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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III SOARING FLIGHT Soaring flight may be defined as being that kind of flight practised by birds in which they keep aloft, without loss of headway, while using none of their own energy beyond that necessary to actuate their rudders, elevators, and warping, or bird equivalents.1 In fig. 5 a simple relative gravity diagram is again drawn, but for a weak gust; and since the virtual level plane has superseded the true level during the gust, textit{the bird, if frictionless, may glide in any direction in the virtual level, including direction AE, without loss of headway and without effort, because it is then running neither uphill nor downhill with respect to the relative gravity AD. Now AE has been arranged tilted at an angle of 1 in 5 to AF, by making BD exactly one-fifth of gravity AB, and if the bird is one having a gliding angle of 1 in 5, instead of being frictionless, it obviously cannot glide nearer to AE than the truly level path AF. Thus, a gust of strength textit{gl$ will keep a " 1 in 5 " bird afloat in level flight at constant headway. The rule is quite general, so that, calling the horizontal gustacceleration textit{a ft. p.s. p.s., and assuming the bird to textit{be acting as a 1 in textit{n bird, it will be maintained in at least level soaring, when : — 1 This defines textit{complete soaring flight, but it will be seen, after the matter has been discussed further, that flapping and soaring may proceed simultaneously as independent partial causes of the whole ascent or support of the bird. Now the air never accelerates for many seconds in one direction, because of the limits to wind velocity, but at every point it is always accelerating more or less in some direction, in a manner that is represented by the vector BD continually altering its magnitu...