# Flying High: How Do Airplanes Stay in the Air?

At any given point in time, there are more than five thousand airplanes flying over the United States. How do these machines, weighing thousands of pounds, ever leave the ground?

According to Yale geophysicist, Professor Ron Smith, airplanes stay in the air by shifting to maintain various states of equilibrium. It is all a matter of thrust and lift: the amount of thrust from the engines must overcome the drag force resisting motion, while the lifting force must counteract the effects of gravity.

Thrust from the engines is simple enough, but “lift” deserves some special treatment. Lift occurs when the special curvature of an aircraft’s wing parts the air into portions above and below it. The wing is designed so that the air above the wing must cover a longer distance in the same time that the air below moves a shorter distance. More distance over a given time means the “top air” has a faster velocity. Thanks to Bernoulli’s Principle we know that faster air particles have lower pressures. It is this imbalance of pressure above and below the wings that causes the plane to rise in altitude..

While lift may not seem intuitive, examples of this force in action can be found in everyday life. If you stick your hand out the window of a moving car, for example, palm parallel to the ground with a slight rounded edge towards the incoming air, an unmistakable force pushes your hand upwards.

An airplane’s wings have special folds called flaps that rotate to change the wing’s curvature. These flaps are used when an airplane needs to maximize its lift at slower speeds, such as takeoffs and landings. It is all a game of manipulating air flow to maintain all-important lift. As Professor Smith says, “It’s simple, if you lose the lift, you crash—fast.”