inherent lateral stability has always been attended
by a considerable loss in the efficiency of the aeroplane and speed due
to the peculiar arrangements in the main lifting surfaces. At present we
must make a decision between efficiency and stability, for one feature
must be attained at a sacrifice in the other. Contrary to the general
opinion, perfect stability is not desirable, for almost invariably it
affects the control of a machine and makes it difficult to maneuver.
Should the stability appliances be arranged so that they can be cut out
of action at will, as in the case of the Sperry Gyroscopic Stabilizer,
they will fulfill the needs of the aviator much more fully than those of
the fixed inherent type. The first thoroughly stable machine, both
longitudinally and laterally, was that designed by Lieutenant Dunne, and
this obtained its distinctive feature by a very peculiar arrangement of
the wing surfaces. It was excessively stable, and as with all very
stable machines, was difficult to steer in a straight line in windy
weather, and was correspondingly difficult to land.

[Illustration: Fig. 12.A. Diagram of the Tractor Biplane]

The first machine of the ordinary biplane type that proved inherently
stable was the R. E.-1 designed in England by Edward Busk. This machine
was flown from Farnborough to Salisbury Plain, and during this flight
the only control touched was the vertical rudder used in steering. Since
then, all English machines have been made at least partially stable, the
degree depending upon the service for which it was intended. It has been
found that in fighting, a very controllable machine is necessary, hence
stability must be sacrificed, or the control surfaces must be made
sufficiently powerful to overcome the stable tendency of the machine.
War machines are made to be just comfortably stable over the range of
ordinary flight speeds, and with controls powerful enough so that the
inherent stability can be overcome when maneuvering in battle. The
present war machine always contains an element of danger for the
unskilled pilot.

*Dihedral Angle*. This was the first lateral stability arrangement to be
applied to an aeroplane, but is only effective in still air. In rough
weather its general tendency is to destroy stability by allowing
dangerous oscillations to take place. Fig. 13 is a front view of a
monoplane in which the wings (w) and (w’) are set at an angle (d), this
angle being known as the "dihedral angle." The dotted line (m-m) shows
the line of a pair of perfectly horizontal wings and aids in
illustrating the dihedral. Assuming the center of lift at CL on the
wings, it will be seen that an increase in the dihedral raises the
center of lift above the center of gravity line C. G. by the amount (h).
With the center of gravity below the center of lift it is evident that
the weight of the machine would tend to keep it on a level keel,
although the same effect could, of course, be attained in another way.
The principal righting effect of the dihedral is shown by Fig. 14 in
which the wings (w) and (w’) are set as before. The machine is tipped or
"listed" toward the left (seen from aviator’s seat) so that wing (w’) is
down. By drawing vertical lines down until they intersect the horizontal
line X-X (the line of equilibrium), it will be seen that wing (w’)
presents more horizontal lift surface than (w) since the projected or
effective wing length (C) is greater than (b). Since (w’) presents the
greater surface, the lift (L) tends to restore the machine to its
original level position. If the wings were both set on the same straight
line, the projected lengths (b) and (c) would be the same and there
would be no restoring effect.

The dihedral would be very effective in still air, but in turbulent air,
and with the body swinging back and forth, the dihedral would act in the
nature of a circular guiding path, and thus tend to allow the swinging
to persist or increase rather than to damp it down, as would be the case
with level straight wings. Again, with the wing bent up at a
considerable angle, a side gust as at (S) would tend to throw the
machine still further over, and thus increase rather than diminish the
difficulty. In practical machines, the dihedral is usually made very
small (d = 176 degrees), the angle of each wing with the horizontal
being about 2 degrees, or even less. I think the advantage of such a
small angle is rather more imaginary than actual, and at present the
greater number of war machines have no dihedral at all. In the older
monoplanes the angle was very pronounced.

Fig. 15 shows the dihedral applied both to the upper wing (U) and the
lower wing (L), the usual method of applying dihedral to large biplanes.
Fig. 16 shows the method of applying the dihedral to small, fast
machines, such as speed scouts, the dihedral in this case being used
only on the lower wing. The dihedral on the bottom wing is usually for
the purpose of clearing the wing tips when turning on the ground rather
than for stability. A lower wing with a dihedral is less likely to
strike the ground or to become fouled when it encounters a side gust in
landing or "getting off." The use of straight upper wings makes the
construction much simpler, especially on the small machines where it is
possible to make the wing in one continuous length.

*Ailerons and Wing Warping*. Since the dihedral is not effective in
producing lateral stability, some other method must be used that is
powerful enough to overcome both the upsetting movements and the lateral
oscillations caused by the pendulum effect of a low center of gravity.
In the ordinary type of aeroplane this righting effect is performed by
movable surfaces that increase the lift on the lower wing tip, and
decrease the lift on the high side. In Some cases the lateral control
surfaces are separate from the wing proper (Ailerons), and in some the
tip of the wing is twisted or "warped" so as to produce the same effect.
These control surfaces may be operated manually by the pilot or by some
type of mechanism, such as the gyroscope, although the former is the
method most used. The lateral control, or side to side balancing of an
aeroplane, can be compared to the side to side balancing of a bicycle in
which the unbalance is continually, being corrected by the movement of
the handle bars.

Fig. 17 shows the control surfaces or "Ailerons" (A-A’), mounted near
the tips, and at the rear edge of the wing W. As shown, they are cut
into and hinged to the main wings so that they are free to move up and
down through a total angle of about 60 degrees. In a biplane they may be
fitted to the upper wing alone or to both top and bottom wings,
according to conditions. For simplicity we will consider only the
monoplane in the present instance.

In Fig. 18, a front view of the monoplane, the machine is shown "heeled
over" so that the wing tip (w) is low. To correct this displacement, the
aileron (A) on the low side, is pulled down and the aileron (A’) on the
opposite end is pulled up. This, of course, increases the lift on the
low end (w) and decreases the lift on the high side (w’). The righting
forces exerted are shown by L-L’. The increased angles made by A-A’ with
the wind stream affects the forces acting on the wings, although in
opposite directions, causing a left hand rotation of the whole machine.
In Fig. 19, conditions are normal with the machine on an even keel and
with both ailerons brought back to a point where they are level with the
surface of the wing, or in "neutral." Fig. 20 shows the machine canted
in the opposite direction with (w’) low and (w) high. This is corrected
by bringing down aileron (A’) and raising (A), the forces L-L’ showing
the rotation direction. By alternately raising and lowering the ailerons
we can correspondingly raise or lower the wing tips. It should be noted
here that in some machines the ailerons are only single acting, that is,
the aileron on the low side can be pulled down to increase the lift, but
the opposite aileron remains in the plane of the wings, and does not
tend to "push down" the high side. Since all of the aileron resistance
in a horizontal direction is now confined to the low side, it turns the
machine from its path, the high wing swinging about the lower tip with
the latter as a pivot. In the double acting control as shown in Figs. 17
to 23, the resistance is nearly equal at both tips and hence there is no
tendency to disturb the flight direction. With single acting ailerons,
the directional disturbance is corrected with the rudder so that when
the aileron is pulled down it is necessary to set the rudder to oppose
the turn. On early Wright machines the rudder and lateral controls were
interconnected so that the rudder automatically responded to the action
of the ailerons.

Fig. 21 is a detailed front elevation of the machine and shows the
control wheel (C) and cable connections between the wheel and the
ailerons A-A’. When the wheel C is turned in the direction of the arrow
K, the aileron A’ is pulled down by the flexible cable (i), and a
corresponding amount of cable (h) is paid off the wheel to the rising
aileron A. Aileron A is pulled up by the connecting cable (e) which is
attached to A’ at one end and to A at the other. Pulleys (f) and (g)
guide the interconnection cable. On turning the wheel in the opposite
direction, aileron A is pulled down and A’ is elevated. In flight,
especially in rough weather, there is almost continuous operation of the
control wheel. Figs. 22 and 23 are sections taken through the wing W and
the ailerons, showing the method of hinging and travel. Fig. 22 shows
the aileron depressed for raising the wing in the direction of L, while
Fig. 23 shows the aileron lifted to lower the wing. In normal flight,
with the machine level, the aileron forms a part of the wing outline (in
neutral position).

In the original Wright aeroplane, and in the majority of monoplanes, no
ailerons are used, the rear of main wing tip being bent down bodily to
increase the lift. This is known as "wing warping," and is practically a
single acting process since the depressing force on the high tip is
seldom as effective as the lift on the low. Warping is not generally
used on modern biplanes since it is impossible to maintain a strong
rigid structure with flexible tips. The control warping and twisting of
such wings soon loosens them up and destroys what remaining strength
they may have had.

[Illustration: Figs. 17-23. Showing Use of Ailerons in Maintaining
Lateral Balance.]

*Banking and Turning*. In making a sharp turn the outer wing tip must be
elevated to prevent slipping sidewise through the effects of the
centrifugal force (side slip). This is known as "banking." The faster
and sharper the turn, the steeper must be the "bank," or the angle of
the wings, until in some cases of "stunt" flying the wings stand almost
straight up and down. Should the bank be too steep there will be an
equal tendency to slip down, and inwardly, since the end resistance
against side slip is very slight. Some aeroplanes assume the correct
angle of bank automatically without attention from the pilot since the
extra lift due to the rapid motion of the outer tip causes it to rise.
On other machines the natural banking effort of the machine is not
sufficient, and this must be increased by pulling down the aileron on
the outer wing tip. Machines that have a tendency to "over-bank" must
have the ailerons applied in the reverse direction so as to depress the
outer tip. In cases of under, or overbanking machines it formerly
required experience and judgment on the part of the pilot to obtain the
correct banking angle. There are now "banking indicators" on the market
that show whether the machine is correctly banked or is side-slipping.

[Illustration: Fig. 24. A Deperdussin Monoplane Banking Around a Sharp
Turn at High Speed. Note the Elevation of the Outer Wing Tip and the
Angle Made with the Horizontal by the Wings. Speed, 105 Miles Per Hour.]

[Illustration: Standard "H-3" Training Biplane.]

CHAPTER II. TYPES OF MILITARY AEROPLANES.

*Divisions of Service*. In the army and navy, aeroplanes are used both
for offensive and defensive operations. They must carry out their own
work and intentions and prevent hostile craft from carrying out theirs.
In offensive operations the machines fly continuously over the enemy’s
country and attack every hostile craft sighted, thus creating a danger
zone within the enemy’s lines where no opposing machine can work without
being threatened with an overwhelming attack. The offensive also
includes bombing operations and the destruction of supply depots and
transportation centers. Defensive aerial operation consists in driving
out the enemy craft from our own lines, and in protecting working
machines when on photographing or observation trips. With a powerful
offensive there is of course little need for defense. The former method
is a costly one, and is productive of severe material and personal
losses.

At the present time there are eight principle functions performed by
military aeroplanes:

  1. Offensive operations against enemy machines.
  2. Reconnaissance, observation, special missions.
  3. Bombing supply centers, railways, etc.
  4. Photography.
  5. Spotting for the artillery.
  6. Signalling for infantry operations.
  7. Submarine hunting.
  8. Patrol and barrage.

[Illustration: Fig. 1. Curtiss "Baby". Biplane Speed Scout. Equipped
with 100 Horsepower Water Cooled Motor.]

Probably the most important service of all is performed by machines
under heading (1). If a successful offensive can be maintained over the
enemy’s lines he is unable to intelligently direct his artillery fire,
and can obtain no information regarding reinforcements, or troop
concentrations for an impending attack. With fighting aeroplanes
clearing the way for our own observation machines and artillery
spotters, the enemy is not only blinded, but is blocked in any attempt
to attack or concentrate his forces. The fact that the French aerial
offensive at Verdun was so efficiently and well maintained accounts for
the failure of the heavy German artillery. Driven far back over their
own lines, the German aviators were seldom able to observe the placing
of the shells, and as a result their gunners were practically trusting
to luck in reaching their target. An immediate and accurate bombardment
always followed one of the very infrequent German air raids over the
French lines. Whenever the French, partially abandoned their aerial
offensive in favor of a defensive campaign, they soon lost their mastery
of the air. As long as enemy machines can be kept back of their own
lines, new trench systems can be constructed, transportation lines can
be extended and ammunition dumps arranged, undertakings that would be
highly precarious with enemy observation machines continually passing
overhead.

[Illustration: Fig. 1. Italian "Pomilio" Two Seater Biplane. Courtesy
"_Flying_."]

To maintain an effective offensive places a tremendous strain on both
the men and the machines, for though the aeroplanes do not penetrate far
beyond the lines they usually meet with superior numbers, and in
addition are continually in range of the anti-aircraft guns. In an
attack over hostile country a slight mishap may cause the loss of a
’plane, for usually the distance from its base is so great as to prevent
a gliding return. Over its own lines an engine failure is usually only a
temporary inconvenience. Fighting aeroplanes, for the offensive, are
small high powered machines generally of the single seater type, and are
capable of high horizontal and climbing speeds. The armament consists of
a machine gun of the Lewis type, and occasionally a few light bombs may
be included in the equipment. As they do not carry out operations far to
the rear of the enemy’s lines they are provided with fuel for only two
or three hours, and this reduced fuel load is necessary for the high
speeds that must necessarily be attained. The area is limited to permit
of quick maneuvers in attack and escape, and at the same time to reduce
the head resistance and weight. The horizontal speed may run up to 150
miles per hour, with a climbing velocity that may exceed 1,000 feet per
minute. Such machines are variously known as "Speed Scouts," "Chasers,"
or "Pursuit Type" (French "DeChasse"). At the beginning of the war the
chasers were largely of the monoplane type, but at present the biplane
is in almost exclusive use.

[Illustration: Fig. 2. Machine Gun Mounting on Morane Monoplane. Gun
Fires Directly Through the Propeller Disc. The Deflecting Plate Attached
to the Root of the Propeller Blade Protects the Propeller When in Line
of Fire. Ammunition in This Gun Is Furnished in Straight Strips or
"Clips."]

The aeroplane employed for surveys of the enemy country and battle front
(2) are of an entirely different type and are much larger and slower.
These "Reconnaissance" machines are generally of the two-seater type,
the personnel consisting of an observer and the pilot, although in some
cases a third man is carried as an assistant to the observer, or to
handle a machine gun against an attack. Since their speed is
comparatively low, they are generally provided with an escort of
chasers, especially when employed on distant missions, this escort
repelling attacks while the observations are being made.

[Illustration: Fig. 2-a. Machine Gun Mounting on S. P. A. D. Biplane.
Gun Is Rigidly Attached to Fuselage Top in Front of Pilot.]

For accurate observation and mapping, the speed of an observation
machine must be necessarily low, and as they are additionally burdened
with a wireless set, an observer, a large fuel reserve, and other
impedimenta, they have a comparatively great area and are therefore
lacking in the maneuvering qualities of the chaser. The span will
average about 40 feet, and the weight carried per horsepower is greatly
in excess of that of the chaser. From a number of examples, the
reconnaissance type will average from 16 to 18 pounds per horsepower,
while the loading of the scout is from 8 to 12. This means that the
former has comparatively little reserve power for rapid climbing. The
present reconnaissance type is always armed, and must not be confused
with the early machine by that name, which, in fact, was merely an
enlarged training machine and had neither offensive nor defensive
powers. The Observer acts as gunner, and is located at a point where he
has the greatest possible range of vision, and where the angle of fire
is as little obstructed as possible.

The radius of action, or the distance traveled per tank of fuel, is
greater with the reconnaissance than with the chaser, present machines
having a capacity of from 10 to 12 hours on a single filling at normal
flight speed.

In bombing operations (3), the loading is very heavy and consequently a
"Bomber" must be a weight lifter to the exclusion of all other
qualities. Not only is the bomb load requirement severe, but the fuel
load is also of great importance, since bombing is usually carried out
at considerable distances from the base. Such machines may carry from
three to six men. All this calls for a tremendous area and a large power
plant. The Handley-Page "Giant," and the Caproni Triplane are examples
of Allied machines of this type while the German "Gotha," used in the
London air raids, is an equivalent enemy machine. As an example of the
weight carrying capacity of a typical bomber, the Handley-Page has
carried a test crew of 21 men, or a personnel load of 3,570 pounds. The
total weight, fully loaded, has been given as 11,500 pounds with a power
plant of 540 horsepower. The maximum speed is 90 miles per hour with a
climbing velocity of about 330 feet per minute. Duration is about 5%
hours at normal speed and full load.

[Illustration: Fig. 3. Handley-Page "Giant" Bombing Type Biplane.
Courtesy "_The Aeroplane_."]

[Illustration: Fig. 4. Curtiss Twin Motor Biplane-Type JN.]

Bombing is of great importance, not only because of the damage caused to
munition factories, transportation lines, store houses, etc., but also
because of the moral effect on both the enemy troops and the civil
population. A well-timed bombing raid will do more to disorganize an
army than almost any other form of attack, and this is attended with a
much less loss of life, and with less cost and equipment. Points in
enemy territory that could be reached in no other way are readily
attacked by bombing planes with all the disastrous effects of heavy
artillery fire. The aeroplane is better adapted for this service than
dirigibles of the Zeppelin type, for they require fewer men for their
operation, and in addition cost less to operate and build.

[Illustration: Fig. 4a. Curtiss "Wireless" Speed Scouts (S-2). By an
Ingenious Arrangement of the Interplane Struts There Is No Exposed Wire
or Cable.]

Bombing operations against well protected objectives are best made at
night since there is less chance of loss through anti-aircraft gun fire,
and also because of the difficulty that the defense machines have in
locating the raiders. Even when well equipped with searchlights and
listening stations, it is not the easiest thing in the world to pick out
and hold the location of an attacking squadron, for the searchlights
immediately betray themselves and can then be put out of action by fire
from the invaders. With the searchlights out of commission, it is almost
impossible for the defending chasers to locate and engage the raiders,
even before the bombs have been dropped. After the bomb dropping has
been accomplished (and with comparative accuracy because of the flares
dropped by the bombing party), the raiders are lightened of a
considerable portion of their load, and are correspondingly increased in
their ability to climb and to evade the enemy chasers.

[Illustration: Fig. 5. Sopwith Speed Scout or "Chaser."]

[Illustration: Fig. 6. Nieuport Biplane Scout with Machine Gun Pivoted
Above the Upper Wing. This Gun Fires Above the Propeller.]

Night flying in squadrons always introduces the danger of collision, and
to minimize this danger, by decreasing the number of machines, the size
and carrying capacity of the bombers has been continually increased.
Again, bombing requires the steady platform that only a large machine
can give, and for accuracy the span and area must be greater than that
of the reconnaissance type. In night flying a large machine is safer to
handle owing to its lower landing speed and ability to come to rest
quickly after landing, and this is of the greatest importance when
landing outside of the aerodrome. For daylight work at comparatively
short distances the smaller bomb carrier used at the beginning of the
war is probably preferable as it has better maneuvering qualities, and
as the bombs are divided among a greater number of machines they are not
so likely to be defeated before accomplishing their object. Because of
their great size, these bombing aeroplanes are nearly always of the
"twin motor" type with two, or even three, independent power plants. The
use of a twin power plant is an added insurance against forced landings
in hostile country, or over unsuitable ground, and even with one dead
engine the machine can be flown home at a fair speed.

[Illustration: Fig. 8. Fokker Synchronized Machine Gun. The Gun Is
Driven by the Motor in Such a Way That the Bullets Pass Between the
Propeller Blades. "_L’Aerophile_."]

"Spotting" for the guidance of the artillery is a duty usually performed
by the reconnaissance type, or small bombing type, and is usually done
under the escort of chasers. Their duty is to direct the battery as to
the placing of shots. The ideal machine for such a purpose would be the
direct lift type similar to a helicopter which could hover over one
particular spot until its object had been accomplished in making
measurements, and plotting enemy positions. Since no such machine is at
present available, the duty must be performed by a low speed aeroplane,
that is large enough to provide a fairly steady platform and at the same
time has sufficient speed for a quick getaway. A dirigible has the
necessary hovering qualities but lacks the speed necessary for avoiding
attack from even the slowest of aeroplanes, and in addition is a
magnificent target for anti-craft guns if kept at an altitude low enough
for accurate observation. A large speed range is a desirable
characteristic in such service.

Photography is of the greatest importance in reconnaissance, since the
camera distinctly records objects on the terrain, so small and obscure
that they may entirely escape the eye of the observer. Again, the
photograph is a permanent record that may be studied at leisure in
headquarters, or may be used in comparisons with photographs taken at an
earlier date in the same territory. Thus changes in the disposition of
enemy batteries, trenches, and troops can be quickly identified. With
modern aeroplane photographic equipment, a vast territory may be
investigated and mapped out by a single machine in a few hours.
Camouflage has but few terrors for the camera, and the photographs often
clearly reveal that which has been passed over time and time again by
the observers. When sent out on a specific mission, the aeroplane
returns the films in an amazingly short length of time, and within a few
minutes they are developed and are ready for the inspection of the
officers in charge. The analysis of these photographs has rapidly
developed into a science well worthy of a Sherlock Holmes. Changes in
the position of shadows, suspiciously sudden growths of underbrush,
changes in the direction of paths, and fresh mounds of earth all have a
definite meaning to the photographic expert.

[Illustration: Fig. 9. Types of Aeroplane Bombs. The Tail Surfaces Guide
the Bomb So That It Strikes on the Firing Pin and at the Same Time
"Safeties" the Bomb So That it Will Not Explode Until it Has Fallen for
Some Distance. In Falling, the Tail Blades Rotate and Release the Firing
Mechanism After the Bomb Has Fallen Clear of the Aeroplane. Courtesy of
"_Flying_."]

[Illustration: Fig. 9-a. Curtiss "JN" Twin Motor Biplane. Observer Is
Seated in Front.]

In the navy the aeroplane has proved of much value in scouting and
particularly in defense against the submarine. Because of its great
speed it has a daily radius of action many times that of a torpedo boat,
and because of its altitude the effective range of vision is still
further increased. At a fair height the observer can easily detect a
submarine even when submerged to a considerable depth, a feat impossible
when near the sea level. For disclosing the conditions existing in an
enemy harbor the aeroplane is fully the equal of the dirigible since it
can approach and retreat rapidly, and without much danger at
comparatively low altitudes. While the dirigible can float indefinitely
at one point, it must be done at an altitude that is safely out of range
of the enemy guns, and this is usually at a point where observation is a
difficult proposition. It does not take long to get the range of such a
target as a hovering dirigible, yet at a much lower altitude it is
difficult to handle naval anti-aircraft guns effectively against a
speeding aeroplane. The smaller scouting seaplanes can report the
position of a submarine to a torpedo boat or "sub-chaser," while the
larger machines are perfectly capable of dealing with the submarine at
first hand. On the large bombing type, a three-pounder gun and a number
of large bombs can be carried, either of which would be sufficient for
the purpose.

[Illustration: Fig. 10. Explosion of a German Aeroplane Bomb Near
Mesopotamia. Courtesy of "_Flying_."]

In land defense chasers and fighters are used for patrol, and to
maintain a barrage against the entrance of enemy machines into our
lines. The patrol machines work along the front line trenches, while the
machines maintaining the barrage are generally arranged in two parallel
lines back of the trenches, the first being about five miles, and the
second about ten miles from the front. All three lines are generally
placed between the enemy and the principal stations and railroad centers
to insure protection from enemy bombers and reconnaissance machines.
Should the first line patrol fail to keep raiders from crossing the
first line trenches, they will have to pass through at least two more
zones of organized fighting squadrons before reaching a vulnerable spot
in our lines. The machines used for patrol and barrage are of the high
speed and fast climbing chaser type. The response to an attack involves
rapid climbing, and a high degree of maneuvering.

[Illustration: Fig. 11. Caproni Triplane with Three Independent Power
Plants. The Motor in the Central Body Drives a Pusher Propeller, While
the Other Power Plants Are Mounted in the Two Outer Bodies and Drive
Tractor Screws. This is an Example of the Larger Bombing Aeroplanes. The
Gun Is Mounted in the Front of the Center Body. Courtesy "_The
Aeroplane_."]

Except for the bombers and battle planes, the machine gun or
"Mitraleuse" has been the only form of arm in common use on aeroplanes.
These use ammunition approximating service rifle caliber and are
furnished in bands, strips or drums according to the type of gun. With
larger guns, the weight of the ammunition has been found excessive with
all but the largest bombing machines, and the recoil of a large caliber
gun has also been difficult to overcome. In a modern American aeroplane
gun of large caliber the recoil has been reduced to almost a negligible
degree, even up to the four-pounder size, by a system of balanced
projectile reactions. This gun has met successful tests, but whether it
has met with general adoption would be difficult to say at the present
time. In Europe, large caliber aeroplane guns have been used on large
"battle planes" or "gun planes" for shelling dirigibles, or in
destroying searchlight stations in bombing raids. The battle planes are
nearly always of the "Twin" type with the gun mounted in the front end
of the fuselage.

*Summary of Types*. To sum up the types required in military operations,
we have: (1) High speed "Chaser" or "Scout" (Single seater), (2) High
speed "Chaser" (Two-seat type), (3) Reconnaissance type, (4) Bombing
type, (5) Gun or Battle Planes. This does not include the training
machines of the two place and "Penguin" types, but as these are simply
unarmed modifications of the two place reconnaissance and single seat
machine, respectively, we will not go into further details at this point
regarding their construction.

*The Chaser or Pursuit Type*. The most important factors in the design
of a chaser are speed and maneuvering ability. The speed must be at a
maximum in both the horizontal and vertical directions, for climbing
ability is fully of as much importance as horizontal speed. Second in
importance is the maximum altitude or "Height of ceiling" to which the
machine can ascend. This maximum "Ceiling" generally goes hand in hand
with the climbing speed, since a fast climber generally has a
correspondingly high maximum altitude. The combination of weight and
head resistance must be such that the climb interferes as little as
possible with the forward velocity.

[Illustration: De Havilland. V. Single Seat "Chaser" or "Speed Scout"
with a Single Rigidly Mounted Machine Gun on Top of Hood (To Left). It
will Be Noted That the Top Wing is Staggered Back Instead of Forward as
in the Usual Type, Thus Allowing the Pilot to Look Directly Up and in
Front of the Top Wing. Dimensions Are in Millimeters.]

Great climbing ability means a large power reserve, hence the weight
carried per horsepower is reduced to from 8 to 12 pounds on the fastest
machines, against the 16 to 18 pounds carried on the larger and slower
reconnaissance types. Strength must be sacrificed to meet these
conditions, so that instead of having a safety factor of from 8 to 12 as
in the larger machines, it is cut to about 5.5, or in other words, the
strength is relatively only half that of the usual type of aeroplane.
This great reduction in strength calls for careful handling, especially
in landing, and also painstaking care in the design and choice of
materials.

High speeds and maneuvering ability both call for small wing areas and
short spans, the areas being so adjusted that the resistance is at a
minimum at the highest speeds. The short spans have a minimum of exposed
interplane bracing and thus indirectly reduce both the head resistance
and the weight. Unfortunately, the most favorable areas at high speeds
are too small for safe landing speeds. With a fixed area, the minimum
landing speed is only a little less than one-half of the maximum flying
speed, hence with a maximum of 150 miles per hour the minimum will
probably be little less than 70 miles per hour. The most efficient wing
sections, and the greatest refinement in the body design, bracing, and
chassis are necessary at speeds of over 100 miles per hour. All other
conditions being equal, the resistance varies as the square of the
velocity, hence at 150 miles per hour, the resistance is 2.25 times that
at 100 miles per hour.

The following table gives approximately the apportionment of the head
resistance producing items in a typical speed scout or chaser.

    ─────────────────────────────────────────────────────────────────
    Body (Fuselage)                                     68 per cent
    ─────────────────────────────────────────────────────────────────
    Chassis, wheels, struts, etc.                       15 per cent
    ─────────────────────────────────────────────────────────────────
    Tail, rudder, fin, elevator                         5 per cent
    ─────────────────────────────────────────────────────────────────
    Wing structure, struts, wire, fittings.             12 per cent
    ─────────────────────────────────────────────────────────────────

The aerodynamic drag due to the lift of the wings is not included in the
above, the useless or parasitic resistance alone being considered. It
will be noted that the body causes by far the greater part of the
resistance, and as a result, the body of the speed scout requires the
most careful attention in regard to streamline form. Fortunately this is
possible with the short stumpy body of the chaser, since a true
streamline form approximates the average body dimensions of the scout.
In the larger machines, the body resistance is not as great in
proportion to the other items since there are more struts and stay
wires, the chassis is larger, and the tail surfaces are of greater area.
The chassis is the next largest item and is one of the most difficult
items to reduce. It has been suggested by several people that the
chassis could be stored away in the body while in flight, but this adds
additional mechanism and weight, and any automatic mechanism for folding
up the chassis members would likely prove unreliable.

*Chaser Armament*. A single seat chaser is provided with one or two
machine guns mounted on top of the fuselage, and directly in front of
the pilot, the length of the barrel being parallel with the fore and aft
center line. They may either be fixed rigidly to the fuselage top, or so
that they can be pointed up, and over the top of the upper wing. With
the machine guns fixed rigidly to the body, as in the early chaser
monoplanes used by Garros and Vedrines, it was necessary at all times to
fire directly through the disc area swept out by the propeller.

Two plans were tried for preventing the propeller from being broken by
the bullets. The first consisted of a device operated by the motor that
stopped the gun whenever the propeller blade came within the path of the
bullets. This early mechanism proved unreliable, since the frequent
stopping, with the propellers running 1200 revolutions per minute, soon
put the apparatus out of order. Soon after the failure of this method,
designers mounted curved protective steel plates on the inner portions
of the propeller blades at points where they were likely to be struck
with bullets. According to calculations in probability and chance, only
one bullet out of every eighteen will strike the protective plate on the
propeller blade, and hence only one out of eighteen bullets will be
wasted. This, however, was a makeshift, and on modern machines the gun
is driven, or "Synchronized" with the motor so that the bullets pass
between the blades.

[Illustration: Curtiss Biplane in Flight. Taken from Another Machine.
Courtesy "_Aerial Age_."]

Many modern single seat chasers have the gun pivoted to the top of the
fuselage so that the pilot can fire above the top plane and to either
side of the body. This does away with the difficulty of keeping the
machine headed directly at the enemy when in action, a method that is
imperative with the fixed type of gun. Two seater chasers are generally
arranged so that the gunner is seated back of the pilot, and the gun is
so pivoted and supported that it can be swung through a wide radius both
toward the front and on either side. This freedom of gun action at least
partly compensates for the slower maneuvering qualities of the two
seater type, since the gun may be swung with the target through quite a
range of field, and without changing the flight direction of the
machine. A gun of this type is provided with stops which prevent the
gunner from shooting into the outlying parts of his own machine. The gun
mounting in many cases of two seater construction consists of a light
circular track that runs around the edge of the cockpit opening. The gun
standard runs on this track, and the gun is pivoted at the upper end of
the standard so that the muzzle can be raised or lowered. The gun turns
in a horizontal plane by sliding on the track, and can be followed
around by the gunner who is seated in the center on a pivoted seat. With
this mounting it is possible to guard against a rear attack, to shoot
straight up, or nearly straight down over the sides of the fuselage.

In a few machines of the two seater type, two machine guns are provided,
one pivoted gun in the rear, and one gun rigidly fastened to the
fuselage in front of the pilot. It is very seldom that both guns can be
brought into action at once unless engaged with a number of enemy
machines, although the front gun is handy in pursuit, and at a time when
the rear gun is ineffective because of the pilot in front of him. Even
with the double equipment, the superior maneuvering qualities of the
single seater makes matters more even than would commonly be supposed.
An added advantage of the single seater is that it is smaller and
therefore more difficult to hit.

English speed scouts have largely adopted the American Lewis gun. The
cartridges in this gun are arranged radially in a circular drum, and are
fed to the gun as the drum revolves. The drum is mounted on the barrel
near the breech and is operated automatically by the successive
explosions. This feeds the cartridges and rejects the empty shells
without the attention of the pilot. It fires about 600 shots per minute.
When one drum is exhausted, another drum of new cartridges can be
quickly and easily inserted. The French use the belt system to a large
extent. In this system the cartridges are attached side by side on a
cotton web belt as in the older types of army machine guns. As in the
Lewis gun, the cartridges are fed automatically by the recoil of the
explosions, and the belt moves through the breech with a step by step
movement until the ammunition is exhausted. This is not nearly as
compact an arrangement as the Lewis gun, and is more difficult to pivot
on account of the dangling belts.

On the right hand side of the Nieuport body there is a drum on which the
belt with the loaded cartridges is wound. The empty end of the belt is
wound on a drum at the left, this drum being provided with a spring to
keep the belt taut. The empty cartridges are discharged through a tube
that passes through the side of the body. On the 1916 Fokker the gun is
of the Maxim type, and is immovably fastened above the engine cowl and
slightly to the right. To fire the gun, the pilot presses down a small
lever fastened to the control column, and from this lever the connecting
Bowden wire closes the motor clutch and starts the gun. A cam is fixed
to the motor shaft in relation to the propeller blades. When firing, the
elevator control is locked fore and aft, while the lateral control
movement is operated by the pilot’s knees. Steering is by the action of
his feet on the rudder bar. Thus the pilot can balance laterally, and
steer with his hands free for the manipulation of the gun, but he cannot
change his elevation.

[Illustration: Aeromarine Training Seaplane]

*Power Plant of the Chaser*. In the smaller speed scouts, the motor is
of the rotary air cooled type, the output ranging from 80 to 110
horsepower, but as the power demands increased the water-cooled motor
came into use, and at the present time has found favor with a large
number of builders. When the power exceeds about 120 horsepower it is
difficult to thoroughly cool the rotary engine, and although the Gnome,
Clerget and Le Rhone are extremely desirable on a chaser because of
their light weight, they cannot be used profitably on the larger scouts.
Up to the present time, the Nieuport and Sopwith use the Clerget and Le
Rhone rotary motors, but the S. P. A. D. and several others have adopted
the water-cooled type. Nearly all of the German chasers, such as the
Roland and Albatros, are water-cooled. Such motors must weigh well below
3 pounds per horsepower if there is to be sufficient power reserve for
fast climbing. The Curtiss scouts are also water-cooled, although the
rating is only 100 horsepower. The French and German machines are very
heavily powered, motors of 175 horsepower being very common, even on
single seaters. The fuel capacity is very limited, probably not
exceeding 2.5 to 3 hours in any case.

*General Dimensions of Scouts*. The following table will give a better
idea of the principal characteristics of these machines. It gives the
overall dimensions, power, speed, climb, etc. It will be noted that the
Nieuport biplane scouts have a smaller lower chord (*). The speeds given
are the sea level speeds since a great change in altitude affects the
performance to a marked degree.

*Reconnaissance Type Arrangement*. These machines are almost invariably
of the two seater type, and are equipped either with one machine gun for
the observer, or with a rigidly fixed gun for the pilot and a pivoted
gun at the rear for the observer. In the majority of cases the observer
is seated in the rear cockpit (Tractor types), and at a point where he
has a greater visual radius and field of fire. With the pusher type, the
observer is, of course, seated in the extreme front of the body, where
he has an extremely wide angle of vision. The pilot in the rear seat of
the pusher is effectually screened from any gun action, either from the
front, side or rear, as the propeller cuts off the field at the back and
the observer and interplane bracing blocks the way at the front and
sides. The observer’s cock-pit is equipped with the signalling
apparatus, photographic equipment, map boards, etc., as well as the
ammunition for the gun. The pilot’s compartment contains the navigating
instruments and controls.

*Armament*. At the beginning of the war nearly all of the French two
seaters were of the pusher type, this arrangement, of course, resulted
in almost a completely dead angle of fire in the rear, and a front
horizontal angle that was practically restricted to 160 degrees. Owing
to the forward position of the gun the vertical angle was quite good,
230 degrees or even better. In the tractor two seater, with a single
movable gun mounted "En barbette" at the rear, the horizontal angle is
about 180 degrees, but the vertical angle is less than with the pusher
type. When the rear gun is supplemented with a front rigidly mounted
gun, there is some protection at the front, but the rigid gun is far
from being as effective as the pivoted rear gun. The front gun of course
fires through the propeller. This armament is used by the German
machines "Aviatic," "Rumpler," "Albatros," and "L. V. G." The forward
rigid gun is usually of the infantry type, while the movable rear gun is
lighter. The latter is fed by drums, or rolled bands on spools, so that
reloading can be performed in the wind stream.

With the two seater type used in reconnaissance, artillery spotting, or
photography, the power is generally in the neighborhood of 220-260
horsepower, and the speed varies from 85 to 100 miles per hour. The area
is approximately 400 to 480 square feet. A single engine is generally
used.

*General Dimensions and Speeds*. Reconnaissance machines of various
types and makes are listed in the following table. A pusher is indicated
by (P) and a tractor by (T). The German aeroplanes (G), and the Allied
aeroplanes (A), are both listed for comparison: It will be noted that
several types of machines have been made by the same firms, and that in
some cases the same machines have different power plants. The Albatros
C-III has been furnished with both the 170 and 220 Mercedes motor. The
Ago biplane has a tapering wing, and the chord width (*) given is taken
at the body. While very recent machines cannot be described, because of
certain restrictions, the horsepower of the latest two seaters will
average about 240 horsepower. If the dates and power items are noted, it
will be seen that the machines used in 1917 have much  larger motors
than those built in 1916. The weight per square foot of surface will
average about 6.5 pounds. The loading per horsepower rarely exceeds 17.0
pounds.

*Bombing Type Aeroplanes*. These large aeroplanes are fitted with either
two or three independent power plants. The German bombers are
represented by the Gotha, A. E. G., Friedrichshafen, and Rumpler G,
while the Allied bombers are the Caproni, Handley-Page, Farman, Voisin,
etc. The speed is about that of the reconnaissance type, and will seat
three or more men. The motors average 500 560 horsepower per power
plant, and the wing area is usually well over 1,000 square feet. The
small two seaters are generally equipped with two pivoted machine guns,
while the three seaters have a third machine gun arranged so that it can
be lowered and fired through a trap door in the bottom of the body.
Defense may thus be had from the rear, or below. In some of the pusher
types, a rapid fire gun of comparatively heavy caliber is mounted at the
front of the body in place of the usual machine gun. This is usually the
case with the sea planes used for submarine chasing.

In addition to bombing operations, these large machines are also used
for the protection of "spotting" aeroplanes, or for the direct
protection of the lines against land attacks. These heavily armed
bombers are very difficult to attack, even for the smaller and more
agile "Chasers," as they can fire from below as well as from the front,
top, or sides. In the bombers which have only a single gun in the rear,
the gunner is working at a disadvantage if his adversary forces him to
continually raise and lower his gun from the top of the body to the
lower trap door. This is very tiring to the rear gunner, and if the
chaser’s tactics are carried out for a sufficient length of time, it can
wear out the gunner by continually rising and dropping at the tail of
the bombing plane. In regard to the front gun, the twin motor type
offers many of the advantages of the pusher, and as a whole, the twin
arrangement will nearly double the field of fire of either the tractor
or pusher.

The bombing planes must have a very large radius of action, particularly
those that are used in night bombing operations. The Gothas in bombing
London fly several hundred miles from their base, and recently a
Handley-Page bombing plane flew from London to Constantinople, Turkey,
making only a few stops on the way. Starting out from Hendon, England,
the Handley-Page machine flew to Paris, down the Rhone valley to Lyons
and Marseilles, and then to Pisa, and Rome (Italy), where they landed.
From Rome the machine passed over Naples, over Oranto and then over the
Albanian Alps to the base at Salonica. Making preparations at this base
they flew the final stage of the trip to Constantinople, a distance of
250 miles over hostile country. The bombing of the Turkish capital was
done at night after a flight of 2 1/2 hours from Salonica. When over the
sea of Marmora, the ship "Goeben" was bombed, and in addition a hit was
scored on the two submarines lying at her side. Four bombs struck the
"Goeben" directly, from an altitude of 800 feet. Two more bombs were
dropped on the German ship, "General," which was the headquarters of the
German staff. Finally, after 30 minutes over the city of Constantinople,
the Turkish War Office was the recipient of two more bombs. In the words
of the Turkish communiqué this "Was not entirely destroyed." On its
return to Salonica it was found that the machine had been struck by 26
shrapnel bullets. This disabled one of the power plants so that the
greater part of the return journey was made on a single motor.

From London to Salonica five men were carried. In addition was their
luggage, bedding, two tool boxes, spare parts equivalent in weight to
one engine, and two 11’-6" spare propellers. Complete, the machine
weighed over 6 tons, with a useful load of about 6,000 pounds. In
crossing the Albanian Alps the machine frequently was at an altitude of
10,000 feet. The power plant consisted of two 275 horsepower Rolls-Royce
motors, and even at this high altitude, and with the heavy loading, no
trouble was experienced. During the bombing, only three men were
carried, the remainder of the useful weight being made up of bombs and
other ammunition. While this record will probably be beaten before this
book goes to press, it will at least give an idea as to the requirements
and capabilities of the bombing type aeroplane.

*Military Training Machines*. The military training machines used in the
United States are generally of the two seater tractor type, similar in
external appearance to the reconnaissance type machines already
described. They are low powered, 90 to 125 horsepower, and will have an
average span of 40’-0". The controls are in duplicate so that the
student’s controls move in unison with the instructor’s.

CHAPTER III. ELEMENTARY AERODYNAMICS

*Definition*. Aerodynamics treats of the forces produced by air in
motion, and is the basic subject in the study of the aeroplane. It is
the purpose of this chapter to describe in detail the action of the wing
in flight, and the aerodynamic behavior of the other bodies that enter
into the construction of the aeroplane. At present, aerodynamic data is
almost entirely based on experimental investigations. The motions and
reactions produced by disturbed air are so complex and involved that no
complete mathematical theory has yet been advanced that permits of
direct calculation.

*Properties of Air*. Air being a material substance, possesses the
properties of volume, weight, viscosity and compressibility. It is a
mechanical mixture of the two elementary gases, oxygen and nitrogen, in
the proportion of 23 per cent of oxygen to 77 per cent of nitrogen. It
is the oxygen element that produces combustion, while the nitrogen is
inert and does not readily enter into combination with other elements,
its evident function being to act as a dilutant for the energetic
oxygen. In combustion, the oxygen enters into a chemical combination
with the fuel while the nitrogen passes off with the products of
combustion unchanged.

Air is considered as a fluid since it is capable of flowing like water,
but unlike water, it is highly compressible. Owing to the difference
between air and water in regard to compressibility, they do not follow
exactly the same laws, but at ordinary flight speeds and in the open
air, the variations in the pressure are so slight as to cause little
difference in the density. Hence for *flight alone*, air may be
considered as incompressible. It should be noted that a compressible
fluid is changed in density by variations in the pressure, that is, by
applying pressure the weight of a cubic foot of a compressible fluid is
greater than the same fluid under a lighter pressure. This is an
important consideration since the density of the air greatly affects the
forces that set it in motion, and for this reason the density (weight
per cubic foot) is always specified in a test.

Every existing fluid resists the motion of a body, the opposition to the
motion being commonly known as "resistance." This is due to the cohesion
between the fluid particles and the resistance is the actual force
required to break them apart and make room for the moving body. Fluids
exhibiting resistance are said to have "viscosity." In early aerodynamic
researches, and in the study of hydrodynamics, the mathematical theory
is based on a "perfect fluid," that is, on a theoretical fluid
possessing no viscosity, and while this conception is an aid in studying
the reactions, the actual laboratory results are far from the computed
values. Such theory would assume that a body could move in a fluid
without encountering resistance, which in practice is, of course,
impossible.

In regard to viscosity, it may be noted that air is highly
viscuous—relatively much higher than water. Density for density, the
viscosity of air is about 14 times that of water, and consequently the
effects of viscosity in air are of the utmost importance in the
calculation of resistance of moving parts.

Atmospheric air at sea level is about 1/800 of the density of water. Its
density varies with the altitude and with various atmospheric
conditions, and for this reason the density is usually specified "at sea
level" as this altitude gives a constant base of measurement for all
parts of the world. As the density is also affected by changes in
temperature, a standard temperature is also specified. Experimental
results, whatever the pressure and temperature at which they were made,
are reduced to the corresponding values at standard temperature and at
the normal sea level pressure, in order that these results may be
readily comparable with other data. The normal (average) pressure at sea
level is 14.7 pounds per square inch, or 2,119 pounds per square foot at
a temperature of 60° Fahrenheit. At this temperature 1 pound of air
occupies a volume of 13.141 cubic feet, while at 0° F. the volume
shrinks to 11.58 cubic feet, the corresponding densities being 0.07610
and 0.08633 pounds per cubic foot, respectively. This refers to dry air
only as the presence of water vapor makes a change in the density. With
a reduction in temperature the pressure increases with the density
increase so that the effect of heat is twofold in its effect.

With a constant temperature, the pressure and density both decrease as
the altitude increases, a density at sea level of 0.07610 pounds per
cubic foot is reduced to 0.0357 pounds per cubic foot at an altitude of
20,000 feet. During this increase in altitude, the pressure drops from
14.7 pounds per square inch to 6.87 pounds per square inch. This
variation, of course, greatly affects the performance of aeroplanes
flying at different altitudes, and still more affects the performance of
the motor, since the latter cannot take in as much fuel per stroke at
high altitudes as at low, and as a result the power is diminished as we
gain in altitude. The following table gives the power variations at
different heights above sea level.

This air table also gives the properties of air through the usual range
of flight altitudes. The pressures corresponding to the altitudes are
given both in pounds per square inch and in inches of mercury so that
barometer and pressure readings can be compared. In the fourth column is
the percentage of the horsepower available at different altitudes, the
horsepower at sea level being taken as unity. For example, if an engine
develops 100 horsepower at sea level, it will develop 100 × 0.66=66
horsepower at an altitude of 10,000 feet above sea level. The barometric
pressure in pounds per square inch can be obtained by multiplying the
pressure in inches of mercury by the factor 0.4905, this being the
weight of a mercury column 1 inch high.

[Illustration: NOTE.-Densities marked * are interpolated from a graph,
but are close enough for all ordinary purposes.]

In aerodynamic laboratory reports, the standard density of air is
0.07608 pounds per cubic foot at sea level, the temperature being 15
degrees Centigrade (59 degrees Fahrenheit). This standard density will
be assumed throughout the book, and hence for any other altitude or
density the corresponding corrections must be made. Owing to the fact
that the temperature decreases as we gain altitude, further corrections
must be made in the tabular values, but as the changes are rather
difficult to make and are relatively small we will not take the matter
up at this point.

[Illustration: Fig. 1. Air Flow About a Flat Normal Plate. Pressure Zone
at Front and .#. Turbulent Zone at Rear (H). Arrows Show Direction O
OW.]

*Air Pressure on Normal Flat Plates*. When a flat plate or "plane" is
held at right angles or "normal" to an air stream, it obstructs the flow
and a force is produced that tends to move it with the stream. The
stream divides,as shown in Fig. 1 and passes all around the edges of the
plate (P-R), the stream reuniting at a point (M) far in the rear.
Assuming the air flow from left to right, as in the figure, it will be
noted that the rear of the plate at (H) is under a slight vacuum, and
that it is filled with a complicated whirling mass of air. The general
trend of the eddy paths are indicated by the arrows. At the front where
the air current first strikes the plate there is a considerable pressure
due to the impact of the air particles. In the figure, pressure above
the atmospheric is indicated by ***, while the vacuous space at the rear
is indicated by fine dots. As the pressure in front, and the vacuum in
the rear, both tend to move the surface to the right in the direction of
the air stream, the total force tending to move the plate will be the
difference of pressure on the front and rear faces multiplied by the
area of the plate. Thus if F is the force due to the impact pressure at
the front, and G is the force due to the vacuum at the rear, then the
total resistance (D) or "Drag" is the sum of the two forces.

Contrary to the common opinion, the vacuous part of the drag is by far
the greater, say in the neighborhood of from 60 to 75 per cent of the
total. When a body experiences pressure due to the breaking up of an air
stream, as in the present case, the pressure is said to be due to
"turbulence," and the body is said to produce "turbulent flow." This is
to distinguish the forces due to impact and suction, from the forces due
to the frictional drag produced by the air stream rubbing over the
surface.

Forces due to turbulent flow do not vary directly as the velocity of the
air past the plate, but at a much higher rate. If the velocity is
doubled, the plate not only meets with twice the volume of air, but it
also meets it twice as fast. The total effect is four times as great as
in the first place. The forces due to turbulent flow therefore vary as
the square of the velocity, and the pressure increases very rapidly with
a small increase in the velocity. The force exerted on a plate also
increases directly with the area, and to a lesser extent the drag is
also affected by the shape and proportions. Expressed as a formula, the
total resistance (D) becomes: D = KAV², where K = co-efficient of
resistance determined by experiment, A = area of plate in square feet,
and V= velocity in miles per hour. The value of K takes the shape and
proportion of the plate into consideration, and also the air density.

*Example*. If the area of a flat plate is 6 square feet, the
co-efficient K = 0.003, and the velocity is 60 miles per hour, what is
the drag of the plate in pounds? *Solution*. D = KAV² = 0.003 × 6 × (60
X 60) = 64.80 pounds drag. For a square flat plate, the co-efficient K
can be taken as 0.003.

*Aspect Ratio*. The aspect ratio of a plate is the ratio of the length
to the width. Thus, with an aspect ratio of 2.0, we understand that the
plate is twice as long as it is wide. The ratio of the length to the
width has a very considerable influence of the resistance or drag, this
increasing as the ratio is made greater. If the resistance of a square
plate is taken as 1.00, the resistance of a plate with an aspect ratio
of 20 will be about 1.34 times as great. The following table will give
the effects of aspect ratio on the resistance of a flat plane.

EFFECTS OF ASPECT RATIO ON FLAT PLATES.

    ─────────────────────────────────────────────────────────────────
    Aspect Ratio.             Resistance K as a Multiple of a Square
    Length/Width              Plate.
    ─────────────────────────────────────────────────────────────────
    1.00 (square)                                               1.00
    ─────────────────────────────────────────────────────────────────
    1.50                                                        1.04
    ─────────────────────────────────────────────────────────────────
    2.00                                                        1.05
    ─────────────────────────────────────────────────────────────────
    3.00                                                        1.07
    ─────────────────────────────────────────────────────────────────
    4.00                                                        1.08
    ─────────────────────────────────────────────────────────────────
    5.00                                                        1.09
    ─────────────────────────────────────────────────────────────────
    6.00                                                        1.10
    ─────────────────────────────────────────────────────────────────
    7.00                                                        1.12
    ─────────────────────────────────────────────────────────────────
    9.00                                                        1.14
    ─────────────────────────────────────────────────────────────────
    10.00                                                       1.15
    ─────────────────────────────────────────────────────────────────
    15.00                                                       1.26
    ─────────────────────────────────────────────────────────────────
    20.00                                                       1.34
    ─────────────────────────────────────────────────────────────────
    30.00                                                       1.40
    ─────────────────────────────────────────────────────────────────

To convert the values of a square plate into a flat plate of given
aspect ratio, multiply the resistance of the square plate by the factor
under the "K" heading. For example: The resistance of a certain square
plate is 20 pounds, find the resistance of a plate of the same area, but
with an aspect ratio of 15. *Solution*. The factor for a ratio of 15
will be found to be 1.26, hence the resistance of the required plate
will be 20 × 1.26=25.2 pounds.

[Illustration: Fig. 2. Air Flow About a Streamline Body Showing an
Almost Complete Absence of Turbulence Except at the Extreme Rear Edge.
Resistance Is Principally Due to Skin Friction.]

*Streamline Forms*. When a body is of such form that it does not cause
turbulence when moved through the air, the drag is entirely due to skin
friction. Such a body is known as a "streamline form" and approximations
are used for the exposed structural parts of aeroplanes in order to
reduce the resistance. Streamline bodies are fishlike or torpedo-shaped,
as shown by Fig. 2, and it will be noted that the air stream hangs
closely to the outline through nearly its entire length. The drag is
therefore entirely due to the friction of the air on the sides of the
body since there is no turbulence or "discontinuity." In practical
bodies it is impossible to prevent the small turbulence (I), but in
well-designed forms its effect is almost negligible.

In poor attempts at streamline form, the flow discontinues its adherence
to the body at a point near the tail. The poorer the streamline, and the
higher the resistance, the sooner the stream starts to break away from
the body and cause a turbulent region. The resistance now becomes partly
turbulent and partly frictional, with the resistance increasing rapidly
as the percentage of the turbulent region is increased.

The fact that the resistance is due to two factors, makes the resistance
of an approximate streamline body very difficult to calculate, as the
frictional drag and the turbulent drag do not increase at the same rate
for different speeds. The drag due to turbulence varies as V squared
while the frictional resistance only varies at the rate of V to the
1.86th power, hence the drag due to turbulence increases much faster
with the velocity than the frictional component. If we could foretell
the percentage of friction, it would be fairly easy to calculate the
total effect, but this percentage is exactly what we do not know. The
only sure method is to take the results of a full size test.

Fig. 2 gives the approximate section through a streamline strut such as
used in the interplane bracing of a biplane. The length is (L) and the
width is (d), the latter being measured at the widest point. The
relation of the length to the width is known as the "fineness ratio" and
in interplane struts this may vary from 2.5 to 4.5, that is, the length
of the section ranges from 2.5 to 4.5 times the width. The ideal
streamline form has a ratio of from 5. to 5.75. Such large ratios are
difficult to obtain with economy on practical struts as the small width
would result in a weak strut unless the weight were unduly increased.
Interplane struts reach a maximum fineness ratio at about 3.5 to 4.5.
Fig. 3 shows the result of a small fineness ratio, the short, stubby
body causing the stream to break away near the front and form a large
turbulent region in the rear.

An approximate formula showing the relation of fineness ratio and
resistance (curvature equal) was developed by A. E. Berriman, and
published in "Flight" Nov. 12, 1915. Let D = resistance of a flat plate
at a given speed, and R = resistance of a strut at the same speed and of
the same area, then the relation between the resistance of the flat
plate, and the strut will be expressed by the formula R/D=4L/300d, where
L = length of section and d = width as in Fig. 2. This can be transposed
for convenience, by assuming the drag of a flat plate as D = 0.003AV²,
where A = area in square feet, and V = velocity in miles per hour. The
ratio of the strut resistance to the flat plate resistance, given by
Berriman’s formula, can now be multiplied by the flat plate resistance,
or strut resistance = R = 0.003AV² X 4L/300d. = 0.012LAV²/300d. It
should be understood that the area mentioned above is the greatest area
presented to the wind in square feet, and hence is equal to the length
of the strut (not section) multiplied by the width (d).

[Illustration: Fig. 3. Imperfect Streamline Body with a Considerable
Turbulence Due to the Short, Stubby Form. Fig. 4 Shows the Flow About a
Circular Rod or Cylinder.]

Assuming the length (L) of the section as 7.5 inches, and the width (d)
as one inch, the fineness ratio will be 7.5. Using the Berriman formula
in its original form, the relative resistance of the strut and flat
plate of same area will be found as R/D=4L/3000 = 0.1, that is, the
resistance of a streamline form strut of above fineness ratio will be
about 0.1 of a flat plate of the same area. It should be understood that
this is only an approximate formula since even struts of the same
fineness vary among themselves according to the outline. Results
published by the National Physical Laboratory show streamline sections
giving 0.07 of the resistance of a flat plate of the same area, with
fineness ratio = 6.5. In Fig. 4 the effects of flow about a circular rod
is shown, a case where the fineness ratio is 1. The stream follows the
body through less than one-half of its circumference, and the turbulent
region is very large; almost as great as with the flat plate. A circular
rod is far from being even an approach to a perfect form.

In all the cases shown, Figs. 1-2-3-4, it will be noticed that the air
is affected for a considerable distance in front of the plane, as it
rises to pass over the obstruction before it actually reaches it. The
front compression may be perceptible for 6 diameters of the object. From
the examination of several good low-resistance streamline forms it seems
that the best results are obtained with the blunt nose forward and the
thin end aft. The best position for the point of greatest thickness lies
from 0.25 to 0.33 per cent of the length from the front end. From the
thickest part it tapers out gradually to nothing at the rear end. That
portion to the rear of the maximum width is the most important from the
standpoint of resistance, for any irregularity in this region causes the
stream to break away into a turbulent space. From experiments it has
been found that as much as one-half of the entering nose can be cut away
without materially increasing the resistance. The cut-off nose may be
left flat, and still the loss is only in the neighborhood of 5 per cent.

*Resistance Calculations (Turbulency)*. In any plate or body where the
resistance is principally due to turbulent action, as in the flat plate,
sphere, cone, etc., the resistance can be computed from the formula R =
KAV², where R is the resistance in pounds and K, A, and V are as before.
The resistance co-efficient (K) depends upon the shape of the object
under standard air conditions, and differs greatly with flat plates,
cones, sphere, etc. The area (A) is the area presented to the wind, or
is the greatest area that faces the wind, and is taken at right angles
to its direction. The following table gives the value of K for the more
common forms of objects. See Figs. 4 to 12, inclusive:

There are almost an infinite number of different forms, but for the
present the above examples will fill our purpose. As an example in
showing how greatly the form of an object influences its resistance, we
will work out the resistance of a flat plate and a spherical ended cone,
both having the same presented diameter. The cone is placed so that the
spherical end will face the air stream. The area A of both objects will
be: 0.7854 × 2 × 2 = 3.1416 square feet. With an assumed wind velocity
of 100 miles per hour, the resistance of the circular flat disc will be:
R= KAV²=000282 × 3.1416 × (100×100) = 87.96 lbs. For the cone, R =
KAV²=0.000222 × 3.1416 × (100 × 100) = 6.97 lbs. From this calculation
it will be seen that it is advisable to surround the object with a
spherical cone shaped body rather than to present the flat surface to
the wind. In the above table the value of K is given for two positions
of the spherical based cone, the first is with the apex toward the wind,
and the second condition gives the value with the base to the wind. With
the blunt end forward, the resistance is about one-half that when the
pointed apex enters the air stream. This is due to the taper closing up
the stream without causing turbulence.

[Illustration: Figs. 4a-5-6-7-8-9-10-11-12. The Values of the Resistance
Co-efficient K for Different Forms and Positions of Solid Objects.
Arrows Indicate the Direction of the Relative Wind. (Eiffel.)]

With the apex forward there is nothing to fill up the vacuous space when
the air passes over the large diameter of the base as the curve of the
spherical end is too short to accomplish much in this direction.

*Skin Friction*. The air in rubbing over a surface experiences a
frictional resistance similar to water. At the present time the accepted
experiments are those of Dr. Zahm but these are still in some question
as to accuracy. It was found in these experiments that there was
practically no difference caused by the material of the surfaces, as
long as they were equally smooth. Linen or cotton gave the same results
as smooth wood or zinc as long as there was no nap or lint upon the
surface. With a fuzzy surface the friction increased rapidly. This is
undoubtedly due to a minute turbulence caused by the uneven surface, and
hence the increase was not purely frictional, but also due to
turbulence. In the tests, the air current was led parallel to the
surface in such a way that only the friction could move the surface. The
surface was freely suspended, and as the wind moved it edgewise, the
movement was measured by a sharp pointer. End shields prevented impact
of the air on the end of the test piece so that there was no error from
this source. The complete formula given by Dr. Zahm is rather
complicated for ordinary use, especially for those not used to
mathematical computations. If Rf = resistance due to friction on one
side of surface, L= length in direction of wind in feet, b = width of
surface in feet, and V= velocity in feet per second, then

Rf = 0.00000778L⁰.⁹³V¹.⁸⁶b.

It will be noted that the resistance increases at a lower rate than the
velocity squared, and at a less rate than the area. That is to say, that
doubling the area will not double the resistance, but will be less than
twice the amount. Giving the formula in terms of area and miles per hour
units, we have: Rf = 0.0000167A⁰.⁹³V¹.⁸⁶. Where A = area in square feet
and V = miles per hour. The area is for one side of the surface only. A
rough approximation to Zahm’s equation has been proposed by a writer in
"Flight," the intention being to avoid the complicated formula and yet
come close enough to the original for practical purposes. The latter
formula reads: Rf = 0.000009V² where Rf and V are as above. Up to 40
miles per hour the results are very close to Zahm’s formula, and are
fairly close from 60 to 90 miles per hour. This approximation is only
justified when the length in the direction of the wind is nearly equal
to the length. If the length is much greater, there is a serious error
introduced.

This formula is applied to surfaces parallel to the wind such as the
sides of the body, rudder, stabilizer, and elevator surfaces (when in
neutral). A second important feature of the friction formula is that it
illustrates the law of "similitude" or the results of a change in scale
and velocity, hence it outlines what we must expect when we compute a
full size aeroplane from the results of a model test.

*The Inclined Plane*. When a flat plate is inclined with the wind, the
resistance or drag will be broken up into two components, one at right
angles to the air stream, and one parallel to it. If the plate is
properly inclined, the right angled component can be utilized in
obtaining lift as with an aeroplane wing. This is shown in Fig. 13 where
L is the vertical lift force at right angles to the air stream and D is
the horizontal drag acting in the direction of the wind. As in the case
of the plate placed normal to the wind, there is pressure at the front
of the plate and a partial vacuum behind. The resultant force will be
determined by the difference in pressure between the front and the back
of the plate. The forces will vary as V² since the reaction is caused by
turbulent flow. Both the lift and drag will vary with the angle made
with the stream, and there will be a different value for the
co-efficient K for each change in the angle. The angle made with the air
stream is known as the "Angle of incidence" or the "Angle of attack."
The change of drag and lift does not vary at a regular rate with the
angle.

[Illustration: Fig. 13. Flow About, Inclined Plane and Forces Produced
by Stream. Fig. 14. Normal Plane with C.P. at center of Plate. Fig. 15..
C.P. Moves Toward Entering Edge When Plate Is Inclined to Wind.]

A line OR is the resultant of the lift and drag forces L and D, this
resultant being the force necessary to balance the two forces L-D. It is
on the point of application O that the plate balances, and this point is
sometimes known as the "Center of pressure." The center of pressure is
therefore the point at which the resultant intersects the surface of the
flat plate. The resultant OR is approximately at right angles to the
surface at small incident angles, and the point O is nearer the front or
"leading edge" (A) of the plane. The smaller the angle of incidence the
nearer will the point O approach the leading edge A. By drawing OL to
scale, representing the lift, and OD to scale representing the drag, we
can find the resultant OR by drawing LR parallel to the drag OD and DR
parallel to the lift line OL. All lines drawn through the intersection
of LR and DR will give the resultant OR to scale. All of the lines must
be started from the center of pressure at O.

The least resultant will, of course, occur when the plane is parallel to
the air stream. The maximum resultant will occur when the angle of
incidence is about 40 degrees, and on a further increase in the angle,
the value of the resultant will gradually decrease. When the plane is
parallel with the stream, the resultant is parallel to the plate, but
rapidly approaches a position at right angles at about an incidence of 6
to 10 degrees. Beyond 10 degrees incidence the angle of the resultant
increases past the normal.

The center of pressure (O), or the point where the resultant force
intersects the plane, moves forward as the angle of incidence is
decreased from 90°. When at right angles to the air current, the center
of pressure is exactly in the center of the plane as shown by Fig. 14.
In this case the drag (D) is the resultant, and acting in the center,
exactly balances the air forces. In Fig. 15 the angle of incidence is
reduced, consequently the center of pressure moves nearer the leading
edge (A). As the angle continues to decrease, the C. P. moves still
further forward until it lies directly on the front edge when the plate
becomes parallel with the air stream. The center of pressure movement is
due to the fact that more and more work is done by the front part of the
surface as the angle is decreased. Consequently the point of support, or
C. P., must move forward to come under the load. It should be understood
that the plane will balance about the C. P. if a knife edge bearing were
applied as at R in Fig. 15.

*Calculation of Inclined Planes*. We will now consider the inclined
plane as a lifting surface for an aeroplane, and make the elementary
calculations for such purpose. The lift will first be calculated for the
support of the given load, at the given velocity, and then the drag. For
several reasons, that will afterwards be explained, the flat plate or
plane is not used for the main lifting surfaces, but the experience
gained in computing the plate will be of great assistance when we start
calculating actual wings.

*Lift and Drag Co-efficients*. The lift component (L) of the inclined
flat plate depends on the velocity, area, aspect ratio and angle of
incidence. Instead of using the co-efficient (K) formerly used for the
total drag, we will use the lift co-efficient Ky. The formula for lift
now becomes: L = KyAV² where A = area in square feet, and V = velocity
in miles per hour. The lift co-efficient Ky, depends upon the angle of
incidence. The horizontal drag D will be calculated from the drag
co-efficient Kx, which is used in the same way as the co-efficient K in
the case of the normal plate. The subscript (x) is used to distinguish
it from the lift co-efficient. Both Ky and Kx must be corrected for
aspect ratio. The drag can be calculated from the formula: D = KxAV²
where the letters A and V are the same as above.

For the calculation of the drag, we will use a new expression—the
"Lift-Drag Ratio"—or as more commonly given, "L/D." This shows the
relation between the lift and drag, so that by knowing the lift and the
ratio for any particular case, we can compute the drag without the
necessity of going through the tedious calculation D = KxAV². The
lift-drag ratio for a flat plate varies with the angle of incidence, and
the aspect ratio, and hence a separate value must be used for every
inclination and change in aspect. To obtain the drag, divide the lift by
the lift-drag ratio. Hence if the lift is 1200 pounds, and the ratio
equals 6.00, the drag will be: 1200/6=200 pounds, or in other words, the
lift is 6 times the drag force. Changing the angle of incidence through
angles ranging from 1 degree to 7 degrees, the lift-drag ratio of a flat
plate will vary from 1.5 to 7.5. When the plane is parallel to the wind
stream and gives no lift, the drag is computed from Zahm’s skin friction
formula.

The following tables give the values of Ky, Kx, L/D, and center of
pressure movement for flat plates of various aspect ratios. The center
of pressure (C. P.) for each angle is given as a decimal fraction of its
distance from the leading edge, in terms of the width or "Chord."

[Illustration: Fig. 16. Plan View of Plate with Long Edge to Wind. Fig.
17. Plate with Narrow Edge to Wind, Showing Loss in Lift. 17a Shows
Effect of Raked Tips.]

Fig. 16 shows the top view or plan of a lifting surface, with the
direction of the wind stream indicated by the arrows w-w-w = w. The
longer side or "span" is indicated by S, while the width or chord is C.
Main lifting surfaces, or wings, have the long side at right angles to
the wind as shown. When in this position, the surface is said to be in
"Pterygoid Aspect," and when the narrow edge is presented to the wind,
the wing is in "Apteroid Aspect." The word "Pterygoid" means "Bird
like," and was chosen for the condition in Fig. 16, as this is the
method in which a bird’s wing meets the air. Contrary to the case with
true curved aeroplane wings, flat planes usually give better lift in
apteroid than in pterygoid aspect at high angles. The aspect ratio will
be the span (S) divided by the chord (C), or Aspect ratio = S/C.

It will be seen from the above that the lift coefficient Ky increases
with the aspect ratio, and that it generally declines after an angle of
30 degrees. The center of pressure moves steadily back with an increase
in angle.

*Example for Lifts*. A certain flat plane has an area of 200 square
feet, and moves at 50 miles per hour. The angle of incidence is 10
degrees, and the aspect ratio is 6. Find the total lift and the drag in
pounds. Also the location of the center of pressure in regard to the
leading edge, if the chord is 5.8 feet.

*Solution*. Under the table headed, "Aspect Ratio = 6" we find that Ky
at 10° = 0.00173, and that the lift drag ratio is 5.2. The center of
pressure is 0.333 of the chord from the front edge. The total lift then
becomes: L = KyAV² = 0.00173 x 200 x (50 x 50) = 865 pounds. Since the
lift drag ratio is 5.2, the drag = D = 865/5.2 = 166.3 pounds. The
center of pressure will be located 5.8 x 0.333 = 1.4 feet from the
leading edge.

Under the same conditions, but with an aspect ratio of 3, the lift will
become: L = KyAV² = 0.0014 x 200 x(50 x 50) = 700 pounds. In this case
the lift drag ratio is 5.1, so that the drag will be 137.8 pounds. Even
with the same area, the aspect ratio makes a difference of 865–700 = 165
pounds. If we were compelled to carry the original 865 pounds with
aspect 3 wing, we would also be compelled to increase the area, angle,
or speed. If the speed were to be kept constant, we would be limited to
a change in area or angle. In the latter case it would be preferable to
increase the area, since a sufficient increase in the angle would
greatly increase the drag. It will be noted that the lift-drag ratio
decreases rapidly with an increase in the angle.

[Illustration: Burgess Seaplane Scout.]

*Calculation of Area*: Let us assume that we are confined to the use of
an aspect ratio of 6, a speed of 50 miles per hour, weight = 2500
pounds, and wish to obtain the area that will give the most efficient
surface (Least lift-drag ratio.) The equation can be now transposed so
that the area = A = KyV². On examination of the table it will be seen
that the greatest lift-drag ratio is 6.4 at 5 degrees, and that the Ky
at this angle is 0.00103. Substituting these values in the equation for
area, we have A = L/KyV² = 2500/000103 x (50 x 50) = 971 square feet.

[Illustration: Wind Tunnel at Washington Navy Yard in Which the Air
Circulates Continuously Through a Closed Circuit]

CHAPTER IV. EXPERIMENTAL LABORATORIES.

*Test Methods in General*. As already explained, the behavior of a body
in an air stream cannot be predicted with any certainty by direct
mathematical calculation, and for this reason, each and every
aerodynamic body must be tested under conditions that are as nearly
similar to the actual working conditions as possible. Prior to Professor
Langley’s first experiments in 1887, mechanical flight with a heavier
than air machine was derided as an impossibility, even by such
scientists as Navier, Von Helmotz, Gay-Lussac, and others, who proved by
the most intricate calculations that a body larger than a bird could not
be supported by its own energy. Such calculations were, of course, based
on a wrong understanding of air flow, and as no experimental work had
been done up to that time, the flow was assumed according to the
individual taste and belief of the demonstrator. The presence of a
vacuum on the back of a plate was not understood, and as this
contributes full two-thirds of the lift, it is an easy matter to see why
all of the early predictions fell short of the actual lifting forces. To
quote one classic absurdity, the scientist Navier proved mathematically
that if mechanical flight were possible, then 17 swallows would be
capable of developing one horsepower.

In spite of these discouraging computations, Langley proceeded with a
very carefully conducted series of experiments, first investigating the
laws of surface sustenation on various forms of plates, and when the
data collected was sufficient for his needs, he started to construct a
number of model flyers with various wing arrangements and aerofoil
forms. It was Langley’s experiments upon aerofoils that cleared the way
for the Wright Brothers, who started a further and more complete
investigation in 1896. Experiments were made on the effect of curvature,
aspect ratio and angle of incidence, and the results obtained in their
"wind tunnel" were afterwards applied to their successful full size
machine. During 1901 to 1902 the Wrights investigated the properties of
at least 100 different aerofoil forms. Both Langley’s and Wrights’
experiments were with models, although they were made in a different
manner. It was in this way that experimental evidence gained precedence
over theory.

Langley’s specimens were mounted at the end of a revolving arm, so that
with the arm revolving, a relative air stream of known velocity could be
had. The aerofoil was mounted in such a way that the lift and drag could
be measured. In the early experiments of the Wrights, the models were
placed in an enclosed channel through which a stream of air was
maintained by a fan. The model was attached to a balance system so that
the lift and resistance could be measured. This is what is now known as
a "wind tunnel," and at present is almost exclusively used in model
tests. Several investigators immersed their model aerofoils in running
water so that the direction of flow could be visibly observed. While
this latter method is of great service in determining disturbances,
stream line flow, and general characteristics, it is qualitative rather
than quantitative, and cannot be used in obtaining accurate numerical
results. A more accurate method of mapping out the direction of flow,
eddies, etc., is to introduce smoke into the air stream.

*Full Size Experiments*. The old "rule of the thumb" method of building
a full size machine without model test data or other experimental
evidence to begin with has seen its day. It is not only exceedingly
expensive, but is highly dangerous, and many a flyer has met his death
in the endeavor to work out untried principles on a full size machine.
The first cost of the machine, the continual breakage and operating
expense, to say nothing of the damage suits and loss of time, make a
preliminary full size tryout an absurdity at the present time. Again,
the results of full size experiments are not always reliable, as so much
depends upon the pilot and weather conditions. The instruments used on a
large machine are far from being as accurate as those used in model
tests. These are also likely to be thrown out of adjustment unknowingly
by falls or collisions. The great number of variables that enter into
such a test make it almost an impossibility to obtain accurate data on
the result of minor alterations, and, in fact, it is almost impossible