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. 

Wilbur’s notebook, 1900

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.

Wilbur’s deposition, 1909

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 [of flight]. . . . 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.

Wilbur to Octave Chanute, May 13, 1900

For some years I have been afflicted with the belief that flight is possible to man.  My disease has increased in severity and I feel that it will soon cost me an increased amount of money if not my life.  I have been trying to arrange my affairs in such a way that I can devote my entire time for a few months to experiment in this field.

My general ideas of the subject are similar [to] those held by most practical experimenters, to wit:  that what is chiefly needed is skill rather than machinery.  The flight of the buzzard and similar sailors is a convincing demonstration of the value of skill and the partial needlessness of motors.

It is possible to fly without motors, but not without knowledge and skill.  This I conceive to be fortunate, for man, by reason of his greater intellect, can more reasonably hope to equal birds in knowledge than to equal nature in the perfection of her machinery. . . .

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 torsion of the tips of the wings.  If the rear edge of the right wing tip is twisted upward and the left downward the bird becomes an animated windmill and instantly begins to turn, a line from its head to its tail being the axis.  It thus regains its level even if thrown on its beam’s end, so to speak, as I have frequently seen them.  I think the bird also in general retains its lateral equilibrium, partly by presenting its two wings at different angles to the wind, and partly by drawing in one wing, thus reducing its area.  I incline to the belief that the first is the more important and usual method. . . .

My business [the bicycle shop] requires that my experimental work be confined to the months between September and January and I would be particularly thankful for advice as to a suitable locality where I could depend on winds of about 15 miles per hour without rain or too inclement weather.  I am certain that such localities are rare.

Wilbur to Octave Chanute, June 1, 1900

[German experimenter Otto] Lilienthal’s enthusiastic efforts to arouse others may yet prove his most valuable contribution to the solution of the problem.  What one man can do himself directly is but little.  If however he can stir up ten others to take up the task he has accomplished much.

Wilbur in a speech published in  Annual Report of the Board of Regents of the Smithsonian Institution, 1902

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.