Mad About Science:

By Ben Olson
Reader Staff

We human beings have accomplished a lot in our short time on Earth. We’ve harnessed the forces of nature to work for us, achieved technological advances that blow other species out of the water — sometimes literally — but one ability we’ve never been able to accomplish is flying (unless you’re Superman or Neo from The Matrix trilogy).

To overcome this, humans have engineered fantastic machines that take us to the air in everything from hot air balloons to supersonic jets. But how exactly is flight achieved?

Put simply, flying is nothing more than overcoming the forces that keep us tethered to the ground. A force is anything that pushes or pulls. Unbalanced forces produce an acceleration of an object in the direction of the resultant force. Four main forces that affect the flight abilities of birds and airplanes are weight, lift, thrust and drag.

An illustration showing the Bernoulli Principle providing lift. Courtesy image.

With the exception of those unfortunate ones who honestly believe the Earth is flat, most of us know about gravity. It’s the force that pulls everything toward the surface of the Earth. In order to achieve flight, airplanes and birds must be able to provide enough lifting force to oppose gravity, also known as the weight force. Lift is a force that acts upwards against weight and is caused by the air moving over and under the wings.

The power source of a flying object provides the thrust, which moves an object forward. With birds and other flying animals, muscles provide the thrust. For flying machines like airplanes, thrust comes from the engine power. For gliders, which fly by always diving at a very shallow angle (birds also do this when they glide), thrust is provided by gravity itself.

Finally, the force working against thrust is called drag, which is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the shape and speed of the object, as well as the density of the air through which it moves. The correct amount of thrust can overcome or counteract the force of drag. 

Imagine driving on a paved surface, then quickly turning onto a gravel road. The “drag” of the gravel will slow down the vehicle because it “grabs” the tires with more force. To retain the same speed as the paved road, a vehicle must then increase its power. Heavier air will drag the bird or plane more, so it takes more thrust to overcome this drag.

When an object is in flight, it is constantly engaging in a tug of war between these opposing forces. To achieve flight, the lift force must be greater than the weight force, and the thrust must be greater than the drag force.

Consider a modern jetliner, which weighs anywhere from 300,000 to 400,000 pounds. To get this behemoth airborne — that is, overcoming the weight force pulling it to the ground — jet engines (or propellers) create enough thrust to oppose the drag caused by air resistance. During takeoff, thrust must counteract drag and lift must counteract the weight before the plane can become airborne. If these forces are not equal or balanced, the object will speed up, slow down or change direction toward the greatest force.

When a plane’s engine causes it to accelerate, the acceleration increases air speed past the wing, which increases lift so the jet gains altitude; but, because the plane is moving faster, drag is increased from air resistance, which slows the plane from speeding up as quickly until thrust and drag are again equal. By balancing these forces, a plane can remain at a constant but greater height.

Approaching the runway, an airplane must lose altitude by reducing thrust. The pilot intentionally allows the drag to become greater than the thrust and the plane slows, losing altitude. A landing is nothing more than a very controlled crash to the Earth.

The wings of an airplane are specially designed to produce enough lift to equal its weight. The shape of wings are called aerofoils, which generally have a flat bottom and curved, tear-drop shape along the top.

There is some debate over what actually produces lift. The traditional explanation of lift has been attributed to a Swiss mathematician named Daniel Bernoulli, who found in 1738 that when a gas (like air) moves, it exerts less pressure. According to Bernoulli’s principle, the faster that air moves, the less air pressure it exerts because the molecules in the air become more spread out.

Because the airflow is disturbed by the aerofoil shape, it divides when flowing around a wing. The top curved surface means air flows faster along the top of the wing than the air flowing along the bottom, flat part of the wing. Because it’s moving faster on the top of the wing, the air pressure is weaker on top of the wing than it is below, which causes the “stronger” air below to “push” the wing more than the air above, creating lift.

In recent years, scientists have debated whether “Bernoulli’s principle” is really what causes lift. Some scientists believe the angle of attack is actually what lifts the wing. This can be explained by Sir. Isaac Newton’s third law of motion, which states that for every action, there is an equal and opposite reaction. Based on this law, wings are forced upward because they are tilted, pushing air downward so the wings get pushed upward. The angle of attack is the angle at which the wing meets the airflow. 

The amount of lift depends on the speed of the air around the wing and the density of the air, so birds and planes will change their angle of attack as they slow to land. This means both birds and planes need wings that are moveable, enabling their shapes to be changed to control their flight.

Flying may look effortless when we see birds soaring through the air, but behind it all is a lot of cool science that allows us — with some deft accompaniment from machinery — to defy our biology and soar like the birds.

Stay curious, 7B.