by J. S. Zerbe
Scanned by Charles Keller with OmniPage Professional OCR software
This work is not intended to set forth the exploits of aviators nor to give a history of the Art. It is a book of instructions intended to point out the theories of flying, as given by the pioneers, the practical application of power to the various flying structures; how they are built, the different methods of controlling them; the advantages and disadvantages of the types now in use; and suggestions as to the directions in which improvements are required.
It distinctly points out wherein mechanical flight differs from bird flight, and what are the relations of shape, form, size and weight. It treats of kites, gliders and model aeroplanes, and has an Interesting chapter on the aeroplane and its uses In the great war. All the illustrations have been specially prepared for the work.
Every Boy's Mechanical Library
BY J. S. ZERBE, M. E. Author of Automobiles—Motors
COPYRIGHT, 1915, BY CUPPLES & LEON COMPANY NY
CHAPTER I. THEORIES AND FACTS ABOUT FLYING
The "Science" of Aviation. Machine Types. Shape or Form not Essential. A Stone as a Flying Machine. Power the Great Element. Gravity as Power. Mass and Element in Flying. Momentum a Factor. Resistance. How Resistance Affects Shape. Mass and Resistance. The Early Tendency to Eliminate Momentum. Light Machines Unstable. The Application of Power. The Supporting Surfaces. Area not the Essential Thing. The Law of Gravity. Gravity. Indestructibility of Gravitation. Distance Reduces Gravitational Pull. How Motion Antagonizes Gravity. A Tangent. Tangential Motion Represents Centrifugal Pull. Equalizing the Two Motions. Lift and Drift. Normal Pressure. Head Resistance. Measuring Lift and Drift. Pressure at Different Angles. Difference Between Lift and Drift in Motion. Tables of Lift and Drift. Why Tables of Lift and Drift are Wrong. Langley's Law. Moving Planes vs. Winds. Momentum not Considered. The Flight of Birds. The Downward Beat. The Concaved Wing. Feather Structure Considered. Webbed Wings. The Angle of Movement. An Initial Movement or Impulse Necessary. A Wedging Motion. No Mystery in the Wave Motion. How Birds Poise with Flapping Wings. Narrow- winged Birds. Initial Movement of Soaring Birds. Soaring Birds Move Swiftly. Muscular Energy Exerted by Soaring Birds. Wings not Motionless.
CHAPTER II. PRINCIPLES OF AEROPLANE FLIGHT Speed as one of the Elements. Shape and Speed. What "Square of the Speed" Means. Action of a "Skipper." Angle of Incidence. Speed and Surface. Control of the Direction of Flight. Vertical Planes.
CHAPTER III. THE FORM OR SHAPE OF FLYING MACHINES The Theory of Copying Nature. Hulls of Vessels. Man Does not Copy Nature. Principles Essential, not Forms. Nature not the Guide as to Forms. The Propeller Type. Why Specially-designed Forms Improve Natural Structures. Mechanism Devoid of Intelligence. A Machine Must Have a Substitute for Intelligence. Study of Bird Flight Useless. Shape of Supporting Surface. The Trouble Arising From Outstretched Wings. Density of the Atmosphere. Elasticity of the Air. "Air Holes." Responsibility for Accidents. The Turning Movement. Centrifugal Action: The Warping Planes.
CHAPTER IV. FORE AND AFT CONTROL The Bird Type of Fore and Aft Control. Angle and Direction of Flight. Why Should the Angle of the Body Change. Changing Angle of Body not Safe. A Non-changing Body. Descending Positions by Power Control. Cutting off the Power. The Starting Movement. The Suggested Type. The Low Center of Gravity. Fore and Aft Oscillations. Application of the New Principle. Low Weight not Necessary with Synchronously- moving wings.
CHAPTEB V. DIFFERENT MACHINE TYPES AND THEIR CHARACTERISTICS The Helicopter. Aeroplanes. The Monoplane. Its Advantages. Its Disadvantages. The Bi-plane. Stability in Bi-planes. The Orthopter. Nature's Type not Uniform. Theories About Flight of Birds. Instinct. The Mode of Motion. The Wing Structure. The Wing Movement. The Helicopter Motion.
CHAPTER VI. THE LIFTING SURFACES OF AEROPLANES Relative Speed and Angle. Narrow Planes Most Effective. Stream Lines Along a Plane. The Center of Pressure. Air Lines on the Upper Side of a Plane. Rarefied Area. Rarefaction Produced by Motion. The Concaved Plane. The Center of Pressure. Utilizing the Rarefied Area. Changing Center of Pressure. Plane Monstrosities. The Bird Wing Structure. Torsion. The Bat's Wing. An Abnormal Shape. The Tail as a Monitor.
CHAPTER VII. ABNORMAL FLYING STUNTS AND SPEEDS Lack of Improvements in Machines. Men Exploited and not Machines. Abnormal Flying of no Value. The Art of Juggling. Practical Uses the Best Test. Concaved and Convex Planes. How Momentum is a Factor in Inverted Flying. The Turning Movement. When Concaved Planes are Desirable. The Speed Mania. Uses of Flying Machines. Perfection in Machines Must Come Before Speed. The Range of its Uses. Commercial Utility.
CHAPTER VIII. KITES AND GLIDERS The Dragon Kite. Its Construction. The Malay Kite. Dihedral Angle. The Common Kite. The Bow Kite. The Box Kite. The Voison Bi-plane. Lateral Stability in Kites, not Conclusive as to Planes. The Spear Kite. The Cellular Kite. Tetrahedral Kite. The Deltoid. The Dunne Flying Machine. Rotating Kite. Kite Principles. Lateral Stability in Kites. Similarity of Fore and Aft Control. Gliding Flight One of the Uses of Glider Experiments. Hints in Gliding.
CHAPTER IX. AEROPLANE CONSTRUCTION Lateral and Fore and Aft. Transverse. Stability and Stabilization. The Wright System. Controlling the Warping Ends. The Curtiss Wings. The Farman Ailerons. Features Well Developed. Depressing the Rear End. Determining the Size. Rule for Placing the Planes. Elevating Plane. Action in Alighting. The Monoplane. The Common Fly. Stream Lines. The Monoplane Form.
CHAPTER X. POWER AND ITS APPLICATION Features in Power Application. Amount of Power Necessary. The Pull of the Propeller. Foot Pounds Small Amount of Power Available. High Propeller Speed Important. Width and Pitch of Blades. Effect of Increasing Propeller Pull. Disposition of the Planes. Different Speeds with Same Power. Increase of Speed Adds to Resistance. How Power Decreases with Speed. How to Calculate the Power Applied. Pulling Against an Angle. The Horizontal and the Vertical Pull. The Power Mounting. Securing the Propeller to the Shaft. Vibrations. Weaknesses in Mounting. The Gasoline Tank. Where to Locate the Tank. The Danger to the Pilot. The Closed-in Body. Starting the Machine. Propellers with Varying Pitch.
CHAPTER XI. FLYING MACHINE ACCESSORIES The Anemometer. The Anemograph. The Anemometrograph. The Speed Indicator. Air Pressure Indicator. Determining the Pressure From the Speed. Calculating Pressure From Speed. How the Figures are Determined. Converting Hours Into Minutes. Changing Speed Hours to Seconds. Pressure as the Square of the Speed. Gyroscopic:Balance. The Principles Involved. The Application of the Gyroscope. Fore and Aft Gyroscopic Control. Angle Indicator. Pendulum Stabilizer. Steering and Controlling Wheel. Automatic Stabilizing Wings. Barometers. Aneroid Barometer. Hydroplanes. Sustaining Weight of Pontoons. Shape of the Pontoon.
CHAPTER XII. EXPERIMENTAL WORK IN FLYING Certain Conditions in Flying. Heat in Air. Motion When in Flight. Changing Atmosphere. "Ascending Currents." "Aspirate Currents." Outstretched Wings. The Starting Point. The Vital Part of the Machine. Studying the Action of the Machine. Elevating the Machine. How to Practice. The First Stage. Patience the Most Difficult Thing. The Second Stage. The Third Stage. Observations While in Flight. Flying in a Wind. First Trials in a Quiet Atmosphere. Making Turns. The Fourth Stage. The Figure 8. The Vol Plane. The Landing. Flying Altitudes.
CHAPTER XIII. THE PROPELLER Propeller Changes. Propeller Shape. The Diameter. Pitch. Laying Out the Pitch. Pitch Rule. Laminated Construction. Laying up a Propeller Form. Making Wide Blades. Propeller Outline. For High Speeds. Increasing Propeller Efficiency.
CHAPTER XIV. EXPERIMENTAL GLIDERS AND MODEL AEROPLANES The Relation of Models to Flying Machines. Lessons From Models. Flying Model Aeroplanes. An Efficient Glider. The Deltoid Formation. Racing Models. The Power for Model Aeroplanes. Making the Propeller. Material for the Propeller. Rubber. Propeller Shape and Size. Supporting Surfaces.
CHAPTER XV. THE AEROPLANE IN THE GREAT WAR Balloon Observations. Changed Conditions in Warfare. The Effort to Conceal Combatants. Smokeless Powder. Inventions to Attack Aerial Craft. Functions of the Aeroplane in War. Bomb-throwing Tests. Method for Determining the Movement of a Bomb. The Great Extent of Modern Battle Lines. The Aeroplane Detecting the Movements of Armies. The Effective Height for Scouting. Sizes of Objects at Great Distances. Some Daring Feats in War. The German Taube. How Aeroplanes Report Observations. Signal Flags. How Used. Casualties Due to Bombs From Aeroplanes.
In preparing this volume on Flying Machines the aim has been to present the subject in such a manner as will appeal to boys, or beginners, in this field of human activity.
The art of aviation is in a most primitive state. So many curious theories have been brought out that, while they furnish food for thought, do not, in any way, advance or improve the structure of the machine itself, nor are they of any service in teaching the novice how to fly.
The author considers it of far more importance to teach right principles, and correct reasoning than to furnish complete diagrams of the details of a machine. The former teach the art, whereas the latter merely point out the mechanical arrangements, independently of the reasons for making the structures in that particular way.
Relating the history of an art, while it may be interesting reading, does not even lay the foundations of a knowledge of the subject, hence that field has been left to others.
The boy is naturally inquisitive, and he is interested in knowing WHY certain things are necessary, and the reasons for making structures in particular ways. That is the void into which these pages are placed.
The author knows from practical experience, while experimenting with and building aeroplanes, how eagerly every boy inquires into details. They want the reasons for things.
One such instance is related to evidence this spirit of inquiry. Some boys were discussing the curved plane structure. One of them ventured the opinion that birds' wings were concaved on the lower side. "But," retorted another, "why are birds' wings hollowed?"
This was going back to first principles at one leap. It was not satisfying enough to know that man was copying nature. It was more important to know why nature originated that type of formation, because, it is obvious, that if such structures are universal in the kingdom of flying creatures, there must be some underlying principle which accounted for it.
It is not the aim of the book to teach the art of flying, but rather to show how and why the present machines fly. The making and the using are separate and independent functions, and of the two the more important is the knowledge how to make a correct machine.
Hundreds of workmen may contribute to the building of a locomotive, but one man, not a builder, knows better how to handle it. To manipulate a flying machine is more difficult to navigate than such a ponderous machine, because it requires peculiar talents, and the building is still more important and complicated, and requires the exercise of a kind of skill not necessary in the locomotive.
The art is still very young; so much is done which arises from speculation and theories; too much dependence is placed on the aviator; the desire in the present condition of the art is to exploit the man and not the machine; dare-devil exhibitions seem to be more important than perfecting the mechanism; and such useless attempts as flying upside down, looping the loop, and characteristic displays of that kind, are of no value to the art. THE AUTHOR.
THEORIES AND FACTS ABOUT FLYING
THE "SCIENCE" OF AVIATION.—It may be doubted whether there is such a thing as a "science of aviation." Since Langley, on May 6, 1896, flew a motor-propelled tandem monoplane for a minute and an half, without a pilot, and the Wright Brothers in 1903 succeeded in flying a bi-plane with a pilot aboard, the universal opinion has been, that flying machines, to be successful, must follow the structural form of birds, and that shape has everything to do with flying.
We may be able to learn something by carefully examining the different views presented by those interested in the art, and then see how they conform to the facts as brought out by the actual experiments.
MACHINE TYPES.—There is really but one type of plane machine. While technically two forms are known, namely, the monoplane and the bi-plane, they are both dependent on outstretched wings, longer transversely than fore and aft, so far as the supporting surfaces are concerned, and with the main weight high in the structure, thus, in every particular, conforming to the form pointed out by nature as the apparently correct type of a flying structure.
SHAPE OR FORM NOT ESSENTIAL.—It may be stated with perfect confidence, that shape or form has nothing to do with the mere act of flying. It is simply a question of power. This is a broad assertion, and its meaning may be better understood by examining the question of flight in a broad sense.
A STONE AS A FLYING MACHINE.—When a stone is propelled through space, shape is of no importance. If it has rough and jagged sides its speed or its distance may be limited, as compared with a perfectly rounded form. It may be made in such a shape as will offer less resistance to the air in flight, but its actual propulsion through space does not depend on how it is made, but on the power which propelled it, and such a missile is a true heavier-than-air machine.
A flying object of this kind may be so constructed that it will go a greater distance, or require less power, or maintain itself in space at less speed; but it is a flying machine, nevertheless, in the sense that it moves horizontally through the air.
POWER THE GREAT ELEMENT.—Now, let us examine the question of this power which is able to set gravity at naught. The quality called energy resides in material itself. It is something within matter, and does not come from without. The power derived from the explosion of a charge of powder comes from within the substance; and so with falling water, or the expansive force of steam.
GRAVITY AS POWER.—Indeed, the very act of the ball gradually moving toward the earth, by the force of gravity, is an illustration of a power within the object itself. Long after Galileo firmly established the law of falling bodies it began to dawn on scientists that weight is force. After Newton established the law of gravitation the old idea, that power was a property of each body, passed away.
In its stead we now have the firmly established view, that power is something which must have at least two parts, or consist in pairs, or two elements acting together. Thus, a stone poised on a cliff, while it exerts no power which can be utilized, has, nevertheless, what is called potential energy. When it is pushed from its lodging place kinetic energy is developed. In both cases, gravity, acting in conjunction with the mass of the stone, produced power.
So in the case of gunpowder. It is the unity of two or more substances, that causes the expansion called power. The heat of the fuel converting water into steam, is another illustration of the unity of two or more elements, which are necessary to produce energy.
MASS AN ELEMENT IN FLYING.—The boy who reads this will smile, as he tells us that the power which propelled the ball through the air came from the thrower and not from the ball itself. Let us examine this claim, which came from a real boy, and is another illustration how acute his mind is on subjects of this character.
We have two balls the same diameter, one of iron weighing a half pound, and the other of cotton weighing a half ounce. The weight of one is, therefore, sixteen times greater than the other.
Suppose these two balls are thrown with the expenditure of the same power. What will be the result! The iron ball will go much farther, or, if projected against a wall will strike a harder blow than the cotton ball.
MOMENTUM A FACTOR.—Each had transferred to it a motion. The initial speed was the same, and the power set up equal in the two. Why this difference, The answer is, that it is in the material itself. It was the mass or density which accounted for the difference. It was mass multiplied by speed which gave it the power, called, in this case, momentum.
The iron ball weighing eight ounces, multiplied by the assumed speed of 50 feet per second, equals 400 units of work. The cotton ball, weighing 1/2 ounce, with the same initial speed, represents 25 units of work. The term "unit of work" means a measurement, or a factor which may be used to measure force.
It will thus be seen that it was not the thrower which gave the power, but the article itself. A feather ball thrown under the same conditions, would produce a half unit of work, and the iron ball, therefore, produced 800 times more energy.
RESISTANCE.—Now, in the movement of any body through space, it meets with an enemy at every step, and that is air resistance. This is much more effective against the cotton than the iron ball: or, it might be expressed in another way: The momentum, or the power, residing in the metal ball, is so much greater than that within the cotton ball that it travels farther, or strikes a more effective blow on impact with the wall.
HOW RESISTANCE AFFECTS THE SHAPE.—It is because of this counterforce, resistance, that shape becomes important in a flying object. The metal ball may be flattened out into a thin disk, and now, when the same force is applied, to project it forwardly, it will go as much farther as the difference in the air impact against the two forms.
MASS AND RESISTANCE.—Owing to the fact that resistance acts with such a retarding force on an object of small mass, and it is difficult to set up a rapid motion in an object of great density, lightness in flying machine structures has been considered, in the past, the principal thing necessary.
THE EARLY TENDENCY TO ELIMINATE MOMENTUM.— Builders of flying machines, for several years, sought to eliminate the very thing which gives energy to a horizontally-movable body, namely, momentum.
Instead of momentum, something had to be substituted. This was found in so arranging the machine that its weight, or a portion of it, would be sustained in space by the very element which seeks to retard its flight, namely, the atmosphere.
If there should be no material substance, like air, then the only way in which a heavier-than-air machine could ever fly, would be by propelling it through space, like the ball was thrown, or by some sort of impulse or reaction mechanism on the air-ship itself. It could get no support from the atmosphere.
LIGHT MACHINES UNSTABLE.—Gradually the question of weight is solving itself. Aviators are beginning to realize that momentum is a wonderful property, and a most important element in flying. The safest machines are those which have weight. The light, willowy machines are subject to every caprice of the wind. They are notoriously unstable in flight, and are dangerous even in the hands of experts.
THE APPLICATION OF POWER.—The thing now to consider is not form, or shape, or the distribution of the supporting surfaces, but HOW to apply the power so that it will rapidly transfer a machine at rest to one in motion, and thereby get the proper support on the atmosphere to hold it in flight.
THE SUPPORTING SURFACES.—This brings us to the consideration of one of the first great problems in flying machines, namely, the supporting surfaces,—not its form, shape or arrangement, (which will be taken up in their proper places), but the area, the dimensions, and the angle necessary for flight.
AREA NOT THE ESSENTIAL THING.—The history of flying machines, short as it is, furnishes many examples of one striking fact: That area has but little to do with sustaining an aeroplane when once in flight. The first Wright flyer weighed 741 pounds, had about 400 square feet of plane surface, and was maintained in the air with a 12 horse power engine.
True, that machine was shot into the air by a catapult. Motion having once been imparted to it, the only thing necessary for the motor was to maintain the speed.
There are many instances to show that when once in flight, one horse power will sustain over 100 pounds, and each square foot of supporting surface will maintain 90 pounds in flight.
THE LAW OF GRAVITY.—As the effort to fly may be considered in the light of a struggle to avoid the laws of nature with respect to matter, it may be well to consider this great force as a fitting prelude to the study of our subject.
Proper understanding, and use of terms is very desirable, so that we must not confuse them. Thus, weight and mass are not the same. Weight varies with the latitude, and it is different at various altitudes; but mass is always the same.
If projected through space, a certain mass would move so as to produce momentum, which would be equal at all places on the earth's surface, or at any altitude.
Gravity has been called weight, and weight gravity. The real difference is plain if gravity is considered as the attraction of mass for mass. Gravity is generally known and considered as a force which seeks to draw things to the earth. This is too narrow.
Gravity acts in all directions. Two balls suspended from strings and hung in close proximity to each other will mutually attract each other. If one has double the mass it will have twice the attractive power. If one is doubled and the other tripled, the attraction would be increased six times. But if the distance should be doubled the attraction would be reduced to one-fourth; and if the distance should be tripled then the pull would be only one-ninth.
The foregoing is the substance of the law, namely, that all bodies attract all other bodies with a force directly in proportion to their mass, and inversely as the square of their distance from one another.
To explain this we cite the following illustration: Two bodies, each having a mass of 4 pounds, and one inch apart, are attracted toward each other, so they touch. If one has twice the mass of the other, the smaller will draw the larger only one-quarter of an inch, and the large one will draw the other three-quarters of an inch, thus confirming the law that two bodies will attract each other in proportion to their mass.
Suppose, now, that these balls are placed two inches apart,—that is, twice the distance. As each is, we shall say, four pounds in weight, the square of each would be 16. This does not mean that there would be sixteen times the attraction, but, as the law says, inversely as the square of the distance, so that at two inches there is only one-sixteenth the attraction as at one inch.
If the cord of one of the balls should be cut, it would fall to the earth, for the reason that the attractive force of the great mass of the earth is so much greater than the force of attraction in its companion ball.
INDESTRUCTIBILITY OF GRAVITATION.—Gravity cannot be produced or destroyed. It acts between all parts of bodies equally; the force being proportioned to their mass. It is not affected by any intervening substance; and is transmitted instantaneously, whatever the distance may be.
While, therefore, it is impossible to divest matter of this property, there are two conditions which neutralize its effect. The first of these is position. Let us take two balls, one solid and the other hollow, but of the same mass, or density. If the cavity of the one is large enough to receive the other, it is obvious that while gravity is still present the lines of attraction being equal at all points, and radially, there can be no pull which moves them together.
DISTANCE REDUCES GRAVITATIONAL PULL.—Or the balls may be such distance apart that the attractive force ceases. At the center of the earth an object would not weigh anything. A pound of iron and an ounce of wood, one sixteen times the mass of the other, would be the same,—absolutely without weight.
If the object should be far away in space it would not be influenced by the earth's gravity; so it will be understood that position plays an important part in the attraction of mass for mass.
HOW MOTION ANTAGONIZES GRAVITY.—The second way to neutralize gravity, is by motion. A ball thrown upwardly, antagonizes the force of gravity during the period of its ascent. In like manner, when an object is projected horizontally, while its mass is still the same, its weight is less.
Motion is that which is constantly combating the action of gravity. A body moving in a circle must be acted upon by two forces, one which tends to draw it inwardly, and the other which seeks to throw it outwardly.
The former is called centripetal, and the latter centrifugal motion. Gravity, therefore, represents centripetal, and motion centrifugal force.
If the rotative speed of the earth should be retarded, all objects on the earth would be increased in weight, and if the motion should be accelerated objects would become lighter, and if sufficient speed should be attained all matter would fly off the surface, just as dirt dies off the rim of a wheel at certain speeds.
A TANGENT.—When an object is thrown horizontally the line of flight is tangential to the earth, or at right angles to the force of gravity. Such a course in a flying machine finds less resistance than if it should be projected upwardly, or directly opposite the centripetal pull.
Fig 1. Tangential Flight
TANGENTIAL MOTION REPRESENTS CENTRIFUGAL PULL.—A tangential motion, or a horizontal movement, seeks to move matter away from the center of the earth, and any force which imparts a horizontal motion to an object exerts a centrifugal pull for that reason.
In Fig. 1, let A represent the surface of the earth, B the starting point of the flight of an object, and C the line of flight. That represents a tangential line. For the purpose of explaining the phenomena of tangential flight, we will assume that the missile was projected with a sufficient force to reach the vertical point D, which is 4000 miles from the starting point B.
In such a case it would now be over 5500 miles from the center of the earth, and the centrifugal pull would be decreased to such an extent that the ball would go on and on until it came within the sphere of influence from some other celestial body.
EQUALIZING THE TWO MOTIONS.—But now let us assume that the line of flight is like that shown at E, in Fig. 2, where it travels along parallel with the surface of the earth. In this case the force of the ball equals the centripetal pull,—or, to put it differently, the centrifugal equals the gravitational pull.
The constant tendency of the ball to fly off at a tangent, and the equally powerful pull of gravity acting against each other, produce a motion which is like that of the earth, revolving around the sun once every three hundred and sixty-five days.
It is a curious thing that neither Langley, nor any of the scientists, in treating of the matter of flight, have taken into consideration this quality of momentum, in their calculations of the elements of flight.
Fig. 2 Horizontal Flight
All have treated the subject as though the whole problem rested on the angle at which the planes were placed. At 45 degrees the lift and drift are assumed to be equal.
LIFT AND DRIFT.—The terms should be explained, in view of the frequent allusion which will be made to the terms hereinafter. Lift is the word employed to indicate the amount which a plane surface will support while in flight. Drift is the term used to indicate the resistance which is offered to a plane moving forwardly against the atmosphere.
Fig. 3. Lift and Drift
In Fig. 3 the plane A is assumed to be moving forwardly in the direction of the arrow B. This indicates the resistance. The vertical arrow C shows the direction of lift, which is the weight held up by the plane.
NORMAL PRESSURE.—Now there is another term much used which needs explanation, and that is normal pressure. A pressure of this kind against a plane is where the wind strikes it at right angles. This is illustrated in Fig. 4, in which the plane is shown with the wind striking it squarely.
It is obvious that the wind will exert a greater force against a plane when at its normal. On the other hand, the least pressure against a plane is when it is in a horizontal position, because then the wind has no force against the surfaces, and the only effect on the drift is that which takes place when the wind strikes its forward edge.
Fig. 4. Normal Air Pressure
Fig. 5. Edge Resistance
HEAD RESISTANCE.—Fig. 5 shows such a plane, the only resistance being the thickness of the plane as at A. This is called head resistance, and on this subject there has been much controversy, and many theories, which will be considered under the proper headings.
If a plane is placed at an angle of 45 degrees the lift and the drift are the same, assumedly, because, if we were to measure the power required to drive it forwardly, it would be found to equal the weight necessary to lift it. That is, suppose we should hold a plane at that angle with a heavy wind blowing against it, and attach two pairs of scales to the plane, both would show the same pull.
Fig. 6. Measuring Lift and Drift
MEASURING LIFT AND DRIFT.—In Fig. 6, A is the plane, B the horizontal line which attaches the plane to a scale C, and D the line attaching it to the scale E. When the wind is of sufficient force to hold up the plane, the scales will show the same pull, neglecting, of course, the weight of the plane itself.
PRESSURE AT DIFFERENT ANGLES.—What every one wants to know, and a subject on which a great deal of experiment and time have been expended, is to determine what the pressures are at the different angles between the horizontal, and laws have been formulated which enable the pressures to be calculated.
DIFFERENCE BETWEEN LIFT AND DRIFT IN MOTION.—The first observation is directed to the differences that exist between the lift and drift, when the plane is placed at an angle of less than 45 degrees. A machine weighing 1000 pounds has always the same lift. Its mass does not change. Remember, now, we allude to its mass, or density.
We are not now referring to weight, because that must be taken into consideration, in the problem. As heretofore stated, when an object moves horizontally, it has less weight than when at rest. If it had the same weight it would not move forwardly, but come to rest.
When in motion, therefore, while the lift, so far as its mass is concerned, does not change, the drift does decrease, or the forward pull is less than when at 45 degrees, and the decrease is less and less until the plane assumes a horizontal position, where it is absolutely nil, if we do not consider head resistance.
TABLES OF LIFT AND DRIFT.—All tables of Lift and Drift consider only the air pressures. They do not take into account the fact that momentum takes an important part in the translation of an object, like a flying machine.
A mass of material, weighing 1000 pounds while at rest, sets up an enormous energy when moving through the air at fifty, seventy-five, or one hundred miles an hour. At the latter speed the movement is about 160 feet per second, a motion which is nearly sufficient to maintain it in horizontal flight, independently of any plane surface.
Such being the case, why take into account only the angle of the plane? It is no wonder that aviators have not been able to make the theoretical considerations and the practical demonstrations agree.
WHY TABLES OF LIFT AND DRIFT ARE WRONG.— A little reflection will show why such tables are wrong. They were prepared by using a plane surface at rest, and forcing a blast of air against the plane placed at different angles; and for determining air pressures, this is, no doubt, correct. But it does not represent actual flying conditions. It does not show the conditions existing in an aeroplane while in flight.
To determine this, short of actual experiments with a machine in horizontal translation, is impossible, unless it is done by taking into account the factor due to momentum and the element attributable to the lift of the plane itself due to its impact against the atmosphere.
LANGLEY'S LAW.—The law enunciated by Langley is, that the greater the speed the less the power required to propel it. Water as a propelling medium has over seven hundred times more force than air. A vessel having, for instance, twenty horse power, and a speed of ten miles per hour, would require four times that power to drive it through the water at double the speed. The power is as the square of the speed.
With air the conditions are entirely different. The boat submergence in the water is practically the same, whether going ten or twenty miles an hour. The head resistance is the same, substantially, at all times in the case of the boat; with the flying machine the resistance of its sustaining surfaces decreases.
Without going into a too technical description of the reasoning which led to the discovery of the law of air pressures, let us try and understand it by examining the diagram, Fig. 7.
A represents a plane at an angle of 45 degrees, moving forwardly into the atmosphere in the direction of the arrows B. The measurement across the plane vertically, along the line B, which is called the sine of the angle, represents the surface impact of air against the plane.
In Fig. 8 the plane is at an angle of 27 degrees, which makes the distance in height across the line C just one-half the length of the line B of Fig. 7, hence the surface impact of the air is one-half that of Fig. 7, and the drift is correspondingly decreased.
Fig. 7. Equal Lift and Drift in Flight.
Fig. 8. Unequal Lift and Drift.
MOVING PLANES VS. WINDS.—In this way Boisset, Duchemin, Langley, and others, determined the comparative drift, and those results have been largely relied upon by aviators, and assumed to be correct when applied to flying machines.
That they are not correct has been proven by the Wrights and others, the only explanation being that some errors had been made in the calculations, or that aviators were liable to commit errors in observing the true angle of the planes while in flight.
MOMENTUM NOT CONSIDERED.—The great factor of momentum has been entirely ignored, and it is our desire to press the important point on those who begin to study the question of flying machines.
THE FLIGHT OF BIRDS.—Volumes have been written concerning observations on the flight of birds. The marvel has been why do soaring birds maintain themselves in space without flapping their wings. In fact, it is a much more remarkable thing to contemplate why birds which depend on flapping wings can fly.
THE DOWNWARD BEAT.—It is argued that the downward beat of the wings is so much more rapid than the upward motion, that it gets an action on the air so as to force the body upwardly. This is disposed of by the wing motion of many birds, notoriously the crow, whose lazily-flapping wings can be readily followed by the eye, and the difference in movement, if any, is not perceptible.
THE CONCAVED WING.—It is also urged that the concave on the under side of the wing gives the quality of lift. Certain kinds of beetles, and particularly the common house fly, disprove that theory, as their wings are perfectly flat.
FEATHER STRUCTURE CONSIDERED.—Then the feather argument is advanced, which seeks to show that as each wing is made up of a plurality of feathers, overlapping each other, they form a sort of a valved surface, opening so as to permit air to pass through them during the period of their upward movement, and closing up as the wing descends.
It is difficult to perform this experiment with wings, so as to show such an individual feather movement. It is certain that there is nothing in the structure of the wing bone and the feather connection which points to any individual feather movement, and our observation is, that each feather is entirely too rigid to permit of such an opening up between them.
It is obvious that the wing is built up in that way for an entirely different reason. Soaring birds, which do not depend on the flapping motion, have the same overlapping feather formation.
WEBBED WINGS.—Furthermore, there are numerous flying creatures which do not have feathered wings, but web-like structures, or like the house fly, in one continuous and unbroken plane.
That birds which fly with flapping wings derive their support from the air, is undoubtedly true, and that the lift produced is due, not to the form, or shape, or area of the wing, is also beyond question. The records show that every conceivable type of outlined structure is used by nature; the material and texture of the wings themselves differ to such a degree that there is absolutely no similarity; some have concaved under surfaces, and others have not; some fly with rapidly beating wings, and others with slow and measured movements; many of them fly with equal facility without flapping movements; and the proportions of weight to wing surface vary to such an extent that it is utterly impossible to use such data as a guide in calculating what the proper surface should be for a correct flying machine.
THE ANGLE OF MOVEMENT.—How, then, it may be asked, do they get their support? There must be something, in all this variety and diversity of form, of motion, and of characteristics, which supplies the true answer. The answer lies in the angle of movement of every wing motion, which is at the control of the bird, and if this is examined it will be found that it supplies the correct answer to every type of wing which nature has made.
AN INITIAL IMPULSE OR MOVEMENT NECESSARY.— Let A, Fig. 9, represent the section of a bird's wing. All birds, whether of the soaring or the flapping kind, must have an initial forward movement in order to attain flight. This impulse is acquired either by running along the ground, or by a leap, or in dropping from a perch. Soaring birds cannot, by any possibility, begin flight, unless there is such a movement to change from a position of rest to one of motion.
Fig. 9. Wing Movement in Flight.
In the diagram, therefore, the bird, in moving forwardly, while raising the wing upwardly, depresses the rear edge of the wing, as in position 1, and when the wing beats downwardly the rear margin is raised, in relation to its front margin, as shown in position 2.
A WEDGING MOTION.—Thus the bird, by a wedge-like motion, gives a forwardly-propelling action, and as the rear margin has more or less flexure, its action against the air is less during its upward beat, and this also adds to the upward lift of the body of the bird.
NO MYSTERY IN THE WAVE MOTION.—There is no mystery in the effect of such a wave-like motion, and it must be obvious that the humming bird, and like flyers, which poise at one spot, are able to do so because, instead of moving forwardly, or changing the position of its body horizontally, in performing the undulatory motion of the wing, it causes the body to rock, so that at the point where the wing joins the body, an elliptical motion is produced.
Fig. 10. Evolution of Humming-Bird's Wing.
HOW BIRDS POISE WITH FLAPPING WINGS.—This is shown in Fig. 10, in which eight successive positions of the wing are shown, and wherein four of the position, namely, 1, 2, 3, and 4, represent the downward movement, and 6, 7, 8, and 9, the upward beat.
All the wing angles are such that whether the suspension point of each wing is moving downwardly, or upwardly, a support is found in some part of the wing.
NARROW-WINGED BIRDS.—Birds with rapid flapping motions have comparatively narrow wings, fore and aft. Those which flap slowly, and are not swift flyers, have correspondingly broader wings. The broad wing is also typical of the soaring birds.
But how do the latter overcome gravitation without exercising some sort of wing movement?
INITIAL MOVEMENT OF SOARING BIRDS.—Acute observations show that during the early stages of flight, before speed is acquired, they depend on the undulating movement of the wings, and some of them acquire the initial motion by flapping. When speed is finally attained it is difficult for the eye to note the motion of the wings.
SOARING BIRDS MOVE SWIFTLY.—Now, the first observation is, that soaring birds are swiftly- moving creatures. As they sail overhead majestically they seem to be moving slowly. But distance is deceptive. The soaring bird travels at great speeds, and this in itself should be sufficient to enable us to cease wondering, when it is remembered that swift translation decreases weight, so that this factor does not, under those conditions, operate against flight.
MUSCULAR ENERGY EXERTED BY SOARING BIRDS. —It is not conceivable that the mere will of the bird would impel it forwardly, without it exerted some muscular energy to keep up its speed. The distance at which the bird performs this wonderful evolution is at such heights from the observer that the eye cannot detect a movement.
WINGS NOT MOTIONLESS.—While the wings appear to be absolutely motionless, it is more reasonable to assume that a slight sinuous movement, or a rocking motion is constantly kept up, which wedges forwardly with sufficient speed to compel momentum to maintain it in flight. To do so requires but a small amount of energy. The head resistance of the bird formation is reduced to a minimum, and at such high speeds the angle of incidence of the wings is very small, requiring but little aid to maintain it in horizontal flight.
PRINCIPLES OF AEROPLANE FLIGHT
FROM the foregoing chapter, while it may be rightly inferred that power is the true secret of aeroplane flight, it is desirable to point out certain other things which must be considered.
SPEED AS ONE OF THE ELEMENTS—Every boy, probably, has at some time or other thrown small flat stones, called "skippers." He has noticed that if they are particularly thin, and large in diameter, that there is a peculiar sailing motion, and that they move through the air in an undulating or wave-like path.
Two things contribute to this motion; one is the size of the skipper, relative to its weight, and the other is its speed. If the speed is slow it will quickly wend its way to the earth in a gradual curve. This curved line is called its trajectory. If it is not very large diametrically, in proportion to its weight, it will also make a gradual curve in descending, without "skimming" up and down in its flight.
SHAPE AND SPEED.—It has been observed, also, that a round ball, or an object not flattened out, will make a regular curved path, whatever the speed may be.
It may be assumed, therefore, that the shape alone does not account for this sinuous motion; but that speed is the element which accounts for it. Such being the case it may be well to inquire into the peculiar action which causes a skipper to dart up and down, and why the path thus formed grows more and more accentuated as the speed increases.
As will be more fully described in a later chapter, the impact of air against a moving body does not increase in proportion to its speed, but in the ratio of the square of the speed.
WHAT SQUARE OF THE SPEED MEANS.—In mathematics a figure is squared when it is multiplied by itself. Thus, 4 X 4= 16; 5 X 5 = 25; and so on, so that 16 is the square of 4, and 25 the square of 5. It has been found that a wind moving at the speed of 20 miles an hour has a striking or pushing force of 2 pounds on every square foot of surface.
If the wind travels twice as fast, or 40 miles an hour, the pushing force is not 4 pounds, but 8 pounds. If the speed is 60 miles an hour the pushing force increases to 18 pounds.
ACTION OF A SKIPPER.—When the skipper leaves the hands of the thrower it goes through the air in such a way that its fiat surface is absolutely on a line with the direction in which it is projected.
At first it moves through the air solely by force of the power which impels it, and does not in any way depend on the air to hold it up. See Fig. 1, in which A represents the line of projection, and B the disk in its flight.
Fig. 11. A Skipper in Flight.
After it has traveled a certain distance, and the force decreases, it begins to descend, thus describing the line C, Fig. 1, the disk B, in this case descending, without changing its position, which might be described by saying that it merely settles down to the earth without changing its plane.
The skipper still remains horizontal, so that as it moves toward the earth its flat surface, which is now exposed to the action of the air, meets with a resistance, and this changes the angle of the disk, so that it will not be horizontal. Instead it assumes the position as indicated at D, and this impinging effect against the air causes the skipper to move upwardly along the line E, and having reached a certain limit, as at, say E, it automatically again changes its angle and moves downwardly along the path F, and thus continues to undulate, more or less, dependent on the combined action of the power and weight, or momentum, until it reaches the earth.
It is, therefore, clear that the atmosphere has an action on a plane surface, and that the extent of the action, to sustain it in flight, depends on two things, surface and speed.
Furthermore, the greater the speed the less the necessity for surface, and that for gliding purposes speed may be sacrificed, in a large measure, where there is a large surface.
This very action of the skipper is utilized by the aviator in volplaning,—that is, where the power of the engine is cut off, either by accident, or designedly, and the machine descends to the earth, whether in a long straight glide, or in a great circle.
As the machine nears the earth it is caused to change the angle of flight by the control mechanism so that it will dart upwardly at an angle, or downwardly, and thus enable the pilot to sail to another point beyond where he may safely land. This changing the course of the machine so that it will glide upwardly, means that the incidence of the planes has been changed to a positive angle.
ANGLE OF INCIDENCE.—In aviation this is a term given to the position of a plane, relative to the air against which it impinges. If, for instance, an aeroplane is moving through the air with the front margin of the planes higher than their rear margins, it is said to have the planes at a positive angle of incidence. If the rear margins are higher than the front, then the planes have a negative angle of incidence.
The word incidence really means, a falling upon, or against; and it will be seen, therefore, that the angle of incidence means the tilt of the planes in relation to the air which strikes it.
Having in view, therefore, that the two qualities, namely, speed and surface, bear an intimate relation with each other, it may be understood wherein mechanical flight is supposed to be analogous to bird flight.
SPEED AND SURFACE.—Birds which poise in the air, like the humming bird, do so because they beat their wings with great rapidity. Those which soar, as stated, can do so only by moving through the atmosphere rapidly, or by having a large wing spread relative to the weight. It will thus be seen that speed and surface become the controlling factors in flight, and that while the latter may be entirely eliminated from the problem, speed is absolutely necessary under any and all conditions.
By speed in this connection is not meant high velocity, but that a movement, produced by power expressed in some form, is the sole and most necessary requisite to movement through the air with all heavier-than-air machines.
If sufficient power can be applied to an aeroplane, surface is of no consequence; shape need not be considered, and any sort of contrivance will move through the air horizontally.
CONTROL OF THE DIRECTION OF FLIGHT.—But the control of such a body, when propelled through space by force alone, is a different matter. To change the machine from a straight path to a curved one, means that it must be acted upon by some external force.
We have explained that power is something which is inherent in the thing itself. Now, in order that there may be a change imparted to a moving mass, advantage must be taken of the medium through which it moves,—the atmosphere.
VERTICAL CONTROL PLANES.—If vertically-arranged planes are provided, either fore or aft of the machine, or at both ends, the angles of incidence may be such as to cause the machine to turn from its straight course.
In practice, therefore, since it is difficult to supply sufficient power to a machine to keep it in motion horizontally, at all times, aeroplanes are provided with supporting surfaces, and this aid in holding it up grows less and less as its speed increases.
But, however strong the power, or great the speed, its control from side to side is not dependent on the power of the engine, or the speed at which it travels through the air.
Here the size of the vertical planes, and their angles, are the only factors to be considered, and these questions will be considered in their proper places.
THE FORM OR SHAPE OF FLYING MACHINES
EVERY investigator, experimenter, and scientist, who has given the subject of flight study, proceeds on the theory that in order to fly man must copy nature, and make the machine similar to the type so provided.
THE THEORY OF COPYING NATURE.—If such is the case then it is pertinent to inquire which bird is the proper example to use for mechanical flight. We have shown that they differ so radically in every essential, that what would be correct in one thing would be entirely wrong in another.
The bi-plane is certainly not a true copy. The only thing in the Wright machine which in any way resembles the bird's wing, is the rounded end of the planes, and judging from other machines, which have square ends, this slight similarity does not contribute to its stability or otherwise help the structure.
The monoplane, which is much nearer the bird type, has also sounded wing ends, made not so much for the purpose of imitating the wing of the bird, as for structural reasons.
HULLS OF VESSELS.—If some marine architect should come forward and assert that he intended to follow nature by making a boat with a hull of the shape or outline of a duck, or other swimming fowl, he would be laughed at, and justly so, because the lines of vessels which are most efficient are not made like those of a duck or other swimming creatures.
MAN DOES NOT COPY NATURE.—Look about you, and see how many mechanical devices follow the forms laid down by nature, or in what respect man uses the types which nature provides in devising the many inventions which ingenuity has brought forth.
PRINCIPLES ESSENTIAL, NOT FORMS.—It is essential that man shall follow nature's laws. He cannot evade the principles on which the operations of mechanism depend; but in doing so he has, in nearly every instance, departed from the form which nature has suggested, and made the machine irrespective of nature's type.
Let us consider some of these striking differences to illustrate this fact. Originally pins were stuck upon a paper web by hand, and placed in rows, equidistant from each other. This necessitates the cooperative function of the fingers and the eye. An expert pin sticker could thus assemble from four to five thousand pins a day.
The first mechanical pinsticker placed over 500,000 pins a day on the web, rejecting every bent or headless pin, and did the work with greater accuracy than it was possible to do it by hand. There was not the suggestion of an eye, or a finger in the entire machine, to show that nature furnished the type.
NATURE NOT THE GUIDE AS TO FORMS.—Nature does not furnish a wheel in any of its mechanical expressions. If man followed nature's form in the building of the locomotive, it would move along on four legs like an elephant. Curiously enough, one of the first road wagons had "push legs,"—an instance where the mechanic tried to copy nature,—and failed.
THE PROPELLER TYPE.—The well known propeller is a type of wheel which has no prototype in nature. It is maintained that the tail of a fish in its movement suggested the propeller, but the latter is a long departure from it.
The Venetian rower, who stands at the stern, and with a long-bladed oar, fulcrumed to the boat's extremity, in making his graceful lateral oscillations, simulates the propelling motion of the tail in an absolutely perfect manner, but it is not a propeller, by any means comparable to the kind mounted on a shaft, and revoluble.
How much more efficient are the spirally-formed blades of the propeller than any wing or fin movement, in air or sea. There is no comparison between the two forms in utility or value.
Again, the connecting points of the arms and legs with the trunk of a human body afford the most perfect types of universal joints which nature has produced. The man-made universal joint has a wider range of movement, possesses greater strength, and is more perfect mechanically. A universal joint is a piece of mechanism between two elements, which enables them to be turned, or moved, at any angle relative to each other.
But why multiply these instances. Like samples will be found on every hand, and in all directions, and man, the greatest of all of nature's products, while imperfect in himself, is improving and adapting the things he sees about him.
WHY SPECIALLY-DESIGNED FORMS IMPROVE NATURAL STRUCTURES.—The reason for this is, primarily, that the inventor must design the article for its special work, and in doing so makes it better adapted to do that particular thing. The hands and fingers can do a multiplicity of things, but it cannot do any particular work with the facility or the degree of perfection that is possible with the machine made for that purpose.
The hands and fingers will bind a sheaf of wheat, but it cannot compete with the special machine made for that purpose. On the other hand the binder has no capacity to do anything else than what it was specially made for.
In applying the same sort of reasoning to the building of flying machines we must be led to the conclusion that the inventor can, and will, eventually, bring out a form which is as far superior to the form which nature has taught us to use as the wonderful machines we see all about us are superior to carry out the special work they were designed to do.
On land, man has shown this superiority over matter, and so on the sea. Singularly, the submarines, which go beneath the sea, are very far from that perfected state which have been attained by vessels sailing on the surface; and while the means of transportation on land are arriving at points where the developments are swift and remarkable, the space above the earth has not yet been conquered, but is going through that same period of development which precedes the production of the true form itself.
MECHANISM DEVOID OF INTELLIGENCE.—The great error, however, in seeking to copy nature's form in a flying machine is, that we cannot invest the mechanism with that which the bird has, namely, a guiding intelligence to direct it instinctively, as the flying creature does.
A MACHINE MUST HAVE A SUBSTITUTE FOR INTELLIGENCE. —Such being the case it must be endowed with something which is a substitute. A bird is a supple, pliant organism; a machine is a rigid structure. One is capable of being directed by a mind which is a part of the thing itself; while the other must depend on an intelligence which is separate from it, and not responsive in feeling or movement.
For the foregoing reasons success can never be attained until some structural form is devised which will consider the flying machine independently of the prototypes pointed out as the correct things to follow. It does not, necessarily, have to be unlike the bird form, but we do know that the present structures have been made and insisted upon blindly, because of this wrong insistence on forms.
STUDY OF BIRD FLIGHT USELESS.—The study of the flight of birds has never been of any special value to the art. Volumes have been written on the subject. The Seventh Duke of Argyle, and later, Pettigrew, an Englishman, contributed a vast amount of written matter on the subject of bird flight, in which it was sought to show that soaring birds did not exert any power in flying.
Writers and experimenters do not agree on the question of the propulsive power, or on the form or shape of the wing which is most effective, or in the matter of the relation of surface to weight, nor do they agree in any particular as to the effect and action of matter in the soaring principle.
Only a small percentage of flying creatures use motionless wings as in soaring. By far, the greater majority use beating wings, a method of translation in air which has not met with success in any attempts on the part of the inventor.
Nevertheless, experimenting has proceeded on lines which seek to recognize nature's form only, while avoiding the best known and most persistent type.
SHAPE OF SUPPORTING SURFACES.—When we examine the prevailing type of supporting surfaces we cannot fail to be impressed with one feature, namely, the determination to insist on a broad spread of plane surface, in imitation of the bird with outstretched wings.
THE TROUBLE ARISING FROM OUTSTRETCHED WINGS.—This form of construction is what brings all the troubles in its train. The literature on aviation is full of arguments on this subject, all declaring that a wide spread is essential, because, —birds fly that way.
These assertions are made notwithstanding the fact that only a few years ago, in the great exhibit of aeroplanes in Paris, many unique forms of machines were shown, all of them capable of flying, as proven by numerous experiments, and among them were a half dozen types whose length fore and aft were much greater than transversely, and it was particularly noted that they had most wonderful stability.
DENSITY OF THE ATMOSPHERE.—Experts declare that the density of the atmosphere varies throughout, —that it has spots here and there which are, apparently, like holes, so that one side or the other of the machine will, unaccountably, tilt, and sometimes the entire machine will suddenly drop for many feet, while in flight.
ELASTICITY OF THE AIR.—Air is the most elastic substance known. The particles constituting it are constantly in motion. When heat or cold penetrate the mass it does so, in a general way, so as to permeate the entire body, but the conductivity of the atmospheric gases is such that the heat does not reach all parts at the same time.
AIR HOLES.—The result is that varying strata of heat and cold seem to be superposed, and also distributed along the route taken by a machine, causing air currents which vary in direction and intensity. When, therefore, a rapidly-moving machine passes through an atmosphere so disturbed, the surfaces of the planes strike a mass of air moving, we may say, first toward the plane, and the next instant the current is reversed, and the machine drops, because its support is temporarily gone, and the aviator experiences the sensation of going into a "hole."
RESPONSIBILITY FOR ACCIDENTS.—These so-called "holes" are responsible for many accidents. The outstretched wings, many of them over forty feet from tip to tip, offer opportunities for a tilt at one end or the other, which has sent so many machines to destruction.
The high center of gravity in all machines makes the weight useless to counterbalance the rising end or to hold up the depressed wing.
All aviators agree that these unequal areas of density extend over small spaces, and it is, therefore, obvious that a machine which is of such a structure that it moves through the air broadside on, will be more liable to meet these inequalities than one which is narrow and does not take in such a wide path.
Why, therefore, persist in making a form which, by its very nature, invites danger? Because birds fly that way!
THE TURNING MOVEMENT.—This structural arrangement accentuates the difficulty when the machine turns. The air pressure against the wing surface is dependent on the speed. The broad outstretched surfaces compel the wing at the outer side of the circle to travel faster than the inner one. As a result, the outer end of the aeroplane is elevated.
CENTRIFUGAL ACTION.—At the same time the running gear, and the frame which carries it and supports the machine while at rest, being below the planes, a centrifugal force is exerted, when turning a circle, which tends to swing the wheels and frame outwardly, and thereby still further elevating the outer end of the plane.
THE WARPING PLANES.—The only remedy to meet this condition is expressed in the mechanism which wraps or twists the outer ends of the planes, as constructed in the Wright machine, or the ailerons, or small wings at the rear margins of the planes, as illustrated by the Farman machine. The object of this arrangement is to decrease the angle of incidence at the rising end, and increase the angle at the depressed end, and thus, by manually- operated means keep the machine on an even keel.
FORE AND AFT CONTROL
THERE is no phase of the art of flying more important than the fore and aft control of an airship. Lateral stability is secondary to this feature, for reasons which will appear as we develop the subject.
THE BIRD TYPE OF FORE AND AFT CONTROL.— Every aeroplane follows the type set by nature in the particular that the body is caused to oscillate on a vertical fore and aft plane while in flight. The bird has one important advantage, however, in structure. Its wing has a flexure at the joint, so that its body can so oscillate independently of the angle of the wings.
The aeroplane has the wing firmly fixed to the body, hence the only way in which it is possible to effect a change in the angle of the wing is by changing the angle of the body. To be consistent the aeroplane should be so constructed that the angle of the supporting surfaces should be movable, and not controllable by the body.
The bird, in initiating flight from a perch, darts downwardly, and changes the angle of the body to correspond with the direction of the flying start. When it alights the body is thrown so that its breast banks against the air, but in ordinary flight its wings only are used to change the angle of flight.
ANGLE AND DIRECTION OF FLIGHT.—In order to become familiar with terms which will be frequently used throughout the book, care should be taken to distinguish between the terms angle and direction of flight. The former has reference to the up and down movement of an aeroplane, whereas the latter is used to designate a turning movement to the right or to the left.
WHY SHOULD THE ANGLE OF THE BODY CHANGE? —The first question that presents itself is, why should the angle of the aeroplane body change? Why should it be made to dart up and down and produce a sinuous motion? Why should its nose tilt toward the earth, when it is descending, and raise the forward part of the structure while ascending?
The ready answer on the part of the bird-form advocate is, that nature has so designed a flying structure. The argument is not consistent, because in this respect, as in every other, it is not made to conform to the structure which they seek to copy.
CHANGING ANGLE OF BODY NOT SAFE.—Furthermore, there is not a single argument which can be advanced in behalf of that method of building, which proves it to be correct. Contrariwise, an analysis of the flying movement will show that it is the one feature which has militated against safety, and that machines will never be safe so long as the angle of the body must be depended upon to control the angle of flying.
Fig. 11a Monoplane in Flight.
In Fig. 11a three positions of a monoplane are shown, each in horizontal flight. Let us say that the first figure A is going at 40 miles per hour, the second, B, at 50, and the third, C, at 60 miles. The body in A is nearly horizontal, the angle of the plane D being such that, with the tail E also horizontal, an even flight is maintained.
When the speed increases to 50 miles an hour, the angle of incidence in the plane D must be decreased, so that the rear end of the frame must be raised, which is done by giving the tail an angle of incidence, otherwise, as the upper side of the tail should meet the air it would drive the rear end of the frame down, and thus defeat the attempt to elevate that part.
Fig. 12. Angles of Flight.
As the speed increases ten miles more, the tail is swung down still further and the rear end of the frame is now actually above the plane of flight. In order, now, to change the angle of flight, without altering the speed of the machine, the tail is used to effect the control.
Examine the first diagram in Fig. 12. This shows the tail E still further depressed, and the air striking its lower side, causes an upward movement of the frame at that end, which so much decreases the angle of incidence that the aeroplane darts downwardly.
In order to ascend, the tail, as shown in the second diagram, is elevated so as to depress the rear end, and now the sustaining surface shoots upwardly.
Suppose that in either of the positions 1 or 2, thus described, the aviator should lose control of the mechanism, or it should become deranged or "stick," conditions which have existed in the history of the art, what is there to prevent an accident?
In the first case, if there is room, the machine will loop the loop, and in the second case the machine will move upwardly until it is vertical, and then, in all probability, as its propelling power is not sufficient to hold it in that position, like a helicopter, and having absolutely no wing supporting surface when in that position, it will dart down tail foremost.
A NON-CHANGING BODY.—We may contrast the foregoing instances of flight with a machine having the sustaining planes hinged to the body in such a manner as to make the disposition of its angles synchronous with the tail. In other words, see how a machine acts that has the angle of flight controllable by both planes,—that is, the sustaining planes, as well as the tail.
Fig. 13. Planes on Non-changing Body.
In Fig. 13 let the body of the aeroplane be horizontal, and the sustaining planes B disposed at the same angle, which we will assume to be 15 degrees, this being the imaginary angle for illustrative purposes, with the power of the machine to drive it along horizontally, as shown in position 1.
In position 2 the angles of both planes are now at 10 degrees, and the speed 60 miles an hour, which still drives the machine forward horizontally.
In position 3 the angle is still less, being now only 5 degrees but the speed is increased to 80 miles per hour, but in each instance the body of the machine is horizontal.
Now it is obvious that in order to ascend, in either case, the changing of the planes to a greater angle would raise the machine, but at the same time keep the body on an even keel.
Fig. 14. Descent with Non-changing Body.
DESCENDING POSITIONS BY POWER CONTROL.—In Fig. 14 the planes are the same angles in the three positions respectively, as in Fig. 13, but now the power has been reduced, and the speeds are 30, 25, and 20 miles per hour, in positions A, B and C.
Suppose that in either position the power should cease, and the control broken, so that it would be impossible to move the planes. When the machine begins to lose its momentum it will descend on a curve shown, for instance, in Fig. 15, where position 1 of Fig. 14 is taken as the speed and angles of the plane when the power ceased.
Fig. 15. Utilizing Momentum.
CUTTING OFF THE POWER.—This curve, A, may reach that point where momentum has ceased as a forwardly-propelling factor, and the machine now begins to travel rearwardly. (Fig. 16.) It has still the entire supporting surfaces of the planes. It cannot loop-the-loop, as in the instance where the planes are fixed immovably to the body.
Carefully study the foregoing arrangement, and it will be seen that it is more nearly in accord with the true flying principle as given by nature than the vaunted theories and practices now indulged in and so persistently adhered to.
The body of a flying machine should not be oscillated like a lever. The support of the aeroplane should never be taken from it. While it may be impossible to prevent a machine from coming down, it can be prevented from overturning, and this can be done without in the least detracting from it structurally.
Fig. 16. Reversing Motion.
The plan suggested has one great fault, however. It will be impossible with such a structure to cause it to fly upside down. It does not present any means whereby dare-devil stunts can be performed to edify the grandstand. In this respect it is not in the same class with the present types.
THE STARTING MOVEMENT.—Examine this plan from the position of starting, and see the advantages it possesses. In these illustrations we have used, for convenience only, the monoplane type, and it is obvious that the same remarks apply to the bi-plane.
Fig. 17 shows the starting position of the stock monoplane, in position 1, while it is being initially run over the ground, preparatory to launching. Position 2 represents the negative angle at which the tail is thrown, which movement depresses the rear end of the frame and thus gives the supporting planes the proper angle to raise the machine, through a positive angle of incidence, of the plane.
Fig. 17. Showing changing angle of body.
THE SUGGESTED TYPE.—In Fig. 18 the suggested type is shown with the body normally in a horizontal position, and the planes in a neutral position, as represented in position 1. When sufficient speed had been attained both planes are turned to the same angle, as in position 2, and flight is initiated without the abnormal oscillating motion of the body.
But now let us see what takes place the moment the present type is launched. If, by any error on the part of the aviator, he should fail to readjust the tail to a neutral or to a proper angle of incidence, after leaving the ground, the machine would try to perform an over-head loop.
The suggested plan does not require this caution. The machine may rise too rapidly, or its planes may be at too great an angle for the power or the speed, or the planes may be at too small an angle, but in either case, neglect would not turn the machine to a dangerous position.
These suggestions are offered to the novice, because they go to the very foundation of a correct understanding of the principles involved in the building and in the manipulation of flying machines and while they are counter to the beliefs of aviators, as is shown by the persistency in adhering to the old methods, are believed to be mechanically correct, and worthy of consideration.
THE LOW CENTER OF GRAVITY.—But we have still to examine another feature which shows the wrong principle in the fixed planes. The question is often asked, why do the builders of aeroplanes place most of the weight up close to the planes? It must be obvious to the novice that the lower the weight the less liability of overturning.
FORE AND AFT OSCILLATIONS.—The answer is, that when the weight is placed below the planes it acts like a pendulum. When the machine is traveling forward, and the propeller ceases its motion, as it usually does instantaneously, the weight, being below, and having a certain momentum, continues to move on, and the plane surface meeting the resistance just the same, and having no means to push it forward, a greater angle of resistance is formed.
In Fig. 19 this action of the two forces is illustrated. The plane at the speed of 30 miles is at an angle of 15 degrees, the body B of the machine being horizontal, and the weight C suspended directly below the supporting surfaces.
The moment the power ceases the weight continues moving forwardly, and it swings the forward end of the frame upwardly, Fig. 20, and we now have, as in the second figure, a new angle of incidence, which is 30 degrees, instead of 12. It will be understood that in order to effect a change in the position of the machine, the forward end ascends, as shown by the dotted line A.
Fig. 20. Action when Propeller ceases to pull.
The weight a having now ascended as far as possible forward in its swing, and its motion checked by the banking action of the plan it will again swing back, and again carry with it the frame, thus setting up an oscillation, which is extremely dangerous.
The tail E, with its unchanged angle, does not, in any degree, aid in maintaining the frame on an even keel. Being nearly horizontal while in flight, if not at a negative angle, it actually assists the forward end of the frame to ascend.
APPLICATION OF THE NEW PRINCIPLE.—Extending the application of the suggested form, let us see wherein it will prevent this pendulous motion at the moment the power ceases to exert a forwardly- propelling force.
Fig. 21. Synchronously moving Planes.
In Fig. 21 the body A is shown to be equipped with the supporting plane B and the tail a, so they are adjustable simultaneously at the same angle, and the weight D is placed below, similar to the other structure.
At every moment during the forward movement of this type of structure, the rear end of the machine has a tendency to move upwardly, the same as the forward end, hence, when the weight seeks, in this case to go on, it acts on the rear plane, or tail, and causes that end to raise, and thus by mutual action, prevents any pendulous swing.
LOW WEIGHT NOT NECESSARY WITH SYNCHRONOUSLY-MOVING WINGS. —A little reflection will convince any one that if the two wings move in harmony, the weight does not have to be placed low, and thus still further aid in making a compact machine. By increasing the area of the tail, and making that a true supporting surface, instead of a mere idler, the weight can be moved further back, the distance transversely across the planes may be shortened, and in that way still further increase the lateral stability.
DIFFERENT MACHINE TYPES AND THEIR CHARACTERISTICS
THERE are three distinct types of heavier-than- air machines, which are widely separated in all their characteristics, so that there is scarcely a single feature in common.
Two of them, the aeroplane, and the orthopter, have prototypes in nature, and are distinguished by their respective similarities to the soaring birds, and those with flapping wings.
The Helicopter, on the other hand, has no antecedent type, but is dependent for its raising powers on the pull of a propeller, or a plurality of them, constructed, as will be pointed out hereinafter.
AEROPLANES.—The only form which has met with any success is the aeroplane, which, in practice, is made in two distinct forms, one with a single set of supporting planes, in imitation of birds, and called a monoplane; and the other having two wings, one above the other, and called the bi-plane, or two-planes.
All machines now on the market which do not depend on wing oscillations come under those types.
THE MONOPLANE.—The single plane type has some strong claims for support. First of these is the comparatively small head resistance, due to the entire absence of vertical supporting posts, which latter are necessary with the biplane type. The bracing supports which hold the outer ends of the planes are composed of wires, which offer but little resistance, comparatively, in flight.
ITS ADVANTAGES.—Then the vertical height of the machine is much less than in the biplane. As a result the weight, which is farther below the supporting surface than in the biplane, aids in maintaining the lateral stability, particularly since the supporting frame is higher.
Usually, for the same wing spread, the monoplane is narrower, laterally, which is a further aid to prevent tilting.
ITS DISADVANTAGES.—But it also has disadvantages which must be apparent from its structure. As all the supporting surface is concentrated in half the number of planes, they must be made of greater width fore and aft, and this, as we shall see, later on, proves to be a disadvantage.
It is also doubted whether the monoplane can be made as strong structurally as the other form, owing to the lack of the truss formation which is the strong point with the superposed frame. A truss is a form of construction where braces can be used from one member to the next, so as to brace and stiffen the whole.
THE BIPLANE.—Nature does not furnish a type of creature which has superposed wings. In this particular the inventor surely did not follow nature. The reasons which led man to employ this type may be summarized as follows:
In experimenting with planes it is found that a broad fore and aft surface will not lift as much as a narrow plane. This subject is fully explained in the chapter on The Lifting Surfaces of Planes. In view of that the technical descriptions of the operation will not be touched upon at this place, except so far as it may be necessary to set forth the present subject.
This peculiarity is due to the accumulation of a mass of moving air at the rear end of the plane, which detracts from its lifting power. As it would be a point of structural weakness to make the wings narrow and very long, Wenham many years ago suggested the idea of placing one plane above the other, and later on Chanute, an engineer, used that form almost exclusively, in experimenting with his gliders.
It was due to his influence that the Wrights adopted that form in their gliding experiments, and later on constructed their successful flyers in that manner. Originally the monoplane was the type generally employed by experimenters, such as Lilienthal, and others.
STABILITY IN BIPLANES.—Biplanes are not naturally as stable laterally as the monoplane. The reason is, that a downward tilt has the benefit of only a narrow surface, comparable with the monoplane, which has broadness of wing.
To illustrate this, let us assume that we have a biplane with planes five feet from front to rear, and thirty-six feet in length. This would give two planes with a sustaining surface of 360 square feet. The monoplane would, probably, divide this area into one plane eight and a half feet from front to rear, and 42 feet in length.
In the monoplane each wing would project out about three feet more on each side, but it would have eight and a half feet fore and aft spread to the biplane's five feet, and thus act as a greater support.
THE ORTHOPTER.—The term orthopter, or ornithopter, meaning bird wing, is applied to such flying machines as depend on wing motion to support them in the air.
Unquestionably, a support can be obtained by beating on the air but to do so it is necessary to adopt the principle employed by nature to secure an upward propulsion. As pointed out elsewhere, it cannot be the concaved type of wing, or its shape, or relative size to the weight it must carry.
As nature has furnished such a variety of data on these points, all varying to such a remarkable degree, we must look elsewhere to find the secret. Only one other direction offers any opportunity, and that is in the individual wing movement.
NATURE'S TYPE NOT UNIFORM.—When this is examined, the same obscurity surrounds the issue. Even the speeds vary to such an extent that when it is tried to differentiate them, in comparison with form, shape, and construction, the experimenter finds himself wrapt in doubt and perplexity.
But birds do fly, notwithstanding this wonderful array of contradictory exhibitions. Observation has not enabled us to learn why these things are so. High authorities, and men who are expert aviators, tell us that the bird flies because it is able to pick out ascending air currents.
THEORIES ABOUT FLIGHT OF BIRDS.—Then we are offered the theory that the bird has an instinct which tells it just how to balance in the air when its wings are once set in motion. Frequently, what is taken for instinct, is something entirely different.
It has been assumed, for instance, that a cyclist making a turn at a rapid speed, and a bird flying around a circle will throw the upper part of the body inwardly to counteract the centrifugal force which tends to throw it outwardly.
Experiments with the monorail car, which is equipped with a gyroscope to hold it in a vertical position, show that when the car approaches a curve the car will lean inwardly, exactly the same as a bird, or a cyclist, and when a straight stretch is reached, it will again straighten up.
INSTINCT.—Now, either the car, so equipped possesses instinct, or there must be a principle in the laws of nature which produces the similarity of action.
In like manner there must be some principle that is entirely independent of the form of matter, or its arrangement, which enables the bird to perform its evolutions. We are led to believe from all the foregoing considerations that it is the manner or the form of the motion.
MODE OF MOTION.—In this respect it seems to be comparable in every respect to the great and universal law of the motions in the universe. Thus, light, heat and electricity are the same, the manifestations being unlike only because they have different modes of motion.
Everything in nature manifests itself by motion. It is the only way in which nature acts. Every transformation from one thing to another, is by way of a movement which is characteristic in itself.
Why, then, should this great mystery of nature, act unlike the other portions of which it is a part?
THE WING STRUCTURE.—The wing structure of every flying creature that man has examined, has one universal point of similarity, and that is the manner of its connection with the body. It is a sort of universal joint, which permits the wing to swing up and down, perform a gyratory movement while doing so, and folds to the rear when at rest.
Some have these movements in a greater or less degree, or capable of a greater range; but the joint is the same, with scarcely an exception. When the stroke of the wing is downwardly the rear margin is higher than the front edge, so that the downward beat not only raises the body upwardly, but also propels it forwardly.
THE WING MOVEMENT.—The moment the wing starts to swing upwardly the rear end is depressed, and now, as the bird is moving forwardly, the wing surface has a positive angle of incidence, and as the wing rises while the forward motion is taking place, there is no resistance which is effective enough to counteract the momentum which has been set up.
The great problem is to put this motion into a mechanical form. The trouble is not ascribable to the inability of the mechanic to describe this movement. It is an exceedingly simple one. The first difficulty is in the material that must be used. Lightness and strength for the wing itself are the first requirements. Then rigidity in the joint and in the main rib of the wing, are the next considerations.
In these respects the ability of man is limited. The wing ligatures of flying creatures is exceedingly strong, and flexible; the hollow bone formation and the feathers are extremely light, compared with their sustaining powers.
THE HELICOPTER MOTION.—The helicopter, or helix-wing, is a form of flying machine which depends on revolving screws to maintain it in the air. Many propellers are now made, six feet in length, which have a pull of from 400 to 500 pounds. If these are placed on vertically-disposed shafts they would exert a like power to raise a machine from the earth.
Obviously, it is difficult to equip such a machine with planes for sustaining it in flight, after it is once in the air, and unless such means are provided the propellers themselves must be the mechanism to propel it horizontally.
This means a change of direction of the shafts which support the propellers, and the construction is necessarily more complicated than if they were held within non-changeable bearings.
This principle, however, affords a safer means of navigating than the orthopter type, because the blades of such an instrument can be forced through the air with infinitely greater speed than beating wings, and it devolves on the inventor to devise some form of apparatus which will permit the change of pull from a vertical to a horizontal direction while in flight.
THE LIFTING SURFACES OF AEROPLANES
THIS subject includes the form, shape and angle of planes, used in flight. It is the direction in which most of the energy has been expended in developing machines, and the true form is still involved in doubt and uncertainty.
RELATIVE SPEED AND ANGLE.—The relative speed and angle, and the camber, or the curved formation of the plane, have been considered in all their aspects, so that the art in this respect has advanced with rapid strides.
NARROW PLATES MOST EFFECTIVE.—It was learned, in the early stages of the development by practical experiments, that a narrow plane, fore and aft, produces a greater lift than a wide one, so that, assuming the plane has 100 square feet of sustaining surface, it is far better to make the shape five feet by twenty than ten by ten.
However, it must be observed, that to use the narrow blade effectively, it must be projected through the air with the long margin forwardly. Its sustaining power per square foot of surface is much less if forced through the air lengthwise.
Experiments have shown why a narrow blade has proportionally a greater lift, and this may be more clearly understood by examining the illustrations which show the movement of planes through the air at appropriate angles.
Fig. 22. Stream lines along a plane.
STREAM LINES ALONG A PLANE.—In Fig. 22, A is a flat plane, which we will assume is 10 feet from the front to the rear margin. For convenience seven stream lines of air are shown, which contact with this inclined surface. The first line 1, after the contact at the forward end, is driven downwardly along the surface, so that it forms what we might term a moving film.
The second air stream 2, strikes the first stream, followed successively by the other streams, 3, 4, and so on, each succeeding stream being compelled to ride over, or along on the preceding mass of cushioned air, the last lines, near the lower end, being, therefore, at such angles, and contacting with such a rapidly-moving column, that it produces but little lift in comparison with the 1st, 2d and 3d stream lines. These stream lines are taken by imagining that the air approaches and contacts with the plane only along the lines indicated in the sketch, although they also in practice are active against every part of the plane.