What force is causing the wind to move downwards at the top? There's something called "no-slip condition". Fluid flowing past a solid surface tends to stick to it. Hold your finger near water flowing from tap (flow should be very less), water sticks to your finger and runs down along it.

Similarly, in case of wing, air above it sticks to surface and flows along it till 'separation point'. The curvature of wings works as guide to air and throws it downward.

The understanding that "the wing is exerting a downwards force on the wind(above the wing)" in my opinion is due to ignorance of no slip condition.

"Bernoullis principle, viscosity, and pressure can all explain this, but how do we do this soley in terms of forces?"

In terms of forces: Air above the wing is deflected downwards by the wing. Hence by Newton's 3rd Law the wind must exert an equal and opposite force on the wing.

In terms of viscosity: You simply can't

In terms of pressure: The stream lines above the wing are being thrown downward the streamlines above them must move down to occupy vacant position, this causes a low pressure zone above the wing hence the lift

Bernoulli's principle: It's a myth, the assumption that fluid should flow faster above the wing to match the time required (to move pass the wing) by wind below the wing is invalid and lacks evidence. The correct way will be to use streamline curvature theorem. More about it

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