Tuesday, March 6, 2007

What We ve Learned From Birds About Flight - and Why It Took So Long

Eons ago, Zork stood outside his cave and watched a vulture spread his wings and soar off a cliff. Zork decided to try this himself. Too late, he realized that his lift to drag ratio was about zero - no lift and lots of drag - and he had the glide ratio of a brick. Zork's decedents gave up on aviation for 50,000 years.

Since humans first looked up at birds in flight, we have been trying to fly like them, often with painful or even fatal consequences. Even with the technology available today, we are just beginning to be able to apply some of the technology that has existed in birds for, oh, 150 million years, such as variable geometry wings, vectored thrust, and fly-by-wire.

To accomplish sustained flight, an object must have some means of generating lift. In airplanes, this is done by moving an airfoil shape through the air with some means of propulsion. The amount of lift generated is a function of the airfoil shape, the angle at which the airfoil encounters the air stream, and the speed at which the airfoil moves through the air.

Birds, however, have combined their lift generating and thrust producing functions into one assembly - the wings. Early observers believed that birds simply flapped their wings up and down, and that bird-like flight could be duplicated by simply flapping their arms vigorously - like Zork - or by building some kind of contraption that flapped its wings. They assumed that if a bird flew by flapping its wings, that they could too by duplicating the motion. However, they failed to understand how a bird actually flies and how much energy it would take. So "flapping" never got off the ground. Sorry about that.

Even the great Leonardo DaVinci spent many years working on totally impractical ornithopters (flapping wing) aircraft, in part because he, too, failed to understand how birds flew.

Birds fly by "flapping" their wings, true, but the actual motion and physics are much more complex. As a bird's wing moves down, the feathers also rotate downward automatically. Air is pushed to the rear (thrust) and lift is generated on the top surface. As the wing transitions to move upward, the feathers rotate in the opposite direction, producing thrust from the top surface of the wing and lift. Think of their wings as a propeller that goes 180 degrees then reverses.

Toward the end of his life, Leonardo did design a device that used a rotating screw-like structure. Igor Sikorsky, it is said, was inspired by Leonardo's work to become a world leader in helicopter development. It's worth noting that helicopter blades and propellers are really a form of screw, so Leonardo was not far off on that one. He also designed a glider which could be turned by the pilot shifting his weight, foretelling the hang glider.

Leonardo - and Zork - also failed to understand how much energy is required to take off and sustain flight. Aircraft - and birds - must generate more lift than they weigh just in order to take off. This requires the development of several enabling technologies.

The first is a light flying structure that can support its own weight while in the air. Many early craft could not. Birds have a very light structure (Thanksgiving Turkeys not withstanding) and extremely powerful muscles to power their wings. A voracious appetite for high-energy foods provides the power.

A second enabling technology was the development of an internal combustion engine that was light, yet produced enough power to move the machine fast enough to generate lift greater than weight. And higher performing engines could not be developed until higher octane fuels became available.

To illustrate the problem, the Wright Brothers' 1903 Flyer weighed 605 pounds without a pilot, and had a 12 horsepower engine. With a pilot, that's over 60 pounds per horsepower. The Cessna 172, a popular, but hardly spectacular performer, runs about ten pounds per horsepower.

But getting airborne and staying there are two different things. Whether bird or machine, a flying object must be controllable or it simply falls out of the sky. A future article will look at stability and controllability.

Bottom line - we have learned a lot from birds about flying. It just took 150 million years.

Article Source: http://www.articlesarea.com/