The Flying Machine: A Review of Airplanes

If you’ve ever looked at the sky, you’ve probably seen planes streaking across the sky, leaving a trail of white smoke. What allows these planes to become airborne and fly through the sky? Birds have light, feathered wings that are flexible, allowing them to pump air under them to take flight. But how does a ginormous hunk of metal with stationary wings suddenly lift itself off the ground and take to the sky?

Planes utilize the fundamentals of physics to achieve flight: Newton’s First and Third Laws of Motion and the forces of flight. Newton’s First Law states that an object at rest will remain at rest and an object in motion will stay in motion with the same speed and direction unless acted on by a net external force. Essentially, if an originally stationary marble was pushed by a person, Newton’s First Law would be implemented because the marble changed its motion after being acted upon by an outside force, the person. Newton’s Third Law states that for every action, there is a reaction that is equal in magnitude and opposite in direction. To exemplify, if a person were to sit in a chair, they would be exerting a downward force on the chair. At the same time, the chair would exert an upwards force that is equal in magnitude on the person. This is what makes it possible for you to sit on the chair without falling through it. 

The forces of flight include four forces that dictate how an airplane moves through the air. These forces include weight, lift, thrust, and drag. Weight is the force on an object due to gravity. Lift is the force that opposes weight. Thrust is the force that pushes an airplane forward. Drag is the force that opposes forward motions. These four forces combined enable an aircraft to be able to move in multiple directions because of its relation with Newton’s Laws. When the forces are balanced, the airplane moves at a constant velocity. When the forces become unbalanced, the airplane changes in motion and direction depending on the amount of force of each of the four forces of flight. 

When an airplane increases in altitude, this motion is referred to as climbing. For an airplane to climb, there must be an imbalance between the four forces where the combination of thrust and lift is greater than the combination of drag and weight. When thrust is greater than drag and lift is greater than weight, the airplane tilts and accelerates upwards.

When an airplane remains at the same altitude and speed, this motion is referred to as cruising. For an airplane to cruise, the forces of flight must be balanced. In this case, the combination of thrust and lift should be equal to the combination of drag and weight. When the forces are balanced, there is a net force of 0 and the airplane will continue in the same direction and at the same speed until some external and unbalanced force acts on it.

When an airplane decreases in altitude, this motion is referred to as descending. For an airplane to descend, there must be an imbalance between the four forces where the combination of thrust and lift is less than the combination of drag and weight. When thrust is less than drag and lift is less than weight, the airplane tilts and accelerates downwards.

It is known that gravity is responsible for the weight force that pulls the plane down, the engines provide the thrust force that moves it forward, and friction creates the drag force that resists the motion. But what about lift—the force that helps an airplane rise and stay in the air? The secret lies in the wings. Designed with an airfoil shape, airplane wings are curved on top and flatter underneath, much like the shape of a whale's fin. There are two main contributors to the lift force: difference in air pressure above and below the wing and the direction of airflow.

When an airplane's engine moves the plane forward at a high speed, the air splits at the curved front of the wing, altering the air pressure above and below the wing. The air above the wing has less pressure because the air that splits above occupies more space. This happens because as air moves over the curved front of the wing, it naturally tries to move in a straight line, up and back. Normally, the airflow would not reach the area near the bottom end of the top surface. However, because of the curvature, the airplane wing pulls the flowing air downward to follow the shape of the airfoil. This causes the same number of air molecules to occupy a large amount of space, leading to a lower pressure. This is the opposite case for the air that is split beneath the wing. Air is more packed together in a smaller space, leading to a greater pressure. Since the air pressure above the wing is lower, the airplane moves upwards.

This airflow that follows the shape of the wing leads to the second contributor to lift. When the tail end of the cross-section of the wing is lower than the curved front, the airflow rushes downwards. Through the third law of Newton’s Laws of Motion, it is known that for every action, there is an equal and opposite reaction. Because the airplane wings are actively pushing air downwards, the air is actively pushing the plane upwards at the same time. This is called a downwash.

These aviation physics concepts were first implemented by the Wright brothers back in the 1900s. On December 17, 1903, the Wright brothers launched the first ever plane with powered flight at Kitty Hawk. While their plane’s flight only lasted up to around a minute, their creation laid the groundwork for human flight to be possible.

Next time you spot a plane in the sky or are up in the air yourself, don’t forget about the groundbreaking discoveries in aviation physics that allows you to travel across the world. I hope you learned something new! Keep a lookout for the next post!

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References

Let’s Talk Science. (2021 Dec. 7). How Planes Fly. Retrieved May 21, 2023, from https://letstalkscience.ca/educational-resources/backgrounders/how-planes-fly

Space Center Houston. (2019, Dec. 18). A look back at the Wright brothers‘ first flight. Retrieved May 21, 2023, from https://spacecenter.org/a-look-back-at-the-wright-brothers-first-flight/

Woodford, C. (2022 Jan. 30). Airplanes. Explainthatstuff. Retrieved May 21, 2023, from https://www.explainthatstuff.com/howplaneswork.html