MUSCULAR ENERGY EXERTED BY SOARING BIRDS. —It is not conceivable that the mere will of the bird would impel it forwardly, without it exerted some muscular energy to keep up its speed. The distance at which the bird performs this wonderful evolution is at such heights from the observer that the eye cannot detect a movement.
WINGS NOT MOTIONLESS.—While the wings appear to be absolutely motionless, it is more reasonable to assume that a slight sinuous movement, or a rocking motion is constantly kept up, which wedges forwardly with sufficient speed to compel momentum to maintain it in flight. To do so requires but a small amount of energy. The head resistance of the bird formation is reduced to a minimum, and at such high speeds the angle of incidence of the wings is very small, requiring but little aid to maintain it in horizontal flight.
CHAPTER II
PRINCIPLES OF AEROPLANE FLIGHT
FROM the foregoing chapter, while it may be rightly inferred that power is the true secret of aeroplane flight, it is desirable to point out certain other things which must be considered.
SPEED AS ONE OF THE ELEMENTS—Every boy, probably, has at some time or other thrown small flat stones, called "skippers." He has noticed that if they are particularly thin, and large in diameter, that there is a peculiar sailing motion, and that they move through the air in an undulating or wave-like path.
Two things contribute to this motion; one is the size of the skipper, relative to its weight, and the other is its speed. If the speed is slow it will quickly wend its way to the earth in a gradual curve. This curved line is called its trajectory. If it is not very large diametrically, in proportion to its weight, it will also make a gradual curve in descending, without "skimming" up and down in its flight.
SHAPE AND SPEED.—It has been observed, also, that a round ball, or an object not flattened out, will make a regular curved path, whatever the speed may be.
It may be assumed, therefore, that the shape alone does not account for this sinuous motion; but that speed is the element which accounts for it. Such being the case it may be well to inquire into the peculiar action which causes a skipper to dart up and down, and why the path thus formed grows more and more accentuated as the speed increases.
As will be more fully described in a later chapter, the impact of air against a moving body does not increase in proportion to its speed, but in the ratio of the square of the speed.