from the side view here https://www.tandfonline.com/doi/full/10.1080/22797254.2018.1537684 https://www.tandfonline.com/doi/full/10.1080/22797254.2018.1... It's

from the side view here https://www.tandfonline.com/doi/full/10.1080/22797254.2018.1537684 https://www.tandfonline.com/doi/full/10.1080/22797254.2018.1... It's clear they follow the "funnel" shape. Optimized for laminar (air) flow. Not for hauling (which you'd want a fixed slope for.