How to Build a Bird
Leonardo da Vinci wanted to build a flying machine, and he had one enormous advantage over almost everyone who would attempt the problem after him: working examples were everywhere. They were birds.
If you wanted to know how to fly in the fifteenth century, what else would you study? There were no airplanes, no wind tunnels, and no aerodynamic textbooks. But birds could take off, climb, descend, turn, glide, and land, often with an ease that made the whole business look considerably simpler than humans would eventually discover it to be.
Leonardo studied them with characteristic intensity. He examined the anatomy of bird wings and filled pages with observations about feathers, joints, balance, wind, soaring, and flapping flight. His early flying-machine designs followed naturally from what he could see: articulated wings operated by human muscle through arrangements of pedals, levers, and pulleys. If a bird flew by moving its wings, perhaps a sufficiently clever machine could do the same.¹
Leonardo was hardly alone. For centuries, inventors returned to variations of the ornithopter, a machine intended to fly by flapping wings. The reasoning was difficult to fault. Nature had already demonstrated that heavier-than-air flight was possible, so the obvious first step was to copy the machine that was doing it.
The problem was that the obvious part of bird flight was not necessarily the important part. A bird’s wing is not simply a lifting surface that flaps; it is part of an extraordinarily adaptable biological system. A bird can alter the shape and orientation of its wings, change the position of individual feathers, redistribute aerodynamic loads, adjust its tail, and coordinate those changes almost continuously. The visible flapping was only one part of a much more complicated solution.
Leonardo’s own work began moving in this direction. His later studies increasingly considered not merely how birds moved their wings, but how they glided, balanced themselves, responded to the wind, and controlled their flight. His human-powered ornithopter schemes did not become airplanes, but his study of birds was increasingly directed toward understanding the principles of flight rather than simply reproducing its visible motions.²
Over the centuries that followed, aviation would make progress by doing something that at first seems backward: becoming less like a bird. Sir George Cayley supplied one of the decisive conceptual breaks around the beginning of the nineteenth century. Rather than asking a single pair of moving wings to do everything, as a bird’s wings appeared to do, Cayley separated flight into different engineering problems. A fixed wing could provide lift, a separate means of propulsion could move the machine forward, and other structures could provide stability and control.³
Once flight had been decomposed into functions, each function could be investigated separately. Engineers no longer needed to reproduce the astonishing integration of a bird before they could get something into the air.
Yet birds never quite disappeared from the story. Otto Lilienthal, whose experiments in the late nineteenth century profoundly influenced the Wright brothers, devoted years to studying them. His 1889 book was titled Birdflight as the Basis of Aviation, but the machines with which Lilienthal made nearly two thousand flights were not mechanical birds. They were fixed-wing gliders, reflecting an approach in which birds remained an important source of aerodynamic knowledge without determining the mechanical form the aircraft itself would take.⁴
That distinction became especially important when Wilbur and Orville Wright confronted what they considered one of the great unsolved problems of flight: control. They were not trying to make an airplane flap. By the end of the nineteenth century, the basic Cayley architecture—a lifting wing, separate propulsion, and some means of controlling the machine—was well established. Keeping an aircraft under command in three dimensions was another matter.
Wilbur’s observations returned to birds. In a May 1900 letter to Octave Chanute, he described watching buzzards restore their lateral balance by twisting their wingtips in opposite directions. By then, the Wrights had already tested wing warping with their 1899 kite, which used a system of lines to warp the wings, changing their geometry so that one side could develop a different aerodynamic force from the other.⁵
This was bird emulation, but of a very different kind from the ornithopter. The Wrights were no longer trying to reproduce the bird itself. Instead, they were looking more closely at how birds controlled their flight.
The distinction is easy to miss because wing warping and wing flapping both involve moving a wing. Aerodynamically, however, the ideas are quite different. The Wrights were not asking their wings to beat through the air to produce both lift and propulsion. They were recognizing that changing the geometry of a lifting surface could change the forces acting upon it—and therefore control the aircraft.
Wing warping itself would not remain the standard solution. As airplanes developed, engineers increasingly assigned individual jobs to individual devices. Ailerons helped control roll, elevators controlled pitch, and rudders controlled yaw, while wings acquired flaps to alter their lifting characteristics at lower speeds, leading-edge devices to extend their operating range, and spoilers that could reduce lift, increase drag, and assist with roll control. Where a bird could continuously adapt one organic structure, engineers surrounded a comparatively rigid wing with specialized moving pieces. For most of aviation history, that was a very good trade.
Modern aeronautical research has occasionally returned more directly to controlled wing deformation. In the early 2000s, NASA’s Active Aeroelastic Wing program used a modified F/A-18 to investigate whether aerodynamic forces and carefully controlled structural flexibility could twist a wing for maneuvering—the same broad principle that connected birds to the Wrights more than a century earlier.⁶
You do not need a research airplane, though, to watch the larger story continue. The next time you find yourself sitting over the wing of a Boeing 737 MAX, look outside during approach. The apparently rigid wing begins changing as leading-edge devices and trailing-edge flaps extend for slower flight, ailerons make corrections in roll, and flight spoilers can rise asymmetrically to supplement those ailerons. The same general family of spoiler surfaces can serve other purposes when commanded as speed brakes, and after touchdown, spoilers rise prominently across the wing, reducing lift and helping transfer the airplane’s weight onto its wheels.
None of this makes a 737 MAX wing mechanically equivalent to a bird’s wing, and the comparison becomes more interesting if we resist making that claim. On the earlier 737 Next Generation, spoiler commands were coordinated substantially through a mechanical system of cables and a spoiler mixer. For the MAX, Boeing replaced that mechanical spoiler-control architecture with fly-by-wire electronic control. The individual surfaces remain specialized mechanical devices, but some of their coordination now occurs electronically.⁷
Five centuries after Leonardo tried to reproduce the visible machinery of a bird, engineers are still trying to achieve some of what made that machinery remarkable in the first place: a wing whose aerodynamic behavior can change according to what flight requires.
The route between the two could hardly have been less direct. Aviation advanced by separating lift from propulsion, stability from control, and eventually one kind of wing control from another. Humans learned to build successful airplanes not by reproducing the bird as a whole, but by taking the problem apart. Somewhere along the way, that made it possible to appreciate the bird differently.
The early inventors saw wings that flapped and tried to copy the motion. Cayley separated the functions. Lilienthal studied birds but flew fixed-wing gliders. The Wrights returned to birds and noticed something less obvious: the wing itself could change. Modern aircraft distribute that adaptability among an assortment of carefully engineered surfaces and increasingly sophisticated systems that coordinate them.
The history of flight suggests that birds were never an inappropriate model for understanding aviation. What changed was not the value of observing them, but the ability to distinguish between imitating their visible motions and understanding the aerodynamic functions those motions accomplished.
Notes:
National Air and Space Museum, “Leonardo da Vinci and Flight.” The Smithsonian notes that most of Leonardo’s aeronautical designs were ornithopters employing flapping wings for lift and propulsion and that his designs used the pilot’s arms or legs to actuate them. The Museo Leonardiano’s reconstruction of Leonardo’s late-1480s Manuscript B design further documents its human-powered system of cords, pulleys, and foot-operated mechanisms.
National Air and Space Museum, “Leonardo da Vinci and Flight”; Metropolitan Museum of Art, Leonardo da Vinci: Anatomical Drawings from the Royal Library, Windsor Castle, discussion of Leonardo’s studies of the bones and musculature of a bird’s wing. The Smithsonian dates the Codex on the Flight of Birds to 1505–1506 and emphasizes Leonardo’s later observations of gliding, balance, wind, centers of gravity and pressure, and the use of wings and tail in control. The Metropolitan Museum notes that his earlier human-powered ornithopter projects belonged principally to the 1480s and 1490s and that by the period of his later anatomical wing studies he had abandoned those particular schemes.
National Air and Space Museum, “Flight Before the Airplane.” The Smithsonian identifies Cayley as the first to conceive the flying machine as a fixed-wing craft with separate systems for lift, propulsion, and control.
National Air and Space Museum, “Lilienthal Glider.” Lilienthal began investigating the mechanics and aerodynamics of bird flight with his brother Gustav in the late 1860s, published Der Vogelflug als Grundlage der Fliegekunst (Birdflight as the Basis of Aviation) in 1889, and made close to 2,000 flights in sixteen glider designs between 1891 and 1896.
Wilbur Wright to Octave Chanute, May 13, 1900; Library of Congress, “The Wilbur and Orville Wright Timeline, 1846 to 1948” and “The Achievement—The Dream of Flight”; National Air and Space Museum, “1899 Wright Kite” and “1900 Wright Glider.” The Library of Congress dates the Wrights’ wing-warping kite experiment to July–August 1899. In his May 1900 letter to Chanute, Wilbur subsequently described his observations of buzzards using “torsion of the tips of the wings” to restore lateral balance.
NASA Armstrong Flight Research Center, “F/A-18 Active Aeroelastic Wing.” The joint NASA, U.S. Air Force Research Laboratory, and Boeing program used a modified F/A-18A to investigate roll control through aerodynamically induced twist of a flexible wing; flight research concluded in 2005.
Federal Aviation Administration, “Boeing 737-700,” discussion of the 737 speed-brake and spoiler system; Boeing, “Boeing Makes 737 MAX Design Decisions,” April 11, 2012; BAE Systems, “737 MAX Will Optimize Aerodynamics with BAE Systems’ Electronics,” July 2, 2013. The FAA documents the earlier 737’s hydraulically powered flight spoilers serving both speed-brake and asymmetric roll-control functions. Boeing stated during MAX development that its fly-by-wire spoilers would replace a mechanical system, and BAE Systems identifies itself as the supplier of the MAX spoiler-control electronics.