What force causes the wind to move downwards at the top of a wing?
The phenomenon known as the "no-slip condition" explains that a fluid flowing past a solid surface tends to adhere to it. For instance, if you hold your finger under a gently running tap, the water will cling to your finger and flow down along it.
Similarly, for an airplane wing, the air above it adheres to the surface and follows its contour until the separation point. The wing's curvature acts as a guide, directing the airflow downwards.
The misconception that "the wing exerts a downward force on the wind above it" likely stems from a misunderstanding of the no-slip condition.
How can we explain this solely in terms of forces?
In terms of forces, the air above the wing is deflected downwards by the wing. Consequently, by Newton's third law, the wind must exert an equal and opposite force on the wing.
In terms of viscosity: Viscosity alone cannot explain this phenomenon.
In terms of pressure: As the air streamlines above the wing are directed downward, the streamlines above must descend to fill the vacated space, resulting in a low-pressure zone above the wing and thus generating lift.
Regarding Bernoulli's principle: The assumption that fluid must flow faster over the wing to match the transit time of the wind below is a misconception. Instead, the streamline curvature theorem provides a more accurate explanation of aerodynamic lift. More