In 1917, Albert Einstein had already explained the relationship between space and time. However, that same year, he designed a flawed airplane wing. His attempt was based on an incomplete theory of flight. Insufficient and inaccurate explanations of flight still circulate today, leading us to question where Einstein went wrong and how planes actually fly.
Though we don’t always think of it this way, air is a fluid medium—it’s just less dense than liquids like water. Things that are lighter than air are buoyant within it, while heavier objects require an upward force, called lift, to stay aloft. For planes, this force is mostly generated by the wings.
One especially pervasive false description of lift is the “Longer Path” or “Equal Transit Time” explanation. It states that air molecules traveling over the top of a curved wing cover a longer distance than those traveling underneath. This explanation has been thoroughly debunked. Air molecules floating above and below the wing don’t need to meet back up. In reality, the air traveling above reaches the wing’s trailing edge much faster than the air beneath.
To understand how lift is actually generated, let’s simulate an airplane wing in motion. As it moves forward, the wing affects the movement of the air around it. As air meets the wing’s solid surface, a thin layer sticks to the wing. This layer pulls the surrounding air with it. The air splits into pathways above and below the wing, following the wing’s contour. As the air that’s routed above makes its way around the nose of the wing, it experiences centripetal acceleration. The air above therefore gathers more speed than the air traveling below. This increased speed is coupled with a decrease in pressure above the wing, which pulls even more air across the wing’s upper surface. The air flowing across the lower surface, meanwhile, experiences less of a change in direction and speed. The pressure across the wing’s lower surface is thus higher than that above the upper surface. This pressure difference results in the upward force of lift. The faster the plane travels, the greater the pressure difference, and the greater that force. Once it overcomes the downward force of gravity, the plane takes off.
Air flows smoothly around curved wings. But a wing’s curvature is not the cause of lift. In fact, a flat wing that’s tilted upwards can also create lift—as long as the air bends around it, contributing to and reinforcing the pressure difference. Meanwhile, having a wing that’s too curved or steeply angled can be disastrous: the airflow above may detach from the wing and become turbulent. This is probably what happened with Einstein’s wing design, nicknamed “the cat’s back.” By increasing the wing’s curvature, Einstein thought it would generate more lift. But one test pilot reported that the plane wobbled like “a pregnant duck” in flight.
Our explanation is still a simplified description of this nuanced, complex process. Other factors, like the air that’s flowing meters beyond the wing’s surface—being swept up, then down—as well as air vortices formed at the wing’s tips, all influence lift. And, while experts agree that the pressure difference generates lift, their explanations for how can vary. Some might emphasize the air’s behavior at the wing’s surface, others the upward force created as the air is deflected downwards. However, there’s no controversy when it comes to the math. Engineers use a set of formulas called the Navier-Stokes equations to precisely model air’s flow around a wing and detail how lift is generated.
More than a century after Einstein’s foray into aeronautics, the lift retains its reputation as a confounding concept. But when it feels like it’s all going to come crashing down, remember: it’s just the physics of fluid in motion.
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Construct a simple wing model using cardboard or foam. Experiment with different shapes and angles to observe how they affect lift. Record your observations and compare them with the principles discussed in the article.
Research and present on common misconceptions about how airplanes fly, such as the “Longer Path” explanation. Create a presentation or poster that debunks these myths using accurate scientific principles.
Use a computer simulation tool to model the airflow around different wing shapes. Observe how changes in wing curvature and angle affect the airflow and lift. Write a report summarizing your findings.
Design and fly paper airplanes with different wing shapes and angles. Measure the distance and stability of each design. Analyze which designs perform best and explain why based on the concepts of lift and airflow.
Invite an aeronautical engineer or a pilot to speak to your class about the principles of flight and the complexities of lift. Prepare questions in advance to ask the expert, and write a reflection on what you learned from the session.
Fluid medium – A substance, either liquid or gas, that is capable of flowing and takes the shape of its container. – Water is a common fluid medium used in many experiments.
Air – The invisible gaseous substance surrounding the Earth, primarily composed of nitrogen and oxygen. – Birds rely on the movement of air to fly.
Lift – The upward force exerted on an object, especially a wing or an airfoil, to oppose gravity and support it in the air. – The shape of the wing allows an airplane to generate lift and stay airborne.
Airplane wing – The main horizontal surface of an aircraft that provides lift and stability during flight. – The design of the airplane wing affects its aerodynamic performance.
Debunking – The act of exposing or proving false a claim, myth, or misconception. – The scientist dedicated his career to debunking pseudoscientific theories.
Explanation – A statement or account that makes something clear or provides understanding. – The teacher provided a detailed explanation of the scientific concept.
Wing curvature – The shape or curve of an airplane wing from the leading edge to the trailing edge. – The wing curvature affects the airflow and lift generation.
Factors – Conditions or circumstances that contribute to a result or outcome. – There are many factors to consider when analyzing the cause of a phenomenon.
Navier-Stokes equations – A set of mathematical equations that describe the motion of fluid substances, including air and water. – The Navier-Stokes equations are used to model airflow around aircraft.
Understanding lift – Comprehending the principles and mechanisms by which an object, such as an airplane wing, generates lift. – The pilot’s understanding of lift is crucial for safe and efficient flying.
