## Thursday, February 1, 2001

### How Do Airplanes Stay in the Air?

Flying for the holidays? Or, did grandma fly in to visit you for Christmas? Have you ever wondered--while on a plane or watching one--how a heavy, metal plane flies through the air?

Whether you are scared of flying or not, the answer to this question is a little frightening. Physicists and aeronautical engineers continue to debate the basic mechanics of flight. This is even more suprising when you consider that the Wright brothers invented the plane over 100 years ago! In spite of the debate, everyone agrees that flight boils down to physics. Yes, this means that no invisible strings are hanging from space!

Most will go on to explain that a plane's ability to fly results from air traveling faster over the more curvaceous top surface of the wing than under the flatter bottom surface. The quicker a fluid like air moves, the less pressure it exerts. This phenomenon is known as Bernoulli's principle and was discovered by Daniel Bernoulli, an 18th century Swiss mathematician. The principle helps us to understand how slower moving air below the wing exerts more pressure on the wing than the faster moving air above it. This results in an upward force called lift. Lift pushes the aircraft upward against the downward pull of gravity.

This principle is well-accepted and accurate, but it does not help us to understand why the air flowing over the wing moves faster. This lack of explanation causes confusion among physicists and aeronautical engineers. One of the leaders in helping to explain this phenomenon, Jef Raskin, actually started his research by arguing with one of his middle school teachers. He argued that this principle didn't make sense because he had seen planes fly upside down. Raskin felt that Sir Isaac Newton's laws of motion were a better explanation, as "a wing is just a device for forcing air down." According to Newton's third law, for every action there is an equal and opposite reaction, so the downward force that the wing applies to the air produces an upward force of the air on the wing. The amount of air directed downward depends on the angle of the wing (the angle of attack) and not the shape of the wing. This would help explain why a plane can fly upside down.

Although Newton's law helps to explain how planes can fly upside down, most believe that both Bernoulli and Newton help to explain flight. They also recognize that curves on the top of the plane are important and without them an airplane may stall and fall.

Flight is a fascinating science, and many aspects of why we can fly are still under debate. Start asking your teacher questions and maybe you will help get some of the debate of flight mechanics off the ground!

Source: Chang K. "What Does Keep Them Up There?" New York Times. 9 December 2003.

### Little Lion Experiment

An important concept in understanding how objects move is gravity. Gravity is the tendency of matter toward some attracting body, particularly towards the center of the earth. To understand the effect of gravity or a flying objects attraction to the ground, drop varies objects such as a piece of paper, a shoe, a book, and a feather from a table. Guess, watch, time, and test which object falls the fastest.

Gravity adds weight or force - an equal amount to each object. But, gravity increases the force or speed of motion on the heavier objects. Thus, the heavier objects such as a shoe or book would beat a piece of paper or feather. You can also feel this principle at work when you slip on ice with and without your book bag! Be careful this winter!