In this blog post, we’ll explore the principles behind how airplanes—including the massive A380 passenger jet—fly, and examine how lift is actually generated by debunking the commonly held misconceptions surrounding Bernoulli’s principle and the “equal-time theory.”
“Where dreams are made. The A380, the airplane of dreams.” This was the advertising slogan used by Korean Air when it first introduced the Airbus A380 in June 2011. With a two-story cabin capable of accommodating a large number of passengers, this aircraft remains the largest passenger plane currently in service. The cabin features separate areas for duty-free shops and a cocktail lounge, and offers more spacious seating for passengers compared to other commercial aircraft. Korean Air introduced its first A380 on June 1, 2011, and began deploying it on the Incheon–Tokyo route starting on the 17th of that same month. Although the A380 is still considered the world’s largest passenger aircraft, Airbus ceased production of the A380 after delivering the final unit in 2021. The A380 has a wingspan of approximately 79.8 meters and a maximum takeoff weight of 575 metric tons. How does this massive hunk of metal actually fly?
Among the various principles of fluid dynamics, “Bernoulli’s Theorem” is likely the one most widely known to the general public through books and documentaries. Generally, according to Bernoulli’s Theorem, when the speed of a gas or liquid—that is, a fluid—increases, the pressure decreases; when the speed decreases, the pressure increases. When asked, “How do airplanes fly?” many teachers and science books explain it using Bernoulli’s Theorem.
We can think of an airplane in flight as having wings that are stationary, with air flowing around them. In aerodynamics, a cross-section shaped like an aircraft wing is called an “airfoil.” Let’s imagine air flowing around an airfoil. According to the commonly accepted explanation, because the upper surface of the wing is longer, the airflow that splits at the front of the wing must travel faster on the upper surface in order to rejoin at the rear. Therefore, based on Bernoulli’s principle, the pressure on the upper surface—where the speed is higher—becomes lower than that on the lower surface. It is explained that the resulting pressure difference exerts an upward force on the wing, and this is the principle behind the generation of “lift,” the driving force that makes an airplane fly. Furthermore, it is said that airplanes fly stably by utilizing this force along with the thrust from the engines.
Many science books, textbooks, and numerous websites explain the principle of how airplanes fly in this manner. However, this explanation actually contains a flaw from a fluid dynamics perspective. If we examine the airflow around an airfoil in detail, we see that air particles passing over the top of the wing reach the trailing edge of the airfoil before those passing under the bottom. In other words, when air flows through an airfoil, the air that splits into upper and lower streams at the leading edge does not necessarily converge at the trailing edge. Therefore, it cannot be explained that the air flowing over the top of the airfoil travels faster simply because the upper path is longer. NASA also refers to this explanation as the “Longer Path Theory” or the “Equal Transit Time Theory” and points out that it is a classic misconception used to explain lift.
So why is the airflow faster on the upper surface of an airfoil? The secret lies in the shape of the airfoil and the flow of air. If you look at the shape of an airfoil, you’ll see that it tapers smoothly toward the trailing edge, forming a streamlined shape. This shape plays a crucial role in ensuring that air flows smoothly past the trailing edge of the airfoil. Imagine directing a fluid flow over an airfoil. In reality, the flow around an airfoil does not take on a fully formed shape from the start; rather, it undergoes a process of change over a very short period of time before approaching a steady state.
To simplify the understanding of the flow around an airfoil, it may initially appear as if the airflow is bending sharply at the rear of the wing. However, in nature, such abrupt flow patterns do not persist. The flow gradually shifts toward the trailing edge of the airfoil and changes; eventually, vortices form in the flow that has moved to the trailing edge. Subsequently, as these vortices and the flow around the airfoil interact, the air takes on a smoother flow pattern behind the airfoil.
Consider coffee in a mug. If you stir the coffee with a spoon, the coffee inside the mug will rotate. In aerodynamics, “circulation” is defined as a measure of how much a fluid flow rotates along a single closed curve. If we consider the curve formed along the inner surface of the mug, the coffee is rotating in one direction, so the circulation value is not zero. Unless there is a specific external disturbance to the fluid flow, this circulation remains constant over time; in aerodynamics, this is explained in relation to Kelvin’s Circulation Theorem.
Let’s consider the circulation around a stationary airfoil. If there is no fluid flow around the stationary airfoil, the circulation value is 0. When this airfoil begins to move for takeoff, fluid flow develops around it, and a vortex forms at the trailing edge of the airfoil for a brief moment. Now, let’s divide the space around the airfoil into sections. In the region where the vortex forms, the fluid is rotating, so the circulation there is nonzero. However, if the circulation of the entire space remains constant over time, the total circulation must remain zero. Therefore, the circulation formed on the airfoil itself has a value opposite to that of the vortex. In other words, a circulation opposite to that of the vortex is formed on the airfoil. This circulation causes the fluid flow around the airfoil to rotate. If air is blowing toward the front of the airfoil, the air on the upper surface is influenced in the same direction as its original motion, causing its velocity to increase, while the air on the lower surface is influenced in the opposite direction of its original motion, causing its velocity to decrease.
In this way, aerodynamics explains the principle of how an airplane flies through “circulation” and “Bernoulli’s principle.” The problem is not that Bernoulli’s principle itself is incorrect, but rather that explaining lift by combining it with the erroneous assumption that the air passing over and under the wing meets again at the trailing edge at the same time. In reality, when explaining lift, one must examine how the airflow around the wing changes and, accordingly, how velocity and pressure are distributed around the wing. Even NASA explains that lift can be explained using both Bernoulli’s principle and Newton’s laws of motion, and that the key is to correctly understand fluid flow.
Today, airplanes have become one of the world’s most important modes of transportation. Although so many people fly, simplified explanations of how airplanes fly are still widely accepted. In particular, the explanation that the air above the wing travels a greater distance and must therefore arrive at the trailing edge at the same time as the air below—and that this causes the upper air to move faster—is easy to understand intuitively but does not accurately describe the actual airflow. Airplanes fly through the sky, carrying everyone’s memories. I wonder if the flight wouldn’t be even more enjoyable if we truly understood the principles behind how airplanes fly before boarding.
