Airfoils are the cross-sectional shape of a wing, and they determine how air flows around it. Their geometry directly controls how much lift and drag an aircraft produces. Understanding airfoil features and how they affect performance is foundational to everything else in aerodynamics.
Different airfoil shapes serve different purposes. Symmetric airfoils work well for aerobatic planes that fly upside down; cambered airfoils give commercial jets better efficiency at cruise. The design choices all come back to the same physics: pressure distribution over the airfoil surface.
Airfoil Geometry
Key Features of Airfoils
Leading edge is the front-most point of the airfoil, where the airflow first meets the surface. Its shape (sharp vs. rounded) influences how the boundary layer develops and when flow separation begins, which directly affects stall behavior.
Trailing edge is the rear-most point, where airflow from the upper and lower surfaces meets again. The geometry here affects the pressure distribution near the back of the airfoil and the size of the wake that forms downstream, both of which influence lift and drag.
Chord line is the straight line connecting the leading edge to the trailing edge. It's the main reference line for airfoil geometry. Angle of attack, thickness, and camber are all defined relative to the chord line.
Camber is the curvature of the airfoil's mean camber line, which is the line running equidistant between the upper and lower surfaces.
- Positive camber (upper surface more curved than lower) creates an asymmetric pressure distribution that increases the lift coefficient. The NACA 2412 is a classic example.
- Zero camber means the airfoil is symmetric. No lift is produced at zero angle of attack.
- Negative camber is rare but effectively inverts the pressure distribution, which is what happens geometrically when a cambered airfoil flies upside down.
Thickness is the distance between the upper and lower surfaces, measured perpendicular to the chord line. Thicker airfoils are structurally stronger but generally produce more drag because they create a larger wake and a bigger pressure difference between the front and rear of the airfoil.
Angle of attack (AoA) is the angle between the chord line and the direction of the incoming airflow. Increasing AoA increases lift, but only up to a critical point called the stall angle. Beyond this angle (typically around 15–20 degrees, depending on the airfoil), the flow separates from the upper surface and lift drops sharply.
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Airfoil Shape and Lift Generation
The asymmetric flow pattern around an airfoil creates a pressure difference between its upper and lower surfaces:
- Upper surface: Air accelerates as it flows over the curved top, and by Bernoulli's principle, faster-moving air has lower pressure.
- Lower surface: Air moves more slowly, resulting in higher pressure.
This pressure difference produces a net upward force perpendicular to the incoming flow, which is lift.
A cambered airfoil (like the NACA 4412) generates lift even at zero AoA because its shape forces an asymmetric flow pattern regardless of the angle. A symmetric airfoil (like the NACA 0012) produces zero lift at zero AoA, since the flow is identical on both sides. It only generates lift when tilted to a positive or negative angle of attack.
Airfoil Performance
Pressure Distribution on Airfoils
Pressure varies along the airfoil surface because the airflow velocity changes as it moves around the shape:
- The upper surface experiences lower pressure, with the lowest point (called the suction peak) typically near the leading edge.
- The lower surface has higher pressure, especially near the stagnation point, where the airflow velocity drops to nearly zero right at the leading edge.
Lift comes from integrating the pressure difference between the lower and upper surfaces over the entire airfoil:
where and are the pressures on the lower and upper surfaces, and is a small element along the surface.
Drag on an airfoil has two main components:
- Pressure drag (form drag): Caused by the pressure difference between the front and rear of the airfoil. Thicker airfoils and airfoils at high AoA tend to have higher pressure drag.
- Skin friction drag: Caused by shear stress between the moving air and the airfoil surface. This depends on surface roughness and whether the boundary layer is laminar (smooth, lower drag) or turbulent (mixed, higher drag).
Types of Airfoils in Aerospace
Symmetric airfoils (e.g., NACA 0012) have no camber, with equal curvature above and below the chord line. They produce zero lift at zero AoA, which makes them ideal for applications that need equal performance in both upright and inverted orientations. You'll find these on aerobatic aircraft and helicopter rotor blades (where the blade must produce lift in both directions during rotation).
Cambered airfoils (e.g., NACA 2412) have a curved mean camber line that generates lift even at zero AoA. This makes them more efficient for typical flight conditions, which is why they're the standard choice for fixed-wing aircraft.
Laminar flow airfoils (e.g., NACA 6-series) are shaped to keep the boundary layer laminar over a larger portion of the surface, reducing skin friction drag. The maximum thickness is positioned farther back along the chord than on conventional airfoils. These are used in high-performance gliders and some general aviation aircraft where efficiency is the priority.
Supercritical airfoils are designed for transonic flight (roughly Mach 0.7–1.2), where normal airfoils would produce strong shock waves on the upper surface. Their distinctive features include a flattened upper surface, a highly cambered aft section, and a larger leading-edge radius. These design choices delay the onset of wave drag by weakening or eliminating shock waves. Modern commercial aircraft like the Boeing 787 and Airbus A350 use supercritical airfoil designs on their wings.