How Do Planes Fly & What Keeps Them In The Air? (2024)

Have you ever watched a plane take off and wondered how that big metal object full of people actually floats in the air? Although the science behind flight may seem complex, it boils down to the balance of four fundamental forces: lift, weight, thrust, and drag.

Lift is created by pressure differences above and below a wing, while weight pulls the airplane down due to gravity. Thrust moves the airplane forward, countering drag, which is the resistance opposing this motion.

Early scientists like Bernoulli and Newton provided essential principles that helped unlock the mystery of flight. Yet, despite advancements, the precise mechanisms of lift remain a topic of ongoing debate and research in the aviation community.

Photo: Joe Kunzler| Simple Flying

The basics: The four forces

Humankind has long looked to the skies and wondered what it would be like to soar among the clouds. Scientists and inventors studied birds and their flight capabilities for centuries before Orville and Wilbur Wright made history with the first successful airplane in 1903.

Flying, particularly in aircraft weighing tens or even hundreds of thousands of pounds, can feel almost like magic. It appears so improbable, yet we seem to have perfected the art in just over 100 years.

However, if you focus on its basics, the science of flight (yes, despite its allure, it is really just science) is quite simple. Flight is possible because of the balance of four physical forces:

• Lift: The upward force created by the movement of air above and below a wing. Air flows faster above the wing and slower below, creating a difference in pressure that keeps an airplane flying.
• Weight: Weight is caused by gravity and pulls the airplane toward the ground. When lift exceeds this pull, the aircraft will climb.
• Thrust: The force that moves an airplane forward through the air. Thrust is created by a propeller or a jet engine.
• Drag: The air resistance opposite thrust that slows the forward movement of an airplane.

Orville and Wilbur used this knowledge to construct their famous Wright Flyer, proving that the “Four Forces” were indeed the secret to flight. However, none of the Wright brothers’ success (or your summer vacation flights) would be possible without the initial work of Daniel Bernoulli or Sir Isaac Newton.

Bernoulli's principle

Air as a physical substance has been studied since as early as the 15th century. According to NASA, Galileo’s student Evangelista Torricelli discovered that air had weight in 1640.

In the 1700s, Swiss mathematician and physicist Daniel Bernoulli began studying fluid dynamics, which can be applied to gases (like air). In 1738, Bernoulli published his groundbreaking principle stating:

“…an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.”

According to this principle, as a fluid (such as air) travels faster, its pressure decreases. Per Skybrary, Bernoulli’s principle is important in aviation because it can be used to calculate the amount of lift on an airfoil.

For example, if the air at the top surface of an aircraft’s wing is moving faster than the air at the bottom surface, Bernoulli’s principle tells us that the pressure on the two surfaces will be different. The air at the bottom of the wing will have more pressure, creating an upward-lifting force. Today, Bernoulli’s principle is used to calibrate a plane’s airspeed indicator to ensure it displays the indicated airspeed appropriate to the dynamic pressure.

Newton’s third law of motion

Although it provides essential insights into airflow and its effects on aircraft, Bernoulli’s principle alone is not enough to solve the mystery of flight. However, by combining his research with that of Sir Isaac Newton, a clear solution was discovered.

In 1686, Newton presented his groundbreaking three laws of motion, per Britannica. While each of these laws can be applied to aviation, Newton’s third law is the most impactful. This law, usually called the law of action and reaction, states:

To every action (force applied) there is an equal but opposite reaction (equal force applied in the opposite direction).

According to this law, as the wings apply a downward force to the air, the resultant equal and opposite force pushes the wings up. This lift is what keeps the aircraft in the sky. Thrust, which is provided by the aircraft’s engines, is what makes the plane move forward through the air.

Photo: Wikimedia Commons

When the air is pushed out the back of an aircraft’s engine, a reaction force occurs (thrust), and the airplane is pushed forward. When an airplane flies through the air, its shape pushes the surrounding air out of the way. As the air pushes back, drag is produced. By understanding Newton’s third law, aircraft designs can be created to reduce drag and improve overall flight performance.

The aerodynamic drag coefficient of an object can be calculated to quantify its resistance as it moves through a fluid. An object with a lower drag coefficient implies that its body shape allows it to move more easily through the surrounding viscous air, per the Physics Hypertextbook.

Yet the mystery continues

By combining the findings of Newton and Bernoulli, early aviation pioneers unlocked the secret of human flight. However, despite the obvious understanding that aviation professionals now have of the science behind the phenomenon, some questions remain.

In 2003, the New York Times ran an article celebrating the anniversary of the Wright Flyer’s first successful flight. In the piece, John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum, is interviewed regarding the science behind flight. According to him, there is actually no agreement on what generates the aerodynamic force of lift.

Anderson said that people, many very knowledgeable in the field of aviation, give different answers to what keeps airplanes in the sky. He says that some hold by their responses with “religious fervor.”

Photo: Joe Kunzler| Simple Flying

While the mathematical theory behind flight is obviously accurate and cannot be argued, many people point out that the equations themselves are not explanations. A non-technical approach to flight, which examines the factors at work and does not deal with numbers or equations, results in controversies and unanswered questions.

According to author Ed Regis in Scientific American, Bernoulli and Newton’s theories contain incomplete explanations of lift. While both are correct and do not contradict one another, certain gaps exist. Bernoulli’s principle, for example, does not explain why air moves faster across a curved surface or why the higher velocity above a wing exists.

Similarly, Newton’s third law, while more comprehensive in its explanation of lift than Bernoulli’s principle, also falls short. Taken by itself, Newton’s principle of action and reaction still does not explain the lower pressure that exists above a wing, regardless of the wing’s curve.

Today’s approaches to the science of flight involve computational fluid dynamics (CFD) simulations and equations that consider the viscosity of air and many other factors. While we are closer to a complete understanding of flight, some believe we have a way to go. As stated by Ed Regis:

Although the current understanding may not be perfect, it seems to be working well enough. Per the Medium,

“…through the use of math and science, we are able to build airplanes that are safe and efficient, even if we don’t completely understand the process behind why it [lift] works.”

As new discoveries and research in the field are presented, scientists and aviation professionals will continue to pursue a better understanding of the mystery of flight. With this constant goal in mind, airlines can continue providing safe, efficient air travel.