My observation of the flight of buzzards leads me to believe that they regain their lateral balance when partly overturned by a gust of wind, by a to… - Wilbur Wright

Herr Otto Lilienthal seems to have been the first man who really comprehended that balancing was the first instead of the last of the great problems in connection with human flight. He began where others left off, and thus saved the many thousands of dollars that it had theretofore been customary to spend in building and fitting expensive engines to machines which were uncontrollable when tried. He built a pair of wings of a size suitable to sustain his own weight, and made use of gravity as his motor. This motor not only cost him nothing to begin with, but it required no expensive fuel while in operation, and never had to be sent to the shop for repairs. It had one serious drawback, however, in that it always insisted on fixing the conditions under which it would work. These were, that the man should first betake himself and machine to the top of a hill and fly with a downward as well as a forward motion. Unless these conditions were complied with, gravity served no better than a balky horse — it would not work at all...
We figured that Lilienthal in five years of time had spent only about five hours in actual gliding through the air. The wonder was not that he had done so little, but that he had accomplished so much. It would not be considered at all safe for a bicycle rider to attempt to ride through a crowded city street after only five hours’ practice, spread out in bits of ten seconds each over a period of five years; yet Lilienthal with this brief practice was remarkably successful in meeting the fluctuations and eddies of wind gusts. We thought that if some method could be found by which it would be possible to practice by the hour instead of by the second there would be hope of advancing the solution of a very difficult problem.

My brother and I became seriously interested in the problem of human flight in 1899 ... We knew that men had by common consent adopted human flight as the standard of impossibility. When a man said, “It can’t be done; a man might as well try to fly,” he was understood as expressing the final limit of impossibility. Our own growing belief that man might nevertheless learn to fly was based on the idea that while thousands of the most dissimilar body structures, such as insects, fish, reptiles, birds and mammals, were flying every day at pleasure, it was reasonable to suppose that man might also fly... We accordingly decided to write to the Smithsonian Institution and inquire for the best books relating to the subject.... Contrary to our previous impression, we found that men of the very highest standing in the profession of science and invention had attempted to solve the problem... But one by one, they had been compelled to confess themselves beaten, and had discontinued their efforts. In studying their failures we found many points of interest to us. At that time there was no flying art in the proper sense of the word, but only a flying problem. Thousands of men had thought about flying machines and a few had even built machines which they called flying machines, but these were guilty of almost everything except flying. Thousands of pages had been written on the so-called science of flying, but for the most part the ideas set forth, like the designs for machines, were mere speculations and probably ninety per cent was false. Consequently those who tried to study the science of aerodynamics knew not what to believe and what not to believe. Things which seemed reasonable were often found to be untrue, and things which seemed unreasonable were sometimes true. Under this condition of affairs students were accustomed to pay little attention to things that they had not personally tested.

Hawks are better soarers than buzzards but more often resort to flapping because they wish greater speed. A damp day is unfavorable for soaring unless there is a high wind. No bird soars in a calm. The object of the tail is to increase the spread of surface in the rear when the wings are moved forward in light winds and thus preserve the centre of pressure at about the same spot. It seems to be used as a rudder very little. In high winds it is folded up very narrow.