The Mystery of Flight: A Bird Is Not A Plane
We know how to fly, right? Then why are scientists only now on the verge of figuring out how animals take to the air?
Biologists wouldn't have nearly so much trouble studying how a bird flies through the air if air weren't invisible. Even when they study the flapping of the bird's wings on high- speed movie film, they're hard- pressed to see precisely what the wings are accomplishing aerodynamically. For a flight researcher, ingenuity is called for, which is why Geoff Spedding turned to soap bubbles filled with helium.
If a bubble of helium is small enough, about as wide as the head of a pin, the weight of the soap film is offset by the buoyancy of the helium inside it. "You get a cloud of the stuff that just sits there," says Spedding, an aerodynamics researcher at the University of Southern California. Spedding's bubble- blowing days were in the early 1980s, when he was doing doctoral research on how pigeons fly. "I trained the birds to fly through these clouds in a darkened room; then I photographed the bubbles in their wake." The resulting quadruple- exposures revealed the air swirling in ways not predicted by conventional aerodynamic theory. Spedding might have used streams of smoke to trace the eddies instead, but bubbles gave him something tangible to track. "Also, you don't gas the bird this way," he says.
Any Sunday- morning bird- watcher can offer a general description of what wings do, but describing isn't the same as explaining. In the five centuries since Leonardo da Vinci labored to unravel the mystery of how birds fly, researchers have amassed information about avian physiology, energy use, migration patterns and the like. Yet despite the scientific scrutiny- - and our own flight machines- - just what an animal does when it flies has remained elusive. "Flight in general is hard to study," says Vance Tucker, a Duke University zoologist who studies the aerodynamics of large gliding birds. "The poor observer has to study something that's going by overhead at high speed. Also, the pressures and forces around the bird are hard to monitor. As a result, the ratio of verbiage to facts in this field has been very high."
To analyze bird flight, researchers have traditionally relied on aerodynamic theory derived from studying fixed- wing aircraft. When applied to animals, however, those rules don't work. Birds are more complicated- - and more accomplished- - than even the most exotic aircraft. To look beyond the bird- as- airplane model, researchers today are scrutinizing birds in wind tunnels, enlisting medical scanners to see bones at work, even building robotic wings- - all to measure exactly what a bird does when it flies. That understanding, in turn, may help humans design better flying machines- - especially new kinds of maneuverable military aircraft.
To humans stuck on the ground, muscle- powered flight seems a miraculous form of locomotion. Indeed, for a human to mimic Daedalus, the mythical Greek hero who escaped from Crete on prosthetic wings of feathers and wax, would take a miracle. Wings big enough to hoist a man aloft would measure as much as 140 feet across. Daedalus' chest would have had to be 6 feet thick to house pectoral muscles powerful enough to flap such wings. And these calculations assume the new physiological changes would add no weight.
Birds, like airplanes, must be lightweight as well as powerful. They can fly only because evolution slimmed down their entire anatomy from their reptilian ancestors. Over time, the bones of birds have become lighter and, in the case of many "finger bones," some have disappeared altogether. The apex of ultra- light construction is the body of the magnificent frigatebird: despite its 7- foot wingspan, its skeleton weighs less than 4 ounces- - half as much as its feathers. (Bats, which, along with birds and flying insects are nature's only true fliers, also have evolved super- lightweight bones. That's why bats hang from their feet when not flying: their leg bones are usually too thin to support standing.) Birds' skulls are surprisingly thin, more eggshell than armor. Their tails are little more than pin cushions for feathers. Wings are mostly feathers, and the feathers themselves are masterpieces of engineering: airy and flexible, yet nearly indestructible. Light as it is, a bird's wing provides both propulsion and lift. Lift comes from air flowing smoothly over the wing's curved surfaces. Propulsion comes from flapping, and flapping is what has so confounded flight researchers. A wing isn't only an oar for "rowing" a bird through the air, as Leonardo surmised. Nor is it simply a canoe paddle. "You read in the literature that birds turn by rolling their inside wing upwards to create drag on that side, the way you drag a canoe paddle sideways in the water to turn," says biologist Ken Dial of the University of Montana's flight laboratory. But drag slows a bird, and speed can mean life or death. Dial has discovered rather that birds turn by dipping the inside wing downward, something like the aileron of a plane. "Only they're doing this with the entire wing," he says, "which is one reason they can turn so much faster than an airplane." With colleagues from three other universities, Dial has used high- speed X- ray movies to examine birds flying freely in wind tunnels. The movies helped them study the movement of a bird's skeleton during flight. Eventually, he wants to use a hospital- style magnetic resonance imaging (MRI) machine to record movements of soft tissue like lungs. Lately, to study motions like banking and turning, Dial has been following his pigeons and magpies with a high- speed movie camera as they fly through a zigzagging obstacle course of hanging curtains. "They fly down the hallway to my office, which we set up as a sort of slalom," he says. The curtains are made of see- through acetate. "This lets us film the birds all the way through their turns."
A maneuvering bird must coordinate a large number of fine movements, from flexing and twisting its wings to varying their sweep through the air. "A bird is inherently unstable," Dial says. "What keeps it stable in flight is its central nervous system, which controls its muscles." A bird is much like a state- of- the- art jet fighter, which is also highly unstable and thus highly maneuverable. Such fighters rely on computerized sensors and controls to make split- second adjustments in midair. Birds, of course, have neither computers nor especially large brains, but, says Dial, "they have a disproportionately large cerebellum, which is known to be involved in coordination."
If birds only traveled long distance, flapping mile after mile in one direction, they wouldn't need to turn on a dime. But birds like the purple martin survive by intercepting flying insects in midair. Even a stork, large as it is, can manage a gentle two- point landing on the branch of a tree in full foliage on a gusty day. To avoid stalling- - the point when a flying object loses lift and becomes a dead weight tumbling toward the ground- - birds somehow gather instantaneous feedback about the airflow over their wings and body.
Pilot and veterinarian- turned- zoologist Richard Brown, now at Sweden's University of Goteborg, thinks he knows how they do it. On a sailplane, a short length of yarn on the pilot's canopy flows smoothly aft when the plane is flying well. As the plane approaches a stall, eddies of air lift the yarn up or even push it forward, warning the pilot. Similarly, says Brown, the thousands of feathers covering a bird's wings and body may double as airflow sensors. The feathers act as mechanical levers that lift up slightly when the flow of air along them is disrupted- - while the bird is banking sharply or gliding near stall speed, for example.
"Next to the base of feathers are specialized nerve endings," he says, "so when the feather moves, the bird can sense it." Brown has found sensors in the wing muscles as well. He wonders if the wing muscles may be acting more as transducers- - passive information- gatherers for the bird's nervous system- - than as active locomotors. (A bird's huge breast muscles are all it needs for steady flight.) Sensors on the wing probably detect turbulence and cause the bird to modify its wing beat in the course of a single downstroke. "The airflow sensor may work at the level of the spinal cord," Brown says, because there may not be time for the bird's brain to get involved. The aerobatic ability of different birds varies dramatically, of course. A swallow will spend eight hours a day soaring and diving, gobbling insects. A robin may take to the air for only a few minutes a day, in noisy bursts of a second or two. Yet the robin is as likely as the swallow to be exhausted by the effort. In part, this disparity is because the robin spends so much of its flight time taking off and landing, rarely building up enough speed to let its wings act as an airfoil. Moreover, compared to the elongated swallow, the robin has a squat, chesty body and stubby wings, which makes flying more laborious. The tradeoff is that robins don't waste time swooping this way and that. Rather, they dart purposefully from some low perch only upon spying a worm or other morsel on the ground.
Takeoffs and landings are, in fact, the most taxing parts of any flight, so many larger birds perform them as seldom as possible. Vultures, hawks, albatrosses and many other large birds spend much if not most of their time in the air soaring in air currents, their wings held outstretched and nearly motionless. The wandering albatross, 10 feet or more from wingtip to wingtip, saves energy by locking its wings into place while soaring; it probably even snoozes while aloft.
To fly efficiently, birds deftly manipulate features of their wings. In slow flight, as when circling to gain altitude in an updraft, a vulture holds its wings out straight and fans out the long, stiff feathers on its wingtips, effectively creating slots. The slots minimize how energetically the air behind the bird is stirred up. The result is less drag, and for a bird moving forward, less drag means more lift. By contrast, a raptor in a controlled dive will fold its wings in partway to shrink their surface area, as the bird needs speed, not lift, when it closes on its prey. Working with an experienced falconer, Duke University's Tucker is using a device like a souped- up optical rangefinder to plot the paths of birds diving at speeds of up to 200 miles an hour. Such tracking is an arduous task, which is one reason the top speed of diving birds has always been a matter of conjecture. Tucker hopes one day to develop a formula applying to birds of any shape and size. "Give me a hawk or a falcon's measurements and how high it climbs," he says, " and I could tell you how fast it can dive at a given angle."
At the other extreme from big- winged gliders are fast- flapping insects. The tiniest wasps and beetles do row through the air, using drag to their advantage, since to them air feels as viscous as syrup. They don't need much lift because if they stopped moving altogether, they'd drop no faster than a mote of dust. They "swim" through the air using wings covered with drag- inducing bristles. On the return stroke, the bristles collapse momentarily like an oar being feathered.
For bigger bugs, flight is more complicated. At Cambridge University, zoologist Charles Ellington has been particularly interested in bumblebees. "A few years ago, one of my students and I published a paper that actually proved that bumblebees can't fly," he says, laughing, "- - that is, by the conventional laws of aerodynamics." A bumblebee's wings, and those of large flying insects generally, really do produce more lift than theory predicts, he says. "We've been trying to find out how they do, and after about 10 years, I think we've got the answer."
Ellington got his results by switching from bumblebees to an easier- to- study surrogate, the enormous Florida tobacco hornworm hawk moth, which has a 4- inch wingspan. By flying the moths through smoke (the smoke didn't bother them), he could see air swirling from the moth's body out to its wingtips- - instead of flowing across its wings from leading edge to trailing edge. Then he built a big mechanical moth with pivoting, brass- and- fabric wings, and Robo- Moth produced the same sideways vortices. Ellington isn't prepared to announce his conclusions yet, but he believes these unexpected eddies will explain much of the insects' extra lift.
Biologists are also closing in on another secret: how insects and small birds create so much lift from a given amount of energy. The wings of fast- flapping species, including fruit flies and hummingbirds, apparently capture and reuse energy from one wingbeat to the next. Power from the end of the wing's downstroke somehow squeezes, stretches, or otherwise deforms something elastic- - perhaps a tendon, possibly muscle tissue itself- - which recoils a moment later to help get the wing moving back upward. (The same principle is what keeps the tines of a tuning fork vibrating.) "A bird that doesn't do that would simply be decelerating [or slowing down] the wing and wasting all of that energy," says Dominic Wells, a physiologist at London's Royal Veterinary College.
"The tuning- fork analogy is actually quite a good one," says Wells. "It suggests that if you have a structure that can store energy elastically, then maybe there is an optimal frequency at which it does that." Experimental evidence supports that idea, finding that birds tend not to vary their flaps-per-second. They're one-gear animals. In one study, Wells tried to force hummingbirds to alter their wingbeat rate, but the birds resisted. (Their flapping frequency - 80 wingbeats a second or more - is what makes hummingbirds hum, of course.)
Despite such energy-saving measures, flying remains the most strenuous activity an animal can perform. So even though natural selection has on rare occasions chosen birds with less flight-worthy bodies and stronger legs, ostriches and pheasants are the exception, not the rule. You could say that birds learned to fly because their nonflying ancestors spent so much of their time being hungry and being chased. Compared to birds in the wild, we humans have plenty to eat and little to fear. No wonder we can't fly. And come to think of it, maybe we should rethink our envy of the world's winged creatures.
Doug Stewart, a freelance writer in Massachusetts and a frequent flier, has ridden in gliders, helicopters and jets, but even houseflies among live fliers now impress him more.