You all know that most species of animals can fly -- but not humans. There are more than a million species of flying insects, but do you know that among the 13,000 warm-blood vertebrate species (which include birds and mammals), about 10,000 of them are able to fly (9,000 birds and 1,000 bats)? Many of them have millions of years of experience, and aerospace engineers are trying to learn from birds, bats and insects to design very efficient micro flapping-wing aircraft for the U.S. Air Force to be used as surveillance tools. Natural flyers are much more efficient than man-made aircraft, states one aerospace engineer at the University of Michigan (U-M), who adds that 'natural flyers obviously have some highly varied mechanical properties that we really have not incorporated in engineering.' But read more...
You can see on the left three examples of birds flapping their wings. (Credit: University of Michigan) These pictures have been extracted from a short video, The Aerodynamics of Birds (1 minute and 12 seconds).
The pictures have been taken by Wei Shyy, chair of the Aerospace Engineering department at U-M and a photographer of birds. "He studies them to help engineers design small, flapping-wing aircraft."
The U-M news release starts with giving some surprising facts about flying. "A Blackbird jet flying nearly 2,000 miles per hour covers 32 body lengths per second. But a common pigeon flying at 50 miles per hour covers 75. The roll rate of the aerobatic A-4 Skyhawk plane is about 720 degrees per second. The roll rate of a barn swallow exceeds 5,000 degrees per second. Select military aircraft can withstand gravitational forces of 8-10 G. Many birds routinely experience positive G-forces greater than 10 G and up to 14 G."
So it's not surprising that Wei Shyy and his lab are looking at birds for designing micro planes. After all, "Shyy and his colleagues have several grants from the Air Force totaling more than $1 million a year to research small flapping wing aircraft." Here is a quote from Shyy about birds. "They're not only lighter, but also have much more adaptive structures as well as capabilities of integrating aerodynamics with wing and body shapes, which change all the time. Natural flyers have outstanding capabilities to remain airborne through wind gusts, rain, and snow."
For more information, let's visit the Micro Air Vehicles research section at Shyy's lab. Here is the introduction. "Micro air vehicles (MAVs), with a maximal size of 15 cm and flight speed around 10 m/s, have gained growing interest from both military as well as civilian community. Equipped with a video camera or a sensor, these vehicles can perform surveillance and reconnaissance, targeting, and bio-chemical sensing at hazardous location. With the rapid progress made in structural and material technologies, miniaturization of power plants, communication, visualization, and control devices, numerous groups have developed successful MAVs."
This second quote is a little bit more scientific. "Because of its small size and low flight speed, an MAV operates under low Reynolds number conditions (104-105). It is well known that as the Reynolds number drops from 106 to 105, the aerodynamic performance drops dramatically due to flow separation. Low Reynolds number airfoils demonstrate characteristics different from high Reynolds number ones. First, the low Reynolds number condition favors a thin airfoil with modest camber; if the Reynolds number decrease further, an airfoil with rough surface will be better than a smooth one. Second, the low Reynolds number airfoil may show zigzag behavior in the lift-drag polar."
If you really want to learn more about this topic, you can buy the book "Aerodynamics of Low Reynolds Number Flyers" written by Wei Shyy with other U-M research scientists, Yongsheng Lian, Jian Tang and Dragos Viieru, and Hao Liu, professor of Biomechanical Engineering at Chiba University in Japan.
The book is available from its publisher, Cambridge University Press for US$80, or from Amazon.com for US$64. There you'll find the complete book introduction including many pictures.
Sources: University of Michigan news release, February 4, 2008; and various websites
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