Aeronautics is divided into two fundamentally different branches--aviatics and aerostatics. The first comprises all types of aeroplanes and heavier than air flying machines such as the helicopters, kites, etc.; the second includes dirigible balloons, passive balloons and all craft which rise in the air by utilizing the lifting force of gases. Aeroplanes are the only practical form of heavier-than-air machines, as the helicopters (machines intended to be lifted directly into the air by propellers, without the sustaining effect of planes), and ornithopters, or flapping wing types, have not been thoroughly developed, and in fact, there are so many serious mechanical problems to be solved before either of these types of air craft will function properly that experts express grave doubts regarding the practicability of either. Aeroplanes are divided into two main types--monoplanes or single surface forms, and bi-planes or machines having two sets of lifting surfaces, one suspended over the other. A third type, the triplane, is not very widely used.
Dirigible balloons are divided into three classes: the rigid, the semi-rigid, and the non-rigid. The rigid has a frame or skeleton of either wood or metal inside of the bag, to stiffen it; the semi-rigid is reinforced by a wire net and metal attachments; while the non-rigid is just a bag filled with gas. The aeroplane, more than the dirigible and balloon, stands as the emblem of the conquest of the air. Two reasons for this are that power flight is a real conquest of the air, a real victory over the battling elements; secondly, because the aeroplane, or any flying machine that may follow, brings air travel within the reach of everybody. In practical development, the dirigible may be the steamship of the air, which will render invaluable services of a certain kind, and the aeroplane will be the automobile of the air, to be used by the multitude, perhaps for as many purposes as the automobile is now being used.
One of the marked features of aircraft development has been the effect it has had upon the refinement and perfection of the internal combustion motor. Without question gasoline-motors intended for aircraft are the nearest to perfection of any other type yet evolved. Because of the peculiar demands imposed upon the aeronautical motor it must possess all the features of reliability, economy and efficiency now present with automobile or marine engines and then must have distinctive points of its own. Owing to the unstable nature of the medium through which it is operated and the fact that heavier-than-air machines can maintain flight only as long as the power plant is functioning properly, an airship motor must be more reliable than any used on either land or water. While a few pounds of metal more or less makes practically no difference in a marine motor and has very little effect upon the speed or hill-climbing ability of an automobile, an airship motor must be as light as it is possible to make it because every pound counts, whether the motor is to be fitted into an aeroplane or in a dirigible balloon.