How Planes Fly

Theory and Concepts
1

Can you tell me more about the pressure difference between the top and bottom of an airfoil?

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This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
The pressure difference between the top and bottom of an airfoil is a fundamental aspect of how airfoils generate lift. For an airfoil configured to generate lift, the pressure on the bottom surface is higher than the pressure on the top surface. This pressure difference results in a net upward force, which is the lift force.

The pressure difference can be explained using the Bernoulli Equation, which relates the pressure and velocity of a fluid. According to Bernoulli’s principle, in a steady, incompressible flow with negligible viscosity, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure. For an airfoil:

  • The flow over the top surface of the airfoil is faster than the flow beneath it. This is due to the shape of the airfoil and its angle of attack, which cause the streamlines to be closer together on the top surface, increasing the velocity.
  • The increased velocity on the top surface leads to a lower pressure compared to the bottom surface, where the flow is slower.

This pressure difference is what generates lift. The design of the airfoil, including its camber and angle of attack, is crucial in determining the magnitude of this pressure difference and, consequently, the lift generated.

To formulate this response, I reviewed the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I have ranked their significance below. If they are marked as relevant, I encourage you to take a look at them:

Textbook sections:

  1. Chapter 9.1: How Airfoils Work Part 1 (RELEVANT)
  2. Chapter 9.2: Engineering Descriptions of Airfoil Forces Part 2 (RELEVANT)
  3. Chapter 9 Fundamentals: Derivation of Thin Airfoil Theory (RELEVANT)
  4. Chapter 9.1: How Airfoils Work Part 3 (RELEVANT)
  5. Chapter 9.1: How Airfoils Work Part 2 (RELEVANT)

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