Sir George Cayley (1773 – 1857) was among the first true scientific aerial investigators to understand the underlying principles and forces of flight. Cayley described the fundamental challenges to human flight: the ratio of lift to wing area, determination of the center of wing pressure, the importance of streamlined shapes, the recognition that a tail assembly was essential to stability and control, the concept of a braced biplane structure for strength, and most importantly the need for a lightweight source of power to broaden the utility of the simple glider. He built his first aerial device in 1796 – a model helicopter with contra-rotating propellers. In 1810, Cayley published a three-part treatise, "On Aerial Navigation", in which he stated that lift, propulsion, and control are the three requisites of flight. In 1849, Cayley built a large gliding machine similar to his 1799 design and tested the device with a 10-year-old boy aboard. Cayley's design was the first successful human-carrying glider.
Sir George Cayley: 1773 - 1857 (Flying Machines)
Aviation and Aeromodelling - Interdependent Evolutions and Histories: Sir George Cayley Bt. (Monash University)