The Science Behind Lift: Why Pressure Decreases Upwards Over an Aerofoil

When an aerofoil (airfoil) passes through the air, a multitude of factors come into play that affect its lift and the pressure distribution above and below it. This phenomenon is crucial for understanding aerodynamics and how aircraft generate lift. This article delves into the detailed mechanisms that lead to a decrease in pressure over the top of the aerofoil, especially under positive camber conditions.

Understanding the Dynamics of Airflow Around an Aerofoil

It is important to recognize that air is not stationary as an aerofoil enters its vicinity. The air in the vicinity of the aerofoil is in a state of motion, exhibiting complex patterns that significantly influence the pressure distribution. As the aerofoil moves, the air particles are set into motion, starting even before the aerofoil fully enters the airflow.

A diagram of the airflow around a wing shows that the air particles above the aerofoil and those below it are in constant motion. Below the wing, the air is continuously pushed forward due to the shape of the aerofoil. On the other hand, the air above the wing is influenced by the curvature and experiences a significant rearward movement towards the trailing edge of the aerofoil, driven by the lower pressure above it compared to the pressure ahead. After the aerofoil passes through, the air flows downward.

Pressure Changes and Their Impact on Lift

The pressure changes observed over the aerofoil are a direct result of the way the aerofoil interacts with the air. As the aerofoil moves through the air, the air particles must be displaced to make way for the aerofoil. This displacement leads to significant pressure changes on both the upper and lower surfaces of the aerofoil.

On the upper surface of the aerofoil, the curvature causes the air to travel a longer distance, effectively thinning the air flow. This thinning effect reduces the density of the air above the aerofoil, resulting in a lower pressure. The lower pressure above the aerofoil is a direct consequence of the higher velocity of the air particles, which try to follow the curved surface of the aerofoil.

Conservation of Momentum and Air Pressure

When the air particles follow the curvature of the aerofoil, they experience a forward and downward movement. This movement requires a force, given that the air has mass. This force is what we refer to as pressure, which is exerted on the aerofoil, creating an upward lift force.

Below the aerofoil, the situation is reversed. The lower surface of the aerofoil must displace a larger mass of air, leading to a higher pressure compared to the upper surface. The air cannot travel up through the aerofoil, so it accelerates downward, exerting a force that counters the pressure difference, creating lift.

Conclusion

In summary, the decrease in pressure on the upper surface of an aerofoil is not a static phenomenon but a dynamic result of the airflow around the aerofoil. The curvature of the aerofoil dictates the path of the air particles, leading to changes in air density and pressure. These pressure changes ultimately contribute to the lift force that enables an aircraft to fly. Understanding this phenomenon is essential for the design and operation of all aircraft.

For a more comprehensive understanding of lift, review the comprehensive article on understanding lift correctly. This resource delves deeper into the physics of lift and explains the principles in greater detail.

Explore more related topics:

Aerofoil lift Air pressure changes Wing aerodynamics

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