Airflow and Lift Generation

Bernoulli's principle states that faster-moving fluids have lower pressure, and slower-moving fluids have higher pressure.
In the context of an airplane wing, the air flowing over the top surface moves faster than the air flowing under the bottom surface.
This difference in speed results in lower pressure on top of the wing and higher pressure on the bottom.
Pressure is defined as force per unit area, such as Newtons per square meter.

The higher pressure on the bottom of the wing and lower pressure on the top create a pressure difference, resulting in an upward force known as lift.
This lift vector pushes up on the wing, counteracting gravity and allowing the airplane to fly.
The traditional explanation attributes the difference in pressure to the equal transit theory.

The equal transit theory suggests that air flowing over the top and bottom surfaces of the wing must meet at the same time at the trailing edge.
This theory implies that air on top must travel faster due to the longer distance, resulting in lower pressure.
However, there is no law of physics mandating that air must recombine at the same time. This is the core misconeption.
Experimental evidence, such as wind tunnel tests, demonstrates that the air flowing over the top surface reaches the trailing edge before the air flowing under the bottom surface.
The equal transit theory is demonstrably false and cannot be relied upon to explain lift.

Despite the inaccuracy of the equal transit theory, several aspects of wing theory remain valid:
- There is lower pressure on the top of the wing.
- There is higher pressure on the bottom of the wing.
- The difference in pressure generates the lift vector upward.
- Airflow is faster on the top of the wing and slower on the bottom.

To understand lift, it is essential to delve into fundamental physics principles, including:
- Fluid mechanics: How molecules interact in fluid flow.
- Newton's laws of motion:
  - $F = ma$ (Force equals mass times acceleration).
  - Equal and opposite reaction.

Instead of focusing on the teardrop shape of conventional wings, consider a simplified flat wing to understand lift generation.
Early aircraft, such as those of the Wright Brothers, featured mostly flat wings with minimal curvature.

Real airplanes always have an angle of attack. The bottom surface of the wing is usually not designed to be totally parallel to the to the path of flight.
Consider a flat piece of plywood or a flat surface at an angle to the relative wind.
When air molecules impact the bottom of the wing, they bounce off, changing their direction and velocity.
This change in velocity is critical because it means a force has been applied.

According to Newton's third law (action-reaction), when the wing redirects the momentum or velocity of air molecules downward, there is an equal and opposite reaction.
The wing exerts a downward force on the airflow, and the air molecules exert an upward force on the wing, generating lift.
Action-reaction pairs act on opposite bodies; they never act on the same body.
The reaction force from the air molecules pushing up on the wing creates the upward component of lift.

The magnitude of the lift force depends on the angle of attack:
- When the wing is parallel to the wind flow, there is no force.
- As the angle increases, lift increases up to a point.
- Beyond a certain angle, lift decreases because the wing smashes directly into the wind, not redirecting it.
There exists an optimal angle of attack for maximizing lift.

Increasing the angle of attack also generates drag which increases as the the angle of attack increases. Air encounters the leading edge and flows over the surface creating turbulence.
Turbulence disrupts the airflow and causes drag, which opposes the motion of the aircraft. Action and reaction forces are molecule activing on the wind and the wind acting on the molecule, generating drag.

When a wing is at a very steep angle by the leading edge of the wing, the flow will generate vortices which cause spiral out and increase drag.
Stalling occurs when the angle of attack becomes too high, causing the airflow over the wing to separate and become turbulent.
This results in a drastic loss of lift.
Pilots must avoid stalling by maintaining sufficient speed and avoiding excessive angles of attack, especially during landing.