I have been looking for solutions and come up with two popular explanations:

  1. Conservation of mass / Continuity

For a positive-cambered wing, the streamlines would be compressed/converged at the leading edge's upper part, forcing the airflow to accelerate in order to maintain the same mass flow rate at the entry and exit point.

  1. Air circulation

Air circulates around an airfoil in clockwise direction and hence accelerating the flow on the upper part.

Would like to ask:

  1. Is it appropriate to use conservation of mass / continuity to explain.
  2. What exactly causes circulation on airfoil?

Follow up on question 2, some explanations saying because of the sharp trailing edge, starting vortex is formed which forces the formation of bound vortex. If it is the case, how sharp an edge would lead to a starting vortex, and why it flows from the lower wing to upper wing?

Thanks for helping!


2 Answers 2


Static pressure differential is the root cause. Conservation of mass and circulation are mathematical models or descriptions of the end effect rather than physical causes.(Bernouelli's effect is similarly a description of behavior rather than a cause.)

Lower static pressure near the rear of the airfoil and higher pressure at the front is a force and that force accelerates the fluid's mass. More crudely put there is a modest hole in the air above the rear of an airfoil with some positive angle of attack, created by the airfoil deflecting flow down and so other air rushes in to fill the hole.


It may be easier conceptually to consider the movement of the wing as the driver of circulation through static air.

If you ever observed the transom of a moving boat, at slow speeds there is a boiling, turbulent flow with water still attached to the boat. This is fluid viscosity at work.

Now, speed up enough where the boat is going faster than the water can "fill in the hole" and ... a significant reduction in drag.

Wings work in a similar manner, they are literally moving faster than the air can equalize the lower pressure created by its "wake". The air returns to where the wing was resulting in downwash.

Also downwash is created by the bottom of the wing deflecting air downwards. The combination of top lift and bottom lift of the wing (and fuselage) create the necessary opposing force against gravity, and it is the top lift that is lost in the stall.

Conservation of mass is obvious, perhaps better explained as conservation of momentum mv, or by action - reaction. This is suited for air deflected from the bottom of the wing. Circulation, and creation of lower pressure above the wing, is more about what goes on in the wake of the airflow created by the Angle of Attack.


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