Nature’s Flyers: Birds, Insects, and the Biomechanics of Flight , David E. Alexander Johns Hopkins U. Press, Baltimore, 2002. $49.95 (358 pp.). ISBN 0-8018-6756-8
Bound as we are to Earth, we find bird and insect flight endlessly fascinating. Most of us perhaps have wondered, at one time or another, just how birds and insects flap their wings, and how they manage to stay aloft. Scientific work inspired by biological flight dates back to the 1500s, when Leonardo da Vinci designed a number of birdlike flying machines (ornithopters). The quest to achieve aviation based on bird flight continued and reached a climax with Otto Lilienthal, who designed ingenious gliders and daringly flew them. Sadly, before he could test one of his powered-glider designs, Lilienthal was killed in a gliding accident in 1896. A few years later, the success of the Wright brothers changed our view of flight. Abandoning the often unstable ornithopter design, the Wright brothers adopted a fixed-wing design. As we know, the fixed wing works remarkably well at large scales and high speeds.
Although the fixed-wing design has engineering advantages, it cannot be simply scaled down to the size of insects, nor do devices built with fixed wings have the maneuverability that birds and insects have. Recently, scientists have used robotic, computational, and theoretical models to make important advances in the study of flapping flight. So now is an exciting time to seriously explore flapping flight at small scales and to take a closer look at birds and insects.
How do birds and insects maneuver, how do they stay on course, how do they navigate and migrate, and how did they evolve? These are the topics of Nature’s Flyers: Birds, Insects, and the Biomechanics of Flight, by David E. Alexander. An assistant professor in the department of entomology at the University of Kansas, Alexander has been interested in bio-mechanics for more than 20 years.
The book is aimed at a popular audience that does not necessarily have a background in fluid mechanics or biology. Alexander achieves the difficult feat of explaining intrinsically complex phenomena without using mathematical or entomological jargon. As a result, the book is clear, beautifully written, and suitable for people at all levels.
Primarily, the book focuses on the physical aspects of flight. Its first half, devoted to the physics of how a wing works, reviews the canonical example of a classical airfoil moving in fluid. Alexander then discusses different flight styles seen in nature, such as gliding, soaring, and flapping. He explains the maneuvering and power requirements during flight. Although the book includes a brief summary of recent findings in insects’ use of unsteady mechanisms, such as dynamic stall and wing and wake interactions, most of the discussions are based on the classical lift and drag of a fixed wing. Such a treatment is appropriate for the level of the book. But of course, it is risky to deduce results about flapping flight using analogies with airplanes. There is no telling when such analogies will go wrong. The old myth of bumblebee flight was an easy case in which anyone could see that the theorists had made an error. Other cases could be much more deceiving. For example, in low-Reynolds-number flapping flight, lift and drag no longer take their traditional role that lift is good and drag, bad. Flapping flight can make use of both.
The second half of the book moves beyond the detailed physics of flight to insect evolution, migration, and navigation, and to the global impact of animal flight. The discussions are brief and general, but introduce readers to some long-standing puzzles. For example, how do some insects manage to fly nonstop over hundreds of miles? How do migrating birds find their way?
It should be clear from reading Nature’s Flyers that many of the questions it broaches are still open-ended. Perhaps the open-endedness will encourage some readers to take on the challenge of solving puzzles in this rich area of research. The book contains an extensive list of references up to 1999, thus providing a good starting point for further investigations. I recommend the book to anyone who is curious about flight.
