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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.
How do airplanes fly? – Benson, age 10, Rockford, Michigan
Airplane flight is one of the most significant 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 travel by boat and train.
A constant challenge for aerospace engineers, like me, who study and design airplanes, rockets, satellites, helicopters and space capsules is to understand exactly why airplanes fly.
Our job is to ensure that flight 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 is the safest way to travel – safer than cars, buses, trains or boats. But even though aerospace engineers design incredibly sophisticated aircraft, you might be surprised to learn that there are still some details about the physics of flight that we don’t fully understand.
May the force(s) be with you
There are four forces that aerospace engineers consider when designing an airplane: weight, thrust, drag and lift. Engineers use these forces to help design the shape of the plane, the size of the wings, and determine 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. If you watch a plane take off, you will see that the shape of the wings changes with flaps from the back of the wings. The flaps help create more lift, but they also create more drag, so a powerful engine is needed to create more thrust.
When the plane is high enough to cruise to your destination, the lift has to balance the weight, and the thrust has to balance the drag. So the pilot pulls the flaps in and can set the engine to produce less power.
That said, let’s define what force means. According to Newton’s Second Law, force is mass times acceleration, or F = ma.
The force of gravity, which keeps us grounded, is a force that hits everyone every day. When you are weighed at the doctor’s office, they measure the force your body exerts on the scale. When your weight is given in pounds, that is a measure of force.
While a plane is flying, gravity is pulling the plane down. That force is the weight of the plane.
But its engines propel 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 mass is multiplied under acceleration.
According to Newton’s Third Law, every action has an equal and opposite reaction. When the air rushes out the back of the engines, there is a reaction force that pushes the plane forward – called thrust.
As the plane flies through the air, the shape of the plane pushes air out of the way. Again, according to Newton’s Third Law, this air pushes back, causing drag.
You can experience something like a drag while swimming. Paddle through a pool, and your arms and legs provide thrust. Stop paddling, and you will move forward as you are mass, but you will slow down. The reason you’re slowing down is because the water is pushing back on you – that’s drag.
Understanding an elevator
Lift is more complex than the other forces of weight, push and pull. Airplane wings create it, and the shape of the wing is critical; That shape is called an airfoil. Basically it means that the top and bottom of the wing are curved, although the shapes of the curves can be different from each other.
As the air flows around the airfoil, it creates pressure – a force that is spread over a large area. A lower pressure is created at the top of the air compared to the pressure at the bottom. Or to look at it another way, air travels faster over the top of the air than below.
Understanding why the pressure and speeds are different at the top and bottom is essential to understanding lift. By improving our understanding of lift, engineers can design airplanes that are more fuel efficient and give passengers more comfortable flights.
The conundrum
Why air moves at different speeds around an airplane is still mysterious, and scientists are still investigating this question.
Aerospace engineers measured these pressures on a wing in wind tunnel experiments and during flight. We can create models of different wings to predict if they will fly well. We can also change lift by changing the shape of wings to create airplanes that fly long distances or fly very fast.
Although we don’t know exactly why lift occurs, aerospace engineers work with mathematical equations that recreate the different speeds at the top and bottom of the airfoil. These equations describe a process called circulation.
Circulation provides a way for aerospace engineers to model what happens around a wing even if we don’t fully understand why it happens. In other words, through the use of mathematics and science, we are able to build airplanes that are safe and efficient, even if we don’t fully understand the process behind why it works.
Ultimately, if aerospace engineers can figure out why air flows at different speeds depending on which side of the wing it’s on, we can design airplanes that use less fuel and pollute less.
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And since curiosity has no age limit – adults, let us know what you think too. We won’t be able to answer every question, but we’ll do our best.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. 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 engineering professional in Ontario, Canada. Dr. Merrett is an associate professor in the Department of Mechanical and Aerospace Engineering at Clarkson University, Potsdam, NY.
