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.]