The Code is the Design

  The Bernoulli Explanation - Another unnecessarily complicated and possibly wrong explanation taught in academia.  The Newtonian Explanation - A simple and intuitive explanation. An airflow study will probably show a net movement of air downwards behind the wing. The video above shows the NASA link, and speaks of the airflow direction change as well.

      This is just very interesting physics. The moving sand in the initial stage has the same effect as the net mass moving downwards. So the hourglass doesn't start moving up until half the sand has moved down. From another perspective, the force of the falling sand counteracts the force of buoyancy in the initial stage. But as the volume of falling sand reduces, the force too reduces and the hourglass starts moving up. Quoting from the  American Journal of Physics 85, 98 (2017);       Weight of an hourglass—Theory and experiment in quantitative comparison   "A flowing hourglass changes its weight in the course of time because of the accelerated motion of its center of mass. While this insight is not new, it is frequently said that the effect is tiny and hardly measurable. Here, we present a simple experiment that allows the monitoring of weight as a function of time, and that shows that there are different stages of the weight variati...