Imagine stepping into the air and flying under your own power, the way a bird does. This is only an imagined possibility, because people cannot fly that way, and anyone who is curious about it should never jump from a high place to try it. The wish itself is very old. 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. An ornithopter is a bird-like machine that flaps its wings to make lift and thrust, and designs of that kind have been tried ever since.

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. In the 1890s Otto Lilienthal, a German engineer who made more than 2,000 glider flights in that decade. He wrote a book, Bird Flight as the Basis of Aviation, that studied bird wings and applied the data to human flight, and he studied the wings of birds to design his gliders. Lilienthal also carefully measured how much the air pushed on bird-like wing shapes, and from those measurements he decided that a curved wing, not a flat one, was needed for flight.

Lilienthal's gliders were steered by shifting his body weight, in the same way that hang glider pilots steer today. A hang glider is a light aircraft with cloth wings and no engine, and the pilot starts it by running off a hill. In 1893 Lilienthal also built, as another experiment, a model with flapping wings like a bird's, but neither it nor its successor worked well. His glider flights proved far more influential than that model, since the Wright brothers got interested in flying after hearing about them in the 1890s. Beginners are understandably not sent aloft at once: hang gliders are taught at licensed schools, where training begins on the ground and on small hills before higher flights. As of November 2026, at Sleeping Bear Dunes National Lakeshore in the United States, hang glider and paragliding pilots must hold a current proficiency rating before they may launch.

To judge any flying machine, it helps to understand exactly what lift is. Lift is the force that holds an airplane up in the air, and it is made by air moving past the wings. For a wing to generate lift, the solid body must be in contact with a fluid, a liquid or a gas: no fluid, no lift; lift also depends on motion, because it 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. 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. NASA Glenn warns that many explanations of how wings make lift are misleading and incorrect, and that ignoring the upper surface's part in turning the flow gives an incorrect theory of lift.

Lift is only one of the four forces that 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, while also making thrust for forward motion and beating drag, the resistance of the air. For an airplane to take off, thrust must be greater than drag and lift greater than weight; in level flight, lift equals weight and thrust equals drag. The Wright brothers' Flyer, made the first powered, controlled flight of a heavier-than-air flying machine, and their first airplane flight, in 1903, lasted only 12 seconds, and on that flight the airplane flew 120 feet (37 meters) through the air; the Wright brothers made four flights on that day in 1903, and the longest went 852 feet in 59 seconds.

Weight and muscle are the places where the idea of flapping human wings runs into its greatest difficulty. A bird needs a low body weight to fly, because as body weight goes up, the muscle output needed for flying goes up too, and a bird's pectoral muscles, which pull the wings down, make up about 15 to 25 percent of its body weight. In birds the main flight muscles make up a bigger share of body mass than in most mammals, and they are anchored to a deep breastbone called the keel. A human's arm and chest muscles are not strong enough to flap wings big enough to lift a person, and flapping with human legs could work in theory only if the wings spanned at least 80 feet and weighed much less than the person. The Smithsonian summarizes the situation by saying that the basic barrier to a human ornithopter is the limited muscle power and endurance of humans compared to birds.

Size itself sets a further limit, and the square-cube law describes it. If an animal doubles in size, its surface area rises by a factor of four but its volume, and so its weight, rises eightfold. With weight up eightfold but the surface that supports it up only fourfold, the pressure on the body doubles, so bones must become thicker and sturdier as animals get larger. Muscle strength depends on cross-sectional area, so muscle tissue must likewise grow thicker in proportion when body size doubles. Bird bones are lighter in several ways: some are hollow with struts for strength, and the jaw has a light keratin beak instead of heavy jawbones and teeth. Flapping needs more and more energy as a bird gets bigger, and heavier birds, from about the size of a small hawk, rely on soaring and gliding instead of continual flapping.

The largest fliers show these limits at work. Argentavis, a giant bird that lived in Argentina about 6 million years ago, had a wingspan of about 6 to 7 meters (20 to 23 feet), and it weighed six times as much as an Andean condor and had a wingspan over twice as long. Pterosaurs were the first vertebrates to fly under their own power, and only birds and bats are the other flying vertebrates. Quetzalcoatlus was a giant pterosaur with wings as wide as a small airplane's, and the Quetzalcoatlus northropi was a pterosaur with an 11-meter wingspan, and researchers found it would have had to jump up to 2.5 meters into the air and flap hard to take off. Wandering albatrosses, by contrast, spend up to 90 percent of their lives soaring above the ocean, flying while hardly flapping by using dynamic soaring, which exploits wind shear near the ocean surface to gain energy.

In dynamic soaring, albatrosses take advantage of the different speeds and directions the wind has at different heights above the ocean. An albatross starts low, climbs slowly, then turns with the faster wind at high altitude to glide down quickly, building the momentum it needs. A model predicts albatrosses need wind faster than 3 meters per second for dynamic soaring, yet tracked birds flew in winds as low as 2 m/s, probably using updrafts over waves. As of December 2022, GPS tracks show wandering albatross airspeed rises with wind speed to a maximum of about 20 meters per second (45 mph). Researchers think albatrosses limit their speed to keep the force on their wings below what the wings can take. Birds with long, thin wings, such as albatrosses, can glide over the ocean for long periods, and an albatross can stay in the air for hours while hardly flapping its wings.

People did eventually fly on muscle power, as the next two machines show. In 1979 a pilot pedaled the Gossamer Albatross, a human-powered airplane, across the English Channel, about 36 kilometers (22 miles) in 2 hours 49 minutes, and the 1979 Collier Trophy honored it as the first man-powered flight across the English Channel. In 1988 the human-powered airplane Daedalus flew about 115 kilometers (72 miles) across the sea from Crete to Santorini in Greece. Its flight set the all-time records for duration (3 hours 54 minutes) and distance of a human-powered flight, as of November 2026. Daedalus was built largely of carbon-fiber composite and Mylar, and its chain drive spun a large, slow-moving propeller; its wing spans 112 feet and the wing alone weighs just 52 pounds. Because a fixed wing can be built both long enough and light enough, about 100 human-powered airplanes with fixed wings had flown by 2010.

Flapping flight took considerably longer to achieve. In August 2010 the Snowbird became the first human-powered flapping-wing aircraft reported to fly continuously. Built by a University of Toronto team, it had flapping wings 32 meters (105 feet) across, and it reportedly flew for 19.3 seconds and covered about 145 meters on its pilot's muscle power alone, after being towed up. The pilot, Todd Reichert, was towed up and then flew on his own power, and he estimated he could produce about 0.3 horsepower, roughly 220 watts. Although the Snowbird's wingspan is comparable to that of a Boeing 737, it weighs less than all the pillows on board a 737. The Daedalus project's testing helped bring about new technology for high-altitude, long-endurance aircraft, according to NASA. Engines change the picture entirely. As of November 2026, the record for the fastest speed in a body-controlled jet-engine-powered suit is 85.06 mph (136.891 km/h), set by Richard Browning of Gravity Industries on 14 November 2019.

Words to know

• ornithopter: flapping machine
• thrust: forward push
• wingspan: tip-to-tip width
• pectoral: chest
• composite: mixed material