Ornithology - Spring 1899
The History: "In the spring of the year 1899 our interest in the subject was again aroused through the reading of a book on ornithology. We could not understand that there was anything about a bird that would enable it to fly that could not be built on a larger scale used by man. At this time our thought pertained more particularly to gliding flight and soaring. If the bird's wings would sustain it in the air without the use of any muscular effort, we did not see why man could not be sustained by the same means." ... Orville Wright
Ornithology is the study of birds. Years later while describing how he and Wilbur became interested in flight, Orville Wright recalled the brothers' interests and thoughts on bird flight in 1899.
There are three important points to note in Orville Wright's statement.
What is gliding or soaring flight and how is it different from powered flight? In true powered flight (bird, bat, some insect or mechanical) there is a propulsion system - a system that propels the animal or mechanical aircraft in a particular direction. For a bird, bat or insect, flapping or other wing movements generate propulsion. On an aircraft, some form of engine is used for propulsion. Both gliding and soaring refer to flight without the use of a propulsion system.
In the late 1800's and early 1900's, a 100 years before mechanical powered flight had been achieved, different (and often incorrect) theories existed on how to achieve flight. Since no one had successfully build or flown a powered flying machine, scientists, engineers and other researchers studying aeronautics (the science of flight) studied examples from nature, especially bird flight. If they could understand how animal flight was achieved, perhaps they could use the same or similar principles to develop and construct an airplane which could carry humans.
In the 1907 edition of Major Hermann Moedebeck's Pocket Book of Aeronautics five types of bird flight are identified:
Rowing flight was flight created through flapping wings.
Gliding flight was rowing flight with periods of motionless non-flapping flight (the glide). During the glide segment, the flight carries on through the energy produced during the rowing portion of the flight. In gliding flight without the use of propulsion, the airborne object can not maintain its altitude and descends downward.
In soaring, the bird stays in one place over a point on the ground without flapping its wings. The bird is held in the air through upward flowing currents of air. These currents form over "wooded land and on rugged rocks". Although the bird's wings are stretched out, its only muscle activity is to make small balancing movements. Soaring flight would hold a bird at a constant height - the bird would hover without propulsion or loss of altitude.
Sea gulls following ships or advancing water waves could engage in sailing flight. Caused by the wind moving upwards after hitting the sails or crests (tops) of waves, sailing flight would hold "the bird at a constant height and at a constant distance away from the sail or the wave-crest, as the case may be". While both soaring and sailing flight would hold the bird up in the air, sailing flight would also move the bird forward.
So when Orville Wright comments that a bird's wings can sustain it in the air without muscular effort, what did he mean? He meant flapping and the muscles required to flap were not necessary to soar or glide, although they are required for powered flight. There is another observation we can make: in soaring and gliding flight the bird's wings do not collapse; the bird's wings and body are able to maintain an appropriate shape to assist in keeping the bird airborne.
Have you seen old pictures of people attempting to fly with wings they have constructed? They tried to flap these wings to fly, but they were not successful. Why? Even if these wings looked similar to a birds, human's do not have the physiological (a body's processes, muscular and skeletal systems) strength and structures to achieve flight. This is why these early "human-flapping" systems were unsuccessful.
The Wrights had read about Lilienthal's glider flights. Lilienthal's gliders, like modern airplanes, had fixed (non-moving) versus flapping wings. So although the Wrights believed bird mechanisms could be constructed on a larger scale for soaring and gliding flight, this did not mean they believed humans could fly by flapping their arms with artificial wings.