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.