4 - The Infancy of Aerodynamics: To Lilienthal and Langley

Summary

The first great aim of the Society is the connecting of the velocity of the air with its pressure on plane surfaces to various inclinations. There seems to be no prospect of obtaining this relation otherwise than by a careful series of experiments. But little can be expected from the mathematical theory; it is a hundred and forty years since the general differential equations of fluid motion were given to the world by D' Alembert; but although many of the greatest mathematicians have attempted to adduce from them results of a practical value, it cannot be said that any great success has attended their efforts. The progress made has been very slight in the case of water, where the analysis is much simpler than for an elastic fluid like air; and the theory of resistance, which is part of hydromechanics, which has most direct bearing on aerial navigation is, perhaps, the part of the subject about which least is known.

Annual report, Aeronautical Society o/Great Britain (1876)

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.