Direct Lift in Theory: The Need for an Acceptable Basis for Design

THE helicopter was the first method of mechanical flight by which success was achieved—although only in model form. Centuries before a power‐driven model aeroplane flew, toy helicopters with some form of string or rubber “propulsion” were common objects of the nursery. As a scientific model, it dates back at least to the days of Leonardo da Vinci, among whose papers is a sketch of a practical design of a helicopter with which he experimented. This is not altogether surprising because, except for the ornithopter based on the close analogy of bird flight, it provides the most obvious method of getting off the ground, by rising direct from it.