why propellers on all these drones aren't shrouded? For VTOL/hovering machines the increased efficiency benefit usually more than covers the additional weight of the shroud/duct.
When I flew my parrot drone with the bumpers it was noticeably more sluggish, and a slight breeze would push it around. The instructions actually say to remove it for outdoors flight.
As far as I know that usually results in reduced efficiency.
it depends on speed. Before some speed - increase which diminishes with increasing speed and going into the loss after that. So the best is for static thrust - VTOL/hover mode. The other commenter gave the explanation i think i was asking about.