The reason why lift and drag are generated is complex. In this article, we will look at 3 popular explanations (continuity theorem+Bernoulli’s principle, Newton’s third law, and circulation theorem) of how lift is generated and analyze their strengths and weaknesses. Then, we will discuss several major sources of drag of the aircraft and analyze how they are generated.

To begin with, let’s look at the most popular theory for explaining the formation of lift. A lot of textbooks around the world states that the lift is generated by the pressure difference between the upper and lower surface of the wing, and this pressure difference comes from the velocity difference between the air that flows through the upper surface and the air that flows through the lower surface: Bernoulli’s principle states that the higher the velocity is, the less the static pressure is. The velocity difference is typically explained by the shape of the wing. The upper surface of the wing is more curved, so the air accelerate according to the continuity theorem (the law of conservation of mass), creating the velocity difference.

This theory sounds correct, but this theory is FLAWED in 2 aspects!

First, unlike the closed chamber, under the circumstance that the plane is flying in the sky that is broad, the accelerating effect caused by the wing’s curvature is negligible. The wing is not curved enough to create the velocity difference that is strong enough to induce pressure difference that is so large. Second, even though the velocity difference is created, we cannot directly use Bernoulli’s principle to explain the generation of lift, because the Bernoulli’s principle is only applicable to points that are on the same streamline. However, it is obvious that the points on upper surface and the lower surface of the wing are on different streamlines, the Bernoulli’s principle is not directly applicable. Therefore, this explanation is flawed.