Many an elementary school pupil has confounded his or her classmates by declaring,
“Scientists have proved that bumblebees can’t fly.” Indeed, scientists have long been mystified because the equations that describe how fixed-wing aircraft lift off the ground don’t seem to work when applied to bugs. But, of course, something
holds up these creatures, and the scheme appears to be similar to that used by airplanes.
Birds, too, have similarities to airplanes, but as living flyers get smaller they rely on
increasingly exotic aerodynamic tricks to wrest themselves into the air, and until very recently these tricks have eluded scientists’ efforts to find them.
Bird Flight
Some aspects of bird flight mirror airplane flight almost exactly. In a glide, for example, a bird’s wing acts just like an airplane’s wing: a simple airfoil with a curved top producing lift by deflecting air downward. But where airplanes use engines to create forward thrust, birds flap their wings. The motion is complex, with the bird not only flapping but also twisting and folding its wing during part of the stroke. Scientists have tethered birds in wind tunnels,
photographed them with high-speed motion picture cameras, and found that on the upstroke, wings rotate back and up, with the leading edge on top and wingtip feathers open to decrease airflow resistance. On the down-stroke, the leading edge rotates back down and the feathers close, acting as small airfoils, propelling the bird forward.
A bird is an instrument working according to mathematical law.
— Leonardo da Vinci, 1505
A Rüppell’s griffon claimed the bird altitude record in 1975 upon colliding with an airliner at 37,000 feet over Abidjan, Ivory Coast. Although the aircraft survived, the bird was less lucky.
However, one expert disputes this record, suggesting that the griffon, which relies on soaring in updrafts for typical flights, may have been sucked up into a thunderstorm and was already dead, frozen, and falling earthward when the airliner hit it. The next highest record goes to a flock of whooper swans that were observed at about 28,000 feet over the Outer Hebrides.
Birds also turn and twist their wings to maneuver, a process observed by the Wright brothers in their studies of pigeons. In fact, the Wrights modeled the steering mechanism in their first airplane after the twisting motion of a bird’s wing. Soon after, aircraft were designed to roll with ailerons (those panels near the wingtips that can be raised or lowered), as these were much easier to engineer and construct than twistable wings.
Airplanes, like birds, widen their wings during slow flight, generating more lift in situations such as takeoff and landing. Also, the structures and internal systems of birds (at least of those that fly) are optimized for flight—built strong but light, like the best-designed aircraft.
Airplane power systems run fast and hot, burning large quantities of fuel. Birds operate at higher body temperatures than other animals and have racing metabolisms. The metabolic screaming meemie of the bird world, the hummingbird, burns around 4 percent of its body weight per hour. A Boeing 747 burns around 3 percent per hour—over ten tons of fuel.
Wings aren’t the only things that enable birds to fly; the rest of their bodies must be suited to flying, too. For instance, the pectoral muscles, which drive the wings, may account for as much as one-half of the bird’s weight. And bird bones are hollow. Consider the frigate bird: It
has a wingspan of over seven feet, but its skeleton weighs only about four ounces.
During migration, some birds can fly for distances rivaling the range of some airplanes. The ruby-throated hummingbird, which weighs about as much as a penny, flies nonstop over the Gulf of Mexico, a distance of 620 miles. That’s nothing compared to the four-inch-long
blackpoll warbler, which, in its autumn migration from Canada to South America, flies
continuously for ninety hours without midair refueling, a feat that puts all airliners to shame.
Bug Flight
Insects, which first took to the air about 350 million years ago, were considered by early aerodynamicists to be more or less like tiny birds. But after aerodynamic calculations failed to account for enough lift, twentieth-century scientists tethered insects in wind tunnels and
observed that many of them didn’t flap their wings up and down like most birds. Instead, they generally flapped front to back, like a rower with an oar. Plus, aerodynamicists realized that air has a certain viscosity, and if you were the size of a bug, the air would seem thick. The
smaller the creature, the thicker the air feels, so small insects like fruit flies can be thought of as swimming in molasses, rather than flying in air.
Even with this understanding of air viscosity, it wasn’t clear how some insects could pull off the feat of flying. Models of insect flight using supercomputers failed to determine the missing lift sources, so scientists turned to dynamic scaling—constructing large working models of insect wings. The first breakthrough came in the mid-1990s from Charles Ellington, professor of zoology at the University of Cambridge in England. His lab constructed a large set of
mechanical wings based on those of a gray hawkmoth. When the model was set to flapping in a wind tunnel with smoke streams, Ellington was able to observe a vortex—a spinning cylinder of air like a sideways tornado—above the leading edge of the wing.
Curiously, this type of vortex had been observed in wind tunnel tests of airplane wings, but always as a brief, unstable effect that occurred when the wing’s angle of attack (the angle at which the wing meets the oncoming air) was increased to the point of stall (where the wing begins to lose lift). The vortex would appear at the airplane wing’s leading edge and
momentarily increase lift dramatically just before the stall. Ellington found that moths can do what airplanes can’t: hang on the edge of a stall, taking advantage of the added lift of this leading edge vortex and, just before the vortex dissipates, quickly redirect and rotate their stroke to generate the same kind of lift with the wing going the opposite way.
Insects flap their wings much more often than birds. Ruby-throated hummingbirds click in at 70 beats per second, bees at 200 beats per second, and mosquitos at around 600 beats per second (600 Hz), which produces their irritating whine.
The bat is the only mammal capable of true flight.
Unfortunately, this added lift from the delayed stall might be sufficient to explain the
aerodynamics of some larger insects, but it still can’t account for the lift from tiny insect wings, which often flutter forward and backward like oars in a figure-eight pattern. Another solution was supplied by Michael Dickinson at the University of California, Berkeley. By immersing a giant set of Plexiglas fruit fly wings in two tons of mineral oil (which would model the relative thickness of the air that the fly swims in), Dickinson’s lab discovered not one but two
additional sources of lift exploited by the fruit fly. First, at the end of a wing stroke, the wing quickly rotates, and this flip mimics the backspin on a baseball—lowering the pressure on the top of the wing and generating a small amount of lift. Second, as the wing starts its backward stroke, it encounters the remains of the vortex shed from the previous stroke, which acts like a little headwind, generating even more lift with the faster airflow.
There are so many different kinds of insects (7,000 new species are found every year), with so many different types of wings, it may take some time before all their aerodynamic tricks are known. In the meantime this new understanding of microaerodynamics has led researchers to begin developing flying microrobots that might someday be used as ultraminiature spy planes.
Why do jet aircraft fly higher than the highest mountains? First, flying above the troposphere
(the lowest layer of the atmosphere, where almost all bad weather can be found) offers a much smoother ride. Second, the higher the aircraft flies, the less dense the air, meaning less drag. On the other hand, thin air has less oxygen to feed jet engines, and fewer air molecules to maintain the airplane’s lift. Plus, The less dense the air the slower the speed of sound, so flying too high forces pilots to fly slower. Each aircraft model has an optimum cruise level based on its design and the amount of fuel it carries. On long international flights, as heavy fuel slowly burns off, pilots will ascend to a higher cruise altitude every two or three hours.
In America there are two classes of travel: first class, and with children.
— Humorist Robert Benchley If the Wright brothers were alive today Wilbur would have to fire Orville to reduce costs.
— Herb Kelleher, founder, Southwest Airlines, 1994
The world’s largest paper airplane had a wingspan of forty-five feet, ten inches. Built by students and faculty at Holland’s Delft University of Technology in 1995, it flew (indoors) for 114 feet, six feet less than the Wright brothers’ first powered flight.