Direction of the grooves are irrelevant.
The direction of the internal vanes are what matters. On the assumption the vanes are not radial/straight, then they should be facing so they are swept backward relation to the direction of movement.
.... see this link:
https://passionford.com/forum/genera...direction.html
Last edited by Isaac.Hunt; Mar 10, 2011 at 12:57 PM.
Reason: ugh ... can't spell.