Before it was possible to produce a power-driven aëroplane, experiments over a long course of years were made with aëroplanes not provided with propelling apparatus.
In its earliest form the aëroplane consisted of a flat surface moved through the air in a position slightly inclined from the horizontal; in its forward movement the plane experiences resistance from the air. As this resistance is directed partly on the under side, it will be partly converted into a lifting force. Of these two forces—head resistance or drift, and the lifting or sustaining force or lift—the first, being unproductive, must be reduced as far as possible; the second, lift, must, on the other hand, be raised to the highest possible degree.
This end is achieved by employing, instead of flat surfaces or planes, surfaces curved in the direction of flight.
The Two Types.—A monoplane is a machine with a single spread of surface supporting it. The best known example of a monoplane is the Bleriot. Biplanes have two supporting surfaces, the one above the other; the Wright and Farman machines are machines of this type. There are other machines which have been invented which have more supporting surfaces than this, the most successful of them all being the Roe triplane. But at present, at any rate, the advantage lies between the monoplane and the biplane, the other machines not yet having reached a sufficiently high standard to be able to compete with them.
The monoplane and the biplane have both their own special uses. The monoplane is obviously the lighter machine, and its head resistance is much less, hence it follows logically that its speed will be greater than that of a biplane. But the biplane is a much more stable machine than the monoplane; it will therefore probably be safer and will certainly be able to carry a greater weight.
In the making of aëroplanes wood is usually used for the framework. Specially selected wood is taken, usually from the spruce, hickory, ash, or birch, since wood combines in itself the strength and tenacity of metal without its weight. The fabric with which this frame is covered is more difficult to obtain, since it must contain in itself all the qualities of strength, lightness, smoothness, etc., without any tendencies to shrink, or rot, or burn.
The biplane carries a load of from two and one-half to three and one-half pounds per square foot of surface; the monoplane from three and one-half to six pounds per square foot. The load per horse power in each case is from thirty to forty pounds. In speed the biplane ranges from thirty-five to forty-five miles per hour, as against forty to sixty miles per hour attained by the monoplane.
Practical Uses.—From the very first days [903] the value of the aëroplanes, from a military point of view, has been realized, not as a weapon of offense so much as of intelligence. It would, in fact, be difficult to imagine a better means of scouting and reconnoitering than is afforded by the flying-machine. Its gradually increasing radius of action renders it available for strategical no less than for tactical reconnaissance; its easy mode of progress and absence of vibration allow the most accurate observations to be made and sketch-maps to be drawn. For dispatch-carrying over difficult country its usefulness is also considerable. Its employment for purposes of offense is much more hazardous. On the other hand, it is practically immune from artillery or rifle fire from the land, especially when flying at a fair altitude.
As a commercial vehicle, and for transport, the aëroplane, owing to its relatively low carrying power, is restricted in its usefulness. With increasing reliability, however, it may well assume a portion of the functions of the motor car.