June 1, 2003
Make no mistake about it. Mother Nature likes to fly. Of the 13,000 warm-blooded and bony critters she has walking, running, and swimming on the Earth, 10,000 of them — namely birds — fly naturally. Only one — namely us — flies unnaturally.
"We're trying to understand how bird wings work," says Ken Dial. As a scientist at the University of Montana studying how birds fly, Dial knows more than most about natural flight. With his own Cessna 210, and commercial, instrument, and multiengine ratings, he also knows a few things about unnatural flight. Being a pilot and a scientist, he's in a position to appreciate what Mother Nature can do.
With childlike enthusiasm, a trait that he takes great pride in, Dial is unraveling the secrets behind nature's most efficient form of locomotion: flight. With a custom-made wind tunnel and a slew of cutting-edge-technology tools, Dial's work is leading to new understanding of how birds fly. It's knowledge that not only warms the hearts of the Audubon folks, but it's also catching the eyes and ears of scientists at NASA and members of the Society of Experimental Test Pilots.
"They're complex," says Dial of the feathered fliers. "It's not as simple as an aircraft where you're able to dissect the thrust and the lift components out into two different structures. Where you have a propeller creating thrust and counteracting drag, and the wing lifting the aircraft to counteract gravity, the bird is doing both with its wings simultaneously. So in some ways, birds are more analogous to a helicopter except their maneuverability is mind boggling."
Probably since the beginning of time — where time is defined by man's awareness of it — man has gazed at birds and found inspiration. Some of the world's first test pilots, Otto Lilienthal and the Wright brothers, certainly did. In their writings, the Wright brothers recorded keen observations of bird flight in the hope of gaining insight into their quest. Others tried modeling their aircraft after nature's experts, but trying to emulate bird flight was beyond the technology of the time and the attempt to do so actually retarded the development of aviation. The Wright brothers were among the first to realize how pointless such efforts were, and after 1903 aircraft builders began ignoring what could be learned from birds. Some folks, like Dial, think it is time to change that attitude.
It's interesting to note that the first powered aircraft was so aerodynamically unstable it was almost impossible to fly. Today, with 100 years of experience and knowledge to build on, we have aircraft that are even more unstable. Aircraft like the F-117 Nighthawk are so twitchy that without computers to control them, they would tear themselves apart in seconds. Watching hawks floating on thermals, it's easy to think of birds as the epitome of stable flight. But even the most graceful bird is "inherently unstable," says Dial. "Which is why when you shoot a bird it doesn't glide to the ground. It tumbles because you've knocked out the computing system, the nervous-system control."
One reason aircraft have been designed with increasing degrees of instability is the desire for greater aircraft performance. If birds are any indication, the designers are on the right track. "The pure aerobatic ability of birds far exceeds the most sophisticated aircraft," says Dial. Barn swallows have demonstrated roll rates in excess of 5,000 degrees per second. While most GA aircraft are limited to 4 or 5 Gs, insect-eating birds pull as many as 10 to 14 Gs, and they do it hundreds of times a day! In terms of speed, let's first consider ourselves. The best human athletes run at a rate equal to 3 to 4 of their body lengths per second. In the same time, a cheetah can hit 18 body lengths. The fastest known aircraft, the Lockheed SR-71, can make about 32. By contrast, a European starling tops out at 120. In terms of range, birds have been known to fly nonstop from Alaska to the island of Fuji, a 4,000-mile journey across open ocean. As for altitude, at 30,000 feet where the air is so thin that a pilot will pass out without supplemental oxygen, the bar-headed goose is flapping along as it flies over Mount Everest.
For tens of thousands of years, man had only his eyes to understand how birds fly. But their wings move too fast for the naked eye to discern what is happening. It wasn't until the development of motion photography that the complex nature of a beating wing began to unravel.
For a long time it was thought that when a bird wants to change flight direction, it turns one wing into the relative wind and essentially drags its way through a turn. If true, it would mean a lot of forward momentum and energy is lost with every change of direction. But Mother Nature is a miserly old lady. She hates wasting anything, especially hard-to-come-by energy.
To test the turning theory, Dial created an aerial obstacle course inside his lab using curtains of acetate suspended from the ceiling. Pigeons were released and photographed with high-speed film and video cameras as they maneuvered through the course. "Instead of having ailerons that create different aerodynamic forces to turn, these animals actually turn the entire wing assembly asymmetrically," says Dial.
The films showed that birds reduce the angle of attack of the inside wing by twisting it downward and increase it on the outboard wing with an upward twist. "And, boy, you would roll with extraordinary roll rates if we were in a Cessna and we tried that!" It's a fact that NASA can support.
Recent NASA tests have shown that flexing a leading edge just 1 degree produces rolling forces that are an order of magnitude more effective than an equal deflection of traditional ailerons. If the kinks can be worked out, some NASA engineers think ailerons could become things of the past and that future aircraft wings could use warping much the same way as birds do and the Wright brothers did in their 1902 glider and the 1903 Flyer.
During a turn in flight, a bird maintains bank angle through symmetrical flapping. Like an airplane, recovery is initiated with opposite wing inputs. But things get more complicated during a decelerating turn, as in landing or catching an insect in flight.
"Because the wing of a bird is doing both the propeller and fixed-wing component, these animals are generating very complex excursions with a complex airfoil," says Dial. During flight, birds are constantly varying wing length, chord, and angle of attack. "They're also changing it [the wing] from essentially nothing, being sucked right up against their body, to an enormous surface area that extends out as a perfectly gorgeous cambered wing."
Working with a custom-built battery-powered wind tunnel, Dial has spent hundreds of hours watching specially trained birds as they fly inside. Using high-speed video synchronized with cineradiography (X-ray movies), birds are photographed flying under controlled conditions. Video and film cameras use normal light to reveal the visible aspects of flight while X-ray movies simultaneously reveal the hidden movements of bone structure. By using surgically implanted markers at various points within the bird in conjunction with external markers on the bird's feathers, 3-D computer models are built to aid flight analysis. Borrowing from other fields of study, Dial and his collaborators employ tiny surgically implanted accelerometers and strain gauges to measure various forces during flight. These measurements help to determine the amount of "work" or horsepower, required during different phases of flight.
Matt Bundle, a doctoral student working with Dial, has even trained birds to fly in the wind tunnel while wearing tiny helmetlike masks. The masks allow Bundle to study oxygen consumption rates and the amount of carbon dioxide produced during different flight profiles. With this knowledge, Bundle can determine the amount of metabolic energy required for each flight phase. It's the bird version of fuel consumption rates.
Observations have shown that many birds do not continuously flap their wings when flying. To conserve energy, they use different flapping styles: flap-gliding and flap-bounding. Flap-gliding, as seen in hawks and vultures, is used in slow flight, and the bird glides momentarily before beginning another wing flap cycle. Flap-bounding occurs at higher cruise speeds and is associated with smaller birds like those found at a bird feeder. Instead of leaving the wings extended as in a glide, the wings are folded briefly against the body to reduce drag and to maintain momentum. A study of flap-bounding flight using zebra finches has shown that, during the brief period the wings are folded, the bird's body actually produces small amounts of lift. While both forms of flapping save energy, only a limited number of birds are known to use both styles.
Who would have thought that birds suffer from adverse yaw? But physics knows no boundaries and bird tails have evolved accordingly. Swifts, swallows, and nighthawks have been shown to use their tails to counteract yaw and to increase rolling forces. Tails also act as air brakes and flaps and can arguably be thought of as an extension of both wings. And like wings, they radically change surface area, camber, and angle to maintain control. Even during steady-state flight, as in soaring, tails are constantly reacting to changes in airflow and are used to counteract the effects of turbulence.
Wind-tunnel studies also looked at a bird's body pitch with different flight speeds. "They definitely change," says Dial. "They change their angle to the incident air with the tail, body, and wing acting as a total wing." As speed reduces, the body angle increases just as an aircraft pitches upward as it slows. Conversely, both aircraft and birds reduce the angle as they approach maximum speed.
Early aircraft used a wide variety of materials to cover their wings. It didn't take long to discover that untreated fabric was porous and created too much drag. What was needed was a smooth surface that was strong and durable — which is a good definition of feathers.
"These extraordinary surface area features that we call feathers are not fully appreciated in how complex, how strong, and how durable" they are, says Dial. "And yet, you can fold them up and put them away with every single wing beat!" But aiding airflow is not the only flight function feathers perform.
To maintain controlled flight, the F-117 depends on an array of external sensors scattered across its surface to monitor airflow around the aircraft. Without these data points, the aircraft computer doesn't know what needs to be done to keep the aircraft in the air. Birds, being inherently unstable as well, have the same problem and need the same information. But the number of data points in the F-117 pales in comparison to those of a bird because every feather, as many as 10,000, is an airflow sensor.
"We don't fully understand the whole loop," says Dial. "We just know that the base of feathers can act as transducers to the central nervous system. They are getting feedback of some type with airflow over the wing. We don't know exactly how much of it is from the covering feathers and how much is the actual flight feathers." What is clear is that "specialized cells at the base of flight feathers ramp up in frequency of transmission as they are being torqued."
Over hundreds of millions of years birds have evolved into a complex and efficient form of unstable flight with fantastic maneuverability. But beyond flight performance, birds have additional abilities to get aircraft designers thinking.
An ornithologist on a bird-banding project on the Chesapeake Bay attaches "bait" to a light line of rope. Safely out of sight inside a blind, he pulls the rope and the bait is hauled up a makeshift flagpole. In the distance, hardly more than a speck, a hunter takes notice. The bait is a cowbird and its legs are tied to the flagpole's rope. Its frantic and futile flapping to escape has attracted one of nature's fastest fliers: the peregrine falcon.
Reducing the span and length of its wings, the falcon converts altitude to airspeed as it moves in to attack. Through binoculars, one can see it rapidly roll 90 degrees one way and then the other as it weaves through the treetops. But as the body rolls about its axis, the head stays perfectly level and the eyes fixed on its prey.
The head, or cockpit, of a bird, says Dial, is attached to "an independent suspension system: their very flexible neck." The neck flexibility allows "the body to turn, and heave, and even spin upside down, yet still keep the head level." For the peregrine, the independent suspension allows it to chase prey at speeds up to 200 mph even as the prey maneuvers to escape. But the prey have flexible necks as well and use them to aid escape. Knowing that the larger wings of hawks or falcons won't allow them to follow, small birds under attack will fly at maximum speed through trees and bushes. It's a tactic that requires wild and rapid maneuvering in tight quarters.
"There's no question that birds are avoiding some extraordinary crashes because they can keep the world very focused and very stable in their mind's eye," says Dial. "That cockpit story of keeping the head perfectly still is an exciting area" for some researcher to explore.
It's research that could be incorporated into aircraft design. We already see stabilized optics used in military applications and in civilian broadcasts of sports and news. Could stabilized cockpits be in our future? Maybe. But one thing is sure: As we enter the second hundred years of aviation, Mother Nature still has a lot to teach us about flying.
Tim Wright, AOPA 1139149, is a pilot, freelance writer, and photographer who lives in Richmond, Virginia.
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