Windswept_ The Story of Wind and Weather - Marq de Villiers [114]
By contrast with its arm wing, the leading edge of a swift's hand wing is sharp, nothing more than the narrow vane of the outermost primary feathers. A group of Dutch biologists decided to check out how the hand wing works, using what they called "digital particle image velocimetry," not in a wind tunnel but in a water tunnel. Apparently the swifts wouldn't cooperate by flitting about a conventional wind tunnel in a straight line, so the Dutch scientists built an artificial swift wing instead. The results surprised them.
Earlier studies of insect wings had discovered that some creatures, masters as they were of the unconventional lift (remember the old saw about it being physically impossible for a bumblebee to fly?) generated what were called leading-edge vortexes, which greatly exaggerated the upward push of the flowing air during both flapping and gliding. A leading-edge vortex forms when the so-called angle of attack, the angle between the wing and the incoming air, is fairly large. The air flow then separates from the wing at the leading edge, and rolls up into a vortex. To facilitate vortex formation at lower angles of attack, the wing needs a sharp edge. To exploit the vortex, to use it to get lift, the insect or bird must keep this rolled-up vortex close to the wing. Insects typically do this by very rapid wing movement. Swifts do it by sweeping back the angle of the hand wings, almost to a V shape. As a consequence, the leading-edge vortex, or LEV, spirals out toward the tip of the wing, looking for all the world like a tiny tornado. And as with real tornadoes, the air pressure at the core of the vortex is very low, sucking the air beneath the wing upward, giving extraordinary lift. These vortexes are also remarkably stable at both low and high angles of attack—insects generally need an angle of about 25 to 40 degrees, but swifts can operate successfully at angles as low as 5 degrees. This gives them the speed to accompany their agility, and the ability to catch quickly flitting insects. Swifts change the sweep angle of their wings in flight, thus changing the angle of attack of the air flow. They use low angles for speeds, high angles to brake in midair—it gives them lots of drag, but the vortex keeps them from stalling and losing height. Aerospace engineers have copied the principle for certain military aircraft, which must be highly maneuverable and perform well at both subsonic and supersonic speeds; pilots of the newer fighter jets, such as the Tornado, can choose different sweep angles for agility or for cruising speed.
The next challenge, the Dutch scientists say, is to learn more about how swifts use their variable wing sweep to directly control the leading-edge vortexes to increase their flight performance. "The swift's flight control might inspire a new generation of engineers to develop morphing microrobotic vehicles that can fly with the agility, efficiency, and short take-off and landing capabilities of insects and birds."6
If course, this isn't the first time humans have learned from birds—or, in the early days, attempted to learn from birds. Icarus is a notorious example, though he is now remembered mostly for his hubris rather than his flight-control technology. Dreamers from Roger Bacon through Leonardo da Vinci have sketched vehicles that somehow mimicked birds, some of which would no doubt have worked if the engineering skills to make them had existed and if the knotty problem of takeoff had been solved. Takeoff was always the most difficult issue. The