How Do Airplanes Fly?

How do airplanes fly? A brief desripction of the theory of powered flight, and the basic physics of airfoils and why airplanes stay up in the air.

Have you ever watched a big jetliner lumber into position on the runway for takeoff and wonder: "how does that thing ever get off the ground?" You know it's because of the wing that it stays up in the air, but how does it really work?

Man has always watched birds as they take flight and wondered how these creatures can take to the sky while the rest of us remain earthbound. The key has always been figuring out how the wing works in lifting and propelling birds in flight. After centuries of trial and error, the Wright Brothers took their first powered, heavier than air flight in 1903.

What they figured out, and what has since become a major aspect of modern civilization, is the "airfoil". The wing on an airplane is an airfoil. When air flows around the wing, it creates lift. They way it creates lift is based on the wing's movement through the air and the air pressure created around the wing. An airplane's wing, in varying degrees depending on the type and design of the airplane, is curved over the top of the wing and straighter underneath the wing. This shape is key in how lift is created as the wing moves through the air. As air hits the wing, it is "split in two", with air moving both over and under the wing. Since the top of the wing has more curve than the the bottom of the wing, the air moving over the top of the wing has further to travel, and thus must move faster than the air moving underneath the wing. The air moving over the top of the wing now has decreased air pressure than the slower moving air under the wing. Lift is created.



This difference in air pressure is the primary force creating lift on a wing, but one other force exerted on the airfoil helps to produce lift. This is the force of deflection. Air moving along the underside of the wing is deflected downward. We remember that Isaac Newton tells us that for "every action, there is an equal and opposite reaction". Thus, the air that is deflected downward (action), helps to push the wing upward (reaction), producing more lift.

These two natural forces on the wing, pressure and deflection, produce lift. The faster the wing moves through the air and the greater the forces become, the greater the lift.

The physics and math get a little more complicated when figuring just how to build a wing to produce the required lift for a particular airframe, and other components are needed to control and stabilize the airplane in flight. The components include the vertical fin, rudder, horizontal stabilizer, and elevator (which can be collectively termed the "tail" of the airplane). But all these elements, like the wing, are airfoils and operate in accordance with these basic principals of pressure and deflection as they move through the air. A propeller is also an airfoil, and it produces "horizontal" lift, pulling the airplane forward. Depending on choice of viewpoint, it can be said the airplane is drawn forward into the vaccuum of a low pressure area created by the spinning airfoil we know as a propeller.(The reaction being thrust.)The wing of an airplane can also be said to induce a draw of the airplane upward into this airfoil created vaccuum depending on how you care to veiw the pressure difference. The key to understanding more completely what an airfoil is doing is the knowledge that air it's self has a degree of mass. The more condensed the air, the more the airfoil (the wing, the prop, or the jet engine), has to work with. This is what restricts airplanes to altitudes that have suffecient air mass. So although airplanes are lifted and held up by pressure difference they ride on air mass. (jet engines are beyond the scope of this article, but they push air through their internal components at very high pressure, creating the required forward motion)

As that giant jetliner rumbles down the runway, faster and faster, until it finally lifts of the ground and into the sky, you'll be secure in the knowledge that what is lifting the craft into the air are basic laws of physics. The same laws of physics that have lifted our feathered friends into the air for millions of years!

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