Airfoil

An airfoil is a streamlined body designed to generate significantly more lift than drag. Lift is the upward aerodynamic force that acts perpendicular to the direction of the moving fluid. Drag is the force that acts parallel to the fluid flow, pulling against the object.

These shapes are essential for flight and movement through fluids. Examples include airplane wings, helicopter rotor blades, and propeller blades. Sails on boats also function as airfoils. Even when submerged in water, similar shapes are called hydrofoils.

This principle extends to nature, where bird wings, fish bodies, and even sand dollars utilize these efficient shapes.

The mechanism of an airfoil relies on how it deflects passing fluid. When a solid body moves through a fluid at a suitable angle, it deflects the fluid downward. This deflection creates a reaction force on the airfoil in the opposite direction. This upward component is the lift.

This process also creates a lower-pressure region above and behind the airfoil. According to Bernoulli's principle, this pressure difference is accompanied by a velocity difference. The air moves with a higher average velocity on the upper surface than on the lower surface. This difference in pressure and velocity is what drives the lift force.

Airfoils are categorized by their specific geometric shapes and intended flight regimes. A symmetric airfoil has identical upper and lower surfaces. These are often used in aerobatic airplanes to allow for frequent inverted flight. In contrast, cambered airfoils have a curved shape. A cambered airfoil can generate lift even at a zero angle of attack.

The shape also changes based on speed. Subsonic airfoils, used for slower flight, typically have a rounded leading edge. This design makes them less sensitive to the angle of attack. Supersonic airfoils, used for very fast flight, are much slimmer and more angular. They often feature sharp leading edges that are very sensitive to the angle of attack.

Understanding the geometry of an airfoil requires specific technical terms. The leading edge is the front point with maximum curvature. The trailing edge is the point furthest from the front. The chord line is the straight line connecting these two points. The chord length is the measurement of this line.

The mean camber line is a line representing the points midway between the upper and lower surfaces. Thickness can be measured perpendicular to this camber line or the chord line. Engineers also track the aerodynamic center and the center of pressure. The center of pressure is the location where the pitching moment is momentarily zero. On cambered airfoils, this location moves as the angle of attack changes.

History shows how mathematical theories helped refine these designs. In the 1920s, German mathematician Max Munk devised thin airfoil theory. This theory relates the angle of attack to lift for incompressible, inviscid flows. British aerodynamicist Hermann Glauert later refined these ideas. In 1945, Richard von Mises noted that wings must have a sharp trailing edge to obtain well-defined lift.

More recently, NASA conducted vital research during the 1970s and 1980s. This research proved that laminar flow wing designs were practical for modern aircraft. Before this, surface imperfections made such designs difficult to use. New manufacturing methods using machined metal and composites eventually made these efficient wings possible.

Efficiency in an airfoil is often measured by its lift and drag curves. As the angle of attack increases, lift generally increases in a linear relation. However, if the angle becomes too steep, the airfoil may stall. At about 18 degrees in some models, the lift falls off quickly. This happens because the upper-surface boundary layer separates and thickens. This thickened layer changes the effective shape of the airfoil. It reduces the circulation and the lift while causing a sharp increase in pressure drag.

Modern aeronautical engineering uses airfoils in many complex systems. Designers use computer programs to create specific shapes for different functions. For example, a supercritical airfoil has its maximum thickness near the leading edge. This helps manage supersonic flow in transonic aircraft. Some wings even use different airfoil sections along their entire span to optimize performance.

High-lift devices, such as flaps and slats, are also attached to airfoils to assist during takeoff and landing. These technologies allow modern aircraft to operate across a wide range of speeds and conditions.