Imitating nature in flight: University of Michigan research explores flap-wing micro UAVs
By Kyle Sherer
February 19, 2008
February 20, 2008 The University of Michigan is designing a new generation of flapping-wing UAVs with wingspans smaller than a deck of cards, based on the efficient and adaptive movements of birds, bats and insects.
The Air Force has given Wei Shyy, chair of the Aerospace Engineering department at the University of Michigan, grants totalling more that $1 million a year to research small flapping-wing aircraft. These 1 to 3 inch models would be able to hover and even perch, making them of great value to the military, which currently has over 600 UAVs operating in Iraq and Afghanistan.
Researchers have discovered that, on a proportional scale, our triumphs of modern aviation are out-performed by the common pigeon and barn swallow. Flexible, independently manoeuvrable wings allow birds to cover more body lengths per second, withstand higher gravitational forces, and achieve greater roll rates. A Blackbird jet flying at 2000 miles per hour covers 32 body lengths per second, while a common pigeon flying at 50 miles per hour covers 75. The roll rate of a barn swallow exceeds 5000 degrees per second, while the A-4 Skyhawk achieves a roll rate of 720 degrees per second. A video discussing the research is available at the University of Michigan site.
Their superiority in the sky can partly be attributed several millennia of practice. While we humans have dedicated roughly a century to mastering the sky, evolution has honed the sophisticated flying mechanisms of birds for 150 million years. In order to navigate through wind, rain, and snow, birds have evolved malleable wings that deform when exposed to pressure. This change of shape allows flying creatures to instantly adapt to continually changing conditions, and also gives them increased control over their movements, allowing them to stall, enhance stability, and increase thrust.
Although the idea of a flapping-wing aircraft may seem new, the concept is about as old as fixed-wing models. Inventors in the 19th century experimented with ornithopters powered by steam and compressed air, and in 1942 Adalbert Schmid flew a motorized, manned ornithopter for 15 minutes. Going back further in time, Leonardo Da Vinci’s famous helicopter design is an ornithopter and the Greek legend of Daedalus and Icarus also proposed the idea that man could use technology to imitate the flight of birds - but if you wish to fly close to the sun, don’t make your plane out of wax and graft it directly to your body.
Ornithopters were initially researched in an attempt to duplicate the success of nature, but were overlooked in the 20th century because fixed-wing models were far better people carriers. However, now that the age of the UAV is upon us, research is accelerating. US UAVs logged more than 160,000 flight hours in 2006, a fourfold increase from 2003. They are being used in Afghanistan and Iraq for combat, logistics operations, target acquisition, decoy, and reconnaissance. Lacking a pilot means the craft are able to undertake dangerous missions, and lengthy missions that exceed the amount of time a pilot can remain active, but it also means that the crafts need to be efficient, autonomous and adaptable - a profile that could fit miniature ornithopters perfectly.
DARPA started funding research into Micro air vehicles, or UAVs less than 15cm in length, in 1996. Not only did the program result in a resurrection of ornithopter design, but the boost in profile has resulted in many hobbyists and universities continuing the trend. Researchers at the California Institute of technology, Vanderbilt University and Harvard University have experimented with models, while the Georgia Institute of Technology’s chemically powered Entomopter is being considered for both military applications and a possible role as a Mars probe. NASA started funding the project after it was revealed that the unique Mars atmosphere would allow an ornithopter to fly slowly over the landscape, land, and take samples, before taking off and continuing its mission.
Ornithopter MAVs that could hover, perch, and move efficiently through tough weather conditions would be invaluable to the military for data gathering purposes. By mimicking the flight behavior of birds they would greatly cut down on energy consumption, while simultaneously increasing reaction time.
In the quest for UAV autonomy and efficiency, developers are taking many different approaches. Northrop Grumman has integrated a “Sense-and-Avoid” capability into its Global Hawk that allows the UAV to avert collisions based on visual data. NASA is working on an Active Aeroelastic Wing manned craft that automatically morphs itself in response to flight conditions. Gatech’s GT Max rotary wing UAV is able to learn as it flies, manoeuvre aggressively and automatically navigate through obstacles. While these craft don’t use the ornithopter design, they are part of the same trend of scientists once again emulating the extraordinary successes of nature.