foils

A foil is a surface shaped to interact with a moving fluid to produce lift or reduce drag. The term covers both airfoils, which operate in air, and hydrofoils, which operate in water. A foil’s effectiveness arises from differences in pressure created by the flow over and under its curved cross-section as the fluid moves past it. Key factors include angle of attack, camber (the curvature), thickness distribution, and the Reynolds number, which depends on speed, size, and fluid properties.

Airfoils are the primary lifting surfaces of aircraft, propellers, and wind turbine blades. They generate lift when air flows faster over the curved upper surface than along the flatter lower surface, creating lower pressure above the foil. Designers optimize lift-to-drag ratio by shaping camber and thickness, selecting appropriate aspect ratios, and managing wingtip vortices to reduce induced drag. Materials, surface finish, and manufacturing tolerances also affect performance.

Hydrofoils are submerged foils mounted to boats or ships. As speed increases, the foils generate lift, partially raising the hull out of the water and markedly reducing wetted area and drag. Hydrodynamic lift must be balanced against structural loads and potential cavitation. Modern hydrofoils support high-speed craft and improve ride stability in waves, with ongoing developments in materials and foil control systems.

Other foils include wing foils and turbine blades, which rely on airfoil principles to convert fluid flow into thrust or energy. The field combines aerodynamic or hydrodynamic theory with computational methods and experimental testing to optimize efficiency, stability, and safety across diverse applications.