Leonardo da Vinci designed ornithopters to study human flight by imitating the flapping wings of birds.
An ornithopter is an aircraft intended to fly by flapping wings rather than by using fixed wings and a propeller. Leonardo's notebooks contain several studies of bird anatomy, wing movement, and machines in which a human operator would power or control wing-like structures.
His designs were based on careful observation of birds and bats, but human muscles cannot provide enough power for a practical large-scale flapping aircraft. Leonardo therefore did not achieve human-powered flight with these machines. His work nevertheless helped connect biological observation with mechanical design.
An ornithopter is not the same as Leonardo's aerial screw. The aerial screw sought lift from a rotating helical surface, while the ornithopter sought lift and thrust from repeated wing beats. Modern experimental ornithopters still use flapping wings, often driven by motors rather than human strength.