Leonardo da Vinci studied the ornithopter, a flying-machine type intended to imitate the flapping wings of birds or bats.
His notebooks contain many observations of bird flight, including wing movement, balance, air currents, and the relationship between weight and lift. Those observations informed designs in which a human operator would power or control flapping wings. Leonardo also examined bat anatomy, whose wing membrane offered a useful model for a large flexible flying surface.
An ornithopter is not simply any early aircraft. The defining feature is flapping wings that provide propulsion, whereas a glider has fixed wings and a helicopter uses powered rotating blades. Leonardo’s studies included both flapping-wing concepts and other aerodynamic ideas, so not every one of his flying-machine drawings is an ornithopter.
No Leonardo ornithopter is known to have flown. Human muscles cannot supply enough power for a practical man-carrying machine of the proposed scale. Nevertheless, his anatomical and aerodynamic observations made these studies an important part of Renaissance investigations into flight.