What makes an airplane fly? — BO, Pemberton, MN
As an airplane’s wing moves through the air, the airstream approaching the wing separates into a flow over the top of the wing and a flow under the bottom of the wing. The wing is shaped and tilted so that the flow over the wing follows a longer path to arrive at the sharp trailing edge of the wing than the flow under the wing must follow. Because it has a shorter distance to travel, the flow under the wing initially arrives at the trailing edge of the wing first and flows up and around that trailing edge to meet the flow over the wing. This type of flow has a kink in it at the wing’s trail edge and is unstable. A few moments after the wing begins moving through the air, the kink at the trailing edge blows away from the wing altogether. This kink leaves as a vortex—a whirling cyclone of air—and as it does, it causes the flow over the wing to speed up so that the two airflows join together cleanly at the wing’s trailing edge. To increase its speed, the flow over the wing converts some of its pressure energy into kinetic energy. Because the flow over the wing has used up some of its pressure energy, and thus experienced a drop in pressure, there is an unbalanced pressure across the wing: the pressure beneath the wing is greater than the pressure above the wing. This imbalance in pressure leads to an overall upward force on the wing and this upward force is what supports the plane’s weight so that it remains suspended in the air. Overall, the airstream is deflected downward as the result of this complicated flow pattern around the wing and the air pushes the wing upward in response. A nice image of the airstream leaving a plane’s wings can be seen at the Canon website, http://www.usa.canon.com/explorers/flight.html.