How do airplanes fly? An aerospace engineer explains the physics of flight

By | March 18, 2024

Curious Kids is a series for children of all ages. If you have a question that you would like an expert to answer, please send it to Curiouskidsus@theconversation.com.


How do airplanes fly? – Benson, age 10, Rockford, Michigan


Airplane flight is one of the most important technological achievements of the 20th century. The invention of the airplane allows people to travel from one side of the planet to the other in less than a day, compared to weeks of traveling by boat and train.

Understanding exactly why airplanes fly is an ongoing challenge for aerospace engineers like me who study and design airplanes, rockets, satellites, helicopters and space capsules.

Our job is to ensure that flying through the air or in space is safe and reliable, using tools and ideas from science and mathematics, such as computer simulations and experiments.

Because of that work, flying in an airplane is the safest way to travel – safer than cars, buses, trains or boats. But even though aerospace engineers are designing aircraft that are stunningly advanced, you might be surprised to learn that there are still some details about the physics of flight that we don’t fully understand.

A diagram of an airplane showing the four forces of flight.A diagram of an airplane showing the four forces of flight.
The forces of weight, thrust, drag, and lift act on an airplane to keep it aloft and moving. NASA

May the force(s) be with you

There are four forces that aerospace engineers consider when designing an aircraft: weight, thrust, drag and lift. Engineers use these forces to help design the shape of the plane and the size of the wings and to figure out how many passengers the plane can carry.

For example, when an airplane takes off, the thrust must be greater than the drag, and the lift must be greater than the weight. When you watch an airplane take off, you see the wings change shape through flaps at the back of the wings. The flaps provide more lift, but they also create more drag, so a powerful motor is needed to create more thrust.

When the plane is high enough and flying to your destination, the lift must balance the weight and the thrust must balance the drag. So the pilot pulls the flaps in and can adjust the engine so that it produces less power.

That said, let’s define what strength means. According to Newton’s second law, a force is a mass multiplied by an acceleration, or F = ma.

A historic black and white photo of the Wright brothers' first flight.A historic black and white photo of the Wright brothers' first flight.

A force that everyone encounters every day is gravity, which keeps us on the ground. When you get weighed at the doctor’s office, they are actually measuring the amount of force your body is exerting on the scale. When your weight is shown in pounds, it is a measure of strength.

As an airplane flies, gravity pulls the airplane downward. That force is the weight of the aircraft.

But the engines push the plane forward because they create a force called thrust. The engines draw in air, which has mass, and quickly push that air out the back of the engine – so there is mass multiplied by acceleration.

According to Newton’s third law, for every action there is an equal and opposite reaction. When the air flows out the back of the engines, there is a reaction force that pushes the aircraft forward – this is called thrust.

As the plane flies through the air, the shape of the plane pushes the air out of the way. Again, according to Newton’s third law, this pushes air back, creating drag.

You may experience something similar to dragging while swimming. Paddle through a pool and your arms and feet provide momentum. Stop paddling, and you will continue to move forward because you have mass, but you will go slower. The reason you’re going slower is because the water is pushing you back – that’s resistance.

Understanding elevator

Lifting is more complicated than the other forces of weight, thrust and resistance. It is created by the wings of an aircraft, and the shape of the wing is critical; that shape is known as an airfoil. Basically this means that the top and bottom of the wing are curved, although the shapes of the curves can differ from each other.

As air flows around the airfoil, it creates pressure – a force that is spread over a large area. A lower pressure is created on the top of the airfoil than on the bottom. Or to look at it another way, air moves faster over the top of the airfoil than underneath.

Understanding why the pressures and velocities at the top and bottom are different is critical to understanding lift. By improving our understanding of lift, engineers can design more fuel-efficient aircraft and provide passengers with more comfortable flights.

A diagram showing how an aircraft's airfoil works.A diagram showing how an aircraft's airfoil works.

The riddle

The reason why air moves at different speeds around an airfoil remains mysterious, and scientists are still investigating the question.

Aerospace engineers have measured this pressure on a wing in both wind tunnel experiments and in flight. We can make models of different wings to predict whether they will fly well. We can also change lift by changing the shape of a wing, creating airplanes that fly long distances or fly very fast.

Even though we still don’t fully know why lift happens, aerospace engineers work with mathematical equations that simulate the different velocities at the top and bottom of the airfoil. These equations describe a process known as circulation.

Circulation gives aerospace engineers a way to model what’s happening around a wing, even if we don’t fully understand why it’s happening. In other words, through the use of math and science, we are able to build aircraft that are safe and efficient, even if we don’t fully understand the process behind why it works.

If aerospace engineers can figure out why air flows at different speeds depending on which side of the wing the air is on, we can eventually design airplanes that use less fuel and pollute less.


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This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Craig Merrett, Clarkson University

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Craig Merrett receives funding from the Office of Naval Research and L3Harris. He is affiliated with the American Institute for Aeronautics and Astronautics and is a licensed professional engineer in Ontario, Canada. Dr. Merrett is an Associate Professor in the Department of Mechanical and Aerospace Engineering at Clarkson University, Potsdam, NY.

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