In this blog post, we’ll explore how the flying fish—a species of fish—uses fluid dynamics principles such as lift, drag, fin structure, and ground effect to glide long distances for extended periods.
The flying fish, also known as “Flying Fish,” can glide distances of up to 400 meters with a single leap and is known to remain airborne for up to about 40 seconds while traveling at speeds of up to 70 km/h. The flying fish’s ability to fly is remarkable, even when compared to the first powered airplanes built by humans. The first powered airplane, built by the Wright brothers on December 17, 1903, flew approximately 36 meters in 12 seconds during its maiden flight, and on its fourth and final flight that day, it flew approximately 260 meters in 59 seconds. Considering these facts, the flying fish demonstrates gliding capabilities far more efficient than those of the first powered airplane ever built by humans. How can a flying fish—which is a fish, not a bird—glide as far as 400 meters in a single leap and remain airborne for as long as 40 seconds? Understanding various principles of fluid dynamics provides a deeper insight into the flying fish’s flight.
Take a deep breath. Or go outside and feel the wind brushing past your body. You’ll likely sense that the space around you is filled with air. When you stand still, the air around you seems to have no effect on you. Most people go about their lives without even being aware that air surrounds them. However, the story is different when it comes to moving objects. Think about the time you sprinted at full speed. You can feel the resistance of the air with your whole body. You’ll literally feel as if you’re “cutting through the wind” as you move forward. The air acts as a force of resistance, making it difficult for us to move forward. Also, think about when you fold and launch a paper airplane or fly a kite. Riding the wind, paper airplanes and kites can stay aloft in the sky. In these cases, the air plays the role of lifting the paper airplane and the kite. In this way, air can act as a resistance against moving objects, hindering their motion, or it can lift objects, allowing them to float in the air. Fluid dynamics is the field of study that examines the properties of “fluids”—including air and liquids such as water—and the forces generated by them.
The force that a fluid exerts to impede the motion of a moving object—such as the air resistance experienced when running—is called “drag.” There are two main causes of drag: friction and pressure. Just as dragging an object across the ground becomes difficult due to friction between the ground and the object, when an object flies through the air, friction between the surrounding air and the object creates a force that impedes its motion. Meanwhile, as an object flies through the air, the flow of air around it changes, creating differences in pressure around the object. While the pressure distribution depends on many factors—such as the object’s shape, speed, temperature, and pressure—the pressure in front of the object is generally higher than that behind it. Since pressure exerts a force from areas of high pressure to areas of low pressure, a force acts in the direction opposite to the object’s flight path. In other words, drag caused by pressure is generated.
The force that causes a fluid to lift an object upward—as seen when flying a paper airplane or a kite—is called “lift.” Lift is generally a force acting perpendicular to drag and is generated by pressure differences around a moving object. As mentioned earlier, when an object flies, the airflow around it changes; if a pressure difference arises between the top and bottom of the object at this time, an upward force can be generated. In particular, if the pressure beneath the object is higher than that above it, a force directed upward is generated. In other words, lift is generated. It is precisely because of lift that airplanes are able to fly.
Thus, the two main forces generated by a fluid such as air are drag and lift. These forces arise from the airflow and pressure distribution around a moving object. However, the airflow around an object varies depending on the object’s size and shape. A streamlined shape is a classic example of a design that reduces drag. Generally, the smoother and more streamlined an object’s surface is, the more smoothly the airflow passes over it, which helps reduce drag. Therefore, for objects moving at the same speed, those with fewer protrusions and a more streamlined shape experience less air resistance, thereby reducing the force required for movement. This is why unnecessary protrusions, such as screws, are minimized on the exteriors of airplanes and cars, and their cross-sections are designed to be as streamlined as possible.
The airflow around an object is highly complex and influenced by many factors—including not only the object’s size and shape but also surface material, angle, speed, and air temperature and pressure—making it difficult to determine with precision. Consequently, accurately calculating the drag and lift acting on an object is a very challenging task. To calculate these forces, various methods—including experimental methods, dimensional analysis, and computer-based numerical analysis—have been developed, and research to understand fluid flow and the forces acting on objects is still actively underway.
Let’s return to the “flight of the flying fish” mentioned at the beginning of this article. The principles behind the flying fish’s flight were also elucidated through fluid dynamics research employing various methods and are well known as the result of research conducted by Professor Choi Hae-cheon and researcher Park Hyung-min from the Department of Mechanical and Aerospace Engineering at Seoul National University. The flying fish is able to glide long distances for extended periods by utilizing its fin structure to achieve a high lift-to-drag ratio and by reducing drag through ground effect while flying close to the water’s surface. First, flying fish can generate high lift through the arrangement and structure of their fins. They have a unique structure in which the pectoral and ventral fins are arranged one in front of the other, and the angle difference between these two fins affects the airflow. When the body of a swallowtail flying fish is viewed horizontally, the front of the pectoral fin points upward at an angle of approximately 12–15 degrees, while the ventral fin points upward at an angle of approximately 2–5 degrees. This angular difference increases the speed of the air flowing between the two fins, resulting in stronger lift. Through wind tunnel experiments using actual flying fish and visualization of the flow structure, the research team confirmed that this fin arrangement plays a crucial role in enhancing the flying fish’s aerodynamic performance. In particular, the “tandem wing” structure—in which the pectoral and pelvic fins are arranged one behind the other—contributes to the flying fish achieving a high lift-to-drag ratio.
Furthermore, by gliding at a height close to the water’s surface, flying fish can reduce drag and fly long distances with minimal effort. If a flying fish flies at a high altitude, induced drag may increase due to strong vortices generated at the tips of its fins. Conversely, when flying very close to the water’s surface, the influence of the water’s surface alters the formation and development of these vortices; pressure beneath the fins also increases, resulting in the “ground effect,” where lift increases while drag decreases. Ground effect is a phenomenon in which drag decreases and the lift-to-drag ratio increases as the flow structure changes when an aircraft moves close to the ground or water surface. Flying fish can utilize this ground effect to glide efficiently near the water’s surface. The study confirmed that as flying fish fly closer to the water’s surface, their drag decreases and their lift-to-drag ratio increases. When combined with the flying fish’s “taxiing” behavior—in which it moves side-to-side with its tail submerged in the water to regain propulsion—it can remain airborne for extended periods and travel distances of up to 400 meters. In this way, the flying fish achieves efficient gliding by appropriately utilizing the aerodynamic characteristics of its fins, the ground effect that occurs near the water’s surface, and the propulsion it gains underwater.
The principles of fluid dynamics evident in the flying fish can be found in most moving objects around us and are applied in many areas, ranging from cars, trains, and airplanes to full-body swimsuits and golf balls. Furthermore, research aimed at understanding these principles and applying them to everyday life is actively underway, and fluid dynamics is being utilized to improve the performance of not only aircraft and automobiles but also various other vehicles and mechanical systems. Research in biomimetic engineering—which seeks to identify engineering principles in the structure and movement of living organisms, such as the flying fish, and apply them to technology—is also continuing. Therefore, research on the principles of fluid dynamics and their application to everyday life is likely to be utilized in various fields in the future, and the potential applications of fluid dynamics appear to be limitless.
