I'm doing a study comparing the classical anemometer (the "3 cups model") and the Rotavecta system.

I'd like to put in equation their movement to show that the Rotavecta system doesn't work in light wind.

II don't know how to express the couple exerted by the wind because it seems hard for me to express the effort of pressure exerted by the wind of each cup, especially in the case of the Rotavecta system where one of the 3 cups is taller than the other.

Does someone can explain me how I could do such a mechanical study?