An Airplane Is Not A Bird
Of course not—yet the analogy has repeatedly held back design improvement
Undoubtedly it was birds that first inspired man with the notion of flight. If there had been no birds, perhaps insects could have done the same, but insects can hardly match the inspirational value of soaring eagles, diving hawks, or maneuvering swifts or swallows. Wanting to fly meant wanting to fly like birds. Yet birds are terrible models for human flight, and a too slavish attention to their example—often unconscious —has often impeded the development of aircraft. An airplane is not a bird, and designers throughout the history of aircraft development have had a hard time fully realizing this.
Even before the first historical attempts at flight, the mistake was made in myth. In classical legend Daedalus and Icarus attached bird feathers to their arms with beeswax so they could escape from their island imprisonment; Icarus flew too near the sun, it melted the wax, the feathers came off, and he fell into the sea. The Greeks told the story as a kind of parable; no one seems to have noted that the real fault lay in trying to make a flying machine out of inappropriate materials. Yet though the Icarus legend seems preposterous today, heavier-than-air aviation experimenters similarly followed the bird model a century ago, and with equally unimpressive results.
The most obvious direct bird imitations are the many attempts to produce ornithopters, aircraft that fly by flapping their wings. A few people are still trying to make them go; but the old ones didn’t work, and the new ones don’t seem to either. The idea is appealing, of course, but the obstacles are formidable. Birds’ wings are operated by muscles that pull, and the most obvious source of power for an airplane, the internal-combustion engine, produces its power by turning a shaft. Transmitting that power into something simulating the pulling of muscles would almost certainly require unsuitably heavy machinery, like a large gearbox or a hydraulic system. Furthermore, birds’ wings do not simply flap up and down; they move in very complex ways involving many muscles, and to simulate all of them would require multiple transmission systems. The controls alone would require a substantial computer—as in Paul MacCready’s ill-fated radio-controlled flier, the Pterodactyl , which crashed in 1986.
Machines that walk like insects—a far simpler problem—have been built, but they do not work very well, and they are far too heavy to fly. To make a machine that simulates the flapping of bird wings—and all the complex movements of bird wings in taking off, turning, soaring, hovering, and landing—and then making those movements controllable by a pilot would be a task of appalling difficulty. The flapping-wing bird model is unlikely ever to work practically as a man carrier.
When designers recognized the capabilities of a power unit that transmitted its power through the turning of a shaft—something that has no biological equivalent, except for the flagella of certain species of protozoa—they eventually found a way to make a lightweight and efficient power-transmission system by utilizing a propeller—a device that has no avian equivalent. Then aviation progress could be made.
The Cambered Airfoil
Until near the end of World War I, nearly all aircraft used wings with airfoils that were imitations of birds’ wings. On such wings the lower surface is concave (cambered), just like a bird’s wing. Such airfoils are reasonably efficient at low to moderate speeds, but they are difficult to cover with fabric on the concave side, and they produce rather high drag. Even more important, they are very thin top to bottom. As a result, such thin airfoils cannot enclose deep wing spars and so cannot be very strong. That means they have to be externally braced. This need for external bracing is what made the biplane the dominant form of airplane during World War I, even though there are no biplane birds.
Eventually Virginius Clark, of the famous Clark “Y” airfoil, and others began to wonder if the bottom of the airfoil could be flat. They tried it and found that performance was at least as good as with a cambered airfoil. There was some raising of stall speed, but drag was less at operational speeds, so overall performance was improved. Moreover, there now could be room for a deep internal spar with strength enough for the wing to be self-supporting. The cantilever wing, as the self-supporting wing was called, required no drag-producing struts and wires, and their elimination further improved performance; but the coming of the cantilever wing had probably been delayed by a too unquestioning acceptance of the bird-model cambered airfoil.
Most pre-Wright aircraft had trouble with control of movement on the roll axis (the axis that runs from nose to tail). Indeed, one of the most significant of the Wrights’ inventions was a means of controlling the aircraft in roll. The method, however, was bird-derived. What the Wrights did was to arrange a system of wires that allowed the pilot to warp the trailing edge of one wing downward to create a higher angle of attack on that end of the wing and consequently create more lift on that side of the aircraft. As that end of the wing lifted, the aircraft banked, by a system very similar to a bird’s.
The method worked reasonably well, but it required a complex system of controls. Initially the wing-warping controls were attached to the pilot’s belt, and he made the airplane roll by shifting his body. Furthermore, it was usable only on aircraft that had thin and flexible wings. It was utterly unsuited to later high-performance aircraft. The invention of the movable aileron, usually ascribed to Glenn Curtiss, made wing warping obsolete, but it was an advance that could come only with rejection of the bird model.
The Wing on Top
There still persists the notion that the proper place for a wing, at least on a small plane, is at the top of the fuselage. There are advantages, of course, to high-wing design: good downward visibility, inherent stability, wing struts that can be in tension, and so on. There are also corresponding advantages for low-wing and mid-wing designs, but high-wing advocates still bring up the bird model. Usually the argument is phrased, “Did you ever see a low-winged bird?” The same question has been posed in advertisements by at least one small-aircraft manufacturer. It is as valid a question as “Did you ever see a bird with two engines or a propeller?” The bird’s reason for having the wing on top simply does not apply to airplanes.
Birds have the wings on top because their wings are powered with muscles and muscles work by contraction. The primary power stroke of a bird’s wing is downward, and the muscles that pull it downward must be arranged below the wing to do that. The muscles must also have an attachment point—the breastbone, which serves as a keel. A bird’s largest muscles are the pectorals, which are the breast meat on a chicken. The system works fine on a bird, but it is a silly argument for building an airplane with a high wing.
The Vertical Tail
Since birds have no vertical tails, many early aircraft had none. Louis Blériot’s early designs had no vertical tails, and many of the World War I designs had very small tails. Birds, however, can do subtle things with their wings that even wing-warped airplanes cannot do. And they can bow their spines sideways and move their heads to the side and slightly bank their tails and so take care of any need for yaw control in ways that rigid-fuselage aircraft cannot (yaw is movement on the vertical axis). For airplanes the vertical tail is essential, but early designers, looking at birds, had a hard time seeing that.
The Internal Skeleton
The internal construction of aircraft long followed, and to a degree still follows, the bird model. Just as a bird has bones that support its structure, so an airplane has spars, longerons, and ribs. The bird’s bones are covered with muscles to move those bones, but the airplane must support its skin otherwise. Furthermore, the aircraft’s skin is necessarily light and fragile, and the load-carrying capabilities of its bone analogues are not very efficient for their weight, nor are they efficient for loads carried in more than one direction. Monocoque construction, in which the stresses are virtually all borne in the outer shell or skin, is often a better answer. Such construction had to wait until proper building materials like wood laminates, high-strength sheet aluminum, and composites were available; it also had to overcome the persistence of the bird model in the minds of designers, manufacturers, and users. It is interesting to wonder if monocoque construction might have become more prevalent earlier had insects, with their chitinous exoskeletons, been man’s unconscious models for flight.
Tail-Wheel Landing Gear
A bird lands head up and tail down. So did early aircraft, with “conventional” tail-wheel landing gear. On rough landing fields and at low landing speeds, conventional gear worked well enough, but there was always the problem of ground loops and poor visibility while taxiing. While there were always a few tri-gears and quadra-gears, the tail wheel hung on into the 1940s. Even during World War II nearly all the aircraft were still conventional-geared—just possibly because that was how the birds did it.
The Shape of Birds
The latest concept to fall is that of planform— the shape of an aircraft as viewed from above. Many early aircraft really did look like birds. Their wings and tails were shaped like birds’ wings and tails. The German Taube (“dove”), of World War I, is probably the most extreme example of this. Its wing tips were swept back and then rounded on the trailing edge, and the horizontal stabilizer was shaped like a bird’s spread tail feathers. Control was maintained by wing and tail warping, through an unbelievably complex system of external posts and control wires. Apparently the designer was trying very hard to follow the bird model but could not find a way to warp the flying surfaces and still keep the wires inside. The vertical tail was very small—as would be expected.
Most notable of bird-shape ideals is the presumption that the wing belongs in front and the tail behind. A. E. Housman wrote: A tail behind, a trunk in front,/ Complete the usual elephant./ The tail in front, the trunk behind,/ Is what you very seldom find . And tail behind and wings in front compose the usual airplane. True, the Wrights had a horizontal tail (canard) on the Flier, but they also had a vertical one behind. Among the British aircraft of early World War II, there were a few Wright imitators that had both canards and tails, but most designers stuck with the bird pattern. Undeniably there are advantages to having tail planes behind the propeller for fighter aircraft. Such a configuration allows for greater maneuverability. Since the most visible aircraft of World War I were the fighters, the tail-behind configuration had essentially defined the airplane by the time World War I was over. Few designers even considered any other configuration. In World War II, of some eight hundred thousand aircraft built by the warring nations, nearly all were bird configurations—wings in front and tail behind.
One rare instance of the consideration of other layouts occurred in a U.S. Army Air Corps request for proposals in 1939. This astonishing request for fighter aircraft of “unusual configuration” led to such innovative designs as the Vought XP-54 Swoose, the Curtiss XP-55 Ascender, and the Northrop XP-56 Black Bullet. Though none of these became operational, the basic configuration of the XP-55, which had a canard in front, was considerably refined by the modern-day designer Burt Rutan to become the basis for his VariEze. Rutan recognized that the canard of the XP-55 needed to be a true lifting surface as well as a control and trimming surface. By greatly increasing the span of the canard, he solved the basic problem of the configuration. There has been a flood of canard designs in the years since. We might have seen them much sooner had designers not been shackled to the assumption that an airplane should look basically like a bird.
It remains to be seen if aircraft designers will eventually cast aside other bird-model notions. Human creativity seems very often to work by seeking a basic model and then making modifications on that model, and the logic of the innovation involved is much easier to discern once it has been abandoned.
To make new modifications on the model or to adopt a wholly new model or, even more difficult, to create something where there is no applicable model, as was the real case with the airplane—those are the truly difficult tasks of creativity.