In this blog post, we’ll explore how an airplane weighing hundreds of metric tons can fly, understand the principles of flight through the lift generated by the wings, and examine the flaws in the commonly held “long-path theory.”
When I lie in bed scrolling through Facebook before sleep, I’m constantly seeing a flood of photos posted by friends from their overseas trips—featuring blue oceans, famous landmarks, and more. Overseas travel is no longer a leisure activity reserved for a select few; in 2025, the number of South Korean citizens traveling abroad reached approximately 29.58 million. While there are various factors behind overseas travel becoming a common leisure activity, the advancement of aircraft has played a particularly significant role.
The “Airbus A380,” a representative model among the large passenger aircraft currently in service, stands 24.1 meters tall, and depending on the model, its maximum takeoff weight reaches approximately 575 metric tons. How can such a massive aircraft, weighing hundreds of metric tons, fly through the sky?
Let’s think back to when we were little and begged our dads to take us on a plane ride. Dad would hold my hand, lift me up with his feet, and pretend I was on an airplane. The force that pushes an object upward—just like the force Dad’s legs exerted on me—is called “lift.” In other words, to counteract an airplane’s enormous weight and keep it airborne, lift equal to or greater than that weight is required. The heavier the airplane, the greater the lift needed.
Early aircraft engineers faced a major challenge in generating lift for airplanes. Unlike when my dad simply had to lift me up, a real airplane had to move forward to travel from one place to another. After much deliberation, aircraft engineers devised a way for airplanes to generate lift on their own as they moved forward at high speeds. The wings attached to the airplane are what perform this function.
If we simplify the shape of one side of an airplane wing, we can see that it has a unique shape—neither an ellipse nor a rectangle—that is difficult to define. Since the shape of the wings varies from one airplane to another, and the amount of lift generated depends on both the wing shape and flight conditions, wing design is considered the core of the entire airplane’s design. So how does an airplane wing with such a unique shape generate lift—the force that lifts the aircraft?
First, let’s examine the “long path theory,” the most widely recognized explanation for how wings generate lift. The curved length of an airplane wing’s upper surface is often designed to be longer than that of the lower surface, and the long path theory uses this fact to explain lift generation. According to this theory, when air hits an airplane wing, it splits into upper and lower streams as it passes over the wing; to cover the longer path in the same amount of time, the upper stream of air flows around the wing at a faster speed than the lower stream. It further explains that lift is generated by a pressure difference between the lower pressure above the wing and the higher pressure below it, based on “Bernoulli’s principle,” which states that the faster a fluid moves, the lower its pressure, and the slower it moves, the higher its pressure. However, the so-called “long-path theory” or “equal-time-of-passage theory” is now widely recognized as a classic misconception regarding the explanation of lift. The biggest error lies in the assumption that the air flowing over and under the wing reaches the rear of the wing at the same time. If we examine the actual airflow around a wing, the air above and below the wing does not reach the trailing edge simultaneously. Furthermore, while Bernoulli’s principle is an equation describing the relationship between pressure and velocity within a single, uniform flow, the long-path theory compares two distinct airflows based on the erroneous assumption that they reach the trailing edge at the same time. Therefore, the pressure difference between the top and bottom of the wing cannot be explained in this way.
Now, let’s understand how airplanes take flight through the “principle of action and reaction,” a more accurate perspective for explaining the generation of lift. The air flowing around an airplane’s wing changes direction due to the wing’s unique shape and the airplane’s motion, resulting in a curved flow rather than a straight one around the wing. This can be easily understood by imagining a stream of water trickling over a stone bridge. When water flows downhill and encounters a stone bridge, it flows around the stones, and you can see ripples forming around them. Furthermore, the shape of these ripples varies depending on the shape of the stones. Although the air in the sky is invisible, it is a fluid just like water, and when it encounters an airplane’s wing, it flows around the wing. Thinking about this phenomenon again, the air—which was flowing like water—is prevented by the airplane’s wing from traveling in a straight line and instead follows a curved path, creating a specific airflow pattern. From this, we can see that the wing is exerting some kind of force on the air.
So, what kind of force is the wing exerting on the air? If we examine the airflow around an airplane wing, we can see that as the air passes over the wing, its direction gradually shifts downward. Let’s say we roll a ball forward toward the 12 o’clock position. But suppose the ball veers slightly to the right as it rolls and continues toward the 2 o’clock position. From this, we can easily see that the ball was subjected to a force to the right, causing it to change direction. Similarly, the fact that the direction of the airflow changes downward means that the wing is exerting a downward force on the air. An airplane’s lift is closely related to this change in airflow. In fact, lift occurs as the overall direction of the airflow—which includes both the airflow above and below the wing—changes; the wing causes a downward change in the air’s momentum, and as a reaction, it receives an upward force.
If you raise your hand and push against a wall next to you, the wall pushes back against your palm just as much as your palm pushes against it. This is the “principle of action and reaction,” which is the key to understanding how an airplane generates lift. Just as in the relationship between your palm and the wall, the fact that the surrounding air is subjected to a force that directs it downward by the airplane’s wing means that the wing is receiving a force in the opposite direction from the surrounding air.
Similarly, if you examine the airflow beneath the airplane’s wing, you can see that the airflow gradually shifts downward, indicating that the surrounding air is being pushed downward by the wing. Furthermore, according to the “principle of action and reaction,” the airplane wing experiences an upward force from the surrounding air. Thus, when an airplane wing is properly designed to move through the surrounding air at high speed, airflow is generated around the wing, and lift—which causes the airplane to rise—is generated by this flow and the pressure distribution on the wing’s surface. However, lift cannot be explained solely as the phenomenon of air being pushed upward beneath the wing. Both the upper and lower surfaces of the wing alter the flow of air, and the resulting pressure distribution determines the total lift acting on the aircraft. Therefore, rather than being conflicting explanations, Bernoulli’s principle and Newton’s laws of motion can be used together to describe the same airflow from different perspectives.
