What does spanwise flow refer to?

Spanwise flow refers to the movement of airflow that travels along the span of a wing, which is the distance from one wingtip to the other. This flow is typically radial in nature, moving from the root of the wing toward the tip. In the context of wing aerodynamics, understanding spanwise flow is crucial because it impacts lift distribution along the wing’s span and affects stall characteristics.

When airflow encounters a wing, it predominantly moves from the leading edge to the trailing edge, but variations in pressure and angle of attack can cause some of that airflow to also move sideways along the wing’s surface. This sideways deflection can lead to various aerodynamic phenomena, including the development of vortex systems that influence lift and drag.

In contrast, upward and downward deflection refers to the vertical component of airflow relative to the wing's position, while forward flow pertains to how air moves in the direction of flight. None of these address the aspect of lateral movement along the wing span, which is why spanwise flow is specifically defined as the sideways deflection of airflow around a wing.