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.