Transcriber's Note

This book was transcribed from a scan of the original found at Google Books. I have rotated some images and have corrected the misspelling of "aileron" that appeared many times in the original text.

A model aeroplane rising from the water.

THE SECOND BOYS' BOOK

OF MODEL AEROPLANES

BY

FRANCIS A. COLLINS

AUTHOR OF "THE BOYS' BOOK OF MODEL AEROPLANES"

ILLUSTRATED WITH MANY

PHOTOGRAPHS AND DIAGRAMS

BY THE AUTHOR

NEW YORK

THE CENTURY CO.

1911

Copyright 1911, by

The Century Co.

Published October, 1911

THE SECOND BOYS' BOOK OF MODEL AEROPLANES

FOREWORD

It is assumed that the reader is familiar with "The Boys' Book of Model Aeroplanes." Some knowledge of the history of aviation and the early models, big and little, will be found helpful, but not essential, as a preparation for the present volume.

Within the year so much has been learned of the science of model aeroplane construction that an entirely new and more detailed treatment of the subject seems to be justified. Since the length of model aeroplane flights has been increased ten times, their improvement is comparable to that of the large man-carrying machines. The science has become more exact, and the chance of failure reduced, until to-day successful flights are within the reach of all.

In the preparation of this volume thanks are due to the New York Model Aero Club, to Mr. Edward Durrant, Percy Pierce, Cecil Peoli, W. S. Howells, Jr., and to the young gentlemen whose models are illustrated herewith, who, singly and collectively, are doing much for the development of the science in America.

LIST OF ILLUSTRATIONS

CHAPTER I MODEL AEROPLANES OF 1911

For the average boy there is no more stirring music than the brisk, whirring note of his model aeroplane. Let the propellers spin steadily for ten glorious seconds, and the journey spans a couple hundred feet or more. Double the time and the flight becomes a triumph. Out of the ingenuity of thousands of boy aviators, the world over, has come a surprising development of the model aeroplane. The experimental stage is passed. Any bright boy may now build a model aeroplane which is certain to give results. The distance qualities of your model may even rival your endurance as a runner in keeping pace with it.

Working along different lines, the builders of model aeroplanes, widely scattered, seem to be gradually developing much the same type of air craft. The tendency is toward the construction of much lighter and more logical models than last year. In place of the complicated models supported by several broad planes, we find the most successful amateur aviators expending all their ingenuity upon simple monoplane forms. The biplane forms are being abandoned by model builders, as well as the biplane form of elevating planes. In place of the models made from fifty or more members, we now find excellent models, capable of much longer flights, formed of but a dozen pieces. The builders of model aeroplanes are keeping pace with the development of the man-carrying machines, if they are not passing them, in developing the flying machine of the future.

Improvement in the distance qualities of the model aeroplanes, in the past few months, has been remarkable. At one of the first model aeroplane tournaments, held in New York, less than two years since, the longest flight was under sixty feet. In less than one year, flights of more than 200 feet had become common. To-day the improved racing model aeroplanes have flown more than 2,500 feet. As a result of the labors of the boy aviators, it is much easier to build a successful model flying machine to-day than it was a year ago.

What may be called the 1911 type of model aeroplane looks every inch a racer. Every unnecessary stick and string has been cut away. When skids are used they are of the lightest possible material and the simplest construction. The miniature rubber-tired wheels, with ball bearings, which made many of last year's models so attractive, are rarely used. The plane surface has been reduced fully one half. One great secret of success is in the cutting down of weight. When your propeller has but half the work to do, the length of the flight is, of course, greatly increased.

Our amateur aviators are attacking one great problem of aviation which the pilots of man-carrying crafts are perhaps neglecting. Model aeroplanes are built to maintain their equilibrium in the air automatically. They must not only rise from the ground, prepared for a long flight, but must be contrived to resist all manner of baffling air currents aloft. Watch the successful model as it gains its altitude, settles down to a horizontal flight, is perhaps knocked off its course by a cross current, and steadies itself with a graceful curve and proceeds on its way.

All these problems must be anticipated. The young aviator must ingeniously arrange his planes and ballast in advance. The regular sky pilot, on the other hand, meets the problems of the air as he encounters them, by flexing his wings against disturbing currents or by banking to maintain an even keel at a turn. If the man-carrying airship had to be prepared to meet all these problems before it left the ground, the problem would be, of course, much more complicated.

In other words, if the motor of a large machine were started and the aeroplane launched without a pilot, would its chances of flight be as good, in proportion to its size, as those of our best model aeroplanes? A model aeroplane which flies 300 feet performs as remarkable a feat as would a large machine flying, unguided, a mile or more. The progress in the construction of model aeroplanes, in brief, already deserves serious scientific consideration.

The last twelve months have brought out a surprising number of new aeroplanes, while notable progress has been made in the standard types. To realize the immense strides or flights forward in the construction of heavier-than-air machines, one need only set the 1911 models beside the aeroplanes of a year or two years since. Even to the eye of the layman in such matters, the older machines are beginning to appear obsolete. In a previous volume, it was suggested that within a few years the aeroplane of to-day would appear like cumbersome stage coaches to one familiar with racing automobiles, and certainly the prophecy is being quickly realized.

The general tendency is in the direction of greater simplicity in design in passenger-carrying craft, as in model aeroplanes. Both the monoplane and biplane types are being developed side by side, and each continues to have its enthusiastic advocates. The increase in the passenger-carrying qualities is realizing the most sanguine hopes. Aeroplanes have carried fifteen passengers for several miles. The speed qualities of machines have developed correspondingly.

If the development of model aeroplanes leads the way in perfecting heavier-than-air machines, as many believe, the monoplane form seems destined to replace all multiplane types. During the past year practically all of the biplane forms have been abandoned by model builders. As a result of wide experiments, it has been found that the monoplane exerts more sustension per unit of surface than any two or three-plane machines. In theory, it is, of course, possible to increase the sustained force by setting one plane above another, but in practice it has been found that the planes must be set so far apart that the arrangement is impracticable. When planes are separated, they must, of course, be stayed and trussed to keep them rigid, and all this adds to the weight and complexity of the machine.

A good specimen of plane building.

"Finish one end of the blade before cutting away the opposite end."

Model aeroplane. Designed by Cecil Peoli.

The builder of model aeroplanes has a great advantage over the designer of man-carrying crafts. The spread of the wings of his model is comparatively small, and the problem of staying and trussing is greatly simplified. The monoplane, especially in a model, requires practically no staying at all. Then again the skin friction is greatly reduced in the monoplane form. Simple as it is, there are great possibilities in the arrangement of these surfaces. The effect of outline upon resistance again may be more closely observed in the monoplane than in the multiplane forms. In other words, if your model goes wrong, it is far easier to locate the fault and rectify it than in the more complicated arrangement of planes.

The flights of the English models this year are longer than those made in America, but, on the other hand, we are solving many practical problems of aviation, in our model building, which the English have not attempted. Even in the case of our single-stick frames built in America, the tendency is toward more stable construction than abroad. The best English models would not qualify for an American model tournament, since they could not rise from the ground.

The best American models, on the other hand, would be outdistanced in an English meet, but their flights would show them to have far greater automatic stability than their English rivals. It is extremely interesting to speculate whether the American or English types of model aeroplanes will survive, and which is contributing more to the solving of the great mysteries of aviation, but, after all, it is a question which only time can answer.

Compare typical flights of the American and English models, and the contrast becomes obvious. The English model is usually held and thrown forward. The starter thus gives it its altitude and direction. Being extremely light, they gain a great deal from the wind. Their flights are usually in straight lines, or in slightly undulating curves. Under favorable conditions, their distance qualities are remarkable. Flights of six or eight hundred feet are common, while the present record is over 2,500 feet or nearly half a mile.

In an American model tournament, the models are set upon the ground and left to themselves. As a rule, it is not even permitted to give them a slight push. The motor must be powerful enough to carry them onward and upward unassisted. In many cases they must be clear of the ground within twenty feet or the flight is disqualified. It is, of course, obvious that the motors must be far stronger than in the case of the English models, and that their frames must therefore be correspondingly heavier to support the weight. The plane surface, in turn, must be increased to support this weight. The average English models, even with American skids, would not leave the ground at all.

Once in the air, the behavior of the American model, again, is entirely different from its English rival. Our aeroplanes are off with a rush. The first part of the flight is at a more or less sharply drawn angle of elevation. It usually rises to an altitude of from ten to twenty feet in a straight line. To secure a good rise requires a much more scientific adjustment of the planes and weighting than in the case of the English models. As it reaches its altitude, it adjusts itself, and here the problem of stability comes in. The marvelous little craft balances itself with the least possible loss of time and power, comes to a horizontal position, and is off on its flight. If its adjustment is not all it should be, it will, of course, fail to right itself and fall backward, or, as the phrase goes "sit on its tail." It is estimated that one-third of the power of the motors is used up in leaving the ground and rising to its maximum altitude.

Our American model builders believe that their flights are far more scientific than in the case of a hand-launched model, and that they are doing more for the actual development of the art of aviation than their English cousins. Whether one prefers to watch an American or English tournament is, of course, largely a matter of taste; certainly both are fascinating.

Much has been learned about motors. It has been found that the rubber motor is capable of great development. Since a flight of one-half a mile may be made by twisted bands of rubber, the average model builder may be content to let clock work and miniature gasoline engines take care of themselves. By building and flying thousands of models, we have found what form of rubber strand is best, just how heavy the strands should be, and the most efficient point of winding. Instead of short heavy bands, we now use much longer and more slender motors. The efficiency of rubber motors has been greatly increased by arranging them in series and connecting them up by gear wheels. It is even possible to buy miniature gasoline motors suitable for model aeroplanes. Flights of more than one mile have been made in this way.

All the best models this year are equipped with twin propellers. It is very little more trouble to build two motors than one, and the model thus equipped will not only travel much further, but will insure much more stable flights. A common trouble in model building has been the lack of stability. Your model has been likely to capsize, even under favorable conditions, spoiling the flight, while a chance gust of wind would knock it out of its course in spite of everything you could do. To overcome this tendency, the surface of the planes might be increased, but this added to the weight of the model, thus cutting down the length of the flights. The twin propellers cut at the root of the problem. They balance the thrust, thus making the flight even and stable. The planes may also be made much smaller with a gain in weight which, in turn, lengthens the flight.

A variation on a familiar form.

An excellent model designed and built by H. Wakkins.

CHAPTER II MODEL AEROPLANE TOURNAMENTS

Within the year, exhibitions and contests of model aeroplane flights have become an established form of entertainment. The attractions of the flights of man-carrying machines are borrowed in a large measure by the model aeroplanes. The building of models has progressed so rapidly, bringing the little air-craft under such control, that a definite program of flights may now be carried out. The programs may be considerably varied to include distance flights, weight-lifting contests, and spectacular flights in which the models loop the loop and perform other amazing feats.

The first formal exhibition or professional appearance of the model aeroplane in public as an entertainment was made in connection with the first aviation meet held at Asbury Park, New Jersey. Two of the most successful model builders, Percy Pierce and Frank Schoeber, of the New York Model Aero Club, were engaged to give exhibition flights for one hour a day in the intervals between the flights of Arch Hoxey, Johnston and other aviators of the Wright Brothers staff.

An original design by Harry McAllister

An interesting experiment in stability

An early model built by E.G. Halpine

The models were flown for more than 200 feet and were enthusiastically applauded. The aeroplanes in miniature imitated the flights of the man-carrying craft with wonderful fidelity, rising from the ground and soaring aloft in long, graceful curves. They came as a very welcome variety, and could be watched without breaking one's neck gazing aloft, or the unpleasant possibility of a serious accident. The applause of the thousands gathered for the meet may be said to have definitely established the model aeroplane as a feature of these tournaments.

The model aeroplane has one great advantage over the man-carrying machines. It makes possible indoor aviation, and may be enjoyed the year round, and is especially effective for evening entertainment. The fortnightly meets in one of the great New York armories, some time since, attracted the attention of the officers, and the boys were invited to give exhibition flights in connection with athletic games. The first of these meets was held under the auspices of the New York Model Aero Club, in connection with the Greek athletic games, in the interval between the games and the ball which followed.

An audience of fully 3,000 people, crowding the armory, witnessed the flights. Some twenty members of the club entered the contest. In a public contest of this kind, much depends upon the system of flying. The floor must be kept clear and the flights follow one another so quickly that the interest will not lag for a moment, and the audience have no opportunity to tire. The flights on this occasion went with a rush and proved in every way so successful that the rules which made this program are given in full on another page.

Few in the audience had ever seen a model flight, and the contest held the great crowd's attention more closely than had any of the evening's athletic events, which had come before. There was a breathless moment of suspense when the whistle had sounded for the first flight. A beautiful white monoplane led off, but in the excitement of the moment, it had not been properly adjusted, and failing to get its altitude, spun daintily across the floor. The second model yawed sharply and flew into the crowd at the side.

The third model found itself, however, rose perhaps twenty feet and, settling down to a steady horizontal, darted across the arena. Every eye followed it. A burst of handclapping greeted its graceful rise, which increased in volume, and as it reached the farthest corner of the great armory, more than 200 feet distant, there was a perfectly spontaneous cheer.

The program was so well organized and carried on that the flights followed rapidly without a break. There was scarcely a moment when an aeroplane was not aloft, and the interest never faltered. There were Scores of excellent straight-away flights of 200 feet or more, at various altitudes. Occasionally a model would fly wild, even refuse to rise, but the flights followed one another so continuously that a failure was quickly forgotten in the delight of watching the next flight.

The rapid development of the model aeroplane was shown particularly in the spectacular flights. The thrilling volplanes and daring aerial feats of the famous air pilots were imitated by the model aeroplanes. The models were made to dart about at unexpected angles, and, while keeping clear of the ground, perform many astonishing feats. The prize for these spectacular flights was won by Henry Ragot whose aeroplane actually looped the loop repeatedly, in obedience to skilful adjustment of the planes and weights.

In launching the model for this flight, the model was held well above the ground and launched at a sharp upward angle. It rose with astonishing speed, in a vertical line, fully twenty feet, when it turned and descended with accelerated speed. The crowd naturally expected a bad smash, but with a good clearance of the ground the model suddenly swept around in a narrow semicircle, rose and repeated the performance. It seemed to many spectators that the model was enjoying a miracle of good luck, but they were mistaken. The flight was repeated several times. Indoor aviation was an instantaneous success.

Unless well-thought-out rules are carefully observed, a public exhibition may fall into confusion, and be seriously marred. A large audience grows quickly impatient of delays between flights. There is, of course, the danger that the models will follow each other too quickly, perhaps collide in the air. The distance and spectacular flights again must be kept separate.

The rules followed by the New York Model Aero Club in these exhibitions worked well in practice. First of all, the floor was kept absolutely clear except for the director of the flights, who took up a position at the center. The distance flights started from one corner only, and the spectacular nights from the center of one side, the weight-lifting contest from another corner.

An official starter, a measurer, and an entry clerk are stationed at each point from which the flights are started. When a model was wound up ready for a flight, a starter waved a small flag to attract the attention of the director out on the floor. From his vantage point, the director could see if the floor was clear and signaled to the starter to go ahead. He blew a whistle by way of signal, one blast for the start of a weight-lifting contest, two for a distance flight, and three for a spectacular flight.

Instantly the whistle sounded, the model signaled was released without a moment's delay. In this way no two models were ever started at the same time, and all confusion was avoided. The whistle was clearly heard in all parts of the hall, and the audience quickly learned to recognize the signals and look to the point from which the start took place. In the distance flights the one flying the model and the measurer alone were allowed to go after the machine. This was done on the run. It is important that any delay be avoided in measuring, since this does not interest the public in the least, and may make the exhibition drag.

An interesting experiment in stability

An early model built by Monroe Jacobs. Note the Ailerons.

The only other person allowed on the floor while the flights were in progress was the owner of the model, who must follow it and bring it back. He was allowed to cross the floor, but once he had secured his model, he must carry it quickly to the nearest point at the side, and find his way back to the starting point along the outer lines. It is confusing both to the flyer and the spectators to have a single unnecessary figure on the floor during the flights. The crowd is kept back by members of the club, wearing the club colors.

The regular fortnightly model aeroplane meets held in New York are doubtless the most largely-attended and best-organized meets of the kind in the world. The 22nd Regiment armory, a spacious structure admirably suited for indoor aviation, has very courteously been thrown open for the purpose on every other Saturday afternoon.

Throughout the season, each of these meets brings together several hundred boys and spectators, and on the average about 100 model aeroplanes. The meet is conducted with intelligence and sympathy by the Y. M. C. A., and is open to all. Of late these exhibitions have become so popular that the crowds actually threaten the convenience of the flyers, and the boys have been required to present credentials on entering, consisting simply of a model aeroplane.

There are few more animated spectacles than the model aeroplane tournament. There is a great sunlit floor, measuring 250 by 150 feet, roofed with glass. The aviation fields are reproduced here in miniature, without loss of animation. Along the sides are continuous lines of "camps," corresponding to the hangars where scores of boys are busy tuning up their machines. They have brought tools and a variety of extra materials, planes, propellers, motors, and strips, which are spread about them.

In each camp the machines,—and there are no two alike,—are being assembled or repaired. Groups of the boys' friends and admirers are gathered about each camp, earnestly discussing the merits of a particular model and its chances in the approaching contest. To stroll down the line of camps is in itself a liberal education in aeronautics.

The records of all flights are carefully preserved, to be counted against the several important trophies which will be awarded at the end of the season. Any one of the scores of contestants can tell you at any moment how the score stands. During this tuning up process, the galleries have filled and an enthusiastic audience is assured.

One of the great beauties of indoor aviation is that it is entirely independent of the weather. The air of the great armory is practically at rest, and the aeroplanes escape the baffling side currents and air gusts. In England, for instance, indoor aviation is practically unknown.

A whistle sounds above the hum of many voices, and at the signal everyone scurries to the sides, leaving the broad floor clear. The judge, starter, and measurer take their positions, and the aviators, with their models tuned up to concert pitch, stand ready at the starting line. The starter announces whether the flight is "official" and if it is to be counted in the competition for the trophies, or is merely a practice or exhibition flight.

The start is made from the extreme corner diagonally across the armory. Only last year the start was made from a point well out in the middle of the floor, but that was when the flights were much shorter. To-day the boys have actually outgrown the armory, and even by flying from corner to corner there is not enough room. The aeroplanes are no longer launched from the hand or even pushed along the ground. They are required to start without assistance and rise in the air without being touched.

"Official flight."

Everyone's attention is attracted by the announcement. Hundreds of boys crowd to the lines. The starter is doubtless known to all, as well as his record and standing in the various competitions. Hundreds of critical eyes are upon the model. It is a thrilling moment. The propellers are released, and the aeroplane starts forward under its own power.

Some leap into the air, others take the full twenty feet permitted them in getting off the ground. There are surprisingly few failures. The length of the take-off, the angle at which it rises, the altitude in the first rise, are critically observed by the young experts.

To the whir of the propellers, which form two blurred circles in the air, the model quickly climbs upward, rights itself and speeds away on its long flight. The young aviator's skill is revealed to every eye by the angle of the ascent, the altitude and the ability to gain equilibrium aloft. The more you know about aviation, the more absorbing is your interest in a flight.

A good rise is usually observed in silence. By the time the model has reached the middle of the armory, more than one hundred feet from the starting line, enthusiasm is aroused. When two-thirds the distance has been covered, the applause begins. Let the model continue without swerving to the farthest corner, and a perfectly spontaneous cheer sweeps the crowd. It is a well-deserved reward of hours of patient effort.

The official measurers take the floor on the run, dragging their tape after them. The crowd overruns the floor to gain a closer view of the model, and the young aviator receives congratulations. The distance is announced at once, and there are more cheers. There is never a dull moment at the meets. One or more machines are almost always aloft. It is as thrilling as a three-ringed circus.

CHAPTER III PARLOR AVIATION

A model glider, or aeroplane without a motor, will be found perhaps as entertaining a toy as the power-driven machine. It is much simpler, of course, to build and adjust a successful glider even than the most elementary model aeroplane. With the problem of the motor and propeller removed, the cost of construction besides is reduced to practically nothing. Here is excellent entertainment for those who have not the time or patience for model building. A graceful glide of successive waving lines makes a beautiful spectacle. Incidentally it is a good plan to work out the designs of large models in this way.

Fascinating little paper models, reproducing the famous man-carrying machines, the Wright, Bleriot, and others, may be put together in a few minutes. With a little adjustment, they may be made to fly from fifty to one hundred times their length. A paper Bleriot biplane six inches in length, for instance, may be made to sail for from twenty-five to fifty feet, and so on. This will be the actual horizontal distance traversed; the actual distance measured in long, undulating curves may be considerably more. Such flights do not consist merely of a long diagonal to earth, but of several surprising upward sweeps, well worth the trouble of construction. It is interesting to note the remarkable stability of their gliders.

A Simple Model Glider

An hour's entertainment, no less interesting than instructive, may be enjoyed with a series of these paper gliders. A different model might be prepared for each guest, and a prize or favor offered for the longest or best spectacular flight. The little gliders will cross a large room before coming down. The various aeroplanes nowadays are so familiar that in any gathering will be found several who favor, for instance, a Wright over a Curtiss take a lively interest in the rivalry of the various models.

An effective glider built by R.S. Barnaby

An efficient sling-shot glider built by John Roche

Begin with a very simple model. You will soon learn the trick of judging the size of the supporting surfaces and the spacing. The Antoinette aeroplane is probably the easiest one to imitate. From a sheet of ordinary writing paper, cardboard or fine wood, cut the form indicated. If the paper be rather heavy, it may be made six inches in length. By folding the paper and making one cutting, it will be found much easier to make the wings even and symmetrical.

The two sides should be fixed at a broad dihedral angle. To keep the little glider on an even keel you will need to add a weight to the front. A large pin or paper clip will answer. Launch the glider by holding it horizontally and throwing slightly forward. If it darts downward, lighten the ballast. If it falls backward, "sitting on its tail," add more weight at the front or bend the tail up.

Your glider will, of course, travel to the ground along the line of least resistance, and the trick is to adjust the center of gravity and center of pressure that this descent may be as gradual as possible. The center of gravity should come a little in front of the center of pressure. The gliding angle, as it is called, or the angle between the course of the model in flight with the ground should be about one in twelve. In other words, the glider descends one foot for every twelve feet it travels forward. Practically all the famous monoplanes may be reproduced in this way.

A variety of gliders may be made in a general arrow form. These arrows, or darts, as they are called, may be made about a foot in length and three or four inches in width. The horizontal surface, it should be borne in mind, is the supporting surface, while the vertical surface gives the flight direction. These gliders will also require weighting at the forward end. They should be thrown forward with rather more force than in the case of the Antoinette.

The biplanes such as the Wright and Curtiss aeroplanes may be reproduced very easily in paper. They fly best when made about six inches in length. Cut the two sheets of paper for the main planes one inch by six inches and round off the corners on one side. Two similar sheets, one by three inches, will be required for the smaller plane in the rear.

The planes are held in position by a series of paper struts, or toothpicks, and should be separated by a distance equal to their width, in this case one inch. Cut the slips of paper to form the struts one and one-half inches in length and bend over the corners at right angles, one-quarter of an inch from either end. These should be pasted in position, always keeping the edge of the struts lengthwise so that they will offer the least resistance in flight.

Connect the two biplanes by strips of paper six inches in length pasted on the lower planes or main deck of the little aeroplane. The forward planes should be fixed at a slightly elevated angle by running struts from the connecting strips to the upper plane. The accompanying picture will show how simple this all is.

The biplanes as a rule require no weighing. To launch them, hold them high in the air and merely let go. They fly best with their smaller planes forward. By varying the angle of the front plane, you can soon bring it to an even keel. A vertical rudder placed three inches behind the main plane will increase the model's directional stability.

An amazingly clever little glider may be made of a piece of reed or cane, say five inches in length, and a sheet of writing paper. With a pair of scissors cut two planes, one three by one inch and the second two by half an inch. You will also need a vertical rudder one inch square. Round off the corners slightly and glue the planes at either end of the stick and exactly on a level. Now fasten the rudder at right angles to the planes beneath the larger plane. If it dips, the front plane is too far back, while if it rises too quickly and settles back, the front plane must be brought back.

The paper gliders form an excellent kindergarten preparation to the study of aviation, leading up to the construction of large model gliders. You will thus gain a skill in adjusting the planes and fixing the centers of gravity and of pressure, which will prove valuable later on. The possibilities of glider building come as a surprise to the laymen in such matters.

THE SLING-SHOT GLIDER.

A fascinating field of experiment is opened by combining the sling-shot principle with the ordinary glider. The speed with which one can launch a glider from the hand is, of course, limited. Now use a small strand of rubber to launch the planes, and the increased speed will not only lengthen the flight surprisingly but make possible a really remarkable spectacular flight. A small glider may be made to return to the starting point or even loop the loop two or three times before touching the ground. By a simple adjustment of the planes, these curves may be varied indefinitely.

Designs for Sling-Shot Gliders.

When you have adjusted your glider to fly well, try the same arrangement of planes on a piece of reed, say eight inches in length, and bend the end over in the form of a hook. By heating the cane over a flame, you can make it turn without breaking and hold its position. Now loop a single rubber band over your thumb and forefinger, and passing the hook over the rubber, pull back exactly as you would use a sling shot. As you release the glider, pull your other hand quickly out of range. By using a heavier paper, one which will hold its shape, and turning the forward edges up slightly, the glider may be made to travel upward in a variety of graceful curves.

Paper Gliders. Antoinette Monoplane and Wright Biplane

An excellent glider with wooden planes

A covered-frame sling-shot glider

The best glider for launching on the sling-shot principle is made from planes cut from thin metal sheets. Aluminum is the best material, but a very thin wood will answer. A one-foot model glider will be found the easiest size to manage. Cut one plane eight inches in length by three in width, and the second five inches by two inches. Round off the corners on one side of each plane, leaving a straight line for the front or entering edge.

Mount the planes on a strip of reed, cane or bamboo about eighteen inches in length. In all these gliders the forward plane is made the smaller, thereby reducing the head resistance as far as possible. The metal planes should be slightly flexed by bending them to a slight concave above the horizontal and just back of the front edge. The forward end of the stick should be bent into a large hook by heating or first soaking in water. If your glider falls quickly to the ground bend the frame a trifle upward.

Since your glider is intended to travel at a comparatively high speed, the planes may be mounted much further apart than in the case of a glider launched from the hand. Try them first ten inches apart and afterwards adjust them to suit. The rubber used for launching the glider should be fairly heavy, say three strands of one-eighth inch rubber or its equivalent. The end of the hook may need adjusting so that it will escape from the rubber on being released.

It will be found an easy matter to obtain long, graceful glides from this model from the first. By launching it upward, it may rise to a considerable height. When you have caught the trick of launching your glider with sufficient force, try a spectacular flight. Set your forward plane at an angle by inserting a block of wood between the stick. In the case of metal planes, bend up the front edge.

A very slight upward elevation will answer. Gradually increase this angle until the model sweeps upward and turns on itself. You will soon be able to make the glider describe a complete circle or loop the loop twice before landing. When traveling at such a high rate of speed, your glider is likely to be dangerous and might inflict a bad cut, and the flight should only be attempted where one has plenty of room.

These flights may be still further varied by adjusting the rear edge of the vertical plane or rudder. By turning the rudder to the right, for instance, the glider may be made to travel to the right or the direction may be reversed. In this way the glider may be made to describe a complete horizontal circle or several circles. By launching the glider upward with this adjustment, it may be made to fly in a graceful spiral.

The success of a glider depends more upon its modeling and finish of its planes than in the case of the model aeroplane. It must gain as much support as possible from the air, since it has no motive power to keep it aloft. Its head resistance must also be cut down. The ordinary cloth-covered planes, which serve well enough for an ordinary model aeroplane, will not carry a glider far. The planes must, therefore, be of metal or wood, or when built-up planes are used they must be of the most careful workmanship.

The simplest form of glider, excepting, of course, the paper model, is made entirely of wood. A glider two feet in length will be found a good size to experiment with. The model should be much heavier than an aeroplane so that one need not take the care in its construction to reduce weight which may make the construction of a model tedious. A glider of this size may weigh upward of one pound. Under favorable conditions, it will glide for two hundred feet, when launched from the hand, while if it is thrown from an elevation, an upper window or a hill top, it may travel considerably further.

Select a stout stick for your base, one inch square and two feet in length. The main plane should measure fifteen inches in width by six in depth, and the smaller plane ten inches by four inches. A thin board about three-sixteenths of an inch thick may be used for the planes.

The front corners should be slightly rounded, and the rear edges cut sharply away. These planes may be flexed by steaming. Hold the section to be bent over the spout of a tea kettle until the wood is soft and pliable enough to bend. If it does not soften sufficiently, immerse the wood in boiling water. The plane should be flexed slightly upward just back of the forward edge. A good curve may be obtained by heating the under surface over a flame.

To hold it in position until it has dried and assumed shape, bend it over a stick laid on a board and fasten the plane down by driving brads around the edges and bending them over to keep it down. Leave it in this position until it is dry and hard.

Your glider will fly better with a vertical rudder, as in the case of the paper models. The rudder should be cut from a thin board of the same material about six inches square. Round off one corner and plane or sandpaper this front edge, which will be the entering edge. The entering edges of the front plane should be prepared in the same way to reduce the head resistance as much as possible. Nail this rudder to the side of the stick directly beneath the rear or larger plane. It will be still better if you mortise it neatly into the center of the stick.

The glider is thrown with the smaller end forward. For the trial flight, mount the smaller plane at the extreme forward end and then move it backward as you test it out, until the glider moves on an even keel. To launch the glider, grasp the central stick from beneath at the point where it balances, and throw it forward with all your might. Since it travels at a much higher speed than a power-driven model aeroplane, it requires much less supporting surface, while the planes may be spaced much further apart.

When you have adjusted the planes, try throwing your glider at an upward angle of say forty-five degrees. It should rise swiftly to a height of upwards of fifty feet, turn backward on itself, and even describe a graceful upward curve before coming down. Now try throwing it into the wind or against a moderately strong breeze. Its course is likely to be very irregular. It will dip and rise at many unexpected angles, and probably travel several hundred feet in all before landing. During the past year, a model glider has been built by Mr. W. H. Howell, Jr., to glide a horizontal distance of 650 feet, while the actual length of the flights has been upwards of 2,000 feet.

CHAPTER IV TOOLS AND MATERIALS

A well-stocked tool chest will be of great assistance to the builder of model aeroplanes, but it is by no means essential. A few simple tools, easily obtained, will be found to answer. First of these comes a serviceable pair of nippers. You will need them to bend the axles of your propellers, in adjusting the motors, and for a score of uses. A pair of nippers with a cutting edge is best. Always be sure to slip these in your pocket before flying your model, for you are sure to need them.

A fine gimlet, or a needle drill, will be found useful in a score of ways. They cost but a few cents. A handle which may be adjusted to drills of different size is best. A drill one thirty-second of an inch in diameter will be found especially useful. The parts of your model are likely to be delicate and easily split, even while driving a small brad. You can avoid the danger of splitting by first using the needle drill, even for small brads, and then enlarging the hole, if necessary, with a larger drill or a gimlet.

Percy Pierce launching a model

A French model built of aluminium

A fine saw will be found very useful,—the finer the better. The timber used for the frame is so light and soft that it is likely to split. A gig saw will be found just the thing for cutting out propeller blanks and other parts, but it is not essential. If your model be made of metal, a small soldering iron will, of course, be found indispensable,—the smaller the better. The metal parts are very delicate, and the iron should have as fine a point as possible. Such an iron can be obtained at a hardware store for a few cents. If you do not know how to solder neatly consult some tinsmith.

Diagram for making the planes

In addition to good cutting tools, a good half-inch chisel is most important. A concave chisel will be found handy in carving propellers. Some of the best propellers have been whittled out with an ordinary penknife, and sometimes a dull one at that, so that after all a good penknife is the most essential tool of all. With this little handful of tools, you will find you can build up the most delicate models.

The world has been ransacked for material which will give the greatest possible strength for its weight. The use of aluminum is, of course, familiar. The search has also brought out the comparatively unknown metal, "magnalium," which, although a trifle heavier, is believed to be much more desirable on account of its greater strength. In a search for strong, light wood the builders of aeroplanes have searched the tropics.

One of their discoveries has been balsic wood, which is of a feather weight. It is exceedingly soft and easily worked, but has the drawback of being rather pithy and easily split. A severe jar is likely to discover some weak point. It will be found valuable, however, for the shorter members of the model. Some model builders use balsic wood as a filling for hollow sticks. The wood may be strengthened by covering with cloth glued firmly about it. It is also used as a filling for thin aluminum tubing.

In all the search for materials nothing has been found to compare with bamboo for lightness and strength. A number of successful model aeroplanes have been built this year in which the central sticks and frames are built entirely of bamboo. Bamboo is especially valuable in constructing the smaller members. It can be bent either by the dry-heat process, described elsewhere, or by steaming. Bear in mind that the strongest part of the stick lies just beneath the hard glazed outer surface. The only drawback of bamboo is a tendency to split at the ends. The extreme lightness of the material on the other hand makes it possible to make rigid joints by glueing and winding with fine thread touched with glue.

Working drawing of the Flemming Williams model

An imported Flemming Williams model. English record 2600 feet.

The lighter woods, whitewood and poplar, are much used by model builders. They are easy to work, especially whitewood, because of its freedom from knots and cross grains. Some builders prefer ash on account of its strength. Beech has rather less strength, some fifteen per cent, while spruce is little more than half as strong as ash. The quality of the wood varies considerably according to its nearness to the bark of the tree. The wood used for model aeroplanes should be well seasoned; a year is not too long.

Motor bases are of two general divisions. The "single stickers," or bases consisting of one member, are commonly called "spars," while the more complicated frameworks are designated as "built-up" frames. The spar type is, of course, the simplest to construct, and, as many believe, the most efficient of all forms. The simpler the design, as a rule, the less chance will there be of breakage. For the beginner the use of plain, honest sticks is, of course, to be recommended.

The built-up motor bases, on the other hand, make a more attractive model to the eye. There is besides an opportunity to reduce the weight of the frame while retaining the same strength. An examination of the models illustrated elsewhere will show to what an art such construction has been brought. By ingenious bracing it is possible to construct motor bases of strips one-sixteenth of an inch, or even less, in width, and yet render the whole sufficiently rigid to withstand the pull of powerful twin motors.

Your frame may be considerably lightened by using hollow sticks or shafts in place of solid members. A stick three-fourths of an inch wide formed of light lath one-eighth of an inch thick will weigh no more than a solid piece one quarter of an inch square. Such a member is stronger than the small, solid stick; it bends less readily under the pull of your motors, and renders the entire frame far more rigid. It will also be found much more satisfactory to mount the planes upon such a frame.

A little practice will enable you to make a very satisfactory stick of this pattern. Secure a light strip one-eighth of an inch thick and of a width one-eighth of an inch less than that of the stick you intend to build. A one-inch stick is probably larger than you will need. The following directions are intended for a stick three-fourths of an inch square.

First cut three square plugs one-half an inch square and one-fourth of an inch thick, and placing one at either end and one in the middle to form a core, build your frame about them. The edges should overlap and be glued continuously together, and the plugs fastened in position with brads driven from the middle of the four sides. When dry, cut away the glue, sandpaper and varnish.

Some model builders prefer a T-section-shaped spar, which is almost as light as the hollow stick, besides being easier to construct. We assume that you are working with eight-inch strips, which will prove heavy enough for ordinary purposes. Prepare one strip one-half an inch wide, the desired length of your base, and the second strip three-eighths of an inch wide, the thickness and length being the same.

Now fasten the smaller strip at the center of the long strip, glueing it first in position. When dry, drive a series of fine brads along the center of the back of the larger strip. Cut away the glue and sandpaper. The T-shaped stick will be found strong enough for all ordinary demands. The rubber strands of your motor may be carried either above or below it.

The H-shaped-section stick is rather more difficult to build, but it will be found somewhat stronger, weight for weight. If you are using one-eighth strips, cut two lengths one-half an inch wide, and a third length three-eighths of an inch wide. Fasten the smaller pieces to the middle of one of these strips, as in the case of the T stick, with glue and brads. When dry, attach the remaining strip opposite, glueing and nailing as before. Some builders prepare these strips by cutting out the grooves with a chisel, thus making a one-piece strip. This requires very careful workmanship, however, and is scarcely worth the trouble.

The motor frame may be still further lightened by using a trussed frame. The weight of this member may be cut in two in this way without sacrificing its strength. To build such a part secure two strips of wood one-eighth of an inch thick, one inch in width, and cut to the desired length. Now from the same material cut six blocks, one-half an inch in length, and set these at regular intervals along one side of the strip. They may be glued or nailed in position, or both. A small brad will hold them in place. In working with such delicate material it will be well to first drill the holes with a fine drill. Next fasten the second strip above them, nailing and glueing as before.

Storing energy for a long distance flight

CHAPTER V THEORY AND PRACTICE OF PLANE CONSTRUCTION

THE planes of your model aeroplane need no longer be a blind experiment whose merits or defects may only be learned by actual test. The science of wing-building is much better understood to-day than a year ago. Without going into the complicated equations dealing with aspect ratio, pressure, and gravity, it is important that one bear in mind a few definite rules in designing even the simplest planes. A great many useless experiments may be avoided.

In a previous volume, it was pointed out that a narrow plane, or one with a high aspect ratio, driven with its broader side forward, would yield greater support than a square surface of the same area. (The aspect ratio, it may be well to repeat, is the relation between the width and depth of the plane. A wing, for instance, whose width is five times its depth, is said to have an aspect ratio of five.)

It has been found that on small planes the center of pressure is situated about one-third the distance back from its front or entering edge. The center of pressure in flexed planes occupies about the same position.

As long as a plane remains horizontal, or nearly so, a very narrow surface,—one, that is, with a high aspect ratio,—will exert greater lifting power than a deeper plane of the same area. An examination of the successful model aeroplanes of 1911 will show that the depth of the planes has been cut away. Planes are used with an aspect ratio as high as ten. The speed at which such a plane travels is a very important factor. As the speed increases, the efficiency of the plane surface increases, so that a model aeroplane driven rapidly may be sustained by less wing area than in the case of one which flies slowly.

As the front edge of a plane is raised, the center of pressure travels backward. By the time the plane has reached an angle of about fifteen degrees, its lifting power has diminished about one-half. A very narrow plane, or one with a high aspect ratio, should, therefore, be set near the horizontal. The model should, moreover, rest upon skids at as low an angle as possible.

In starting off, the planes will thus exert their maximum lift, or nearly so. If the narrow planes be elevated too much, the center of pressure will come nearer the rear than the front edge, and tend to force the aeroplane downward, or, as the phrase is, make it "sit on its tail." As long as a plane is traveling horizontally, or at low angles, its rear portion exerts very little sustaining power and may be cut away.

A plane with a high aspect ratio is much more stable in flight than a surface of greater depth. The center of gravity of a flat plane would, of course, coincide with the center of the surface when the plane is in motion. When the plane tilts, the center of pressure, as we have seen, moves backward or forward. If the plane has little depth or a high aspect ratio, this center of pressure cannot move far.

It must oscillate back and forth within very narrow limits. A very little tilt up or down will restore it to its normal position, so that a plane with high aspect ratio is more stable than one with a deeper surface.

The efficiency of a curved surface over a flat plane was analyzed in a former volume. Such a curve, when well drawn, adds to the lifting power as well as the stability. Since a curved plane does more work than a flat surface, its size may be reduced and its aspect ratio increased. In other words, the curved plane may be narrower than a flat surface, and may be made thinner in proportion to its width.

The height of the curve, or camber as it is called, has been worked out by elaborate mathematical equations, but we may take the general results without following the calculations. For a plane six inches in depth, the camber should be about one-half an inch, or one in twelve, or in this proportion. The curve should be a parabolic with the highest point well forward, one-third the way from the front edge. The front, or entering edge, of the plane should be the thickest point. It should be tapered off to a thin edge in the rear.

In theory, it is possible to model a plane so delicately that it will fly against the wind by the pressure of the wind itself. It is extremely important that both sides of the plane be brought to this curve as accurately as possible. An efficient plane must, therefore, be covered smoothly on both sides. Such a plane again offers very little skin friction to the wind.

It is difficult to lay down any hard and fast rules for the relation of weight to wing surface, since the types of aeroplanes differ so widely. It has been found, however, that in large models one square foot of surface will support about one-half a pound of weight, when traveling at a high rate of speed.

You will find that your model, if its wings have a spread of thirty inches os thereabouts, will approach one pound in weight. The figuring will show you that two wings, whose combined area is less than 150 square inches, will be comparatively small and certainly well under those generally employed a year ago.

The planes used on this season's models are marvels of lightness and strength. Much has been learned by studying the methods employed by the builders of man-carrying aeroplanes. It is a simple matter to build a three-foot plane which weighs complete less than one ounce, and is strong enough to withstand many a violent shock.

A geared model built by Leslie V. Robinson

An ingenious biplane

It will be found a good plan first to lay out the exact form of your plane on a smooth board. Make the depth of the plane one-fifth of its length. It will be noticed that this plane is much more slender than those used last year. Next draw a line at the center the entire length of the board, and mark off one-tenth of the length of the plane from either end. From this center describe a quarter circle at either end, on the same side of the line. This will form your main or entering wedge. The rear corners should be cut sharply away and only slightly rounded.

There is no better material for the main frame than a thin reed, cane or bamboo. The longer ribs may be made of any light lath and the cross ribs of a thin flat strip of the same material. Soak the reed overnight to make it as pliable as possible, or heat it over a flame. Now lay the reed over the outline of the plane, and hold it in this position by driving thin brads on both sides and bending them over the cane. When the outer edge is complete, mortise the ends slightly and tie and glue firmly together.

With the outer frame held rigidly in position, it will be found a much easier matter to introduce the ribs. If you are building a flexed plane first, insert a stick of wood from end to end before placing your cross ribs in position. The thickness of this temporary stick will, of course, determine the curve of your plane. For a three-foot plane, a height of one-half an inch will answer.

The ribs may now be bent over this obstacle and fastened securely to the outer rim by glueing, tying, or nailing. The cross ribs may also be raised by inserting small wedges between them and the longitudinal ribs. When your frame is complete, loosen it from the board and you will find it regular and rigid. Cover it with a very thin cloth pulled tightly over the frame, and glue or sew it in position. A small plane may be covered only on the under side.

Excellent results are being obtained in England with planes built up entirely of wire. If aluminum wire is used, the weight of the wings is considerably cut down, but even ordinary wire will be found lighter than wood. For a plane thirty inches in width, or thereabouts, the wire used should be at least one-sixteenth of an inch in diameter, and should be soft enough to bend easily and hold its position.

It will be found a good plan to plot out the exact shape of your plane on a sheet of paper, and then bend the wire over this outline. The ends may be fastened together readily by binding tightly with fine wire, such as florists use, and touching the joint with solder. Be careful, of course, to keep the joint smooth. The cross ribs of these metal frames may also be made of wire. Bend the ends at right angles and attach to the inner sides of the plane with fine wire, and touch all the joints with solder.

There are several advantages in the metal planes. It is a very simple matter to flex the plane by bending the cross ribs and the ends upward to the desired curve, much easier than when working with wood. Such a frame will stand almost any amount of knocking about without injury. A swift volplane to earth, which would smash any ordinary wooden frame to "smithereens," would have little effect on a model plane. Such frames again are very easy to cover.

It will be found a good plan to sew the cloth to one edge, draw tightly across and sew fast to the opposite side, while a few stitches around the metal cross ribs will give it any curve you desire. A metal frame makes it possible to experiment with various curves. It is an easy matter to bend the ribs up or down and alter the line of the plane at will.

Small stability or guiding planes may be made of a sheet of metal, although such construction is not advisable for the main plane. When your front or entering plane is the smaller one, it is possible to turn it into a very efficient motor anchorage.

The plane should be cut from a sheet of aluminum, preferably. Fasten this securely to the front of your motor base with nails, or tying in position. The wires of the hooks holding the ends of the motors may be passed through the holes cut to the back of the rear edge of the plane and bent over. Of course it is very simple to anchor double motors, or even multiple motors, in this way.

One of the novelties in plane construction is a narrow wing with ends brought well back. It may be built either flat or flexed, and promises to afford unusual stability. The form is very popular among model builders in England, where it is made very narrow, its depth often equaling its width.

In many of the English models, these planes are placed far forward and raised well above the main stability plane. They are built with the entering edge either straight or slightly curved. Such front planes behave especially well in the open air and even against a considerable wind pressure.

There is still considerable difference of opinion as to the best material for covering planes. Several specially prepared aeroplane cloths have been placed upon the market which are guaranteed to be practically airproof. The cloth is rather heavy, however, and better suited for large machines. A thin silk answers the purpose perhaps as well as anything.

Some model builders select the thinnest possible silk and then render it airproof by varnishing or covering with a thin solution of wax or paraffin. When this is neatly done, the planes are very taut and shipshape. Several preparations are offered for sale for coating planes, which are excellent.

In the search for the lightest possible covering, some builders have gone a little further and use a very thin paper known as bamboo paper. Even the thinnest paper will be found as impervious to air as a rather heavy cloth. Its weight is practically nothing, even for a large plane. It requires no varnishing or preparation, although it is sometimes painted to render it more rigid.

There is, of course, a very obvious objection to paper that it is easily punctured, but on the other hand, such accidents are very easily repaired. A bad rip may be patched up with a touch of paste, or, the plane may be re-covered very quickly. With this paper care must be taken to fasten it to the frame of the plane as securely as possible, as a loose sheet will flutter and increase the head resistance.

A well-proportioned model built by Reginald Overton

A good model intended for long distance work built by A. C. Odom

In order to lighten the plane, the outer frames at the ends and rear may be cut entirely away. An appreciable saving of weight is thus obtained without weakening its structure. This plan is especially to be advised in comparatively small planes. Design your plane and lay out its exact form on a board. A thin strip of wood should be cut the width of the front or entering edge, and similar straight lengths for the longer ribs.

It will be found a good plan to use a heavier piece back of the front edge or at the top of the curve. In building your plane, follow the former directions of laying a stick on the board to give you the height of the curve. The shorter cross ribs may then be fastened by glueing to the longer ribs. By using a light lath or strip for the cross ribs, it will be possible to make them sufficiently rigid merely by glueing without the trouble of nailing. A skeleton frame of this kind has the advantage of being very elastic.

In covering the frame, draw the cloth tightly across the upper side of the frame and touch with glue at regular intervals along the ribs. When dry, trim away the cloth between the points of the ribs and the open ends. The rear edge may be held in position merely by the shorter cross ribs. Trim the cloth along the edge.

In such a plane it is well to stiffen the cloth covering by painting with shellac or varnish. This also lends a semi-transparent effect which improves the general appearance of the plane. By cutting away the side and end pieces of the frame such a plane three feet in width may be made to weigh less than one ounce.

Since it is very important that the covering of the planes may be perfectly smooth, it will be well to experiment with several different methods of attaching the cloth or silk or paper. By covering with paper, a taut surface like a drumhead may be had. Use a rice or fiber paper and moisten the sheets by laying them between damp cloths, as was explained in detail in a previous volume. In drying, the paper contracts and tightens.

In covering a frame with cloth, the angle of the two sides may be altered by stretching the covering over the raised ribs on one side and drawing it tightly from edge to edge on the reverse side. If you have difficulty in making your surface smooth, try lacing it to the sides. You will need a strong hem at the edge. By using a thread, you will be able to pull the cloth taut much the same as tent flaps are tightened.

The proper curve for a flexed plane is still a matter of dispute. The highest part of the curve should come well forward, while the rear surface is drawn straight. A good camber may be plotted very simply. Draw a rectangle with a length sixteen times its height. Now mark off a point on the upper side one-fourth of the way from the left-hand corner and draw diagonal lines from this point to the two lower corners. Next round off the broad angle formed by the two lines and you will have a good curve to imitate in flexing your planes.

CHAPTER VI SCIENTIFIC PROPELLER BUILDING

Ever since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood.

In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane.

A beautiful monoplane built by R. Mungokee

Detail of a model built by R. Mungokee

An ingenious application of the dihedral angle

The propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller's work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute.

There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down.

Diagram Showing How To Make A Propeller From A Wooden Blank

There is much difference of opinion as to the proper modeling of the propeller. It has been worked out by elaborate equations that the blade should be concave and yet in actual tests it has been found that some machines are driven faster by a flat blade propeller. By a flat screw we mean a straight pitch propeller, or one in which the angle does not vary from the hub to the tip. When Mr. Glenn H. Curtiss made his famous record flight at Rheims, he used a straight pitch propeller, and when, later, his machine was equipped with propellers scientifically curved, his aeroplane lost speed. Evidently the exact relation of propeller forms to the machine still remains much of a mystery.

Design of Metal Propeller

A very simple test of the efficiency of propellers of various modeling may be made by running them in heavy smoke. By burning a piece of oily cotton waste, you may watch the action of the propellers on the smoke, while, at the same time, this greasy smoke will leave its mark on the section of the propeller blade which does the most work. The speed of the blades near the hub of the propeller is, of course, much less than at the tips, and consequently the work they perform in sending the aeroplane forward is small. At the extreme end of the propellers, the air, of course, tends to slip off.

The most efficient part of the blade, according to these tests, is about one-third of the radius distant from the center. Less than two-thirds of the propeller seems to do effective driving work. On the propellers driven against greasy smoke, the blades near the hub remain comparatively clean while the portion most soiled extends outward from this point. The test would seem to indicate that a broad blade narrowing to the hub would develop the maximum thrust. It would also seem that it is unnecessary to carry the lines of the blade close to the axle, thereby possibly weakening the propeller.

To understand the theory of the propeller, bear in mind the principle of the action of the wings, for the analogy between the two is very close. The forward, or entering, edge of the propeller is curved so that it will cut its way smoothly and offer less resistance than a straight entering edge. The blade of the propeller is made slightly concave for exactly the same reason that the plane is curved. Like the plane, such a surface takes advantage of the resistance of the air.

The curve of the propeller blade near the hub is made much higher than further on because this part travels more slowly, and it is important that the cylinder of air set in motion by the blade should have the same velocity throughout its diameter. The blade is made widest at its outer end, since this is the most effective surface and is expected to do the greatest amount of work. The other end of the propeller blade is rounded off in order that the air may slip away, thus avoiding skin friction, which at this point is naturally high.

A test of high aspect ratio planes

A modified Bleriot built by Cecil Peoli

The width of the propeller blade has been the subject of an immense amount of investigation and discussion. The friends of both the wide and narrow blade back up their arguments with complicated equations, which it would only be confusing to repeat. It is argued by some authorities that since the narrow blade does not stir up the air as long a time as the wide blade, therefore one blade does not stir up the air enough to interfere with the action of the second blade.

Langley Propeller Blade

A small blade may be driven by a much lighter motor, and is, of course, capable of much higher speed. On the other hand, the wide blade drives the model much further ahead per turn than the narrow blade, while making a much greater demand upon the motor.

Briefly a narrow propeller is best for speed and the wide blade propeller for power. There is an immense amount of difference of opinion concerning the form and position of the propeller so that it is impossible to lay down any hard and fast rules. It is argued by several well-known aviators that a propeller is more effective when driven with its straight edge forward and there is scarcely a point not in dispute.

One of the most novel propeller designs, the Cowley, is a blade bent in the form of an arc of a circle, the radius of the curve being equal to the diameter of the propeller. The propeller is mounted with the convex surface forward. The theory of this propeller is that it focuses the air, as it were, which it throws back forming a cylinder of air which travels at a higher speed than one set in motion by the horizontal blades.

The tendency for the air to slip off the ends of the propeller blades is probably reduced. This form of propeller may be made by steaming the blades and bending them into position. A mould may be prepared and the steamed blades forced to take their shape and held in position until they have dried.

A series of experiments have been made in England with boomerangs to discover the effect of curved surfaces on flight. The Langeley propeller, which embodies the information gained in this way, has a flat back while the face is concave, following the general stream line form. The ends of the propeller blades are practically square. Some of the new propellers are covered with a thin canvass glued smoothly over the greater part of the blade. The covering guards somewhat against splitting and splintering.

In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform.

Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction.

Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem.

Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling.

A combination of several interesting features

A skilful adjustment of the front plane and skid built by Percy Pierce

Propeller blanks are easily prepared in case you find it inconvenient to buy them. The following directions refer to a propeller eight inches in length, but the same proportions hold good for any size. Select a piece of some straight-grained wood, white pine is best, which will not split readily, and is easy to work. The original block for an eight-inch propeller should be eight inches in length, two inches in width, and three-fourths of one inch thick. Now draw a line lengthwise, exactly bisecting the block, and mark off the middle of the line, and two points one inch from either end. With one of these outer points as a center, describe a quadrant of a circle above the line, and from the corresponding point, draw a similar circle below the line. From the center of the blocks draw a complete circle one-half of one inch in diameter. Draw straight lines from the ends of the arcs to the vertical diameters of the circle, and saw away the wood to these lines. In carving your propeller, first cut away the wood from the longer straight lines of the block on opposite sides. The blades should be slightly concave. It will be found a good plan to finish one side of the blade before cutting away the opposite side. Cut away the wood until the blade is less than one-eighth of an inch thick, and sandpaper away all marks of the knife or chisel. The wood should then be oiled or covered with a light coat of varnish. It is very important that the two ends of the propellers should be uniform both as to their modeling and weight. To mount the axle, drill a hole at the center just large enough to admit the wire. The outer end may be bent over and inserted into the hole to keep it rigid. If the axle does not fit tightly, drive in small wedges of wood, such as a toothpick, at both sides.

The propeller used by the Wright Brothers on their machines is very simple to construct. Prepare a propeller blank eight inches in length, two in width and three-fourths of an inch in depth. Draw two lines parallel with the longer sides, the first seven-eighths of an inch and the second one and one-eighth inches back. Now at the upper right-hand corner mark off a point one and one-half inches from the end, and from the opposite corner on the lower base the same dimension. Connect these two points.

Wright Propeller Blade

The blank is completed by cutting away to these lines, leaving the blades each one and one-eighth inches in width. The axle should be left a little full, say three-eighths of an inch across. Round off the outer corners. In modeling your propeller cut away or bevel the sides formed by the two intersecting lines, which will form the entering edge of the propeller. The blade should be cut to a very slight concave, although some prefer a flat blade. The propeller is mounted by drilling a hole at the center and mounting in the usual way.

The propellers of a model aeroplane are subject to more wear and tear than those of a regular passenger-carrying machine. At the end of every flight, they face a possible catastrophe. In the search for some durable form of screw, a number of interesting discoveries have been made. One builder has succeeded in coating a wooden propeller with bronze by subjecting it to an electroplating process, but this is much too complicated for the amateur. The lighter metals, aluminum and magnalium, naturally suggest themselves for the purpose. Such propellers weigh no more than wood and may be readily bent to the required shape.

Procure a thin sheet of aluminum, or, if this cannot be had, a smooth piece of tin will do. It must, however, be heavy enough to hold its shape. The design of the propeller may be laid out on the tin, and the metal trimmed away. To make an eight-inch propeller, draw a rectangle eight inches in length and two inches broad, and draw a line joining the middle of short sides. At the center, draw two vertical lines half an inch on either side of the center lines, half an inch above and below the center, forming a small inner rectangle. Now from a point on the bisecting line, one inch from either end, draw two semicircles. Next, connect the top of one of these circles with the nearest point of the inner rectangle and draw another line from the point below to the corresponding corner of the large rectangle. Repeat the diagram on the other end of the rectangle, reversing the curve as indicated in Fig. A.

In cutting out the design, allow the straight sections running to the sides of the larger rectangle to remain. They will be needed to hold the central piece in position. This consists of a block of wood measuring one inch by one-half an inch and one-quarter of an inch in thickness. The strips at the center should be bent tightly over the corners, overlapped, and nailed firmly down with brads. Next, at the center, punch a small hole and drill through the block a shaft large enough to hold the axle of the propeller which is then firmly imbedded in it. One great advantage of the metal propeller is the fact that you may readily alter its pitch.

An efficient propeller may be made by mounting metal blades on a wooden shaft. Procure a stick one quarter of an inch square and three inches in length, and saw through both ends for a distance of three quarters of an inch. Prepare your propeller blades by plotting them out on a sheet of aluminum, as described above, and cut away the middle section. The blades may then be inserted in the open ends of the stock and nailed securely in position. The edges of the wood may then be rounded off and the axle inserted firmly at the center. The metal sheet should be bent into the proper pitch as in the case of other metal propellers.

FABRIC PROPELLERS

The most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.

An efficient model, showing excellent construction, designed by John Caresi

One of the best minimum plane models of 1911

THE LANGELEY BLANK.

Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.

In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.

CHAPTER VII ASSEMBLING THE MOTORS

In the present stage of model aeroplane building, rubber strand motors satisfy every demand. Models have been flown for more than 2,500 feet by the force of these twisted strands, and doubtless their efficiency will be still further increased. Such motive power is besides very easily obtained and applied. Careful tests have shown that more energy may be stored up in twisted rubber strands than in the same weight of springs of steel or any other metal.

In gauging the strength of your motor, much depends upon whether your model is to rise from the ground or be launched from the hand. In the model tournaments in England, the flying machines are almost invariably thrown across the starting line, while in America they are required to rise unaided. It is obviously unfair, therefore, to compare the distance records of the two countries.

A Metal Motor Anchorage

It requires a comparatively powerful motor to raise a model from the ground, whereas a lighter motor would be sufficient to propel it through the air. Many models, capable of flights of several hundred feet when thrown will refuse to rise, while, on the other hand, some models which rise well enough have poor distance qualities.

It should be borne in mind that the length of the motor, speaking broadly, controls the distance qualities, and its diameter the speed of the model aeroplane. A long slender motor, capable of from five hundred to one thousand turns which will revolve the propellers for thirty seconds or more, should insure a flight of several hundred feet. As you increase the number of strands of rubber, building up the diameter of the motor, you cut down the number of turns and therefore its duration, although you increase its speed.

A motor capable of one thousand turns must be about forty inches in length and consist of but six, or at most eight, of these strands. A model which may be driven by this motor, it will be found, must be very light. A model aeroplane weighing upwards of one pound, on the other hand, will require motors composed of fourteen strands or more to raise it from the ground. It is a very simple matter, of course, to add strands of rubber until your motor develops sufficient energy for the work it is expected to do.

The length and diameter of your motor, again, has a direct influence on the height of the flight. Too much power tends to raise the aeroplane higher than necessary above the ground. Since it requires more energy to drive a model aeroplane upward than along a horizontal direction, this is obviously a waste of energy.

If it is desired to fly the model as far as possible, it must be kept close to the ground. In the case of weight-lifting contests, the problem of altitude is, of course, entirely different. Overwinding is even worse than underwinding, since it shortens the life of the motor.

Try out your aeroplane with ten strands on each motor and increase them later. The motor, as previously explained, is formed by looping the rubber strands loosely between the hooks, just as zephyr is wound on a skein. Keep the strands very loose and fasten them to the hooks by tying with a strand of rubber. In winding, do not turn the propeller after the rubber has a double row of knots for its entire length. Such a motor should take up from three hundred to five hundred turns, perhaps more. Do not keep the elastic wound up too long before starting your flight. The strain is great and it quickly wears out.

A Metal Motor Anchorage

The rubber strands should not be allowed to come in contact with any metal parts of the model. The copper that is often used for wiring is especially injurious and tends to decompose the rubber. The hooks of both the propeller and motor anchorage should be covered with a piece of rubber tubing. This serves a double purpose. With this protection, the rubber when tightly twisted is in no danger of being cut by the wire or of taking up the oxides which quickly eat through it.

A notable model possessing unusual stability. Built by W.S. Howell, Jr.

Front view of model built by W.S. Howell, Jr.

It requires an expert to pick out the best quality of rubber. If the strands be examined under a magnifying glass, it will be found that the edges of fresh rubber of the best quality are clean-cut, whereas the cheaper rubber, and that which is worn, has commenced to granulate, giving the edges a ragged appearance.

A simple test is to stretch the rubber over a ruler. A good rubber, in first-class condition, will stretch about seven times its length, and on being released instantly spring back to its original size. The same rubber should stretch to ten times its length without breaking.

There is a great difference of opinion among the most successful model builders as to the best shape of rubber strands. Some prefer the flat, band rubber, while others are obtaining satisfactory results with rubber cut in square strands. The strand used by the English model builders is seldom more than one-sixteenth of an inch square, while in America one-eighth of an inch strand is commonly used.

Experiments have been made with a single strand of rubber one-fourth of an inch square, but the results have not been satisfactory. One theory is that the corners of the square rubber tend to cut into one another and quickly wear out, and that a perfectly round strand would be the more efficient. At present there are no such strands on the market. It is argued by some that the square strand in twisting must be turned on itself further than the flat strand, and is therefore placed under an unnecessary strain. After all, the advantage of one form over another is fractional, and an extra strand added to the motor will balance any possible defects.

Figures have been prepared giving the exact relation of the size of rubber to the number of turns, although such statistics are elastic. The problem may be worked out with your own motor. Differences of temperature will be quickly noted. The rules prepared by V. E. Johnson, M.A., an English authority on aviation, are as follows: The motive power is doubled by increasing the number of rubber strands one-half; by doubling the number of strands, the motive power is increased more than two times; and the tripling of the strands increases the motive power seven times. As regards the number of turns the same authority states that the doubling of the number of strands diminishes the number of turns by one-third to one-half.