Humanity's first attempts at flight were made with flapping wings strapped to the arms in imitation of birds, but these had no success. Machines designed to fly in this way, called ornithopters, date to antiquity (c.400 B.C.) and models that are capable of flight have been known for more than 100 years. However, there are no practical aircraft based on ornithopter designs, even though an ornithopter—which has no theoretical top speed limit—should be capable at least of efficient low-speed flight. In the 1930s an Italian model weighing approximately 50 lb (110 kg) and powered by a 0.5-hp motor was successfully flown.
Airships and balloons owe their ability to ascend and remain aloft to their inflation with a gas lighter than air; this is an application of Archimedes' principle of flotation, i.e., that a body immersed in a fluid (liquid or gas) is buoyed up by a force equal to the weight of the fluid that it displaces. Aircraft, which are heavier than air, are able to remain aloft because of forces developed by the movement of the craft through the air. Propulsion of most aircraft derives from the rearward acceleration of the air. It is an application of Newton's third law, i.e., that for every action there is an equal and opposite reaction. In propeller aircraft the forward motion is obtained through conversion of engine power to thrust by means of acceleration of air to the rear by the propeller. Lift is obtained largely from the upward pressure of the air against the airfoils (e.g., wings, tail fins, and ailerons), on whose upper surface the pressure becomes lower than that of the atmosphere. In jet-propelled aircraft, propulsion is achieved by heating air that passes through the engine and accelerating the resultant hot exhaust gases rearward at high velocities. Rockets are propelled by the rapid expulsion of gas through vents at the rear of the craft. The high speeds that are produced by jet and rocket engines have brought about substantial changes in the science of flight.