Airfoil

How wings generate lift

The shape of an airfoil forces air to travel different paths above and below. The curved upper surface accelerates the flow, while the lower surface slows it. By Bernoulli's principle, faster-moving air has lower pressure. This pressure difference — low above, high below — creates a net upward force: lift. But Bernoulli is only part of the story. The airfoil also deflects air downward (Newton's third law), and the circulation around the wing (Kutta condition) determines exactly how much lift is generated. The thin-airfoil result C_L = 2πα is a beautiful first approximation that captures the linear relationship between angle of attack and lift.

Angle of attack and stall

The angle of attack (α) is the angle between the wing chord line and the oncoming airflow. As α increases, lift increases — up to a point. Beyond the critical angle (typically 12–18° for most airfoils), the boundary layer on the upper surface can no longer follow the sharp curve. The flow separates: smooth streamlines break apart into turbulent eddies. Lift drops suddenly and drag spikes. This is stall — the most important phenomenon in aerodynamics to understand, because it sets the minimum speed at which an aircraft can fly.

Pressure distribution

The pressure around an airfoil varies continuously. On the upper surface, pressure drops to a minimum near the leading edge (the suction peak) and then recovers toward the trailing edge. This adverse pressure gradient — pressure increasing in the flow direction — is what eventually causes boundary layer separation and stall. The lower surface maintains higher pressure. The integral of this pressure distribution around the entire airfoil gives the total aerodynamic force, which decomposes into lift (perpendicular to the flow) and drag (parallel). The pressure view in this simulation shows low pressure in blue and high pressure in red.