How Can Airplanes Fly in the Sky?

Understanding the mechanisms of how airplanes take off, fly in the air, and land will enhance your experience each time you “travel the skies”.

Principle of Aircraft Operation

Takeoff Mechanism of Airplanes

Airplanes can weigh up to hundreds of tons. Many people wonder what tremendous force can lift this “flying monster” off the ground. The answer lies in aerodynamic lift (also known as Joukowski lift).

Specifically, an airplane in motion is always subject to four forces: thrust, drag, gravity, and lift. As the airplane runs along the runway, the airflow around the wings creates a pressure difference between the lower and upper surfaces of the wings.

Lift Model for Takeoff.

The physical consequence of this phenomenon is that a lifting force appears, pushing the airplane upward from the ground. The faster the airplane moves, the greater this lift force becomes, until it overcomes the Earth’s gravitational force, lifting the massive machine weighing hundreds of tons into the sky.

How Jet Engines Work

In-Flight Operations

In reality, airplanes can fly at any desired altitude. The world record recorded the American SR-71 Blackbird reaching nearly 26 km above sea level in 1976. Meanwhile, typical commercial airplanes usually fly at altitudes between 8.5 km to 10.7 km, which is within the upper part of the troposphere.

Temperature decreases with altitude, reaching as low as approximately -50 degrees Celsius. Therefore, even in the wheel well of the airplane, it is challenging to prevent cold air from entering due to the thickness of insulation materials and the absence of heating systems like in passenger cabins. Additionally, the air in the troposphere moves strongly both vertically and horizontally, causing various weather phenomena such as rain, hail, wind, snow, and fog.

The stable cruising speed of a commercial airplane is about 900 km/h. As altitude increases, air resistance decreases (above 5.5 km, drag is halved), allowing the airplane to fly faster. Furthermore, flying at higher altitudes helps reduce the risk of colliding with migratory birds, one of the causes of catastrophic airborne accidents.

Air pressure inside the airplane significantly decreases at higher altitudes, being only about 1/10 of that at ground level.

Conversely, the air becomes less dense at higher altitudes. The air pressure also drops significantly, only around 1/10 of that at ground level. Consequently, the amount of air present in the wheel well is very low.

For airplanes using propeller engines, the lower oxygen levels make fuel combustion more difficult. In flight, airplanes typically retract their landing gear because leaving it extended would allow air to rush into the wheels, causing them to spin, which increases drag and reduces flight speed.

Landing Mechanism of Airplanes

The flight concludes with the airplane landing on the runway. When approaching the ground, the air beneath the wings is compressed against the ground, causing small vortices and creating a “ground effect” that lifts the airplane briefly before it officially touches down on the runway.