flat ski convex undersurface battons for hangliders for convex undersurface so they wont spin under load
https://youtu.be/eml-IM4H2cQ
<iframe width="560" height="315" src="https://www.youtube.com/embed/eml-IM4H2cQ" frameborder="0" allowfullscreen></iframe>
https://youtu.be/eml-IM4H2cQ
<iframe width="560" height="315" src="https://www.youtube.com/embed/eml-IM4H2cQ" frameborder="0" allowfullscreen></iframe>