Who? George Cayley
What? Many consider him the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight. Designer of the first successful glider to carry a human being aloft, he discovered and identified the four aerodynamic forces of flight-weight,lift,thrust and drag are in effect on any flight vehicle.
When? in 1799 he set forth the concept of the modern aeroplane as a fixed-wing flying machine with separate systems for lift, propulsion, and control
How? He continued his research using models and by 1807 had come to understand that a curved lifting surface would generate more lift than a flat surface of equal area.
Why? Because he had published his now-classic three-part treatise "On Aerial Navigation" which stated that lift, propulsion and control were the three requisite elelments to successful flight, apparently the first person to so realize and so state.