In 1891, the sky over Germany hosted the first successful manned, heavier-than-air flying machine - the hang glider of Otto Lilienthal (Figure 4.1). If we compare it with Cayley's 1804 glider (Figure 3.1), we can detect little fundamental difference between the two flying machines. Both have fixed wings and horizontal and vertical tail structures. Both designers were knowledgeable and concerned about the static stability of their machines, recognizing that the center of gravity should be ahead of the aerodynamically generated aerodynamic center of the vehicle (notice the ballast weight hanging from the nose of Cayley's glider and the position of Lilienthal's body). Both machines were without any mechanical means of flight control, and both were unpowered gliders. However, there was one fundamental difference: The wing of Lilienthal's glider was a rigidly curved (cambered) airfoil shape, whereas the wing of Cayley's 1804 glider was simply a flat surface.