An airfoil is a shaped surface designed to generate lift when air flows around it. Wings, propeller blades, turbine blades, and fan blades all use airfoil physics to guide fluid motion and create useful forces. Lift matters because it allows aircraft to fly, wind turbines to extract energy, and many machines to control moving air or water.
Engineers design airfoils by balancing lift, drag, stability, and structural limits.
Key Facts
- Lift equation: L = 1/2 rho v^2 S CL
- Dynamic pressure: q = 1/2 rho v^2
- Pressure force on a surface acts perpendicular to that surface.
- A positive angle of attack usually increases lift until stall occurs.
- Bernoulli relation along a streamline: P + 1/2 rho v^2 + rho gh = constant
- Circulation form of lift per unit span: L' = rho v Gamma
Vocabulary
- Airfoil
- An airfoil is a curved shape designed to produce lift efficiently as a fluid flows around it.
- Angle of attack
- Angle of attack is the angle between the airfoil chord line and the oncoming airflow direction.
- Camber
- Camber is the curvature of an airfoil measured by how much its mean line bends away from a straight chord.
- Lift coefficient
- The lift coefficient is a dimensionless number that describes how effectively an airfoil produces lift for a given flow condition.
- Stall
- Stall is a loss of lift caused by airflow separation from the airfoil, usually at too large an angle of attack.
Common Mistakes to Avoid
- Saying lift happens only because air travels farther over the top is wrong because lift depends on pressure differences, flow turning, and circulation, not a rule that air packets must meet at the trailing edge.
- Ignoring angle of attack is wrong because the same airfoil can produce different lift, drag, or stall behavior depending on its orientation to the incoming airflow.
- Using Bernoulli's equation without checking flow conditions is wrong because it applies along streamlines under idealized assumptions and must be combined with the full airflow pattern around the wing.
- Assuming more angle of attack always means more lift is wrong because lift increases only up to a critical angle, after which separation causes stall and lift drops.
Practice Questions
- 1 An airfoil has wing area S = 12 m^2, air density rho = 1.2 kg/m^3, speed v = 40 m/s, and lift coefficient CL = 0.80. Calculate the lift using L = 1/2 rho v^2 S CL.
- 2 A small wing produces lift per unit span L' = 300 N/m in air with density rho = 1.2 kg/m^3 and speed v = 25 m/s. Use L' = rho v Gamma to find the circulation Gamma.
- 3 Explain why an airfoil can still produce lift even if its upper and lower surfaces are not perfectly symmetric, and describe how camber and angle of attack affect the pressure field.