In this blog post, we’ll explore the basic principles of fluid dynamics—the study of air and water—and examine how the airflow and pressure differences around an airplane’s wings generate lift, enabling the heavy aircraft to take flight.
If we were to list the substances essential for life on Earth, what would they be? Many people would think of air and water. Air and water play an essential role in the survival of living organisms, and all life on Earth constantly interacts with them. Therefore, living organisms are continuously influenced by the movement of air and water, as well as the forces they generate. Fluid dynamics is the field that studies the movement of gases, such as air, and liquids, such as water, as well as the forces generated during these processes. The movement of fluids is often difficult to observe directly and thus hard to understand, but it is precisely this characteristic that makes many phenomena seem mysterious. A prime example of this is how an airplane stays airborne. How exactly does an airplane moving forward generate an upward force that allows it to fly?
The key reason an airplane can fly is that its wings alter the flow of air, generating an aerodynamic force called lift. Lift acts in a direction that supports the airplane, and in most airplanes, the majority of lift is generated by the wings. Around the wings, the speed and direction of the air change, and consequently, the pressure distribution also changes. Generally, on a wing generating lift, the pressure on the upper surface is lower than that on the lower surface; when this pressure distribution is summed across the entire wing, an upward force is created. At the same time, the wing deflects the passing airflow downward; from the perspective of action and reaction, as the wing deflects the air downward, it receives an upward force in return. Therefore, rather than explaining lift simply as a pressure difference between the top and bottom of the wing, or conversely, as merely the act of pushing air downward, it is more accurate to understand these two phenomena as a single, interconnected aerodynamic process. NASA also explains that when a wing deflects the airflow, lift is generated in the opposite direction, and the U.S. Federal Aviation Administration (FAA) explains that the low pressure on the upper surface of the wing, the relatively high pressure on the lower surface, and the downward flow of air all contribute to the generation of lift.
To easily understand this relationship between forces, imagine a situation where forces are exerted in opposite directions. An airplane’s weight, caused by gravity, acts downward, while the lift generated by the wings acts primarily upward. When an airplane flies level at a constant altitude, the vertical component of lift must balance the airplane’s weight. Conversely, if lift is less than the weight, the airplane struggles to maintain altitude; if the necessary conditions are met and lift increases, the airplane can climb. In actual flight, thrust generated by the engines and drag—the resistance of the air against the aircraft’s forward motion—also come into play. Therefore, flight can be described as a process in which various forces—lift, weight, thrust, and drag—interact with one another. The FAA also explains that maintaining straight-and-level flight requires not only managing the relationship between lift and weight but also appropriately adjusting speed and angle of attack.
While this explanation makes the principle seem relatively simple, actual airplanes are extremely heavy. In the case of large passenger aircraft, the maximum takeoff weight can reach hundreds of metric tons, depending on the aircraft type and operating conditions. To lift such a massive object into the air, the wings must generate a tremendous amount of lift. However, this does not mean that the air simply pushes the underside of the wing upward with great force. When examining the actual pressure distribution around the wing, the low pressure formed on the upper surface of the wing has a significant effect on lift, and the high pressure on the lower surface also contributes to lift. The FAA explains that, based on investigations of pressure distribution on wing surfaces conducted through wind tunnel tests and actual aircraft, under typical lift-generating conditions, the low pressure on the upper surface of the wing generates a considerable force, and the high pressure on the lower surface also contributes to the upward force.
So how do airplane wings create this pressure distribution? To understand this, we must first examine the movement of air as it flows along a curved path. For a fluid flow to move along a curved path rather than in a straight line, a force is required to bend the flow. In this case, a pressure difference can form perpendicular to the direction of flow, and these spatial changes in pressure cause the fluid to accelerate and change its direction of travel. This relationship between fluid motion and pressure can be explained by the fundamental equations of fluid mechanics, including Euler’s equations. Conversely, when the flow of a fluid is artificially deflected by the shape or motion of an object, a pressure distribution associated with that flow is formed. As explained in the original text, the direction of fluid flow can change depending on pressure differences, and conversely, the pressure distribution also changes during the process of deflecting the fluid flow.
This phenomenon also occurs around an airplane wing. The airfoil—the cross-section of an airplane wing—is designed to alter the flow of air. The upper surface of a typical passenger aircraft wing is curved, and during flight, the wing maintains a constant angle of attack relative to the airflow. The air flowing around the wing changes in speed and direction due to the wing’s shape and angle of attack, resulting in different pressure distributions on the upper and lower surfaces of the wing. In particular, during normal flight conditions when lift is generated, a pressure lower than that of the surrounding free flow forms over a significant portion of the wing’s upper surface, while relatively high pressure may form on the lower surface. When these pressure differences are summed across the entire wing surface, they result in lift that lifts the aircraft upward. While the original text focuses on the relationship between the curvature of the upper surface of the wing and the pressure drop as a way to understand this phenomenon, actual lift is not determined by wing curvature alone but is generated by the combined action of various factors, including the wing’s shape, angle of attack, and the speed and density of the air.
There is one point to note here. It is often explained that because the upper surface of an airplane’s wing is longer than the lower surface, the air passing over the upper surface must travel faster to meet the air on the lower surface at the rear at the same time, resulting in lower pressure on the upper surface. However, this is not an accurate explanation. There is no physical law stating that air split at the front of the wing must necessarily reunite at the rear of the wing at the exact same time. In reality, the wing’s shape, angle of attack, air viscosity, circulation patterns, and pressure distribution all interact to alter the speed and direction of the airflow. NASA also clearly distinguishes explanations relying on the so-called “equal transit time” or “longer path” as incorrect theories of lift. Therefore, when understanding the principle of how an airplane takes off, it is more accurate to recognize that the wing alters the entire surrounding airflow—resulting in a pressure distribution and a downward airflow—rather than simply thinking that “the air moves faster because the upper path is longer.”
The magnitude of lift is also closely related to flight speed. Generally, lift is influenced by the lift coefficient, which varies depending on air density, the square of the flight speed, wing area, and factors such as wing shape and angle of attack. Therefore, even with the same wing, an increase in airspeed—all other conditions being equal—can generate greater lift. This is one of the key reasons why an airplane must achieve sufficient speed on the runway during takeoff. Conversely, when airspeed decreases, a higher lift coefficient is required to support the same weight, which generally necessitates increasing the angle of attack. However, the angle of attack cannot be increased indefinitely. If the critical angle of attack is exceeded, the flow over the wing can separate significantly, leading to a stall—a condition in which lift drops sharply. Therefore, the phenomenon of an airplane staying airborne is not achieved by the shape of the wing alone, but is the result of the combined effects of various factors, such as the wing’s shape and area, angle of attack, flight speed, and air density.
Fluids such as air and water are essential substances for living organisms, and at the same time, they constitute the dynamic environment that governs the movement of living beings and objects. Living organisms maintain their vital functions using various substances, including oxygen and water; however, since they live within fluids such as air and water, their movements are constantly influenced by the forces generated by these fluids. Birds are able to fly through the air and airplanes are able to fly through the sky thanks to their interaction with air, just as penguins and turtles move through water by utilizing their body shapes and flippers to exchange forces with the fluid. In this way, air and water are not only essential for survival but also serve as the mechanical environment that enables movements such as walking, flying, and swimming. This is precisely why fluid mechanics—the study of fluid motion and forces—is utilized in the design of countless machines and engineering technologies, including aircraft, ships, automobiles, turbines, and pumps.
