Comment by queuebert
1 day ago
As a physicist, it's not at all clear to me that tighter tolerances would lead to higher efficiency or less noise. I assume it shakes out in the CFD simulations, but I would be curious to know the explanation.
I thought the primary gain in efficiency came from the large blades, with the blade shape the next most important factor. Gaps between the blade and housing feels like a single-digit percent effect.
For the same reason a winglet is used on an airplane, or a ducted fan is more efficient than a propeller: there is a large pressure difference on the end of a wing or propeller, and the high pressure side will jump around to the low pressure side and cause a tip vortex. This causes an induced drag, which moves the lift vector backwards (as drag, but not skin friction drag). Higher aspect (think wind turbine blades or glider wings) minimise this, as do winglets or ducting.