Has anybody done testing on delivered CFM of ceiling fans? Energy star has a test room where the blades are 40 5/8" from the ceiling, but this rarely happens in living rooms. Perhaps manufacturers should list actual CFM output at various mounting heights instead of the laboratory 40 5/8" that almost never exists in the real world.
How is CFM affected when installed at the default 12" from the ceiling? When used with taller ceilings, does installing a 12", 18", or even a 24" downrod significantly affect delivered CFM?
How does blade diameter come into play? Are smaller fans "more tolerant" of being mounted closer to the ceiling? Is a 42" fan a better choice than a 60" for a typical 8' ceiling due to limited space above the blades?
Energy Star Test Room
Interesting question - the only item I know of is the minimum 8" for proper air flow (or 25% reduction - no source on this being tested but seems to be industry thought) --- some say it should only be 8' off the ground at max while others argue 9' to 11' with better occupant comfort being better.
As for tolerant - a lot comes down to design, but I would give the nod to larger ones designed to be mounted close to ceiling - size is based more on room size / movement needed
Take a little smoke and see the throw. Try to make air visible, with RPM and throw, can tell CFM.