Imagine stepping off the ground and flying with wings of your own, the way a bird does without a second thought. This is only an imagined possibility, because people cannot fly that way, and anyone considering the idea should never jump from a high place to try it. To work out what such flying would demand, it is useful to begin with the forces involved, because every flying animal and machine must cope with the same set of them. Four forces act on anything that flies: lift, drag, weight and thrust, and birds and airplanes use the same four. For flight, a person must overcome weight, which is gravity, and create lift to do it. In addition, a person must make thrust for forward motion and beat drag, the resistance of the air.
Of the four forces, lift is the one that works directly against gravity, so it deserves a careful definition. Lift is the force that holds an airplane up in the air, and it is made by air moving past the wings; it also depends entirely on motion, since lift is made by the difference in speed between a wing and the air, so a wing that is not moving through air makes no lift. Without air there is no lift, which is exactly why wings do not work in space. An airplane's lift can be explained by the wing pushing air downward, a push-back described by Newton's third law of action and reaction.
A quadcopter drone stays up in a comparable way, because its four propellers push air down and the air pushes the drone up. A typical airplane wing has a more curved upper surface. Air speeds up as it flows over the curve, its pressure drops, and the higher pressure below pushes the wing up. This is Bernoulli's principle at work, and a bird's wing is shaped like an airplane wing. The shape of a wing was investigated long ago by Otto Lilienthal, who measured how much the air pushed on bird-like wing shapes and decided that a curved wing, not a flat one, was needed for flight. He studied the wings of birds to design his gliders, and he wrote a book, Bird Flight as the Basis of Aviation, that applied the data to human flight.
Lift is only one side of the problem, because weight and muscle power must be balanced against it, and a flier has to manage both together. A bird needs a low body weight to fly, because as body weight goes up, the muscle output needed for flying goes up too. A bird's pectoral muscles, which pull the wings down, make up about 15 to 25 percent of its body weight. In contrast, a human's arm and chest muscles are not strong enough to flap wings big enough to lift a person. The Smithsonian says the basic barrier to a human ornithopter is the limited muscle power and endurance of humans compared to birds.
Inventors have nevertheless tried many flapping designs over the centuries. Leonardo da Vinci sketched flying machines with flapping wings more than 500 years ago, in the late 1400s, and he drew flapping-wing machines with the pilot lying face down, standing, working the wings with arms, or with legs. Most of his bird-like flying-machine designs came before his serious study of bird flight, which began with the Codex on the Flight of Birds in 1505. Reuben Spaulding's 1889 U.S. patent was for a man-powered ornithopter: wings and a tail attached to a man, designed to make the wearer look and fly like a bird.
Human power did eventually succeed in sustained flight. In 1979 a pilot pedaled the Gossamer Albatross, a human-powered airplane, across the English Channel. It needed less than half a horsepower (373 watts) and flew at only about 10 to 15 miles per hour. The 1979 Collier Trophy honored the concept, design and construction of the Gossamer Albatross, the first man-powered flight across the English Channel, with special recognition for pilot Bryan Allen. In 1988 the human-powered airplane Daedalus flew about 115 kilometers (72 miles) across the sea from Crete to Santorini in Greece. As of November 2026, that flight holds the all-time records for duration (3 hours 54 minutes) and distance of a human-powered flight. Daedalus weighed less than 70 pounds.
Words to know
- lift: upward force
- drag: air resistance
- thrust: forward push
- ornithopter: flapping machine
- Bernoulli's: physics principle
