The Second Boys' Book of Model Aeroplanes

by Francis A. Collins

CHAPTER XV CURIOSITIES OF THE AIR

Until the summer of 1911, the longest model aeroplane flights officially recorded in America remained under 300 feet. From England meanwhile came disquieting reports of 1,000 foot flights and better, made by a number of aeroplanes. A comparison of the best American and English models showed that, both as regards form and workmanship, American boys were holding their own against their English cousins, and utterly failed to account for the much greater distance qualities of the foreign models.

In July, 1911, the American distance record was suddenly jumped to 1,691 feet, 6 inches, by Cecil Peoli of New York. The model used had been flown in the regular indoor meets for very much shorter distances. This sensational advance in the distance record was made at an outdoor tournament at Van Courtland Park, New York City. Of the thirty-four models entered for the contest, including the familiar models built by Percy Pierce, H. Watkins and others, several showed a similar increase in distance qualities. The model aeroplanes were the same as had flown but 200 feet indoors, their rubber motors exerted no more power than before, the pitch of the propellers remained unchanged.

What then is the secret of the suddenly acquired distance qualities? Evidently the difference lies in the quality of the air the little ships navigate. It is commonly said that the air indoors is dead as contrasted to the live air found out-of-doors. The variation in the quality and movement of the air forms a very interesting study which no aviator can afford to neglect. To the actual navigator of the air this study is just as important as life and death, while to the designer even of model aeroplanes it is of course of vital importance.

Although the composition of the air and its behavior under various conditions has been the subject of scientific examination for centuries, "it is only within the past few years that it has been studied with the idea of bringing it under control. The long painstaking experiments of Langeley and Lilienthal, referred to in the previous volume, for determining the resistance of the air and its effect on the surface of the aeroplane, opened a new field of scientific research. Within the past few years, even months, the advancement in our knowledge of the air has been greater, it is safe to say, than in the previous century.

It is not generally realized by the laymen how rapid has been the development of "aerology," nor how practical are the results obtained. There have already been established in Germany three scientifically equipped stations for observing air conditions for the benefit of aeronauts, just as the weather bureau observes weather conditions and informs ships at sea of approaching storms. One of these, established by Dr. Polis at Aachen, is now in operation. In addition to this the Public Weather Service Stations of Germany have been equipped recently with the necessary apparatus for making daily observations of the upper air for the special benefit for aerial navigators.

The resistance offered by the air to the passage of an air ship of any type depends upon its density. The air is obviously an exceedingly variable medium, as capricious as quicksilver, as both the sky pilot and the flyer of model aeroplanes have learned. The density and therefore the resistance depends upon the temperature, the pressure and the state of equilibrium. We are in the habit of thinking of density and pressure as affecting enormous volumes or areas miles in extent, such as are reported in the weather forecasts from day to day. To an extent these same conditions are found changing within a few feet. It is this tendency to rapid change of conditions which makes the problem of stability in aeronautics so baffling.

The pressure upon an aeroplane, whether a man-carrying machine or a model, varies considerably between the level of the seashore and the top of a mountain. An aeroplane in rising from the level of the sea for several thousand feet therefore meets new and unexpected conditions. The density is reduced fully one-half at an altitude of 18,000 feet, and since aeroplanes have risen more than 10,000 feet this must be taken into consideration. Even at a height of 300 feet, which is often reached by aeroplanes in flight, the difference in the pressure calls for skilful manipulation on the part of the sky pilot.

The density varies again according to the temperature. Let an aviator suddenly run into a hot or a cold strait of air and the pressure upon his planes will instantly change. The effect of temperature must be taken into consideration by any one flying model aeroplanes. It may happen that a draft of cool or of hot air, by changing the pressure on the planes, will throw a model aeroplane out of balance and mar an otherwise promising flight. A difference of a few degrees of temperature will often affect a very sensitive model. The only way to combat this is of course to build your model with the greatest possible stability.

The presence of high buildings or other violent inequalities will also affect the density of the air and in turn the pressure exerted on the wings of an aeroplane. Among aviators it is generally believed that a great city is one of the most dangerous possible objects to fly above. In the case of New York with its many sky-scrapers, for instance, the danger is vastly increased. Even in the open country the presence of a deep valley or other depression will so affect the density of the atmosphere that an aeroplane is likely to be drawn down from its course. These areas are known among aviators as "pockets," and are often large enough to swallow up a large man-carrying craft, at times with disastrous effects. The chance of your model aeroplane running into such a pocket is of course considerable.

A striking illustration of the effects of these chance currents was afforded during a recent model aero tournament in New York. A model aeroplane which had flown with remarkable steadiness for more than 150 feet chanced to pass over the head of a boy who was walking slowly across the course. This moving object served to set up a small whirlpool of air. The model on striking it was instantly checked, when it turned, skirted the column of air and passed on. In an indoor flying, an open window, merely by changing the temperature slightly in its vicinity, will often cause a model to be seriously deflected, perhaps to be thrown completely off its course. It will often be noticed in outdoor flying that a model, in passing over a stream or body of water or a mass of dense foliage, will encounter a change of temperature which will appreciably affect its course.

In flying model aeroplanes the performance of a machine will often vary unaccountably from day to day. With the same motor and winding, the model will fly much higher and more freely at different seasons of the year. It is well to bear in mind that in the summer months the heat causes a low density. The pressure exerted by the atmosphere is therefore correspondingly small; the model, or for that matter a large machine, travels much faster. A dry day also tends to cause low density. This will account for the excellent flights on warm, dry days and the crankiness of the machine when the weather is damp.

Cold weather on the other hand tends to increase the density of the air. The speed of an aeroplane is materially reduced but on the other hand the air will be found more buoyant. Such weather is better for heavy models, although they fly much more slowly. It will be seen that at high altitudes, where the density is least, models should fly both faster and higher than they will nearer the sea level.

One of the curiosities of the air is the effect of eddies created by the passage of aeroplanes. Let an aeroplane move fast enough through the air and it will create high density above its course, and low density below its line of flight. This condition may also be found in the wake of model aeroplanes. It will often be noticed when two models are flying near together that the disturbance caused by one will seriously interfere with the second machine. The contrast between the air above and below the models in flight in especially noticeable when the planes are flexed.

Two of the earlier Peoli models

A great deal has been learned of the action of the air upon aeroplanes by photographing the air currents. The smallest eddies of the air have been made visible by taking instantaneous photographs of thin smoke as it passes obstacles of various size and form. It has been found that a square object causes a great deal of disturbance when in an air current. The air is compressed in front of it and eddies for some distance in its wake before finally coming to rest. An elliptical object causes less disturbance, but the air continues to splash in its wake for some distance. Even a perfectly spherical object offers a surprising amount of resistance to the air. In the case of a long narrow ellipse the disturbance is considerably reduced. A curved surface however, such as is used for the planes of an aeroplane, cuts the air with practically no resistance, and the air flows smoothly about and joins behind it with very little wake or splashing. A form which suits the air in this way or a "stream line" body as it is called is obviously just the right design for the wings of an aeroplane.

Even the experts in aeronautics differ so widely, however, that it is impossible to lay down any definite rules. One of the greatest authorities on the science of aviation, Mr. Horatio Philips, of England, believes that aeroplanes gain more support from the entering edges of their planes than from the rest of their surface. He argues that it is this edge, meeting the air currents, which serves to hold the aeroplane suspended.

This theory is founded on the experiments made with a machine of original design flown by Mr. Philips as far back as 1890. It consisted of a series of planes mounted one above the other at regular intervals, much the same as the strips of a Venetian blind. The lifting power of this model in proportion to its surface and the power exerted was enormous. This theory is borne out in part by the success of the model aeroplanes with very narrow wings which have been flown with great success during the year. The beautiful model built by Stewart Easter, for instance, which is illustrated on another page, depends for its support on planes which are no wider than ordinary window blinds.

There is an immense difference of opinion again as regards skin friction. Some writers believe the air has a tendency to stick to certain materials more than to others, and that this difference is so great as to materially retard some machines in their passage through the air. A complicate series of tables has been worked out in great detail to show the exact amount of this friction on various bodies. Some aviators go to great pains to make every part of their aeroplanes as slippery as possible. This is done by polishing the surfaces exposed to the air and in some cases enclosing the foreward part of the aeroplane, like a ship's prow, to diminish friction.

On the other hand we find some of the greatest authorities on aviation disregarding this question almost entirely. In the Wright machines, for instance, the surface of the wings is usually left comparatively rough, and the sticks and wires are placed without any attention to diminishing friction. This is true as well of the Delegrange, Voison and Farman machines. Still other aviators design every detail of their machines to cut down this so-called skin friction. The uprights, for instance, are made elliptical in shape, with the sharp edges turned forward so that they will cut their way the more smoothly through the air.

Several interesting attempts have been made to design a prow for an aeroplane which will cut the air with the least possible amount of friction. It is noticeable in these designs that the prows are very blunt, like the prow of a canal boat, and not as might be expected, sharp and narrow like that of a racing craft. The blunt-nosed prow is considered best for the air ship, because the air being one eight hundredth as dense as water offers very little resistance to its entrance. It is so much easier, in other words, to push an air ship through the air than a boat through water that there is no object in sharpening its nose.

The air, however, in flowing along the sides of a rapidly moving body sticks to it, and retards its progress relatively more than water retards a boat. It is important, therefore, that the body of the aeroplane be made very short, so that the sides will offer as little friction as possible. This reverses the proportions of a water-borne ship. We are so accustomed to see fast boats with long, narrow hulls, that it comes as a surprise to find that a fast air ship must have a very broad beam and as short a hull as possible.

It is probable that, as air ships develop, this general characteristic will become more marked. As aeroplanes become larger and faster they will therefore depart further and further from the conventional ideas concerning water-borne craft. It is impossible to prophesy at present what form the great passenger-carrying air ships of the future will take, but it is certain that their hulls or the closed-in portion carrying the machinery and passengers will be very short, snubbed-nose affairs. The world will be obliged to change its mind as to what constitutes a speedy-looking craft.

Man has learned to fly. More than 1,000 aeroplanes have been built which have successfully risen above the earth. It is estimated that these aeroplanes have flown in all more than 250,000 miles, or a distance equal to ten times the circumference of the earth. But no machine has yet been made which will fly alone, without the skilful manipulation of planes and rudders. The model aeroplane which soars gracefully aloft, suiting itself to the varying air conditions, perhaps comes nearest to the automatic flying machine.

A great advance will be made in aviation with the appearance of some practical contrivance for securing automatic stability. The aeroplane as it stands to-day shows a wonderful advance in the improvement of its general lines, and the mechanical perfection of its parts, but the question of stability remains practically the same as it was when the Wright Brothers made their first flights. The machine has been brought under a remarkable control, but only as it is directed by the practised hand of the sky pilot. Let him take his hand even for a moment from the levers, which control the planes and rudders, and there is danger of a bad spill, perhaps a fatal accident. Practically nothing has been accomplished in building machines which will fly unaided.

It has been pointed out, elsewhere in this volume, that the experiments with the model aeroplanes are certain to have an important influence upon the development of aeronautics as a whole, because they address themselves particularly to this problem of stability.

It is believed by many aviators that the problem of automatic stability will be solved by some form of the gyroscope. A great many experiments are being made with various forms of the gyroscope, although no machine has as yet been actually fitted and flown with such a device. The general principle of the gyroscope is very simple. It consists of a wheel which is made to revolve at very high speed. When such a wheel turns fast enough it will remain in a fixed position. Every one is familiar for instance with the gyroscope top. You wind it up and place it at any angle, and it will support itself and retain the position until it has run down.

The gyroscope has been applied for instance to railroad trains and has worked, experimentally at least, with remarkable success. By installing a gyroscope on a car running on a single track the car may be kept upright by the stabilizing force exerted by the revolving wheel. The same principle has been applied to steamships to prevent their rolling in heavy seas. The tipping of a car running on a single track, or of a ship at sea, is of course very much like the rolling of an aeroplane in flight; and it would seem that such a stabilizing device might solve the problem. The gyroscope has a serious disadvantage, however. Since it revolves at enormous speed any breakage would tend to throw the detached part to one side with dangerous force. In the case of the gyroscope used to steady ships at sea this force would be sufficient to send a piece of metal through the hull. On so delicate a craft as an aeroplane such an accident might readily prove disastrous.

It has been found again in experiments with gyroscopes on steamers that the frames have been seriously strained, even when the gyroscope worked smoothly. When a ship rolls and pitches to the motion of the waves, it is of course under a great strain but this balances itself. When it is held in a rigid horizontal position the pitching and rolling exert an exaggerated strain on the hull. In one instance a vessel actually broke its back under such a strain and was lost. An air ship, being at best a very frail structure, could scarcely be expected to stand the strain which has wrecked a steel ship. At the recent Paris Aeronautical show several extremely ingenious combinations of the pendulum and gyroscope principles were illustrated. It is of course possible that some modified form of the gyroscope may solve the problem.

A remarkable series of tests has been made this year in France with an automatic stability device based upon an entirely new principle. The rudders are operated automatically, in this case by a vertical fin opposed to the wind. As the pressure of the air varies, the rudder is forced up or down, thus bringing the aeroplane to an even keel without the assistance of the aviator. It is reported that model aeroplanes equipped with this device have been flown in three hundred experiments without a single accident.

The day is approaching when the air conditions will be observed and announced for the benefit of aviators exactly as the weather is foretold to-day. An aviator who is about to start on a cross-country flight will thus be able to study the conditions of the air lanes and lay out his route just as an automobilist looks up good-roads maps. In crossing a particular piece of country he will therefore know whether to take a low or a high air lane, that is, one of few hundred feet above the earth or the one several thousand feet aloft. The pressure of the air on the regular air lines will be announced as well. In this way aviation will be made much safer than it is to-day, when the aviator must venture without any knowledge of conditions aloft except those he may gain from the ground surface weather maps. During the year 1910, fully a score of lives might have been saved had aviators had such information.

The first of these observation stations is actually in operation in Germany to-day. Each of these stations is supplied with a number of rubber balloons equipped with automatic apparatus for recording atmospheric conditions in the upper air lanes. The stations also contain the proper apparatus for measuring the ascensional force of the balloons, with the gas generators used for inflating the balloons. When the balloons are sent up for great heights their altitude will be measured by means of the theodolite.

The soundings of the air are taken twice daily at eight o'clock in the morning and two in the afternoon whenever the weather permits. In the summer the morning observations are made much earlier. The movements of the balloons are then carefully observed at various altitudes until they are lost. These observations are then telegraphed to the central station at Lindenberg and sent out much the same as the regular weather forecast.