Learning to fly in the U. S. Army; a manual of aviation practice

Front matter

EARNING TO Fly N THE U.S. Army

^UAL OF AyiATiOH PRACTICE

Cornell University Library TL 545.F18

Preface

This book is primarily intended for non-technical readers; and for the student aviators who, called from non-technical occupations, must cram themselves at short notice with the gist of airplane flying; and who must omit everything except the outstanding fundamentals.

The desired essentials are here given without sacrificing accuracy to brevity. It has been necessary to omit many technical details of interest to the aeronautical engineer, to whose needs other larger textbooks are adapted as a complete survey of technical aeronautics. In brief, the book presents the main principles of aviation, such as the aviator must know in order properly to understand his airplane, keep it trued up, and operate it in cross-country flights as well as at the flying field.

Out of the 2000 aeronautical books now in existence, a few are adapted to use as textbooks for the present need, but none give the particular and abridged information in tabloid form such as must be adopted for the best time economy of these students.

It is by the kind permission of Professor E. A. Holbrook, Mr. 0. S. Beyer, and Mr. C. M. Hebbert that chapters VI to XI have been included. Acknowledgment and full credit is due them as co-authors. Chap. VI, VII, and part of Chap. XI were prepared by Prof. E. A. Holbrook; Chap. IX, X, pages 152-157 of Chap. VIII, and pages 173-177 of Chap. XI were prepared by Mr. 0. S. Beyer; Chap. VIII, pages 133-152, was prepared by Mr. C. M. Hebbert.

vii viii PREFACE

In the Chapter on "History of Aviation"' only those experiments are treated which have a bearing on flight today; this chapter is to be used in conjunction with the Chapter on "Principles of Fhght," especially as regards controlling the airplane.

The question of Airplane Motors has not been touched, because to do it justice would unduly increase the size of this volume, and because adequate treatises on the subject are available.

Contents

Page

Preface vii

Chapter

Chapter I History Of Aviation

That part of the history of Aviation which has especial interest for aviators is of recent date, and extends back only two dozen years. Of course efforts have been made toward manflight ever since the early sixteenth century, when Leonardo da Vinci invented the parachute and became the first patron of aeronautics; between the time of this famous artist and the present many experimenters have given their attention to the problem, but previous to the last decade of the nineteenth century nothing practical was achieved. Then, with the perfection of the steam engine and the development of the gasoline engine, there came inducement to sound experimentation, bringing forth such wellknown figures as Maxim, Langley, Lillienthal and Chanute.

The work of each of these men is an interesting story by itself, especially that of Langley, who approached the matter from a strictly scientific viewpoint, established testing apparatus and built successful self-propelled steam models years before the Wright brothers reported their independent successes. He reproduced his models to full scale with every expectation of success, but failed, due to exhaustion of his capital.

Langley's Experiments in Aerial Navigation. — In all the history of aerial navigaition one of the most romantic stories is that describing the scientific researches begun in 1887 by Langley and culminating in 1896 in the first really successful case of mechanical flight using a prime mover; continuing up to 1903 when this first successful machine, a model of 12-ft. span, was reproduced to full scale and manned for its trial flight by a human pilot ; and ending with the destruction of this full-sized machine on launching, so that Langley missed the glory of being the actual discoverer of manflight only by a hair's breadth, dying shortly afterward of a broken heart, as is conceded by those who knew him. If this full-scale machine had performed as successfully in 1903 as it actually did after being rebuilt and partly remodelled' a decade later by the Curtiss company, Langley would have antedated the first successful flight made by the Wright brothers by a narrow margin of about 2 months.

Lillienthal (Germany, 1894). — But omitting details regarding the early experimenters we will consider only that part of the history of aviation most important to the prospective aviator. We will confine ourselves to the sequence of gliding and power experiments begun by Lillienthal, carried forward by Chanute and brought to completion by the Wrights.

Lilhenthal was the first man to accompUsh successful flights through the air by the use of artificial

(Courtesy Jas. Mcn/ts' " .ieronautical Annual.") Fig. 2. — Lillienthal's biplane glider in flight, 1894. Note. — (a) Arched wings; (6) fixed tail; (f) method of balancing by swinging legs wing surfaces. After many years of experiment and study of soaring birds he constructed rigid wings which he held to his shoulders and which, after he had gained considerable velocity by running forward downhill, would catch the air and lift his weight completely off the ground. The wings were arched, for he observed this was the case in all birds; fiat wings prove'd useless in flight, and suggested a reason for the failure of previous experi-

Fig. 3. — Chanute's biplane glider, 1896.

Note improvement in rigidity by bridge-type trussing.

menters. To these rigid wings Lillienthal fastened a rigid tail; the wings and the tail comprised his "glider." There were no control levers and the only way the operator could steer was to shift the balance by swinging his. legs one way or the other.

Lillienthal constructed an artificial hill for his gliding so that he could coast downward for some distance without striking the ground and he was able to accomplish many glides of a couple of hundred yards in length.

Chanute (Chicago, 1896).— Chanute's experiments in gliding were quite similar to Lillienthal's and were made on the sand dunes along Lake Michigan outside of Chicago. His apparatus was more strongly constructed, being of trussed biplane type, a construction suggested to him by his experience in bridge building, and one which persists today as the basis of strength in our present military biplanes.

The Wright Brothers, 1901.— Lillienthal was killed in a glide, having lost control of his apparatus while some distance above the ground. The Wright brothers read of his death and commenced thinking over the whole problem. Lillienthal's method of balancing his large apparatus by the mere effect of swinging his legs appeared to them as a very inadequate means of control. They came to the conclusion that the immediate problem in artificial flight was the problem of stability, which they felt should be solved by an entirely different means than that employed by Lillienthal and Chanute. The work already done had demonstrated without question that support in the air had been established; with the addition of controllability the Wrights looked forward to doing something worth while in the way of artificial flight.

To improve Lillienthal's method of shifting the weight, they conceived the idea of leaving the pilot in an immovable position in the glider, and instead of obliging him to shift his weight this way and that, they proposed to manipulate the sm-faces of the wings themselves by means of levers under the pilot's control, so that the same result of balancing could be obtained by quite a different and superior method.

They set out, therefore, deliberately to solve the whole question of airplane stability. There was the fore and aft or horizontal stability, for which LilUenthal had swung his legs forward and backward; there was in addition the sidewise or lateral stability for which Lillienthal had swung his legs to left and right. The fundamental requirements to be met were that during flight the gUder should be kept in its proper attitude without diving or rearing up, and without rolUng into an attitude where one wing tip was higher than the other, i.e., the machine was to be kept level in both directions.

Fore and Aft Control.^ — ^After some preliminary trials the Wrights found that the fore and aft balance could be controlled by an elevator or horizontal rudder, supported on outriggers on the front of the airplane, and operated by a lever. If the pilot found the glider pitching too much downward, and tending toward a dive, he would tilt the elevator upward by moving the lever, thus tm-ning the gUder back into its proper attitude. This elevator in modern machines is back of the airplane.

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II fi a s a better place for it than was chosen by the Wrights. It may be said that their chief reason for first putting it in front was that they could see it there and observe its effect. They soon realized that the rear location gave easier control, and they acted accordingly.

Lateral Control. — After satisfying themselves regarding fore and aft control, the Wrights took up lateral control. Their problem was to devise a means for keeping the span of the wings level so that when for any reason one wing tip should sink lower than the other, it could be at once raised back to its proper position. Lillienthal had tried to do this by swinging his legs toward the high side; the shifted weight restoring the position. The Wrights, to obviate this inadequate method, bethought themselves to restore equilibrium by means of the wind itself rather than by gravity. They observed an interesting maneuver employed by a pigeon which seemed to secure its lateral balance in exactly the way they wanted; this bird was seen to give its two wings each a different angle of attack, whereat one wing would lift more forcibly than the other, thereby rotating the bird bodily in any desired amount or direction about the line of flight as an axis. To copy this bird apparatus in a Wright glider, it was found sufficient to alter the angle of the wing tips only, leaving the chief part of the supporting siu"face in its original rigid position. In other words, the wing tips were to be warped; the one to present greater angle of attack, the other less angle, exactly as in the case of the pigeon. Suppose the airplane to develop a list to the left, the wing on that side sinking, the pilot was to increase the angle at the tip of this left-hand wing bymoving the warping lever, and at the same time decrease the angle of the right-hand wing by the same lever. He was to hold this position until the airplane was righted and brought back to level position.

This arrangement proved to have the effect anticipated and maintained stability easily on a glider much larger than Lillienthal ever managed with his leg-swinging method.

Directional Control. — We have now followed the development by the Wrights of airplane control as regards :

1. Fore and aft or "pitching" motion, accomplished by an elevator operated by lever.

2. Lateral or "rolling" motion accomplished by wing warping operated by a second lever.

These were the only controls used in the earliest gliders. It remains to consider the third element of control, viz:

3. The directional or "yawing" control, which is accomplished by an ordinary vertical rudder operated by a third lever.

The Wrights found the warping had all the effect anticipated but had also certain secondary and undesirable effects. Whenever they applied the warping lever to correct the rolling motion, the glider responded as far as rolling control was concerned, but at the same time would "yaw" or swerve out of its course to right or left. This was a serious complication. For, in the moment of swerving, the high wing which they desired to depress would advance faster than the low wing, and solely by its higher velocity tended to develop a greater Hft and thereby neutralize the beneficial effect of the warp. In many of their early glides, because of pronounced swerving, the warp effect was entirely counteracted and failed to bring the glider back to level; with the result that one wing tip would sink, at the same time swinging backward until the machine was brought to the ground. No amount of controlling could prevent this.

After much bewilderment on this point, the Wrights observed that whenever a wing tip was warped to a large angle its resistance became relatively greater and it slowed up while the opposite side went ahead. They at once hit upon the idea of a rudder, previously considered unnecessary, which they believed could be turned in each case of yawing just enough to create a new and apposing yawing force of equal magnitude.

They therefore attached a rudder at the rear, connecting its tiller ropes to lever No. 2, and giving this lever a compound motion so that one hand could operate either warp or rudder control independently (or simultaneously in proper proportion to eliminate the yawing tendency above mentioned). This combination is the basis of the Wright patents and is essential in airplanes of today.

Great success now ensued in their gliding experiments; the machine was always in perfect control; c juld be manipulated in any desired manner; tiu-ned to right or left, or brought down to earth with safety.

Thus were the three elements of control applied by the Wrights to their glider and the problem apparent in Lillienthal's death was solved. The next step was to install a power plant able to maintain forward speed without resorting to coasting downhill by gravity; and therefore capable of producing a horizontal flight.

In developing a power flyer aside from the question of control the proper design was arrived at as follows :

Efficiency of Wings. — The Wrights knew from Langley and Chanute that flat wings were inefficient and useless, and curved wings essential; they did not know whether the amount of curvature mattered much. To find this out by trials in gliding would be slow and expensive. They adopted a better way — the wind-tunnel method, wherein smallscale models were tested and coinpared for efficiency in a blast of air. They made their wind tunnel 16 in. in diameter and created a powerful air blast through it by means of an engine-driven fan. Small models of wings were placed in the center of this confined air blast, mounted on a balance arm which projected into the tunnel from the outside. The air forces and efficiency of the models were thus measured. A large variety of shapes were tested and one was selected as best of all from the standpoint of curvatxire and rounded wing tips. This shape was adopted in their flyer, and though on a much larger scale fulfilled the predictions made for i1^^ efficiency in the indoor wind-tunnel experiments.

The Wright glider was, of course, a biplane model. They tested a small 6-in. model biplane and found that the two wings together were less efficient than either wing by itself. However, other considerations, such as rigidity of trussing, decided them to adopt the biplane rather than a monoplane arrangement.

Low Resistance to Forward Motion. — The Wrights used their wind tunnel also in choosing for the struts of their airplane a shape which would present least head resistance to forward motion. They found that a square strut had a resistance which could be decreased by changing the shape to resemble a fish. The resistance of the pilot himself was decreased by making him lie prone, face downward on the bottom wing.

Propeller Efficiency. — Although little data on the subject of propeller efficiency was available to the Wrights, they were able to arrive at a very creditable design wherein two propellers were used, driven from a single motor, and rotating one each side of the pilot. The mechanical difficulties which have since embarrassed the use of two propellers were less with the Wrights because they were dealing with smaller horsepowers than are in use today; they therefore were able to realize a very high propeller efficiency.

Motor. — When the Wrights were ready to apply a motor to their glider, they found it impossible to secure one Ught enough, and had to set about building one themselves. They adopted a foiu-- cylinder type, water-cooled, and their aim was to save weight and complication wherever possible. Their first motor gave about 12 hp., which was raised to a higher and higher figure by subsequent improvements until it reached 20 hp. In itsearliest stages it was able to give sufficient power for short horizontal flights.

Means of Starting and Landing. — One reason the Wrights could use such low horsepower was that they employed auxiliary starting apparatus to get up original speed. They knew that less horsepower was necessary to fly an airplane after it was once in the air than was necessary to get it into the air at the start, and they therefore rigged up a catapult which projected their airplane forward on a rolling carriage with great force at the start, so that all the motor had to do was to maintain the flight in air. The Wright airplane had at first no landing wheels, and was provided only with light skids on which it could make a decent landing. Present-day airplanes, of course, have wheels on which to roll both at starting and at landing and their motors are powerful enough to eliminate the necessity for a starting catapult.

Bleriot's Contribution to Aviation. — Bleriot experimented a great many years before he attained success and did so years after the Wrights had

(.Courtesy American Technical Society and Scientific American SuppUment.) Fig. 5.— Details of Bleriot XI monoplane.

successfully flown. But when he did obtain success, his great ingenuity produced featiu-es of design which were a decided step forward. He added a body to the airplane and produced a machine which instead of being a pair of wings with various appendages, was a body to which wings were attached, giving a more shipshape and convenient arrangement. The motor, instead of being located beside the pilot as in the Wright machine, was put in the very front of the body ahead of the pilot where It was not likely to fall on him in case of a smash. This location of the motor entailed the use of a single propeller at the front, a "tractor" screw as it was called, less efficient than the double propeller of the Wrights, but better from the standpoint of mechanical convenience. The body of a Bleriot, which was quite similar to the body of any bird in' its general arrangement, projected to the rear in a tapering form and carried at the rear a rudder and elevator. The motor, pilot and tanks were thus enclosed within the body and away from the wind. Bleriot's contributions were then, better location of the motor, adaptation of the body or "fuselage," ehmination of the front elevator and substitution of the rear elevator.

Nieuport and Fokker's Contribution to Aviation. — A further advance on Bleriot's design was made by Nieuport and later by Fokker. The former utilized the fuselage principle of Bleriot and enclosed the whole framework, front and back, to give a stream-line form, and eren went so far as to make wind-tunnel experiments from which he was able to choose a very efficient.fuselage shape as well as wing and strut efficiency.

(From HaywarcTs "Practical Aeronautics.") Fig. 6. — Nieuport monoplane.

Representing an advance in speed, due to covered streamline body.

Chapter II Types Of Military Airplanes And Their

USES

Modem Airplanes Combining Best Features of Previous Experiments. — The modern airplane, of which the Curtiss training machine used at the U. S. Aviation Schools is typical, is a combination of the best features referred to above. It is of the biplane type for, as shown by Chanute, rigid trussing is thus possible, an advantage sufficient to offset the slight loss of efficiency which exists in the biplane. The landing gear consists of two wheels provided with shock absorbers; the body is of the general stream-line type, enclosed from front to' back, containing comfortable seats for the passengers and enclosing the motor and tanks away from the wind. The motor is at the front where, in an accident, it will not be on top of the pilot. The warping effect is obtained by hinging flaps at the wing tips, the same effect being obtained while at the same time leaving the whole wing structure rigid and strong rather than flexible and weak, as was the case in the early warping type of machines.

Military Airplanes of Today. — In the modern airplane, therefore, we see that matters of efficiency, to which the Wrights gave great attention, have been sacrificed in favor of convenience, particularly in favor of power and speed. This is the effect of military demands for airplanes where power, speed, and abiUty to climb fast are vital requirements. To escape from or to destroy an enemy, high speed and ability to cUmb fast are, of coiirse, prerequisites. Moreover, from the standpoint of safety in maneuvering it is desirable to have a reserve of power and speed. Therefore, the design of miUtary machines has tended in a given direction up to the present.

New considerations have arisen on this account, such as for instance the question of landing. Fast machines in general make high-speed landings, and are for that reason dangerous. The original Wright machines were built to land at such a slow speed that ordinary skids were sufficient to take the shocks. Nowadays the high-powered airplane is likely to come to grief in landing more than at any other time. The question of stability in flight has of recent years been treated mathematically and experimentally, using of course the fundamental system of "three axes control" first applied by the Wrights. It has been found that by properly proportioning the tail surfaces and properly arranging the wings and center of gravity, any desired degree of stabiUty may be obtained, such that a machine may be made almost self-flying or, if preferred, may be made very sensitive.

All of the above features of design have had consideration in the latest types of military airplanes. Observe the high speed of the latest speed scouts, where power is concentrated exclusively on speed and climbing ability and landing speed is dangerously high. We see the advent of the triplane scout, which is an attempt to secure slow landing speed combined with high flying speed. We see machines with the motor and propeller in the rear, or with two motors, one to each side of the body out in the wings, the object being to avoid interference of the propeller with the range of gun fire. In short, we see the effect of many miUtary considerations on the design of the airplane. It will be interesting at this point to survey what are these military uses of the airplane.

Aerial Fighting. — Fighting in the air is the most spectacular use to which miUtary airplanes have been put. The first requirements in a fighting airplane are speed and climbing ability and these must be obtained at all costs, because speed and climb are weapons of defense and offense second only in value to the gun itself. The concentration of motive power for speed and climb requires that as little weight as possible be used; and therefore the fastest fighters are designed to carry only one person and are very light and of course very small. It is usual to have one gun fixed to the body and firing through the propeller in the case of a tractor, and a second adjustable aim gun pointing upwards over the top wing. This gives the pilot a chance to fire a round at the enemy while "sitting on his tail" or following from behind; and then when diving below the enemy the second gun is available for shooting overhead. These very high-speed fighters are difficult to land, due to their speed, and are suitable only for the highest-trained pilots.

Directing Artillery Fire. — The friendly airplane is sent out over the enemy's positions, soars above the target, sends back signals by wireless to the friendly battery regarding the effect of fire; practically dictating the success of artillery operations.

Reconnaissance. — The friendly airplanes go out, usually in squads for the sake of protection, and observe by means of photographs or vision size of enemy troops, batteries, trenches, lines of communication, etc.; report the situation to headquarters as a source of daily photographic record of the operations of the enemy, to such an extent that any change of the enemy's position can be analyzed. Of course the value of reconnaissance is lessened when the enemy disguises his gun emplacements, etc. In reconnaissance machines it is important to have two persons, one to steer and the other to scan the countryside. The reconnaissance machine is therefore a two-place type which may or may not have armament. It need not be so fast, especially when convoyed by fighting speed scouts. The two-place machines are frequently used for fighting, in which case the pilot will have a gun fixed to the body and shooting through the propeller, and the passenger, especially in German machines, will also have a gun mounted in the turret so that it may be shot in a variety of directions by the passenger.

Bomb Dropping. — This maneuver requires squad flights to be of great value. The fundamental characteristic of a bombing airplane is its ability to carry great weight. Such machines are of comparatively large size and not particularly fast. Weight carrying is of course incompatible with speed and climbing ability and therefore the bombing machine must be a compromise if it is to have any reasonable speed. It may be said that airplanes compare very unfavorably with dirigible balloons for bomb raids because the latter are able to carry several tons of bombs as against the airplane's quarter of a ton.

Locating Submarines. — For coast patrol or submarine spotting, the airplane is an important factor, for from an airplane it is possible to see for a considerable depth into the water, and to locate hostile submarines.

Training Student Aviators. — The training machine on which prospective aviators secure their flying instruction may be considered as a type in which great speed and power is not essential, but in which reUability and ease of control is desirable. The typical military training airplane in this country is a single-motor tractor of moderate horsepower (about 100) having of course the seats in tandem and funished with dual control so that operation may be from either pilot's or passenger's seat. The dual-control system of training which prevails in this country differs from the French method of starting the pupil out alone to try his wings; it enables the pilot to keep a constant eye upon the pupil's control manipulations and to correct them instantly whenever they are in error before any damage is done. A possible improvement in the dual-control training machine will be the substitution of side by side seats for tandem seats. At present, communication is difficult due to the great noise of the motor;

Fig. 7. — U. S. training airplane, dual control (Curtiss JN4).

Speed 43 to 72 mi. per hr. ; climbing ability 300 ft. per min. ; 90 h.p. ; weight

fully loaded 1,890 lbs.

but with the adoption of side by side seats such as is used in naval training schools, the pilot and pupil will be able to communicate to better advantage.

Types of Airplanes. — To suit the foregoing purposes flying machines exist in seven distinct different shapes at the present time, namely: monoplanes, biplanes, triplanes, single-motor tractors, single-motor pushers, double-motor machines and marine airplanes. The last four types may be( either monoplanes, biplanes or triplanes. In order to understand the adoption of one or the other type for military use, it is well to run over the characteristics of the seven types mentioned.

Monoplanes. — The simplest form of airplane is the monoplane which is fashioned after the manner of a bird (see Fig. 34). There are two things to say in favor of the monoplane: first, that the passengers have an unobstructed view forward and range of gun fire upward because there is no wing above them; second, the aerodynamic efficiency of the monoplane is superior to any other type. But when the bird design is applied to a man-carrying apparatus, it becomes impracticable to construct spars to take the place of the bird's wing bones; and therefore to give the wings proper strength it becomes necessary to truss them with numerous tension wires stretching from the running gear out to various portions of the wings. There are also wires running from a vertical mast above the body to a point on the top part of the wing; these wires, while they give the wing no added strength during a flight, are necessary in order that the shock of landing shall not break the wings off sharp at the shoulder. It is characteristic of monoplane construction that from a point below the body and also from a point above the body a number of heavy wires run outward to various points on the wings; and it may be said that the strength to be secured from this construction is not all that could be desired.

Biplanes. — The biplane is an improvement over the monoplane from the latter standpoint; in the biplane there are two parallel surfaces separated by vertical sticks or struts, thus forming parallelograms which are susceptible of being trussed by means of tension- wire diagonals in a manner familiar and well understood in case of bridges. It is possible' to build up biplane wings of great rigidity and strength by this system, much more easily than in case of monoplanes. However, the biplane type is from the standpoint of efficiency inferior to the monoplane. This is due to the fact that the vacuum above the bottom wing which is so necessary for high duty is somewhat interfered with by the upper wing; thus while in a biplane the upper wing operates about as efficiently as it would operate in a monoplane, yet the lower wing has its efficiency materially reduced and the resulting overall efficiency of a biplane compared area for area with the monoplane is about 85 per cent, as great. However, recent developments of the airplane have more or less put efficiency in the background and as a result today the biplane is more popular than the monoplane. In addition to the greater strength of biplane wings theiir span may be less than the monoplane for the same supporting area. This makes them less unwieldy. Moreover, for certain reasons a biplane machine of high speed may be landed at a lower speed than equivalent monoplanes.