Aircraft Lift Explained: Bernoulli vs. Newton's Equations | Fly with Magnar

Summary

In this educational video, Magnanudal, an island captain and instructor, clarifies misconceptions about how wings generate lift. He explains that while Bernoulli's principle is often incorrectly used to describe lift, it's actually the pressure difference above and below the wing that creates it. The video debunks the equal transit time hypothesis and emphasizes Newton's third law of motion as the simplest explanation for lift. It also discusses the importance of wing design, including the role of the leading edge and the impact of angle of attack, and touches on the limitations of flat wings in flight.

Takeaways

• 📚 Bernoulli's Principle states that an increase in fluid velocity leads to a decrease in static pressure and vice versa, with the sum of static and dynamic pressures remaining constant.
• 🌀 The equal transit time hypothesis suggests that air molecules must travel faster over the top of a wing to meet those below, leading to lower pressure and lift.
• 📉 The pressure difference above and below a wing, not the speed of air over the wing alone, creates lift.
• 🔄 Newton's Third Law of Motion explains lift as a reaction to the downward deflection of air by the wing.
• 📐 The Kutta-Joukowski theorem is used by professionals to calculate the lift of a two-dimensional airfoil and the Lanchester-Prandtl equation for three-dimensional wings.
• ✈️ The shape of the leading edge of a wing is crucial for efficient airflow and lift generation.
• 💨 The airflow over the top of a wing is faster than below, contributing to most of the lift.
• 🚫 A flat wing is inefficient for lift generation due to airflow separation at the leading edge.
• 🌬️ The Coanda effect is not the primary reason airflow stays attached to a wing; it's the pressure difference that matters.
• 🔝 Thin wings with sharp leading edges are ideal for supersonic flight but require flaps for efficient low-speed flight.

Q & A

What is Bernoulli's principle as explained in the script?

Bernoulli's principle states that when the velocity of a fluid increases, its static pressure decreases, and the dynamic pressure increases, and vice versa, with the sum of static and dynamic pressures remaining constant according to the principle of conservation of energy.

What is the relationship between static pressure and dynamic pressure in the context of fluid dynamics?

Static pressure is the pressure exerted by a fluid that is not moving, while dynamic pressure is the kinetic energy in a fluid when it is in motion. According to Bernoulli's principle, an increase in dynamic pressure leads to a decrease in static pressure and vice versa, maintaining a constant sum of both pressures.

How does the angle of attack affect the lift generated by a wing?

The angle of attack is crucial for lift generation. As the angle of attack increases, the wing deflects the air downwards more effectively, creating more lift. However, beyond a certain point, the airflow over the wing can separate, leading to a loss of lift, known as a stall.

What is the equal transit time hypothesis mentioned in the script?

The equal transit time hypothesis is the idea that air molecules or particles separated in front of a wing must meet at the same point behind the wing, leading to higher velocities and lower pressures on the upper surface due to the longer path, according to Bernoulli's principle.

Why is the pressure difference above and below a wing important for lift?

The pressure difference above and below a wing is essential for lift because it is this difference that creates the net upward force on the wing. The wing's shape and angle of attack cause air to move faster over the top surface, reducing pressure, while the air below moves slower, maintaining higher pressure.

How does Newton's third law of motion relate to the generation of lift?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. In the context of lift, when a wing pushes air downwards, an equal and opposite force is generated, pushing the wing upwards, which is the lift.

What is the Venturi effect, and how does it relate to lift generation?

The Venturi effect is the reduction in fluid pressure that occurs when a fluid flows through a constricted section of a pipe or channel. In the context of wings, the air is compressed over the leading edge, causing an acceleration and a decrease in static pressure, which contributes to lift.

Why is the shape of the leading edge of a wing important for lift generation?

The shape of the leading edge is important because it influences how air flows around the wing. A curved leading edge allows air to follow a curved path, which, according to Newton's second law, results in acceleration and a consequent decrease in static pressure, contributing to lift.

How do flaps on an aircraft wing contribute to lift?

Flaps on an aircraft wing increase the wing's surface area and camber, which enhances the airflow's adherence to the wing's surface, especially at lower speeds. This increase in curvature over the wing allows for greater lift generation, which is crucial for takeoff and landing.

What is the Coanda effect, and how is it related to lift?

The Coanda effect is the tendency of a fluid jet to stay attached to a convex surface. While it can be used to increase lift in some applications, it is not the primary mechanism for lift generation on a wing. Lift is primarily due to the pressure difference above and below the wing, not just the attachment of the airflow to the wing's surface.