April 2008

Engineering a flying machine by aping birds

Instead of a fixed-winged aircraft that flies straight through all types of weather, how about one that soars with flapping wings like a bird? Aerospace industries divisions await the answer.

The concept is under study because natural flyers like birds, bats, and insects outperform man-made aircraft in aerobatics and efficiency.

University of Michigan engineers are studying these animals so they can design flapping-wing planes with wingspans smaller than a deck of playing cards.

A Blackbird jet flying nearly 2,000 miles per hour covers 32 body lengths per second. However, 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.

"Natural flyers obviously have some highly varied mechanical properties that we really have not incorporated in engineering," said Wei Shyy, chair of the Aerospace Engineering department at Michigan.

Shyy photographs birds to help him understand their aerodynamics.

Pressure generated during flight cause the flapping wings to deform, he said. In turn, the deformed wing tells the air the wing shape is different than it appears in still air. If appropriately handled, this phenomenon can delay stall, enhance stability, and increase thrust.

Flapping flight is inherently unsteady, but that is why it works so well. Birds, bats, and insects fly in a messy environment full of gusts traveling at speeds similar to their own. Yet they can react almost instantaneously and adapt with their flexible wings.

Small flapping wing aircraft would fly slower than their fixed wing counterparts would, and more importantly, they would be able to hover and possibly perch in order to monitor the environment or a hostile area. Shyy's current focus is on the aerodynamics of flexible wings related to micro air vehicles with wingspans between 1 and 3 inches.

"These days, if you want to design a flapping wing vehicle, you could build one with trial and error, but in a controlled environment with no wind gusts," Shyy said. "We are trying to figure out how to design a vehicle that can perform a mission in an uncertain environment. When the wind blows, how do they stay on course?"

A dragonfly, Shyy said, has remarkable resilience to wind, considering how light it is.