Earlier, Cayley studied the center of gravity of flying  machines, as well as the effects of wing dihedral. Furthermore,  he pioneered directional control of aircraft by including the  earliest form of a rudder on his gliders.  [Figure 1-1] In the late 1800s, Otto Lilienthal built upon Cayley’s  discoveries. He manufactured and flew his own gliders  on over 2,000 flights. His willow and cloth aircraft had  wings designed from extensive study of the wings of birds.  Lilienthal also made standard use of vertical and horizontal  fins behind the wings and pilot station. Above all, Lilienthal  proved that man could fly.  [Figure 1-2] Octave Chanute, a retired railroad and bridge engineer,  was active in aviation during the 1890s.  [Figure 1-3]  His  interest was so great that, among other things, he published  a definitive work called “Progress in Flying Machines.” This  was the culmination of his effort to gather and study all the  information available on aviation. With the assistance of  others, he built gliders similar to Lilienthal’s and then his  own. In addition to his publication, Chanute advanced aircraft  structure development by building a glider with stacked wings  incorporating the use of wires as wing supports.  The work of all of these men was known to the Wright  Brothers when they built their successful, powered airplane  in 1903. The first of its kind to carry a man aloft, the Wright  Flyer had thin, cloth-covered wings attached to what was  primarily truss structures made of wood. The wings contained  forward and rear spars and were supported with both struts  and wires. Stacked wings (two sets) were also part of the  Wright Flyer.  [Figure 1-4]  Figure 1-2.  Master of gliding and wing study, Otto Lilienthal (top)  and one of his more than 2,000 glider flights (bottom) Figure 1-1.  George Cayley, the father of aeronautics (top) and a  flying replica of his 1853 glider (bottom). 1-2