In this blog post, we’ll explore the principles of mechanical engineering that enable an airplane weighing over 100 tons to take flight.
The airplane has made the greatest contribution to bringing the world within a day’s reach. Since their invention by the Wright brothers, airplanes have been used in various ways—sometimes as lethal weapons capable of killing people, and other times as humanity’s fastest mode of transportation. Do you know how much these widely used airplanes weigh? In the case of the Boeing 747, which Korean Air primarily uses, the weight—including fuel and passengers—reaches 352 tons. You’ve probably watched Olympic weightlifting competitions. Even the strongest person in the world, capable of lifting the heaviest objects, cannot hold a weight of about 300 kg for even five seconds. So how does an airplane, weighing hundreds of tons, take off and even fly?
For an airplane to fly, two forces are required: lift and thrust. Let’s first look at lift. Lift is the force that lifts the airplane, generated by the pressure difference between the top and bottom of the wings. To understand lift, we first need to understand two properties of fluids. First, fluids flow from areas of high pressure to areas of low pressure. This is easy to understand if you think of a tube. When you open the cap of a tube, air escapes because the pressure inside the tube is greater than the atmospheric pressure outside the tube.
Let’s look at a slightly more complex example. Consider the following setup: a straw is inserted into a container filled with water, and another straw is attached perpendicular to it. Each end of the two straws has a hole, allowing air to pass through or water to be sprayed out. If you blow air into one of the straws, the air passes through it rapidly, lowering the pressure inside the straw. As a result, the pressure in the lower straw becomes greater than that in the upper straw, and due to this pressure difference, the water at the bottom overcomes gravity and flows out. Just like the principle behind water spraying from a spray bottle, lift is also a phenomenon caused by a pressure difference.
Everyone has probably walked down the street with an umbrella during a typhoon. If the wind blows from above the umbrella, will the umbrella fly upward or downward? The umbrella flies upward. When the wind blows, the air above the umbrella is displaced, creating a relatively low-pressure area. Consequently, the umbrella is lifted upward from the relatively high-pressure area below. This is lift.
Now that we’ve examined the concept of lift, let’s see how it’s applied in airplanes. As we saw earlier, for an airplane to receive the force that lifts it, the wind speed above the wing must be faster than the speed below the wing. Here, another property of fluids comes into play. When a fluid flows, the cross-sectional area through which it flows is inversely proportional to its speed. Let’s look at an example to help illustrate this. Imagine holding a hose with water flowing out of it. You’ve probably experienced at least once that when you pinch the end of the hose, the water’s speed increases. This happens because pinching the hose reduces the cross-sectional area through which the water flows, thereby increasing its speed. Generally, the cross-section of an airplane wing is streamlined, with the upper surface curved more convexly than the lower surface. This shape causes air to concentrate on the narrower upper surface, resulting in a faster flow speed there compared to the lower surface. Consequently, the pressure on the upper surface becomes relatively lower than that on the lower surface, allowing the airplane to generate lift and take off.
Next, let’s look at thrust. For an airplane to generate lift, air must flow over the wings. However, the wind blowing unilaterally against a stationary airplane is too slow to provide sufficient lift. In other words, an airplane can only generate lift and take off due to the reaction force of the wind when it has its own forward propulsion. This forward propulsion generated by the airplane is called thrust. Airplanes use jet engines to generate thrust. A jet engine is a device that draws in air from the atmosphere, compresses it using a compressor, and then burns this compressed air to generate power. Let’s use a real-life example to understand this more easily. Imagine a balloon. Everyone has probably experienced trying to squeeze a balloon to make it smaller, only for it to burst. As the balloon shrinks, the internal pressure increases, and once that pressure exceeds the limit that the rubber can withstand, the balloon bursts. The principle used by a jet engine is the same. Compressing air creates pressure. When that compressed air is burned using fuel, the force generated by the pressure and the force generated by the heat are converted into power. That power is what propels the airplane.
So far, we’ve explored the principles behind how airplanes fly through fluid mechanics, a branch of mechanical engineering. As a result, we’ve learned that an airplane takes off and moves forward by generating enough lift to get airborne using the thrust produced by its jet engines. Although airplanes currently require runways tens of kilometers long to take off, cutting-edge technology is developing aircraft capable of taking off without runways. Before long, we may live in a world where, instead of going to an airport, we board an airplane right in front of our homes, just as we take a bus.
