Leonardo's interest in flight appeared during the years he spent in Florence when he was young, but it was after he moved to Milan, in about 1482, that the subject began to assume special importance for him. His observation of birds, albeit non-specialised, persuaded him that there was nothing mysterious about flight - in contrast with the ideas of scientists of the time - but was a purely mechanical phenomenon, due to the beating of the wing on the air. The fact that air is compressible and therefore exerts a resistance capable of supporting a weight was one of Leonardo's fundamental insights, leading him to conclude that men, too, could fly: "...that for these reasons you will be able to know the man with his large constructed wings, creating a force against air resistance and succeeding, will subdue it and rise above it."
One of the first applications of this observation was the famous parachute project of about 1485, consisting of a rigid pyramid-like structure, with a base of 7.2 metres and a height of 7.2 metres, covered with starched linen to make it impermeable and airtight. More or less contemporaneously, there was another application of the principle of air resistance, the so-called "air screw", which can be seen as a prototype of the helicopter. At the basis of da Vinci's machine was the assumption that, in certain determinate conditions, air can behave as a solid body and that an object that spins within itself must lift itself up, like a screw turning and penetrating wood. The machine consists of a worm screw, about 10 cm in diameter, with a cane structure covered in starched linen and reinforced with a metal border. The parachute and "air screw" represent an isolated case in Leonardo's study of flight since most of the flying machines designed by Leonardo are equipped with wings, usually beating. It was on the wing - its form, structure and creation - that he concentrated the wide and detailed research that he carried out over the years, although in a way that is not easy to follow.
After going in various directions, he seems to have turned to a type of wing a sportelli, that is, in the form of a bat's wing, divided into three parts, for which he used different materials according to a criterion of progressive structural lightness. The a sportelli solution, however, did not satisfy him and he eventually abandoned it in favour of a continuous covering which he evidently regarded as more functional. The bat type of wing - a sportelli or with continuous covering - was, however, neither a definitive nor final choice. One of the last machines he designed used a completely different type of wing with a frame, vaguely similar to a hand, and a cover. Designing machines was not Leonardo's exclusive interest in flight but extended to include collateral problems such as support instrumentation. This included instruments for air navigation, such as an inclinometer for checking the trim of the flying machine and, in particular, a series of meteorological instruments - hygroscopes, anemoscopes and anemometers - which testify to the fact that Leonardo did not neglect the importance of atmospheric conditions in successful flight.