In this blog post, I’ve summarized what I learned while reading ‘Metal Wings That Cut Through the Wind’, focusing on the forces acting on an airplane, the principles behind lift, and how lift is controlled by changing the angle of attack.
Understanding How Airplanes Fly
If you’re flying on an airplane for the first time, you might see the wings swaying up and down during the flight and wonder, “Is this plane safe?” or “Can such a heavy object really fly?” I, too, found myself wondering how such a heavy aircraft could remain airborne as I watched it. To understand these questions, we need to examine the field of mechanical and aerospace engineering.
Mechanical and aerospace engineering is a practical applied science that utilizes machines, devices, and artificially processed materials for the benefit of humanity based on the laws of nature; it is founded on thermodynamics, dynamics, solid mechanics, and fluid mechanics. The generation of lift—the principle behind an airplane’s flight—can be analyzed based on fluid mechanics, while thermodynamics is used to manufacture jet engines and understand their operating principles. Furthermore, solid mechanics is necessary for verifying whether an aircraft has sustained damage and for inspecting its stability, while dynamics is used to analyze the movements that occur during an aircraft’s takeoff, landing, and flight. I came to believe that examining the principles of flight and its movements in this way is the most efficient and direct method for understanding the field of mechanical and aerospace engineering. In particular, by examining the forces acting on an airplane and the principles behind the generation of lift from a mechanical engineering perspective, I was able to gain a more concrete understanding of flight.
First, to understand the principles of flight, it is necessary to examine the fundamentals of fluid mechanics. A fluid is a substance that changes shape under the action of shear stress, and the term refers to both liquids and gases. I found it fascinating that the movement of a Boramae aircraft as it cuts through the wind and soars through the sky can also be analyzed based on the properties of fluids. Lift is the force that lifts an object heavier than air into the air. The principle behind the generation of lift can be explained through Bernoulli’s principle and Newton’s Second Law.
The forces acting on an airplane include lift (LIFT), the force that lifts the airplane; gravity (GRAVITY), the force by which the Earth pulls the airplane down; thrust (THRUST), the force generated by the engines that propels the airplane forward; and drag (DRAG), which is the sum of fluid friction and viscous forces acting in the direction opposite to the direction of flight. According to Newton’s Second Law, flight is possible when thrust exceeds drag and lift exceeds gravity. This led me to realize that an airplane does not fly simply “because it has wings,” but rather because flight occurs through the interaction and balance of various forces.
Bernoulli’s Principle and Lift Generated by Wings
Bernoulli’s Principle can be understood through the phenomenon where the same volume of water flows faster through a narrow channel than through a wide one. It is based on the law of conservation of energy, which states that in a closed system with no external energy input, the total energy derived from a fluid’s pressure, velocity, and height remains constant. Therefore, it can be understood as a phenomenon in which pressure decreases when the fluid’s velocity increases at a given point.
This principle can be observed to some extent in everyday life. If you turn on a faucet and bring a stick close to one side of the water stream, you can see the stream bend toward the stick. This can be explained as a phenomenon caused by viscosity between the object and the fluid. Similarly, during flight, viscosity arises between the airplane’s wing and the airflow, causing the air to flow along the wing’s surface.
Generally, the cross-sectional shape of an airplane wing is convex on the upper surface and relatively flat on the lower surface. As the airplane moves forward and cuts through the air, the airflow split by the wing flows along the convex upper surface and the flat lower surface, depending on the wing’s shape. At this point, due to the difference in the wing’s shape, the speed of the airflow over the upper surface increases, and according to Bernoulli’s principle, the pressure on the upper surface of the wing becomes relatively lower. Through this explanation—that lift is generated on the wing, making flight possible—I was able to understand how an airplane’s wings enable it to take to the air.
Does adjusting the angle of attack change the lift?
If so, what is the best way to properly control lift for flight? Generally, to control lift, one changes the shape of the airplane’s wings or adjusts the angle of attack. The angle of attack refers to the angle formed by the wind striking the surface of an airplane’s wing. The fact that lift can be controlled by adjusting the wing’s angle of attack was a crucial aspect of understanding the principles of flight.
To determine the optimal angle of attack, engineers build scaled-down models of actual airplanes and conduct wind tunnel tests to verify the relationship between the angle of attack and the aerodynamic forces (air pressure) experienced by the actual aircraft. Additionally, experiments are conducted to determine the correlation between angle of attack and lift through tests that examine the pressure distribution on the wing surface to verify aerodynamic forces and lift. It was also striking to learn that an airplane’s movement is not determined by theory alone but is verified and adjusted through physical models and experiments.
As the angle of attack increases, lift increases; however, so does drag—the opposing force that impedes the direction of flight. Generally, when the angle of attack is within 20 degrees, the rate of increase in drag is smaller than the rate of increase in lift, making flight possible. On the other hand, when the angle of attack exceeds 20 degrees, the rates of increase in lift and drag diverge sharply, leading to a stall. Ultimately, it is not always best to increase the angle of attack as much as possible; rather, it must be adjusted within a range appropriate for flight. Through the experimental process and scientific reasoning, I learned that lift can be controlled by changing the angle of attack, which demonstrates that both theory and experimentation are necessary for flight.
To what extent can engineering analysis explain flight?
It is clear that, based on the theories outlined above, we can understand and describe the principles that allow an airplane to fly. However, I also came to believe that these theories should not be viewed as immutable truths. Engineering analysis controls various assumptions and variables to solve problems and predicts the correlation between the flow of air—a fluid—and the forces acting on an aircraft’s wings. Yet, in experimental simulations or real-world situations, discrepancies between theoretical predictions and actual results are inevitable.
In particular, there is a limitation in that predicting the properties of a fluid—which changes shape due to shear stress—and its motion is not as straightforward as understanding and describing the dynamic motion of solids. While various scientific laws and engineering analyses are used to explain the principles of aircraft flight, there are aspects of actual natural phenomena that are difficult to fully explain through the theories and calculations we have established alone. In fact, it occurred to me that precisely because of these limitations, the important task of engineering is to continuously conduct experiments and seek new methods.
While reading this work, I came to understand that an airplane is not simply a machine that flies through the sky, but rather the result of the integration of knowledge from various fields, such as fluid mechanics, thermodynamics, solid mechanics, and dynamics.
In particular, by examining the relationships between lift, drag, thrust, and gravity acting on the wings, and by considering how lift varies depending on the angle of attack, I realized just how many scientific principles and experimental processes underlie the phenomenon of flight—something we often take for granted. At the same time, I came to reflect on the fact that engineering theories have limitations that prevent them from perfectly explaining reality, and that overcoming these limitations is a challenge that future students of mechanical and aerospace engineering must address.
