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The Physics Behind Flight

Taking flight has fascinated people for thousands of years. With a yearning to soar through the air like birds, people have long written stories and passed along fairy tales about humans and animals who were able to fly. In reality, people don't have the ability to fly because the human body lacks wings and a suitable source of power. Over history, many people have tried to fly in various ways. From gliders to hot-air balloons to jet airplanes, humans have learned about flight by a process of trial and error. The forces that combine to produce flight are weight, lift, thrust, and drag. These four forces contribute to acceleration of a plane, making flight possible. Because the forces are interconnected, changing any one of them affects the others.

Weight and Lift

The force of gravity pulls everything down toward the surface and ultimately the center of Earth. Scientists call this pull "weight force." Anyone who jumps into the air will immediately be pulled back down to the ground thanks to the force of gravity. Objects and animals that fly have the ability to oppose this weight force with the force of lift. Lift force acts against weight force when air moves both over and under wings. Air will move faster above a wing and slower below the wing, which works to create a pressure difference. This pressure difference is instrumental in keeping a plane or a bird in the air.

Thrust and Drag

The power source needed for propulsion through the air is known as thrust. Thrust force moves an object forward. For birds, thrust is provided by their muscles. For a jet plane, thrust is provided by its engine. Gliders without engines create thrust force with the specific angle of the wings in the air. The force that counteracts thrust is drag, which is created by air resistance. A flying object's drag is dependent on the object's shape, the flying speed, and the air density.

How it Works

As an object or animal flies, a continual counterbalance occurs between the opposing forces of lift and weight and thrust and drag. At takeoff, thrust works against drag and lift works against weight, making it possible for an object to move into the air. To fly successfully, the lift force must be greater than the weight force and the thrust force must be greater than the drag force. Both lift and drag occur when an object moves through the air. During this movement, the weight force pulls down on the object, which opposes the lift that happens as air moves over the wings. Thrust happens either by muscle or engine power, opposing the drag that comes from air resistance.

For a plane or bird to fly straight and maintain a constant speed and height, lift force pulls upward while weight force pulls downward. At the same time, thrust force pulls forward while drag force pulls backward. If these forces are not balanced, the plane or bird will either fly faster or slower or it will move in the direction of the strongest force. An increase in thrust will cause higher speeds. This acceleration will make the air flow over and under the wings faster, which increases the lift force and makes the object fly higher. Faster flight speeds create more drag, which causes a decrease in acceleration until the thrust and drag forces equalize again. This means that the plane keeps a constant speed but at a higher elevation. To decrease the elevation, the pilot would need to reduce the thrust. This results in more drag and less thrust, so the plane flies lower and slower. Aircraft modifications such as increasing weight or wing span that affect one force will affect all of the forces.

Additional Resources

The Physics Behind Flight