Airfoil Shape and Lift

Airfoil Shape and Lift Explained

The shape of an airfoil—the cross-sectional profile of a wing—directly influences how lift is generated. Key parameters include the camber (curvature) and thickness.

Camber and Lift Generation

• Symmetrical Airfoils: These have identical upper and lower surfaces. At zero angle of attack, they produce no lift because the airflow and pressure distribution are the same above and below the wing. Lift only develops as the angle of attack increases.
• Positively Cambered Airfoils: These have a curved upper surface and a flatter lower surface. They generate lift even at zero angle of attack due to the increased airflow speed (and thus reduced pressure) over the upper surface. For a typical cambered airfoil, zero lift occurs at a small negative angle of attack (often around -4°).
• Negatively Cambered Airfoils: These are rare in main wings but may be used for control surfaces. They require a positive angle of attack to reach zero lift.

CL-Alpha Graph and Key Points

• The CL-alpha graph shows how the coefficient of lift (CL) varies with angle of attack (alpha).
• For cambered airfoils, the curve crosses the horizontal axis (zero lift) at a negative angle of attack; for symmetrical airfoils, this point is at zero.
• The maximum lift coefficient (CLmax) is reached just before stall, at a critical angle of attack.

Thin Aerofoils and Reduced Camber

• Thin airfoils with less camber are often used for high-speed aircraft to reduce drag, though they may produce less lift at lower speeds.

Spanwise Lift Distribution and Wing Planform

• The way lift is distributed along the wing (spanwise) depends on both the airfoil shape and the overall planform (wing outline). Elliptical lift distribution is most efficient, but practical designs balance efficiency with structural and operational considerations.

Camber Line

• The camber line is an imaginary curve equidistant from the upper and lower surfaces of the airfoil. Its shape defines the degree and type of camber, influencing lift characteristics.

Operational Relevance

• Pilots must understand how different airfoil shapes affect lift, especially during takeoff, landing, and maneuvering, to avoid stalls and optimize performance.